Embryology (Generalized)

EARLY EMBRYOLOGICAL EVENTS Time 30 hrs after fertilization

Events Zygote undergoes first cleavage to become a 2 cell embryo. Later, with further cell divisions, forming 4-, 8-cell pre-embryo.

Day 4 (72 – 80 hours after fertilization)

12 – 18 cell stage; Enters uterine cavity; Compaction forming inner blastomeres and outer blastomeres; After compaction the pre-embryo can now be referred to as a morula. Note: Compaction occurs around the time where the preembryo enters the uterine cavity. Whether compaction completes before or after entering uterine cavity is debatable. Note: Compaction is the process where two populations of cells are generated, the inner blastomeres and outer blastomeres. At this stage, cell fate is being specified but still reversible, that is, cell fate is not determined yet.

Day 5 (96 hours after fertilization)

32 – 64 cell stage; Cavitation occurs (after compaction and entry into uterine cavity); After cavitation, the pre-embryo can now be referred to as a blastocyst. During cavitation, the inner and outer blastomeres differentiate to form the embryoblast (inner cell mass) and the trophoblast respectively. Cell fate is now determined, that is, not reversible.

Day 6 (120 hours after fertilization)

Hatching (the process where blastocyst loses its zona pellucida) begins.

Day 7 (150 hours after fertilization)

Implantation begins; As soon as blastocyst attaches to the endometrium epithelium: 

trophoblast differentiates into an inner layer of cytotrophoblast and an outer layer of syntiotrophoblast.

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embryoblast (inner cell mass) differentiates into epiblast and hypoblast. This forms the bilaminar disk.

Day 8 (168 hours after fertilization)

Amniotic cavity is formed: On the 8th day, small cavities begin to appear between epiblast and cytotrophoblast. These cavities fuse to form a single amniotic cavity. Cytotrophoblast then develops a layer of cells called amnioblasts which forms the roof of the amniotic cavity known as amnion, while the floor is formed by epiblast. The amniotic cavity is formed very quickly, all these are completed on the same day. Also, on the 8th day, hypoblast cells begins to migrate over the inner surface of the cytotrophoblast. Also, on the day 8, lacunae begin to appear in the syntiotrophoblast.

Day 9

Implantation is completed; A coagulation plug is formed to close off the surface epithelium. Also on day 9, adjacent lacunae fuse to form a lacunar network (there is still no direct contact with the enlarged maternal blood vessels at this stage).

Day 10

On day 10, the hypoblast cells have now fully extended around the circumference of the blastocyst cavity, forming a thin layer of membrane (Heuser’s membrane aka exocoelomic membrane) that lines the inner surface of the cytotrophoblast. The blastocyst cavity is now called the primary yolk sac (aka exocoelomic cavity). Also, on day 10, maternal blood begins to enter the lacunar network.

Day 12

Also, on day 10, extraembryonic mesoderm of uncertain origin appears between the inner surface of the cytotrophoblast and the outer surface of the amnion and Heuser’s membrane. By day 12, an almost completely regenerated uterine epithelium covers the closing plug. On the day 12, cavities appear in the extra-embryonic mesoderm (except for a mesodermal stalk that connects the amnion to the trophoblast), these cavities rapidly then fuse to form a continuous crescrent shaped extraembryonic colelom, aka chorionic cavity (still on day 12). The presence of extraembryonic coelom splits the extraembryonic mesoderm into two layers, the splanchnic mesoderm and somatic mesoderm. Note: The extraembryonic coelom (aka chorionic cavity) is filled with fluids.

Late Day 12

A new wave of hypoblast cell proliferation begins to produce a new membrane that migrates out over the inside of the extraembryonic mesoderm, pushing the primary yolk sac in front of it. This new layer of hypoblast cells will later (shortly) become the lining of the definite (secondary) yolk sac. At the same time (on day 12), the chorionic cavity enlarges. As the chorionic cavity continues to increase in size on day 12, it begins to push on (pinching) the primary yolk sac. Note: the chorion is formed by the extraembryonic mesoderm and two layers of trophoblast. It surrounds the embryo and other membranes.

Day 13

Eventually pinching it into two separate cavities by early day 13. On day 13 after fertilization, cells of the cytotrophoblast proliferate locally and penetrate into the synctiotrophoblast, forming cellular columns of cytotrophoblast surrounded by synctiotrophoblast. These cellular columns within the syncytium are known as primary villi.

Day 14

On day 14, first evidence of an embryonic axis, the primitive streak appears, with the primitive node being locate at is caudal end.

Day 15 - 16

Gastrulation occurs: Epiblast cells near the primitive streak proliferates and migrates through the primitive streak. The first cells will begin to replace the hypoblast to later form the endoderm. The remaining cells form intra-embryonic mesoderm between the other two layers and produce two midline structures between the epiblast and the future endoderm: the prechordal plate and the notochordal process.

Day 17

Trimlaminar disc is formed:  Ectoderm  Mesoderm  Endoderm Notochord is formed. The precordal plate is anchored into the endoderm and is located cephalad to the notochordal process but its fate remain uncertain. Note: The mesoderm separates the ectoderm from the endoderm everywhere except at the site of the future mouth (oropharyngeal membrane) and the site of the future anus (cloacal membrane). Here, the ectoderm remains in contact with the endoderm, but will eventually rupture to form the oral and anal openings.

Day 18

Neural plate is formed: Neuroectoderm (the part of the ectoderm over the notochord which receives signals from the notochord) thickens to form the neural plate.

Pre-embryo: In humans, pre-embryo refers to the product of gametic union after the completion of fertilization to the completion of implantation about the end of the second week (more strictly speaking and correct: the appearance of the primitive streak). The preembryonic forms ranges from the zygote, morula, blastocyst, bilaminar embryonic disc to the appearance of the primitive streak. That is, the term pre-embryo is used to refer to the first three major stages of development: the earliest stages of cell division (the so called cleavage stages), the blastocyst (when the pre-embryo is a hollow ball of cells) and the stages of implantation up to the formation of the primitive streak. Embryo: In humans, embryo refers to the product of gametic union after the completion of implantation about the end of the second week (more strictly speaking and correct: the appearance of the primitive streak) to the completion of organogenesis (about the end of the eighth week from gametic union). That is, after the process of implantation is completed, we have an embryo. Fetus: In human, fetus refers to the product of gametic union after completion of organogenesis (start of the ninth week) to birth. That is, completion of organogenesis is used to signal the transition from embryo to fetus. In other words, the accomplishment of organogenesis ends the period during which the developing organism is called an embryo and begins the period in which the organism is called a fetus. In the fetal stage, all organs (except reproductive system) do not undergo new formation, they simply undergo fine differentiation and rapid cell growth. Newborn (aka neonate): In medical contexts, newborn or neonate (from Latin,neonatus, newborn) refers to an infant in the first 28 days after birth; Infant: young children between the ages 1 month and 12 months; Toddler: when a human child learns to walk.

EMBRYOLOGY OF THE CARDIOVASCULAR SYSTEM The development of the cardiovascular system (the heart and the vascular system) begins in the middle of the 3rd week. This is because in the middle of the third week, the embryo is no longer able to meet its nutritional requirements by diffusion alone. And by the end of the 8th week, the basic structure of the cardiovascular system is in place. That is, by the end of the 8th week, the cardiovascular system possess the definitive structures of that in the adult with only minor changes occurring after this time.

In the middle of week 3 [day 17 and day 18], blood islands appears extraembryonically and intraembryonically as red spots in 3 locations, with extraembryonic vasculogenesis slightly preceding intraembryonic vasculogenesis.

Blood islands: aggregations of blood and/or vessel forming cells. Extraembryonically: outside the embryo

ExtraembryoniImpcally, blood islands appear in the:  Mesoderm of the yolk sac [day 17]  Mesoderm of the connecting stalk and the chorion [day 18] Intraembryonically, blood islands appear in the:  Mesoderm of the embryonic disc [day 18]

Intraembryonically: within the embryo Vasculogenesis: the de novo formation of blood vessels, responsible for the first vessels in life. Note: Blood islands form from mesenchymal cells at the same time other mesoderm is forming.

