Chick Embryo 72 Hours

Lab 8: Chick Chick, 72 hours Nervous system Cranial nerves Digestive system

72 hour chick embryo • Cranial nerves and ganglia are quickly developing, with one new cranial ganglion per neuromere. p • The organs of the digestive system are forming. • At this stage the embryo has a foregut, hindgut, and the beginnings of organs along the midgut.

Neural crest cells • So versatile that they could be called a fourth germ layer. They help form many titissues: – Melanocytes, adrenal glands, peripheral system neurons including spinal ganglia, and bones and cartilage of the head, and parts of the cranial trigeminal and glossopharyngeal and vagus ganglia ganglia...

• They arise along the neural tube starting around 27 hours and are migratory along specific paths. Two movies show the fluorescently labeled cells moving away from the neural tube over 10-12 hrs.

Neural crest cells migrating

Neural crest cells migrating

Neural crest cells can be fluorescently labeled with a specific antibody (HNK-1)

Immunocytochemistry for neural crest cells

HNK-1 HNK 1 expressing cells in color

Neural crest cell derivatives

Origin between ectoderm and neural tube

• Neural crest cells play a large role in vertebrate evolution because they form so many of the structures in the head and pharyngeal arches that are characteristic of a vertebrate body type. • They are therefore the main cells responsible for changes in craniofacial evolution.

The avian bill is formed in part from the cranial neural crest migrating from the fore and midbrain region i tto th the b branchial hi l arches h

(A): Migration of neural crest cells (shown in red) (B) and (C): Maxillary (mx) and mandibular arches (md) (D): Beak bone (red) from NCC NCC. Trainor, P. ( Science 299: 523 2003).

The avian bill is not entirely made from NCCs – they contribute but do they determine its pattern?

A grafting experiment was done to test this. Chick neural crest cells were put in the quail embryo ((“quck”) quck ) or quail neural crest cells were put in the duck embryo (“duail”) ( duail ). The results are on the next slide.

The duck beak of course is longer. The quail beak has a maxillary ill bump near the tip p ((egg gg tooth). )

A. Quail NC cells transplanted in a duck embryo ( quck) (a k) results lt in a stubby quail type bill and having the egg tooth bump.

B. Duck NC cells transplanted in a quail embryo (a duail)) results in a longer duck type bill.

• The results of the reciprocal transplants indicate that it is the neural crest which forms not only the cartilage and bone but directs the overlying ectoderm and the mesoderm as well in the formation of the avian beak beak.

Cranial nerves and ganglia Sensory organs • Most of the paired cranial nerves (bundled collections of neurons) have a cell body containing the nucleus and most of the cytoplasm in ganglia (singular ganglion). The thi d ffourth third, th and d sixth i th h have no ganglia. li • Nerves are made up of neurons extending back to the central nervous system or to some target sensory organ or muscle. • The next slide shows a typical migrating embryonic neuron with a leading edge growth cone which extends the axon from the cell body.

The migrating neuron as shown in lecture.

Cranial nerves • The sources of the neurons can be alongside the neuromeres (rhombomeres), or from ectoderm just under the epidermis (epibranchial placodes) or growth from the developing sensory organ (nose, eye, ear). • The olfactory sensory nerve (I) forms axons growing from the nasal pits toward the cerebral hemispheres. • The optic nerve (II) forms from the sensory retina and later axons grow back to join to the diencephalon. diencephalon • The oculomotor (III) nerves are motor and later innervate the eye y muscles (not ( yet y formed). )

Cranial nerves of the hindbrain • The trigeminal (V) is both sensory and motor with the semilunar ganglion sending branches to the eye (ophthalmic) and maxillary/mandibular processes. Placode cells and neural crest form p this nerve. • The facial nerve (VII with acousticofacialis ganglion) is a mixed nerve for the face face. Trace it to the second mainly, but mandibular arch too. Placode cells and neural crest contribute to this nerve It is close to the auditory nerve (VIII) nerve. which innervates the nearby developing ear (auditory vesicle).

• The glossopharyngeal (IX) and associated superior ganglion is a sensory nerve can be traced to the third arch. Epibranchial placodes and neural crest form this nerve nerve. • The vagus (X) and associated jugular ganglion is a mixed nerve for the fourth arch. Epibranchial placodes contribute to the neural crest forming these neurons neurons.

Spinal cord

Spinal p accessory nerve (XI)

Glossopharyngeal ganglion (IX) Otic vesicle Acousticofacialis ganglia (VII/VIII)

left

right

Neuromeres of myelencephalon Metencephalon

Yolk sac

anterior

Semilunar ganglion g g of trigeminal nerve Mesencephalon (V)

Spinal cord Notochord

Facial nerve

Metencephalon

Semilunar ganglion (V) of trigeminal nerve Isthmus Mesencephalon

Digestive system and endoderm derivatives • At 72 hours the endodermal derivatives such as the lung buds begin to rapidly expand into the mesenchyme of mesoderm origin. • The esophagus, p g , stomach,, liver and duodenum are just beginning to form. Try to trace the connections between these tissues as they all branch off the same original endodermal tube. • Note the closeness of the ductus venosus to the li liver rudiment; di t llater t th the liliver will ill surround d thi this vessel. Why would later much more vitelline blood be directed to the liver?

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Pharyngeal region • The four pharyngeal (branchial) arches are characteristically found in all vertebrates during development whether or not gills are formed. This is an example of the retention of common structures and developmental pathways in vertebrate evolution. l ti • The derivatives of the endoderm and neurall crestt cells ll vary th the mostt iin evolution.

Posterior endoderm derivatives • The hindgut is the region posterior to the caudal intestinal portal portal. The hindgut ends in the cloaca. Note how the allantois branches off ventrally ventrally. The cloaca is a blind ending but will later form an opening at the site of the cloacal membrane (proctodeum). Try to find possible functional openings of the mesonephric ducts into the cloaca.

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Sagittal g section to show how allantois branches off of the hindgut

Neural tube cell organization • The dividing population of cells is closest to the lumen in the ependymal (ventricular) layer as mentioned in the lecture. At the early stages seen in the next slide the division into three layers (ependymal (ependymal, mantle and marginal) is not completed. • Because all tube cells are attached at the ependymal end and extend the full width, the tube is considered to be a pseudostratified epithelium Pseudo meaning that there is only epithelium. one cell layer but because cell bodies are at various positions it looks like there are manyy strata or layers.

Chick embryo neural tube cells before the formation of the three neural layers.

Dividing cells can sometimes be seen close to the lumen in the ventricular zone.

anaphase

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In the spinal cord or brain of the 72 hour chick embryo the thin outer marginal zone sometimes is seen (i.e rarely) as distinct from the two darker and denser inner layers. Try to find it.

In viewing sections note how the thi k thickness off the th layers l varies i between b t brain regions.

• In the next slide the adult spinal cord structure is shown with the dorsal sensory and ventral motor nerves. • In the 72 hr chick the nerve axons are growing toward the targets (such as the limb bud which is very early in its development) but the staining method used for these sections does not allow nerve fibers to be easily seen.

Adult neural tube structure

• The oculomotor nerves (cranial nerve III) can sometimes be seen as thin usually non-continuous streaks extending from the mesencephalon floor toward the internal carotid arteries. These nerves will innervate the muscles of the eyes eyes. (The oculomotor nerves may not be seen in all sets of sections sections.

Body cavities • The coelom is beginning to become divided into the pericardial cavity surrounding the heart, the pleural cavity surrounding the lungs lungs, and the peritoneal cavity around the gut.