Embryology Made Ridiculously Simple Handout

Embryology Made Ridiculously Simple: The Intra-uterine Life:  It is the time that elapses between fertilisation and the birth of a new individual. It is about 9 calendar months.  The intra-uterine life of the individual is divided into 3 stages Period Germinal Period Embryonic Period Foetal Period Duration The 1st 2 weeks From 3rd week to 3rd 3rd month to end of month pregnancy Characteristics Formation of 3 germ Differentiation of the Growth of the various layers: ectoderm, 3 germ layers into organs and systems endoderm and organs and systems mesoderm Congenital Congenital Congenital Congenital anomalies anomalies are liable anomalies are liable anomalies are less to occur to occur liable to occur The 1st week of Pregnancy:

FERTILIZATION: Fusion of the male and female gametes happens in the Fallopian tube Egg complex is surrounded by cells called the Corona Radiata and a Zona Pellucida Radiata is penetrated by sperm and reaches the ZP Three glycoproteins make up the ZP, 1, 2 and 3 Initially, the sperm binds to ZP3 Liberation of enzymes found in sperm sac called acrosome In the acrosome are acid proteinase, collagenase, acrosin amongst others Zona Pellucida binding stimulates this release of enzymes from the sac Acrosomal reation is the name given to this process and it helps the sperm to get through the ZP.

Tail of the sperm also propels it forward Influx of Calcium into the sperm head assist this process One plasma membrane is formed by fusing the sperm and egg membranes Nuclei (haploid/pronuceli) of both join up to form diploid egg. So in summary Fertilisation: Definition: The fusion of male and female gametes Site: Lateral 1/3 of the Fallopian tube Mechanism: The fertilising sperm pierces the corona radiata and Zona Pellucida. The head detaches from the rest of the sperm and forms the male pronucleus which contains half the number of chromosomes. The nucleus of the ovum is also haploid and both fuse to form a dilpoid zygote.

Analogy: There is a ball covered in velcro and a needle and syringe which is to be inserted into the balloon. The ball is like the ovum and the needle is the sperm and the velcro is the Zona Pellucida. The ball has been on the floor for a long time and stuck to the velcro is a lot of fluff from the rug (fluff is the corona radiata). We need to insert the needle through the velcro and into the balloon. So we need to remove the fluff first (corona radiata). The needle needs to then get through the velcro (the ZP) before it gets to the balloon. It attaches to the velcro first (to ZP3) and then penetrates it. In the syringe attached to the needle is a potent acid. We're going to use the syringe and needle to release some acid to dissolve the velcro (the acid in the syringe is like the acrosomal sac contents), so we can penetrate the ball.

How is Polyspermy prevented?

1. The fusion of the sperm and ovum causes a change in the action potential of the plasma membrane. This prevents other sperm attaching. 2. The influx of calcium causes the ZP to separate from the egg, thus preventing any sperm attaching. Past Paper Question (Medicine): Describe the essential steps in fertilisation. Describe cellular events in fertilisation. Buzz Words: Zona Pellucida, Corona Radiata, ZP3, Acrosomal Reaction, Fusion, Pro-nuceli

Cleavage:

Definition: a series of mitotic divisions occurring in the zygote. Each cell that results is called a blastomere. Site: these divisions occur in the zygote as it passes in the fallopian tube to reach the uterine cavity. Steps:  The 2 cell stage appears at 30 hours after fertilisation  The 4 cell stage appears at around 45 hours after fertilisation  The Morula is the 12-16 cell stage and appears about 3rd day  The cells become arranged into an inner cell mass in the centre and an outer cell mass in the periphery  The Blastocyst develops on the 4th day  As the cells of the morula continue to divide, fluid from the uterine cavity enters the spaces between the cells.  These fluid filled spaces join together to form one large cavity called a blastocele and the morula is now called a blastocyst.(the Zona Pellucida disappears completely at day 4)

Blastocyst Differentiation of the Blastocyst: The cells of the outer cell mass form the trophoblast- will form foetal membranes later on The cells of the inner cell mass become located at one pole- called the embryonic pole (the opposite pole is called the abembryonic pole) The blast attaches to the endometrium at 5-6 days after fertilisation. Implantation: Embedding: Definition: It is the penetration of the blastocyst into the superficial compact layer of the endometrium. Time: Begins day6 or 7 and ends by day 11 or 12. Site: endometrium of the posterior wall of the fundus of the uterus Steps:  The blastocyst becomes attached to the endometrium by its embryonic pole

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The trophoblast cells covering the embryonic pole erode the epithelium of the endometrium (possibly by enzymatic action)to allow the blastocyst to penetrate through the defect. After complete embedding of the blastocyst, the penetration defect is closed by a fibrin clot. Implantation is completed by growth of the epithelium to cover the defect.