On day 17-18, the mesoderm layer differentiates into paraxial mesoderm, intermediate mesoderm and lateral plate mesoderm.

In the 3rd week, shortly after the appearance of blood islands, the blood islands begin to develop into blood vessels of the primitive cardiovascular system. Blood vessels formed include:  A pair of vitelline veins that return blood from the yolk sac  A pair of umbilical veins that return blood from the chorion via the connecting stalk  Chorionic vessels  Paired endocardial tubes  Paired dorsal aortae

Note: The paired dorsal aorta give rise to a pair of umbilical arteries and a series of paired vitelline arteries. Note: The vessels developing in the connecting stalk and chorion are called umbilical vessels and chorionic vessels respectively. Note: primitive vitelline and umbilical veins will grow into the cranial end of the embryonic disc. Note: The order in which the blood vessels appear is likely to be vitelline veins < umbilical veins < endocardial tube ~ dorsal aortae < vitelline arteries ~ umbilical arteries ~ intersegmental branches ~ lateral branches (SEE LATER)

In particular the endocardial tubes and dorsal aortae are formed on day 19.

Note: The paired dorsal aortae develops almost simultaneously as the paired endocardial tubes.

On day 18-19, coelomic spaces/intercellular spaces form in the lateral plate mesoderm, thus dividing the lateral plate mesoderm.

On day 20, paired dorsal aortae connect with the rostral ends of the paired endocardial tubes. Shortly after, the connection of dorsal aortae with the endocardial tubes (still on day 20), embryonic folding begins, bringing the bilateral endocardial tubes toward the ventral midline of the embryo. On day 20, the septum transversum also begins to form. On day 20, coelomic spaces in the lateral plate mesoderm merges to form the intraembryonic coelom.

Note: connection of dorsal aortae with endocardial tubes occur prior to embryonic folding.

A number of events happen by the end of the third week [day 21]: 

Embryonic folding which began on day 20 continues to fold on day 21 (folding is complete by day 28). The embryo folds longitudinally so that the heart ends up in the future thorax region. While lateral folding brings the endocardial tubes together in the midline.

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Septum transversum has been formed on day 21.

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By day 21, the paired endocardial tubes, the paired dorsal aortae and blood vessels in the connecting stalk, chorion, and yolk sac are linked up, forming a primitive cardiovascular system. In general, new vessels that will arise later, do so as outgrowths of these existing vessels.

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Endocardial tubes begins to fuse on day 21 (fusion is completed on day 22).

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On day 21 – 22, when the dorsal aortae are still paired, three sets of arterial branches begin to sprout from the paired dorsal aortae: o Ventral branches: including the series of paired vitelline arteries and a pair of umbilical arteries o Dorsolateral branches (aka intersegmental branches): a series of paired arteries arising from the postero-lateral surface of the dorsal aortae. o Lateral branches: a series of paired arteries

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A series of constrictions and dilations appears. These will later become truncus arteriosus, bulbus cordis, primitive ventricle, primitive atria, sinus venosus (after completion of fusion) on day 22.

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The primitive heart may begin to beat on day 21 - 22. This may occur even before the heart tubes are completely fused together.

Note: The primitive vascular system establishes a functional fetal-placental vascular connection. That is, a blood vascular system between the embryo and placenta is established. It is no coincidence that the onset of organogenesis of most organ systems occurs in week 4. Note: Once the system consisting of the endocardial tubes, dorsal aortae, blood vessels of the vitelline system and umbilical system is in place and connected with each other by day 21, any new vessels that arise do so as outgrowths of existing vessels. Note: The two endocardial tubes are forced into the thoracic region due to cephalic and lateral foldings of the embryo where they later (on day 22) fuse together to form a single tube called the primitive heart tube. Note: The inflow and outflow vessels make connection with the endocardial tubes even before the tubes are translocated into the thorax. Note: The series of paired vitelline arteries and the paired umbilical arteries branch off the paired dorsal aortae (that is, arise from the initially paired dorsal aortae). Note: Early vitelline arteries are a series of paired vessels supplying the yolk sac. That is, they are initially paired vessels but will later fuse. Note: The heart starts to beat on day 22, but the circulation does not start until day 27 to day 29. Note: The vitelline veins enter the body of the embryo via the yolk sac stalk, form an anastomotic network around the foregut of the digestive tract, and then enter the septum transversum, which they cross on their way to the sinus venosus.

Note: Prior to looping the heart tubes forms contrictions and dilation which will later become separate components of the heart.

 Neural tube is formed on day 21.  Foregut and hindgut has been formed by day 21 – 22.

Note: During longitudinal folding, a part of the yolk sac (covered by endoderm) is incorporated into the embryo as foregut and hindgut. Note: Foregut and hindgut will be elongated by day 25.

On day 22, fusion of the endocardial tubes complete (thus forming the primitive heart tube). The series of constrictions and dilations now (day 22) form the truncus arteriosus, bulbus cordis, primitive ventricle, primitive atria, sinus venosus.

 Endodermal lining on the ventral side of the caudal foregut differentiates into hepatoblast and begins to proliferate to form a thickening called the hepatic plate on day 22.

A number of important events occur in week 4 (on or prior to day 26): 

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The anterior cardinal veins and posterior cardinal veins appear on about day 22 – day 24. The anterior cardinal veins and the posterior cardinal veins join to form the left and right common cardinal veins that drain into the sinus venosus around day 24 – day 26 (cardinal system is thus formed). The paired dorsal aorta begins migrate toward each other to fuse on day 23 - 25. And on day 25 28, the paired dorsal aorta have fused together caudally from the tenth to sixteenth somite, forming in the region of fusion a single dorsal aorta (median dorsal aorta), which will later become the descending aorta of the adult. As the left and right dorsal aortas begins to fuse, the paired vitelline arteries will begin to fuse as well. By day 25 - 28, the paired vitelline arteries have fused and become unpaired. Most vitelline arteries will begin to degenerate over time. Around day 35 (near the end of the fifth week), only 3 unpaired vitelline arteries persists and they are called: the celiac trunk (to the foregut), the superior mesenteric artery (to the midgut) and the inferior mesenteric artery (to the hindgut).

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Meanwhile (around the time where the aortic arches are reforming at the cephalad end which is also around the time where the vitelline arteries are remodelling), lateral arterial branches and intersegmental arteries (dorsolateral arterial branches) that were formed previously are being further developed.

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Looping and elongation of the primitive heart tube begins on day 23 (and completes on day 28).

 The connection between the dorsal aortae and the paired endocardial tubes are being pulled during embryonic folding until they form a loop called the first aortic arch on day 24 (3mm). In other words, the first pair of aortic arches has formed on day 24 (3mm).

Note: The anterior cardinal vein appear slightly earlier than the posterior cardinal veins. Note: The anterior cardinal veins make their appearance shortly after the umbilical and the vitelline veins on about day 22 to day 24. Soon after the anterior cardinal veins, the posterior cardinal veins appear. Note: On day 22, thee dorsal aortae are still paired. Note: Lateral arterial branches supply the retroperitoneal structures. Note: Dorsolateral branches (intersegmental arteries) supply the head, neck, body wall limbs, and vertebral column. Note: The dorsal aorta issues about 30 pairs of intersegmental arteries. Most of these degenerate, but the adult intercostal arteries, lumbar arteries, and common iliac arteries are remnants of some of the embryonic intersegmental arteries. Note: The anterior division of the primitive carotid artery will later develop into anterior cerebral, middle cerebral and anterior choroidal arteries. Note: The posterior division gives origin to the posterior communicating and posterior cerebral arteries.

 The second pair of aortic arches begins to form on day 24 (3mm).  The primitive internal carotid artery appear around day 24 to day 26. And by day 26 to day 28, the primitive internal carotid artery begins to present an anterior division and a posterior division.

Note: The three vitelline segments that persist are the 10th, 13th and 21st. Note: By the end of day 26, the basic fetal vascular system is almost complete.

 On day 24, midgut is now clearly distinguished.  On day 24 – 25, hepatoblast of the hepatic plate have now proliferated enough such that the BM is penetrated. The hepatic plate is now called the liver bud or hepatic diverticulum.

IMPORTANT EVENTS OCCURING ON OR AFTER DAY 26 Folding (Continue) On day 26, folding has now brought the left and right intraemrbyonic coelom next to each other, separated only by the ventral mesentery.

Note: In order of appearance, we have anterior cardinal veins < posterior cardinal veins < common cardinal veins < subcardinal veins < supracardinal veins.

On day 28, folding completes. Also on day28, the ventral mesentery degenerates resulting the merging of right and left parts of the intraembryonic coelom (body wall closure). The midgut is now suspended from the abdominal wall only by the dorsal mesentery. Note that the midgut is continuous with the yolk sac via the vitelline duct.

Aortic Arch Development On day 26 (3-4mm), the second pair of aortic arches has formed and the third pair of aortic arches begins to regress. On day 28 (4mm), the third pair of aortic arches has formed, the fourth pair of aortic arches begins to form and the first pair of aortic arches begins to regress. On day 29 (5mm), the fourth pair of aortic arches has formed, the sixth pair of aortic arches begins to form and the second pair of aortic arches begins to regress. On day 29 - 30, both the first & second pairs of aortic arches have regressed. On day 32 (6mm), the sixth pair of aortic arches has formed. By day 35, the third, fourth and sixth arches are well developed. Carotid System and Vertebrobasilar System Development 

On day 24, the cervical portions of the internal carotid arteries (aka primitive internal carotid artery) have emerged from the first aortic arch.

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On day 28, intracranial portions of the internal carotid arteries have developed by the extension of the dorsal aortae. That is, the dorsal aortae have extended cranially to form the intracranial portions of the primitive internal carotid arteries.

Note: The inferior vena cava passes nearly vertically upward in a groove on the posterior surface of the liver.

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On day 28, a pair of longitudinal neural arteries have developed from the primitive vascular plexus on the surface of the neural tube. The pair of longitudinal neural arteries are located dorsal and parallel to the internal carotid artery. Note: The longitudinal neural arteries are initially supplied from below by the dorsal aortae via the transient cervical intersegmental arteries (first intersegmental cervical C1 artery?). Note: first intersegmental cervical C1 artery is also known as the proatlantal intersegmental artery. On day 28 – 29, other transient connections between each longitudinal neural artery and its corresponding carotid artery also develop. These are the primitive trigeminal arteries, the primitive otic arteries, and the primitive hypoglossal arteries.

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On day 29 – 30, recall that both the first and second aortic arches have regressed. Thus the primitive internal carotid arteries now appear as extensions of the third aortic arch.

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On day 29 – 30, shortly after the involution of the first and second aortic arches, the rostral (distal) end of the internal carotid arteries have developed cranial (rostral) and caudal divisions.

Note: After the hepatic portion of the inferior vena cava develops from the vitelline veins, the supracardinal veins then develop whereas the postcardinal veins begin to atrophy. Note: The supracardinal vein largely take over the function of the posterior cardinal vein in draining the posterior wall.

Note: The cranial division initially terminates as olfactory arteries whereas the caudal division initially forms an anastomosis (second temporary connection) between the internal carotid arteries and the dorsal longitudinal neural arteries. The portion of the anastomosis that joins to the cranial end of the dorsal longitudinal artery will later give rise to the posterior communicating arteries. The remaining portion of the anastomosis typically later regresses and disappears. 

On day 31 – 32, primitive maxillary arteries and primitive dorsal opthalamic arteries have developed as branches of the internal carotid arteries.

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On day 31 – 32, ventral pharyngeal arteries have developed as extensions of the third aortic arch.

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On day 31 – 32, posterior communicating arteries (PComA) have developed.

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On day 31 – 32, subsequent to the development of the posterior communicating arteries, the primitive connections (primitive trigeminal arteries, the primitive otic arteries, and the primitive hypoglossal arteries) between the internal carotid arteries and the longitudinal neural arteries begin to regress. The otic artery regresses first, followed by the hypoglossal artery, and lastly the trigeminal artery. These primitive connections have regressed in the period between day 32 – day 35. Note: Blood in this period reaches the longitudinal neural arteries via the posterior communicating arteries and the proatlantal intersegmental arteries. The proatlantal intersegmental artery becomes the major blood supply to the posterior circulation until the definitive vertebral arteries form.

Note: Septum primum originates from the superoposterior wall (that is, it originates superoposteriorly). It grows in the inferoanterior direction towards the endocardial cushions with a concave leading edge. Foramen primum: is the gap between the free margin of the septum primum and the endocardial cushions. Note: The foramen primum becomes progressively smaller and finally disappears as the septum primum completely fuses with the endocardial cushions.

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On day 31 – 32, longitudinal neural arteries begin to coalesce in the midline.

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On day 32 – 33, vertebral arteries begin to form from longitudinal anastomoses of the cervical intersegmental arteries.

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On day 33, the longitudinal neural arteries have joined to form a single median artery called the basilar artery. Note: the caudal region remains plexiform and is later replaced by the inferior cerebellar arteries.

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On day 35 – 36, vertebral arteries have formed from the longitudinal anastomoses of the cervical intersegmental arteries. While the portions of the first six cervical intersegmental arteries, that are connected to the dorsal aortae, begin to regress. In particular, the proatlantal intersegemental arteries have regressed around day 36 – 37. Note: The portion of the proatlantal intersegmental artery that remain contributes to the horizontal segment (V3) of the vertebral artery. On day 35 – 36, the common carotid arteries begin to form as the segments of dorsal aortae between 3rd and 4th aortic arches (known as the carotid ducts) begin to regress. On day 37 – 38, anterior cerebral arteries, middle cerebral arteries, posterior cerebral arteries and anterior choroidal arteries have formed (begins to form on day 33 - 35??). Note: The anterior cerebral arteries, middle cerebral arteries and anterior choroidal arteries develop from the rostral/cranial division of the internal carotid arteries. On the other hand, the posterior cerebral arteries arise from the basilar arteries. On day 38 - 39, the ventral pharyngeal arteries have become the external carotid arteries.

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On day 38 – 39, the distal part of the right sixth arch begins to disappear.

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Around day 40, the vertebral arteries finish their maturation. The remnants of the portions of the first six cervical intersegmental arteries, that are connected to the dorsal aortae, are barely visible by this time.

Note: Septum secundum originates in the superoanterior wall (that is, it originates superoanteriorly). It grows in the inferoposterior direction with a concave leading edge (free margin). It grows far enough to cover the ostium secundum but does not become a complete partition (its growth gradually ceases). This leaves a characteristic oval opening called the foramen ovale. Foramen ovale: The process of the development of the septum secundum leaves a foramen at about the midpoint of the septum secundum, this foramen is known as the foramen ovale. That is, the foramen ovale is the opening between the upper and lower limbs of the septum secundum when it is fully developed. Note: Perforations in the septum primum are produced by programmed cell death and appears in the central part of the septum primum. Note: At no time during fetal development are the right and left atria ever completely divided. This ensures passage of maternally oxygenated blood acroos the atria to the fetal systemic circulation. Note: The septum secundum arises from the roof of the atrium on the right side of the septum primum. Note: The septum secundum covers the foramen secundum but forms an incomplete atrial partition.

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On day 42, the segments of dorsal aortae between the 3rd and 4th aortic arches becomes much thinner.

Note: Closure of the foramen primum occurs when the free edge of the septum primum fuses with the endocardial cushions.