So in Summary Implantation: Sixth/Seventh day Fundus of Uterus Embryonic pole attaches Trophoblasts erode endometrium Penetration defect formed Blastocyst enters Embeds completely Closed by a clot Intact epithelium again Sad Forgotten Emos Try Partying But Eat Carrots Instead

THE SECOND WEEK OF PREGNANCY: • 1. 2. 3.

The following things happen: Implantation: is complete (11th/12th day) Trophoblast differentiates into TWO layers (syncytio-trophoblast and cytotrophoblast) Inner cell mass becomes TWO layers (called bilaminar germ disc and composed of Ectodermal layer and Endodermal layer) 4. TWO cavities are formed (amniotic cavity and the yolk sac) NB) there is a rapid rate of growth in the second week compared to the first week

The Eighth Day:  The blastocyst is still partially embedded in the endometrium

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Inner cell mass forms 2 layers- an inner ENDODERM (small polygonal cells) and an outer ECTODERM (tall columnar cells). The trophoblast starts to differentiate into 2 layers: Outer dark zone without cell boundaries called the SYNCYTIOTROPHOBLAST and an inner pale zone with clear boundaries called CYTOTROPHOBLAST. An Amniotic Cavity starts to be formed : small clefts appear between the ectodermal cells and the trophoblast, the clefts join each other to form the amniotic cavity. The cytotrophoblast develops a layer of cells called amnioblasts which form the roof of the amniotic cavity, while the floor is formed by the epiblast or ectoderm.

9th and 10th days:  The blastocyst becomes more deeply implanted and the fibrin clot covers the penetration defect.  The trophoblast becomes fully differentiated into cyto and syncytio-trophoblast.  Spaces called lacunae appear in the syncytio-trophoblast.  The amniotic cavity becomes larger  A second cavity forms called the Primary Yolk Sac at the ventral aspect of the embryonic disc. The Primary Yolk sac is the new name for the old blastocyst cavity. Its roof is formed by the endodermal layer of the hypoblast and the rest of its lining is called Heuser's membrane. 11th and 12th days:  The blastocyst is completely embedded by now  Lacunae in the syncytio-trophoblast formed earlier start communicating with each other to form larger spaces. This is a primitive maternal foetal circulation. 

Lots of interesting things happen to the cyto-trophoblast:

Formation of the extra-embryonic mesoderm: cells of inner surface of the cyto-trophoblast form a loose tissue called extra-embryonic mesoderm Formation of extra-embryonic coelom: Cavities are formed inside the extra-embryonic mesoderm. These cavities fuse together to form the extra-embryonic coelom. However, the coelom doesn't replace the mesoderm completely, rather it divides it into two, SOMATOPLEURE which lines the cyto-trophoblast and the splanchnopleure which covers the yolk sac. NB) The connecting stalk is a mass of mesoderm connecting the roof of the amniotic cavity with the trophoblast. ANALOGY: imagine the extra-embryonic mesoderm is like the a dried up oxbow lake and the yolk sac and amniotic cavity and bilaminar germ disc is an island in the middle. When the water flows in the rainy season. The lake has two river banks or two lake edges on either side. The water is like the extra-embryonic coelom and the banks on either side are the somatopleure (on the outside- has an O) and splanchnopleure (on the inside, has an N). There is a bridge connecting the island to the outer edge/bank. This is the connecting stalk

13th Day: The most prominent changes are the appearance of primary chorionic villi as follows: Parts of the cytotrophoblast at the embryonic pole project into the syncytiotrophoblast forming primary chorionic villi, surrounded by lacunae. The primary yolk sac gets smaller and gets pinched off and is now called the secondary yolk sac. video: http://www.youtube.com/watch?v=f35JpW2DqDU&feature=related

So in Summary: Week 2 of Pregnancy: 2 cavities- yolk sac and amniotic cavity 2 germ cell layers-hypoblast and epiblast 2 trophoblast layers-cytotrophoblast and syncytiotrophoblast

The third week:

Summary:  Changes in the embryonic disc 1. Formation of the intra-embryonic mesoderm- now a TRILAMINAR germ disc 2. Formation of the notochord- which is a temporary supporting structure to the embryonic disc  Changes to the trophoblast (chorion): 3 types of chorionic villi form and cover the whole surface of the chorionic vesicle.

Formation of the intra-embryonic mesoderm:  Formation of the primitive streak:  At the beginning of the third week, ectodermal cells in the caudal part of the bilaminar germ disc migrate to the midline forming a primitive streak (a narrow midline groove)basically the cells of the epiblast migrate downwards forming a groove.  These cells separate from the epiblast (now called ectoderm) and migrate in all directions. This new layer of cells is called the intra-embryonic mesoderm. There is an area in the cranial end that it doesn’t migrate to, that is the propchordal plate and an area called the cloacal membrane, behind the primitive streak. These areas remain BILAMINAR. 