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On day 42, the distal part of the right sixth aortic arch has disappeared while the proximal part has formed the right pulmonary artery. That is, the right sixth aortic arch has lost its connection with the right dorsal aorta.

Aorticopulmonary septum: the portion of the conotruncal septum that divides the truncus arteriosus into ascending aorta and pulmonary trunk.

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On day 42 – 44, plexiform anastomoses have developed across the midline that join the anterior cerebral arteries. This will later give rise to the anterior communicating artery.

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On day 44, the segments of dorsal aortae between the 3rd and 4th aortic arches have disappeared. The common carotid arteries now exist as stems of the internal and external carotid arteries.

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On day 45 - 46, the distal portion (the segment between the origin of the seventh intersegmental artery and the junction with the left dorsal aorta) of the right dorsal aorta begins to disappear. Note: the proximal portion will later remains as part of the right subclavian artery.

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Around day 48 - 49, the vertebral arteries have shifted to its adult position. The cervical intersegmental arteries are obliterated, with the exception of the 7th inter-segmental artery, which gives rise to the subclavian artery.

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Around day 48 – 49, external carotid continues to elaborate with branches for thyroid, lingual, occipital and external maxillary arteries. The more proximal portion of this system displays the primordial internal maxillary artery which will eventually become the middle meningeal artery.

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On day 49 – 51, anterior communicating arteries have developed and the circle of Willis is formed. Note: the Circle of Willis is completed with the formation of the anterior communicating artery.

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On day 51, the distal portion (the segment between the origin of the seventh intersegmental artery and the junction with the left dorsal aorta) of the right dorsal aorta have disappeared. The proximal portion has formed part of the right subclavian artery.

NOTE The basilar system, including stems of the anterior inferior cerebellar arteries and posterior inferior cerebellar arteries with small vessels extending into the developing choroid plexus of the fourth ventricle, can be visible by CS 18 – 19 although I cannot find conclusive information on exactly when and in what order these arteries are formed. Some clues include: (i) At day 35, the only blood supply to the cerebellum is from the superior cerebellar arteries. It is not until day 44 that the PICA (posterior inferior cerebellar arteries) become apparent. (ii) The anterior inferior cerebellar arteries arise just after the longitudinal neural arteries have united to form the basilar artery. Note: AICA = anterior inferior cerebellar artery = anterior and inferior cerebellar artery. PICA = posterior inferior cerebellar artery = posterior and inferior cerebellar artery.

Conal septum: the portion of the conotruncal septum that divides the conus cordis (aka bulbus cordis) into aortic and pulmonary conuses. Conotruncal septum: the septum that divides the conus cordis and truncus arteriosus into outflow tracts. That is, conotruncal septum consists of aorticopulmonary septum and conal septum. Note: The aorticopulmonary septum develops from truncal cushions that appear as right superior and left inferior ridges in the truncus arteriosus. The conal septum similarly develops from the conal cushions which fuse beneath the truncal cushions. Note: During the fifth week, development of the outflow cushion begins. Ridges (cushions) appear in the truncus arteriosus, and concomitantly, there is the development of cushions in the bulbus cordis. The left and right truncal and concal cushions the begin to zipper together superiorly and inferiorly. That is, the fusing septum proceeds spirally in both the inferior and superior directions to completely separate the outflow tract. Note: The muscular part of the interventricular septum does not divide the ventricles completely but leaves a gap called the interventricular foramen. The interventricular foramen is later closed the membranous interventricular septum (aka membranous part of the interventricular septum). Thus, the right and left ventricles are completely separate by the interventricular septum, consisting of muscular and membranous parts. Note: There are different theories as to what the membranous interventricular septum is derived from.  One common theory is that the membranous interventricular septum is an

Development of the Venous System At this stage (day 26), three paired venous system (developed previous) that drains into the sinus venosus are present: The vitelline veins drain the yolk sac, the umbilical veins return oxygenated blood from the placenta, and the cardinal veins drain the embryo. Development of the Cardinal Vein System  Around day 27 – 28, the left and right subcardinal veins begin to sprout from the base of the posterior cardinal veins (parallel to the posterior cardinal veins). 

On day 29 – 30, the left and right subcardinal veins have developed and have connected to each other by a median anastomosis (intersubcardinal anastomosis) in front of the aorta.

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On day 34 – 38, the left and right subcardinal veins have formed lateral anastomoses with the posterior cardinal veins (postcardinal-subcardinal anastomosis).

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On day 34 – 38, the right subcardinal vein has obtained access to the liver where a critical anastomosis (the hepatosubcardial junction) develops between the right subcardinal vein and the hepatic sinusoids. This initially small anastomosis will later (eventually) enlarges to become portion of the inferior vena cava. On day 36 – 40, after the formation of the hepatosubcardial junction, the hepatic sinusoids form a large posterior channel within the liver, which becomes the intrahepatic portion of the inferior vena cava.

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On day 38 – 40, the left and right supracardinal veins begins to sprout from the posterior cardinal vein, developing parallel (dorsolateral) to the posterior cardinal vein and running medial to the subcardinal vein.

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On day 40 - 42, the left and right supracardinal veins are formed. These veins are connected proximally with the intersubcardinal anastomosis via the supracardinal-subcardinal anastomosis. Note: Distally the supracardinal veins will later join the interpostcardinal anastomosis when it the interpostcardinal anastomosis is formed.

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On day 40 – 42, the posterior cardinal veins begin to regress in the middle, whereas the distal posterior cardinal veins begin develop a weblike anastomosis (interpostcardinal anastomosis).

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outgrowth from the inferior endocardial cushion and conus septum. That is, the two together form the membranous part of the interventircular septum, which later fuses with the muscular part. Another theory is that the membranous interventricular septum is derived mainly from the inferior (aka posterior) endocardial cushion. It expands into the interventricular foramen and fuses with the upper edge of the muscular portion of the interventricular septum. Simultaneously, it also fuses with the lower edge of the conus septum.

Note: Posterior endocardial cushion is also known as the inferior endocardial cushion. And the anterior endocardial cushion is also known as the superior endocardial cushion. Note: Atrioventricular valves are developed mainly from the endocardial cushions and the lateral cushions. Some sources suggest that the conotruncal septum (that extends down to the endocardial cushions) may also have some contribution to the atrioventricular valve formation. Note: The semilunar valves (aortic and pulmonary valves) are formed from the distal portion of the truncal ridges. That is, valve develops from the swellings of the truncal tissue at the line of fusion between the truncal septum and the conal septum. Note: Shortly after the appearance of the superior and inferior endocardial cushions, new cushions begin to develop from the left and right lateral walls. These cushions are known as the right and left lateral endocardial cushion.

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On day 40 – 42, the subcardinal veins begins remodelling: The longitudinal segments of the left subcardinal vein begins to regress while the right subcardinal vein begins to lose its connection with the posterior cardinal vein. Note: This will result in the left side of the body served by the subcardinal system drain solely through transverse anastomic channels to the right subcardinal vein.

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On day 42 – 45, interpostcardinal anastomosis has formed distally and has been joined by the supracardinal veins.

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On day 45 – 49, the posterior cardinal veins have nearly completely regressed with the exception of the cranial and caudal (cephelad to the interpostcardinal anastomosis) extent that persists.

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On day 45 – 49, subcardinal veins have been remodelled.

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On day 45 – 49, a brachiocephalic anastomosis has formed between the left and right anterior cardinal vein.

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On day 50 – 54, different segments of IVC has now fully developed from the vitelline, subcardinal and supracardinal veins.  The hepatocardiac segment of IVC (the portion between the liver and the heart) is derived from the right vitelline vein (right hepatocardiac channel).  Hepatic segment of IVC is developed from the hepatic sinusoids.  Suprarenal segment of IVC (above the kidneys) is formed from the right subcardinal vein.  Renal segment of IVC is formed from the right supracardinal-subcardinal anastomosis.  Infrarenal segment of IVC (portion of the IVC distal to the entrance of the renal veins) is formed from the right supracardinal vein.

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On day 50 – 54:  The junction for the renal, adrenal and gonadal veins is developed from the intersubcardinal anastomosis.  The renal veins are also formed from the intersubcardinal anastomosis. Some sources suggest that the left and right supracardinal-subcardinal anastomoses may also be involved.  The gonadal veins are formed from the remnants of parts of the subcardinal veins below the intersubcardinal anastomosis.  The suprarenal veins are formed from the remants of parts of the subcardinal veins above the intersubcardinal anastomosis.

Note: As hepatic diverticulum invades the septum transversum, left and right vitelline plexuses develop in the septum transversum, from the vitelline veins. Note: The development of hepatic sinusoids is only partially understood. Experiments seem to suggest that: Intrahepatic capillary vessels in a developing liver receive blood from vitelline veins. However, vitelline veins do not give rise to the whole vascular system in a maturated liver. Liver sinusoids develop independent of vitelline and umbilical veins. Hammond came to the conclusion that major vascular system development, including sinusoids, occurred extravascularly in the liver, and the invasion of vitelline veins was a secondary phenomenon. The vitelline and umbilical veins that invaded the transverse septum are unlikely to contribute to the sinusoidal system by the sprouting of capillary vessels, but rather they give rise to other liver vasculature systems. Hammond also suggests that mesenchymal cells in the transverse septum differentiate into endothelial cells of the sinusoids. Others suggest the primitive hepatic sinusoids are formed as the capillary plexus derived from the vitelline veins penetrate the developing hepatic sheets within the septum transversum.

Note: At the same time as the cystic diverticulum appears (approximately day 26), a dorsal pancreatic diverticulum forms in the dorsal wall of the duodenum, opposite to the site of origin of the hepatic diverticulum.

Note: By the time the embryo is 11mm in length (day 36 to day 40), the hepatic portion of the inferior vena cava develops from the vitelline veins. The supracardinal veins

 An anastomosis forms below the liver between the two supracardinal veins out of which a part of the hemiazygos vein will later arise. The supracardinal veins give rise to the azygous and hemiazygos veins. 

On day 52 – 56, the superior vena cava develops from the union of the proximal right anterior cardinal vein and the right common cardinal vein.

Cardiac Development (Begins: day 26, Completes: day 56) Division of the Atrioventricular Canal (Begins: day 26 -28, Touch/Appose: 32 – 33. Completion/Fusion: day 35 -40) Endocardial cushion begins to grow on day 26 – 28 and appose each other on day 32 – 36. By day 37 - 42 fusion between opposite endocardial cushion has completed.

Atrial Septation (Begins: day 26 - 28, Completion: day 39 - 42)  On day 26 – 28, Septum primum begins to develop resulting in the formation foramen primum. 

On day 32, Perforations/fernestrations appear in the septum primum before closure of foramen primum.

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On about day 33 – day 35, the perforations/fernestrations have coalesced to form foramen secundum. Concurrently or immediately after that, still on day 33 to day 35, the septum primum merges with the endocardial cushions resulting in the closure of foramen primum. Also concurrently or immediately after, septum secundum begins to develop to the right of the adjacent septum primum.

then develop whereas the postcardinal veins begin to atrophy.

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The septum secundum grows downward and will soon cover/overlap/overlie the foramen secundum on its right side but never forms a complete atrial partition. It forms an incomplete partition. The process of septum secundum development leaves a foramen at about the midpoint of the septum secundum.

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On about day 37 - 42, the septum secundum is fully developed, and the incomplete partition is manifested as a foramen at about the midpoint of the septum secundum, this foramen is known as the foramen ovale. That is, the foramen ovale is roughly formed on day 37 - 42.

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On day 39 – day 42, the upper portion of the septum primum (attached to the cranial portion of the atrium) regresses, whereas the lower segment (attached to the endocardial cushions) remains, serving as a flap valve for the formen ovale. At the same time, the edge of the septum secundum forms the rim of the foramen ovale. That is by about day 39 –day 42, intra-atrial septation is completed.

Conotruncal Septation/Outflow Tract Seperation (Septation Begins: day 29 - 34, Septation Completes: day 43 – 49; Separation Begins: day 42 - 44, Separation Completes: day 49 - 55) Septation of the truncus arteriosus begins around day 29 to day 34 and is completed around day 43 to day 49. Aorta and pulmonary artery starts to separate from each other around day 42 to day 44. By day 49 to day 52, aorta/pulmonary artery separation is completed.

Ventricular Septation (Muscular Begins: day 28 - 30, Muscular Halts: day 42 – 44; Membranous Begins: day 44, Membranous Completes: day 49 – 52) Muscular interventricular septum begins to form on day 28. The growth of the muscular interventricular septum is halted on around day 42 – 44 (leaving open an interventricular foramen that allows communication between the ventricles) to wait for the septation of the outflow track. By day 49 – 52, membranous part of the interventricular septum is completed (that is, the membranous interventricular septum fuses with the muscular interventricular septum). That is, closure of the interventricular foramen occurs around day 49 – 52. Formation of Cardiac Valves (Begins: day 34 – 37, Completes: day 50 - 56)

Note: The normal diaphragm is derived from several components.The central tendon is formed from the septum transversum; small dorsolateral portions are from pleuroperi-toneal membranes; dorsal crura are from the esophageal me-sentery;the posterior lateral muscle is derived from the inter-costal muscle groups. Note: Some authors define the end of embryonic stage by the formation of segmental (tertiary) bronchi which supplies the bronchopulmonary segments. In this case, the embryonic stage ends on day 42.

Hepatic Development + Development of Vitelline Veins & Umbilical Veins (Begins: day 26, Completes: day 52 – 56)  On day 26 – 27, hepatic diverticulum grows into the septum transversum. Shortly thereafter, cells begin to interact through mutual induction.

Note: Distal angiogenesis is the formation of new capillaries from pre-existing ones at the periphery of the lung.

 On day 28, hepatic diverticulum begins to send out hepatic cords to invade septum transversum.  As soon as hepatic cords invade the septum transversum, hepatic sinusoids begins to appear as well as the left and right vitelline plexuses, around day 28 – 29.  On day 29 – 30, the left and right vitelline plexuses have been incorporated with the hepatic sinusoids. That is, the vitelline plexus around the liver has become connected to the hepatic sinusoids.  On day 30 – 31, the umbilical veins have been linked to the hepatic sinusoids. That is, as the liver expands, vascular connections between the umbilical veins and the hepatic sinusoids are established.  On day 32 – 33, proximal part of both left and right umbilical veins begins to degenerate.  On day 34 – 35, distal right umbilical vein begins to degenerate.

 On day 35, left horn of sinus venosus begins to decrease in size. At the same time (on day 35), proximal left vitelline vein begins to degenerate and proximal right vitelline vein begins to enlarge in size.  Also on day 35, distal vitelline veins (left and right) begins modification (and will later form the portal vein).  On day 35 – 36, ductus venosus begins to develop (and can now be distinguished).  On day 36, proximal parts of both left and right umbilical veins have now degenerated.  On day 36 – 40, the hepatic portion of the inferior vena cava develops from the vitelline veins.  On day 38 – 42, hepatic portal vein has now been formed from the distal vitelline veins.

Note: Development of the intrapulmonary vessels occur at all stages (including embryonic stage) and directly corresponds to overall lung growth. Researches suggested that the branching lung buds (with its epithelial lung cells) induce the expansion of the capillary plexus (i.e. through vascular growth factors) and act as a template for pulmonary blood vessel development. In more detail, the distal part of the branching airway is wrapped in a meshwork of capillary that will expand by distal angiogenesis as the lung bud grows. Newly formed capillary plexus will then remodel dynamically, as they

 On day 44, distal right umbilical vein has lost its connection to the hepatic sinusoids. Note that some remnants still persist at this stage.

form part of the vessels of the afferent or efferent component. That is, intrapulmonary vessels originate from the continuous expansion and coalescence and remodelling of the primary capillary plexus.