Because mesoderm forms vascular tissue. These bilaminar areas do not have mesoderm so without a blood supply they will break down. This is important as these areas need to break down to form contact with the external environment:

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Prochordal plate  buccopharyngeal opening

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Cloacal membrane  anus

Pillow and Pillow Cover.

The two pieces of material sewn together to form the pillow cover are the epiblast (ectoderm) and hypoblast (endoderm). When I want to stuff the pillow into the pillow cover, there needs to be an opening, yeh? That opening is like the primitive streak. When I stuff the pillow I make sure the whole of the pillow case is encased with pillow. The stuffing is the mesoderm, the pillow cover represents the ectoderm and mesoderm. Mnemonic: 

Primitive streak, Midline groove of Epiblast cells, forms Mesoderm, Migrates all directions, Trilaminar disc forms

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Prime Ministers of England Meet Many Troubles

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This whole process of mesoderm formation is called GASTRULATION

Mesoderm components MESODERM: 

Mesothelium (peritoneal, pleural, pericardial)/ Muscle (striated, smooth, cardiac)

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Embryologic Spleen/ Soft tissue/ Serous linings/ Sarcoma/ Somite

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Osseous tissue/ Outer layer of suprarenal gland (cortex)/ Ovaries

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Dura/ Ducts of genitalia

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Endothelium

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Renal Microglia

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Mesenchyme/ Male gonad

Formation of the Notochord: 

There is a thickening of the ectoderm at the cephalic end of the primitive streak and the primitive node, remember, is where there is the central depression with the slightly elevated area.

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The cells of this primitive node proliferate and form a solid rod of cells called notochordal process which grows in a cephalic direction between the endoderm and ectoderm.

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Inside the notochordal process Is a small central notochordal canal which passes from the primitive pit anteriorly.

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The notochord is a bit like a hollow metal pipe that passes through the pillow we talked about earlier.

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The notochord gives the embryo structure and helps it to define its axes.

Changes to the trophoblast in the 3rd week

Formation of the chorionic villi

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Primary chorionic villi: begin to appear by the end of 2nd week at the embryonic pole of the chorionic vesicle and increase in number by the beginning of the 3rd week. Each primary villus is made up of a central core of cytotrophoblast and a covering layer of syncytiotrophoblast.

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Secondary chorionic villi: by the beginning of the 3rd week, cells from the extraembryonic mesoderm start to penetrate the primary chorionic villi and forms secondary chorionic villi. Each secondary chorionic villus is made up of a central core of extra-embryonic mesoderm, a middle zone of cytotrophoblast and an outer zone of syncytiotrophoblast.

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Tertiary chorionic villi: by the end of the 3rd week, a loop of afferent and efferent capillaries appears in the mesodermal core of the secondary chorionic villi. The afferent one is connected to the umbilical artery and the efferent one is connected to the umbilical vein.

After the third week 

Differentiation of the Intra-embryonic mesoderm:

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The mesoderm is formed as a loose tissue between the ectoderm and endoderm by day 17 on either end of the notochord.

The three parts: 

As development proceeds, 2 longitudinal grooves appear in the mesoderm on either side of the notochord, dividing it into 3 parts.

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Paraxial mesoderm, on either side of the notochord

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About 20th day, transverse grooves appear dividing the paraxial mesoderm into somites (blocks). The first pair form day 20. 3 additional ones form each day from 21st to 30th day i.e. by the end of 1st month, 30 or 31 pairs of somites are formed. B

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Day 30-40, rate has slowed and finally 42-44 pairs are formed.

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S- structures of the paraxial mesoderm

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O- occipital to coccygeal

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M- mesodermal tissue

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T-TWO parts- ventromedial and dorsolateral

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E- embryo age can be estimated from number of somites

Intermediate Mesoderm: 

This tissue, which temporarily connects the paraxial mesoderm with the lateral plate

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In the cervical and upper thoracicregions it forms segmentally arranged cell clusters (the future nephrotomes), whereas more caudally it forms an unsegmented dmass of tissue known as the nephrogenic cord. From this partly segmented, partly unsegmented intermediate mesoderm developesthe excretory units of the urinary system.

The lateral plate mesoderm: Forms the pleura and peritoneum.

THE PLACENTA:

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Chorion frondosum (bushy or villous chorion)

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Chorion laeve (smooth chorion) Decidua basalis Decidua capsularis Decidua parietalis (decidua vera)

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By the beginning of the 4th month, placenta has 2 components: o Fetal portion – o Maternal portion – 1. Chorionic & decidual plates 2. Junctional zone 3. Decidual septa & cotyledons

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At full term, placenta is discoid with a diameter of 15~25 cm, is approximately 3 cm thick, & weighs about 500~600 g. It has a: Fetal surface Maternal side

Function of the Placenta: Exchange of metabolic & gaseous products between maternal & fetal bloodstreams  Exchange of gases –  Exchange of nutrients & electrolytes –  Transmission of maternal antibodies –  Production of hormones