 On day 49, distal right umbilical vein has now fully degenerated.  Also, on day 49, left proximal vitelline vein has degenerated.  On day 52 – 56, the left sinus horn has become the coronary sinus.  On day 52 – 56, the ductus venosus is completely formed.

Note: Recent evidence, however, suggests that the pulmonary arteries may initially arise from the fourth aortic arch on day 28 – 29 and later merge with the sixth aortic arch.

Note: The third, fourth and sixth aortic arches are welldeveloped by day 35-36. Development Of Other Gastrointestinal Organs Gall Bladder and Cystic Duct:  On day 26, cystic diverticulum appears (this begins the development of the gallbladder and cystic duct). Pancreas:  On day 26, a dorsal pancreatic bud appears in the dorsal wall of the foregut, opposite to the site of origin of the hepatic diverticulum.  On day 28, a ventral pancreatic bud appears from the developing bile duct (caudal to the developing gallbladder.

Note: Before the sixth arch is completely formed, the proximal pulmonary arteries are already present. Note that the pulmonary trunk is formed as the aorticopulmonary septum divides the truncus arteriosus into two major channels. Note: At CS16 (day 37 to day 42), the segments appear much thinner. At CS16 (day 37 to day 42), the distal part of the right sixth aortic arch also starts to regress. At CS 17 (day 42 to day 44), the dorsal segment and the distal part

 On day 32, the ventral pancreatic bud begins to migrate posteriorly.

of the right sixth aortic arch have regressed almost completely.

 On day 35, the ventral pancreatic bud has now migrated posteriorly.  On day 42, the ventral and dorsal pancreatic buds fuse. Spleen: The spleen develops as a condensation of mesenchyme of the dorsal mesentery of the stomach during the 5th week. That is, the spleen develops from mesenchymal cells between the layers of the dorsal mesentery (behind the stomach).

Note: Previously, it has been thought that only conducting airways are formed during the pseudoglandular period. However, this traditional understanding has been challenged. Note: The pseudoglandular stage is characterized by branching of the conducting airways, formation of the peripheral (first appearance) of acinar tubules. Note: Acinar tubules are smooth-walled, blind-ending channels lined by cuboidal epithelium that first arise as distal branches of the terminal bronchioles. Acinar tubules will later give rise to the adult pulmonary acinus.

Note: In the human lung, a second circulatory system,

Development of the Respiratory System

the bronchial circulation, arises from the dorsal aorta supplying systemic blood. The bronchial vasculature

Upper Respiratory Tract: On day 22, the upper respiratory tract arises from bony structures of the head. Lower Respiratory Tract: On day 22-23, a ventral outpouching is formed from the ventral surface of the foregut, this ventral outpouching is called laryngotracheal groove (aka respiratory diverticulum). Shortly after the laryngotracheal groove is formed, the proximal part of the laryngotracheal groove develops into larynx and trachea. 1. Embryonic Stage (Begins: Day 24; Ends: End of week 7)

develops after the pulmonary circulation, with bronchial vessels first apparent by 8 weeks. The network of bronchial vessels is extensive, with bronchial arteries demonstrated as distal as the alveolar ducts in the adult respiratory tree.

Note: Beneath the surface epithelium is the submucosa, which contains loose connective tissue with longitudinally arranged elastic fibers. Bronchial glands are seromucinous glands with ducts opening into the bronchial lumen

Airway Branching: Forming sequential endodermal buds carry with them coast of mesenchyme. o Distally, the laryngotracheal groove grows and develops to become the primitive lung bud on day 24-28. The formation of the lung bud marks the beginning of the embryonic phase of lung development. o The primitive lung bud has bifucrcated laterally to form right and left primary bronchial buds on day 26-30. o The left and right primary bronchial buds branches asymmetrically, forming lobar bronchial buds (aka secondary bronchial buds) with three right and two left on day 31-36. o Continued branching yield segmental bronchial buds (aka tertiary bronchial buds) on day 41-42. o On day 48-49, subsegmental bronchial buds have been formed. The development of subsegmental bronchial buds marks the end of the embryonic stage and the beginning of the pseudoglandular stage.

present within the submucosa. Cartilage plates and smooth muscle bundle lie beneath the mucosa and submucosa.

Trachea and Esophagus Separation o Separation of the trachea and esophagus begins in the 4th week. Several mechanisms have been proposed to explain how the tracheal tube forms and separates from the developing foregut. It is currently accepted that once lung buds form and fuse in the midline, a septum growing from caudal to cranial regions separates tracheal and esophageal compartments. Alternatively, it is thought that separation occurs by fusion of endodermal ridges growing from each side of the foregut; as they meet in the midline, two tubes form. In addition, a mechanism involving local activation of programmed cell death in the endoderm has been proposed. o Complete separation of the trachea and esophagus is achieved on day 36-42.

Note: In the pseudoglandular stage, there are no alveoli, so respiration is not possible and premature infants born during this stage cannot survive.

Note: Clara cells are non-ciliated low-columnar or cuboidal epithelial cells present in the bronchioles (including membranous bronchioles and respiratory bronchioles). Clara cells increase in number along the length of the bronchiole and are found predominantly in the terminal bronchioles and respiratory bronchioles. According to recent data, the main functions of the Clara cells can summarized as (1), the secretion of certain components of the extracellular bronchiolar lining layer (2), metabolism and detoxification of xenobiotics and other toxic compound (3) and participation in the renewal process of the bronchiolar epithelium.

Note: Acinar tubules have cuboidal epithelium, whereas saccules have a flatter epithelium. Note: Acinus is the term applied to the gas exchange unit associated with a single terminal bronchiole.

Diaphragm o Development of the diaphragm which began with the formation of septum transversum (in the third week), is fully complete on day 49-52.

Pulmonary Vasculature o On day 24-26, splanchnic plexus arises around the foregut by proliferation of angioblastic cells probably from the dorsal aortas, ventral aortas, cardinal venous system, umbilical venous system and sinus venosus.

Note: The epithelium of the prospective bronchial system is columnar, whereas that of the pulmonary acinus is initially approximately cuboid. By terminal and lateral budding there is an increase in the number of generations of bronchial and acinar tubules.

o

Recall that on day 26, the lung bud arise as an outgrowth of the primitive foregut and is thus surrounded by the splanchnic plexus. The part of the splanchnic plexus in the region surrounding the lung bud can be called the pulmonary plexus. Therefore, there is already blood circulation to and from the lung (through the primitive connections of the splanchnic plexus) and connection with the rest of the embryonic circulation from the earliest point of lung development. That is, we can say pulmonary vasculature is part of the embryonic circulation from the moment the lung starts to develop.

o

On day 27-29, common pulmonary vein (a small endothelial outgrowth/outpouching) spurts from the posterior superior wall of the primordial left atrium just to the left of the developing septum primum.

o

On day 29-32, the common pulmonary vein (from the back of the left atrium) engages and connects with the pulmonary plexus. Shortly after, four individual

pulmonary veins develops (by coalesce) from the pulmonary plexus and are joined to the common pulmonary vein. Also, the pulmonary plexus gradually loses its connection with the splanchnic plexus.

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o

On day 38-40, the connections between the pulmonary plexus and the splanchnic plexus involute.

o

As the left atrium grows, the transient common pulmonary vein will gradually be incorporated into the wall of the left atrium, forming the smooth part of the left atrium, so that the common pulmonary vein no longer exists as an anatomical structure and the individual pulmonary veins become connected separately and directly to the left atrium. In more detail, by the end of week 5, there are openings of two pulmonary veins into the left atrium. And in week 8, there are four openings.

The proximal left and right pulmonary arteries can be seen to start arise from the sixth aortic arch on day 32-33. Note: the distal parts of the left and right pulmonary arteries (and other pulmonary vessels) arise from distal angiogenesis and remodelling of the pulmonary plexus. By day 44 - 50, the pulmonary arteries have joined the pulmonary plexus that surrounds and infiltrates the lung buds.

Note: Early in the canalicular period the lung have a simple airspace configuration. Potential gas-exchanging structures are smooth-walled, blind-ending channels lined by cuboidal epithelium and supported by abundant loose interstitium and scattered small blood vessels. As the canalicular period progresses, interstitial tissue decreases, capillary growth increases and these channels take on a more complex irregular pattern.

Note: The distinction between the saccular and alveolar phases is somewhat arbitrary. Hislop, Wigglesworth, and Desai cliam that alveoli are present at week 29. Langston and coworkers believe that week 36 is the earliest point at which alveoli can be distinguished. Note: Although some investigators have concluded that the progenitor of type I cells is an undifferentiated epithelial cell, there is more convincing body of evidence suggesting that type I cells develop from differentiated

2. Pseudoglandular Stage (Begins: Beginning of week 7; Ends: End of week 16) Airway Branching: During this stage, the conducting airways continue to develop, forming the remaining conducting airways. That is, the entire air-conducting bronchial tree up to and including the terminal bronchioles (aka all pre-acinar airways or all nonrespiratory airway elements) are set down in this stage. Around week 16, the epithelial cells of the distal air spaces begin to flatten (becoming more cuboidal) and increases in glycogen (which serves as substrate for future surfactant synthesis) forming short acninar tubules. The first appearance of short 1st order acinar tubules (arising as distal branches of the terminal bronchioles) with cuboidal epithelium at the periphery of lungs by the end of week 16 marks the end of the pseudoglandular stage and the beginning of the canalicular stage. Note that gas exchange is not yet possible by the end of the pseudoglandular stage. Premature infants born during the pseudoglandular stage cannot survive.

type II cells. In either case, the growth of capillaries beneath the epithelial cells (either undifferentiated or type II) seems to be the stimulus for type I cell differentiation. Note: In week 20 – 22, the cuboidal cells of the acinar tubules begins to differentiate into type II pneumocytes that, in turn, differentiate into type I pneumocytes. This differentiation into type I pneumocytes leads to a flattening of the epithelium. Note: Type II pneumocytes are responsible for producing surfactant, whereas type I pneumocytes are responsible for gas exchange.

Note: At the sites of apposition (where capillary beds are in close apposition to the overlying epithelium), thinning of the epithelium occurs to form the first sites of the airblood barrier.

Note: the term “saccule” derives from the saclike appearance of the most peripheral airspaces, which represent the future alveolar ducts and alveoli.

Pulmonary Vasculature: Recall that the development of the intrapulmonary vessels occur at all stages and directly corresponds to the overall lung growth. Pulmonary vessels develop in parallel to the development of the conducting airways of the lung and follow the same branching pattern. The vascular growth is in close proximity to the conducting airways. That is, generations of vessels develop alongside sequential generations of

Note: At the beginning of the saccular stage, the peripheral airways form typical clusters of widened airspaces termed saccules or terminal sacs. Each new generation of this pathway is originally formed as a blindending saccule. STRICTLY SPEAKING, as soon as it divides distally, however, it is no longer a saccule, but an openended channel. , the morphology of all these channels and saccules undergoes change until the formation of the alveoli is completed in the postnatal period. Therefore, these structures have been designated as transitory ducts and transitory saccules or, more generally, as transitory

branching airways. All preacinar pulmonary arteries and veins are formed by the end of the pseudoglandular stage. Differentiation of the mesenchyme and epithelial (beginning in the more proximal regions of the airway and progress distally):  Mesenchyme surrounding the branching buds begin to differentiate into blood vessels, lymphatics, cartilage (chondrocytes), smooth muscles (myofibroblast), connective tissue (fibroblast) around the epithelial tubes.  Smooth muscles begins to appear at the end of week 7. It is fully developed during the canalicular stage.  Lymphatics first appear in the hilar region of the lung in week 8 and in the lung itself by week 10. It is fully developed during the canalicular stage.  Cartilage formation (appearing as ringed cartilaginous exoskeleton) begins in the proximal airways (trachea & main bronchi) on week 10-11 and continues to move distally forward from that time forward until it reaches completion. On week 16, cartilage appears in the segmental bronchi. On week 24-25, cartilage formation is completed where cartilage has extended to the most distal bronchi (about 10 – 14 airway generations).  Epithelial differentiation with the appearance of basal cells, goblet cells, pulmonary neuroendocrine cells, ciliated cells and nonciliated columnar (Clara) cells. Most importantly, we will look at the times in which the epithelial cells differentiate into ciliated cells and goblet cells and submucosal glands.  Isolated neuroepithelial cells appear in proximal airways in week 8-9. By week 9-10, they have formed clusters.  Basal cells in the large airways appear as early as week 10.  Cilia begins to appear in week 10 in the trachea & extends to the main stem bronchi in week 12. In week 13, cilia are found in the segmental bronchi.  Goblet cells begins to appear in the epithelium at week 13-14. Note that goblet cells produce airway mucus. Goblet cells extends to the most proximal intrasegmental bronchi by week 24. With increasing gestational age they extend distally down to the bronchioles.  Submucosal gland begins to appear at week 13-15. On week 24-25, submucosal glands have extended as far down the airway as they do in the adult lung.

airways or airspaces, because their morphology changes further until alveoli are formed.

Note: Septation results in smaller spaces termed subsaccules. Exactly when these subsaccular structures become alveoli is a matter of debate. Some have advocated that any structures bordered on three sides should be termed an alveolus. Note: Despite, no apparent morphological differences in their lamellar bodies, immature lungs produce surfactant rich in phosphatidylinositol, whereas lungs late in gestation produce surfactant rich in phosphatidylglycerol. Note: Secondary crests are highly sensitive to hypoxia and positive pressure through mechanical ventilation. If damaged, these secondary crests may fail to develop and may prevent further alveolar growth resulting in decreased overall surface area available for gas exchange in the lung.

Note: It is difficult to draw a distinct separation between the saccular and alveolar stage as they overlap to some degree. Researchers disagree when true alveoli actually exist. It is argued that true alveoli appear around 32 to 34 weeks, but they may be present as early as 29 weeks according to some researchers. Alveoli develop from a thinning of the terminal air saccules and the increased growth of secondary crests. Note: The distinction between the saccular and alveolar phases is somewhat arbitrary. Hishop, Wigglesworth, and Desai claim that alveoli are present at 29 weeks.

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Clara cells in the bronchioles appear as early as week 16-17. These clara cells initially exhibits large glycogen stores (immature Clara cells). Later the glycogen stores are replaced by secretory granules forming mature Clara cells on week 19.

3. Canalicular Stage (Begins: Beginning of week 17; Ends: week 26) Airway Branching: During this canalicular stage, 1st order acinar tubules that were formed previously begins to branch/multiply by subdividing, forming further more generations of acinar tubules. Throughout the canalicular stage, the acinar tubules also grow by lengthening and widening while the surround mesenchyme progressively thins. Evenutally, the terminal acinar tubules undergo marked expansion, dilation and thinning of epithelium forming widened airspaces known as saccules. The first appearance of saccules by week 24 - 26, marks the end of the canalicular stage and the beginning of the saccular stage. Note that it is during canalicular phase that the primitive pulmonary acinus (respiratory portion of the lung) truly begins to develop. It is proved that the respiratory portion of the lung develops by budding of acinar tubules, however, the mode of budding is not restricted to any particular pattern. Vasculature (Development of distal pulmonary circulation): Recall that the development of the intrapulmonary vessels occur at all stages. But it is during this stage that the fetal lung undergoes greatest amount of vascularization. Proliferation of rich vascular supply occurs and by week 20 – 21 a capillary network is formed in close association with the airway epithelium due to a gradual decrease in mesenchymal tissue resulting in close apposition of pulmonary vasculature to the epithlium. That is by week 20 - 21, the capillaries arrange themselves around the airspace and come into close apposition to the overlying epithelium. Apposition of capillaries to the wall of the primitive airspaces results in formation of a primitive potential air-blood barrier. Differentiation of Type I and Type II Pneumocytes: Capillary beneath the cuboidal epithelial cells lining the acinar tubules (in the most peripheral part of the lung) results in thinning of the epithelium (thinning of cytoplasm) and appearance of type I and type II pneumocytes (originating from the differentiation of the cuboidal epithelial cells) on week 20 -22. After differentiation, lamellar bodies associated with surfactant synthesis begin to appear in the cytoplasm of type II cells. Type I alveolar lining cells, which differentiate from type II cells, begin their flattening process and attenuate to provide an air-blood interface. Surfactant production by type II

Note: The beginning of this stage is not sharply defined; some alveolar formation probably begins a few weeks earlier. The three distinguishing features of an alveolus are: (1) It is an open outpouching of an alveolar duct (2) It is lined almost exclusively by the thin processes of type I pneumocytes (3) Its interstitial capillaries are exposed to at least two alveoli simultaneously. Note: The barrier between the gas in the alveoli and the blood in the capillaries is normally composed of three layers – the thin processes of the type I cells; a basement membrane that appears to be common to both the endothelial and alveolar cells; and the thin extensions of the endothelial cells.

Note: The human lung at term contains only a small protion of adult number of alveoli, and the airspace walls are represented by a thick “primary septum” consisting of a central layer off connective tissue surrounded by two capillary beds, each of them facing one alveolar surface. This double capillary netwowrk is not present in the adult lung. As alveolar architecture changes with the appearance of secondary septa, folding of one of the two capillary layers occurs within the secondary septa. Microvascular maturation involves fusion of the double capillary network into a single capillary system. The expansion of surface area and luminal volume compresses the interstitium, bringing

pneumocytes begins around week 22 – 24. The epithelium is thin enough to allow for some gas exchange by week 24 – 25. Sufficient quantities of surfactants are produced by week 24 – 26 for some gas exchange to take place. Note: By the end of the canalicular stage, survival of the fetus becomes possible. Some authors even suggest around week 22 – 24. The respiratory system (the air-blood barrier) at the end of the canalicular stage have the capacity to perform some gas exchange. That is, some respiration may be possible towards the end of this stage. However premature infants born at this stage still have quite poor prognosis. Exogenous surfactant and respiratory support are frequently needed.

4. Saccular Stage (Begins: Week 26; Ends: Week 32 - 36) Airway Branching: During the saccular stage, saccules subdivide, expand in length and width distally. Ingrowth of ridges of mesenchymal tissue divides saccules into smaller units. The detection of shallow cup-shaped indentations (interalveolar septa) in the saccule walls which begins around week 36, marks the end of the saccular stage and the beginning of the alveolar stage. Vasculature: Vascularization continues to increase. Also, as the mesenchymal ridges protrude into the saccules, part of the capillary net is drawn in with them, forming a double capillary layer. The end result of the saccular stage is a rapid increase in the gasexchange surface of the lung. Note: Capillaries in the saccular stage are generally exposed to only one respiratory surface. In the next stage (alveolar stage), each capillary is exposed to at least two respiratory surface simultaneously (mature alveolus). Differentiation: Continual differentiation of type I and II alveolar cells occurs during this period. AT II cells are further differentiated with increment in surfactant containing laminar bodies. Surfactant production thus increases such that airway collapse is less likely. AT II cells become functionally mature around week 32 – 36 (completing the maturity of the lung). There is also further differentiation of type II cells to type I cells.

the capillary networks in close proximity to potential airspaces and thereby promoting both alveolar surface area expansion and capillary bed fusion. By the third postnatal week, lung volume increases by 25 % and there is a 27% decrease in the interstitial tissue volume that is believed to promote microvascular fusions. Note: The primitive alveoli do not become definitive as

true alveoli until after birth, at which time they are only shallow bulges of the walls of the terminal sacs and respiratory bronchioles.

During the saccular stage, there are further compression of the intervening interstitium (that is mesenchyme continues to thin) and thinning of the epithelium. There is a marked decrease in the prominence of the interstitial tissue and a marked thinning of the airspace walls.

5. Alveolar Stage The alveolar stage of lung development extends from approximately week 32 - 36 until 18 postnatal month, but most alveolarization occurs within 5-6 months following delivery at term. During the alveolar stage, alveolarization continues. Saccules develop into alveolar ducts or alveolar sacs with numerous alveoli.

APPENDIX Carnegie Stage 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Day 1 2–3 4–5 6–7 8 – 12 13 – 14 15 – 17 17 – 19 19 – 21 22 – 23 24 – 25 26 – 28 29 – 30 31 – 35 35 – 38 37 – 42 42 – 44 44 – 48 48 – 51 51 – 53 53 – 54 54 – 56 56 – 60

Somites

1–7 7 – 13 13 – 20 20 – 29 29 – 35 35 – 44 > 44 (?) ? ? ? ? ? ? ? ?

Length 0.1 – 0.15mm 0.1 – 0.2mm 0.1 – 0.2mm 0.1 – 0.2mm 0.1 – 0.2mm 0.2mm 0.4mm 1 – 1.5mm 1.5 – 2.0mm 2.0 – 2.5mm 2.5 – 3.0mm 3.0 – 4.0mm 4.0 – 5.0mm 5.0 – 7.0mm 7.0 – 9.0mm 8.0 – 11mm 11 – 14mm 13 – 17mm 16 – 18mm 18 – 22mm 22 – 24mm 23 – 28mm 27 – 31mm

Bronchial Vascular System THE ADULT LUNG has a dual vascular system, the bronchial and the pulmonary system. The bronchial system oxygenates all nonrespiratory structures of the lung, whereas the pulmonary system transports deoxygenated blood to the alveoli for gas exchange. The pulmonary arteries arise from the pulmonary trunk of the heart and closely follow the bronchial tree, giving rise to the alveolar capillary plexus. These capillaries drain into the pulmonary veins, which run through the connective tissue septa back to the left atrium of the heart. The bronchial arterial circulation, arising from the aortic arch and supplying the bronchi, bronchioles and interlobular septa in older children and adults, also contributes significantly (in utero and early infancy) to the circulation to the alveolar ducts and alveoli in the central portions of the lung. Bronchial arteries at this stage have direct anstomotic connections with pulmonary arteries, and some of these connections are maintained into adulthood. The bronchial arteries directly supply pulmonary alveolar capillary beds at this time. There is limited literature available concentrating specifically on the development of the bronchial system. The bronchial arteries develop as sprouts from the descending aorta to the bronchi. The time lag between the formation of the pulmonary vasculature system and the bronchial vasculature system indicates they are indeed two separate systems. It is likely that bronchial vessels arise from the same splanchnic plexus as the pulmonary vessels. In more detail, bronchial vessels arise from the remnants of the splanchnic plexus after the pulmonary vessels are formed. A normal lung is supplied by a pulmonary artery branching from the pulmonary trunk. In its intrapulmonary course, it branches with the airways. Furthermore, there is a varying number of nutritive bronchial arteries that, with a marked variation in their origin and course, mainly originate from the descending thoracic aorta and the upper posterior intercostal arteries. These bronchial arteries supply wall of the bronchi and the media of the larger intrapulmonary arteries. Although the pulmonary and the bronchial arteries basically supply different tissues, anastomoses are common. Larger connections can be found at the level of the segmental pulmonary arteries, whereas many small anastomoses are seen with the pulmonary arterioles at the lobular level. The bronchial arteries in general share their venous drainage with the pulmonary arteries by way of the central pulmonary veins. There is a description of a drainage by separate bronchial vein directly to the left atrium. Literature regarding the bronchial arteries and veins is scant; however, Boyden described the growth of bronchial arteries from the descending aorta in the fetal period after establishment of the pulmonary system, and the venous drainage of the bronchial arteries has been described as remnant of the pulmonary plexus connected once with the systemic veins.