This exam has 5 pages and a total of 50 points.
You will have 90 min. to complete it.

Answer all short answer questions as briefly as possible.

Make sure your name and id number are on all pages (1 point for doing this!)

1. Briefly define each of the following (6 points):

Netrin:

An extracellular protein related to laminin that acts as either a chemoattractant or repellant, depending on the type of neuron.

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Stem Cell:

Cell that can produce more stem cells or go on to differentiate.

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Angiogenetic Cell Cluster:

Clusters of cells embedded in the yolk sac that generate primitive blood cells and some blood vessels. Also called "blood islands".

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Intramembranous Ossification:

Process of direct mesenchymal secretion of bone, as in the plates of the skull, without cartilaginous intermediates as in endochondral ossification.

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Coelom:

Primary body cavity. In vertebrates, the coelom is lined with splanchnopleure-derived tissue.

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Myotome:

Portion of the somite that generates muscle progenitors.

  2. Circle the appropriate response for each of the following statements (5 points):

Melanocytes form from neural crest cells that migrate through the posterior half of somites	F
In mammals, the embryonic endoderm forms from the hypoblast	F
In vertebrates, the lungs originate as simple outpocketings of endodermal tissue	T
The umbilical artery is important in mammals because it carries oxygenated blood from the placenta back to the fetus	F
Apoptotic elimination of neural crest cells in the trunk helps to determine which neural crest cells form cartilage structures	F
Spina bifida results from incomplete closure of the anterior neuropore	F
The steel mutation generates its characteristic phenotype because cells that normally produce melanocytes have insufficient quantities of one type of growth factor receptor	F
Unlike in the Xenopus gastrula, the superficial epithelial cells in a zebrafish gastrula do not involute	T
Lacunae form within the cytotrophoblast, allowing gas exchange between the maternal and fetal blood supplies	F
Erythropoeitin is crucial for the formation of vascular endothelial cells during development of the vasculature in vertebrates	F

3. Complete the following table referring to extraembryonic membranes in amniotes.

Membrane	Type of mesoderm that contributes to it	Major function(s) in birds	Major function(s) in placental mammals
Amnion	somatic	fluid environment around embryo	same
Chorion	somatic	protection	nutrition/gas exchange (placenta)
Yolk sac	splanchnic	nutrition (also blood formation)	blood formation
Allantois	splanchnic	waste disposal	waste disposal/gas exchange (forms part of umbilical cord)

 

4. For each phrase referring to various morphogenetic movements on the left match all appropriate response(s) drawn from the column on the right. Note: in some cases, the correct answer will require more than one choice from the right-hand column. If none of the answers on the right are appropriate, write "none" (6 points).

Undergoes Convergent Extension	1,2,3,5	1. Deep cells of the dorsal non-  involuting marginal zone of Xenopus
Undergoes Invagination	3,6	2. Dorsal embryonic shield of  zebrafish gastrula
Undergoes Ingression	4,5	3. Neural plate of salamanders
Undergoes Involution	2	4. Sea urchin primary mesenchyme  cells
Uses Directed Migration Via  Filopodia Or Lamellipodia	4,7	5. Chick epiblast
		6. Human optical vesicle
		7. Mouse trunk neural crest cells

5. You have discovered that some species of frog that are disappearing from tropical forests worldwide appear to develop defects during gastrulation.

A. Your colleague showed that healthy embryos from these species gastrulate in a manner very similar to Xenopus embryos. Based on your friend's results, describe how the overall shape of the prospective dorsal involuting marginal zone (IMZ) should change during gastrulation (2 points):

It would become narrower and longer along the anterior-posterior axis following its involution. [Also undergoes radial intercalation prior to involution]

B. You believe that fibronectin may play a role similar to that in salamanders. What distribution of fibronectin would you expect to see when you immunostain healthy gastrulae from the tropical species? Assume all appropriate reagents are available (2 points).

FN would be aligned in fiber arrays along the anterior-posterior axis, along the blastocoel roof.

C. Describe an experiment that you could perform to test whether fibronectin is required for successful completion of gastrulation in the tropical species. Assume all appropriate reagents are available (2 points):

Inject anti-FN antibodies or RGDS peptides into the blastocoel. Migration of leading edge mesoderm would be inhibited, preventing involution.

6. Trunk neural crest cells of vertebrates give rise to many different types of tissues in vertebrates.

A. Describe two experiments that suggest that neural crest cells are pluripotent that do not involve the use of cells cultured in vitro (4 points):

1. Transplant cervical crest to trunk or vice versa. The transplanted cells will produce ganglionic structures and neurotransmitters appropriate to their new location.

2. Inject single NC with lineage tracers. Multiple cell types are derived from the progeny of a single injected cell.

B. Mutations in the c-kit homologues in mouse and humans result in defects in neural crest derivatives. Based on what you know about the molecular identity of c-kit, briefly provide a cellular explanation for the observed defects. (2 points)

c-kit is a receptor for the stell survival factor. If NC cells don't make c-kit, they can't respond to the presence of stell in their environment, and they'll die.

7. For each embryonic brain region listed on the right, indicate which structure on the left is generated from it. If none is appropriate, write "none". (3 points)

Structure	Embryonic brain region
1. Cerebellum _____c______	a. Prosencephalon
2. Pons _____c______	b. Mesencephalon
3. Lens of the Eye _____none______	c. Rhombencephalon
4. Neural Crest of the Pharyngeal Arches _____c______
5. Dorsal Root Ganglion _____none______
6. Medulla _____c______

8. Growth factors and hormones are thought to be important for proper formation of the skeleton in vertebrates.

A. Describe one piece of evidence that indicates that the proper formation of skeletal elements depends on peptide growth factor signaling in the vertebrate embryo (2 points)

(1) short ear mouse: mutant for BMP-5, missing ear and rib structures. (2) mutations in FGF receptors result in thanatophoric dysplasia, achrondroplastic dwarfism, etc.

B. You read in the newspaper that Olympic weightlifters in lower Slobovia are using recombinant human growth hormone to improve bone strength. Why might this be a bad idea? (1 point)

Growth factors affects many different processes, so injections could affect many processes, which could be very dangerous.

9. Eph receptor tyrosine kinases and their ligands, the ephrins, are thought to be important during formation of connections between retinal axons and the optic tectum in the chick. The basic structure of the retina and tectum and their normal connections are shown in the diagram for reference.

A. Describe an experiment in vitro that demonstrates that retinal neurons respond to molecules expressed on the surface of tectal cells. Be sure to indicate the nature of the response (i.e., whether it is repulsive or attractive). (3 points)

When membranes are prepared from tectal cells and laid down in strips on nitrocellulose, explanted temporal axons avoid the posterior membrane strips.

B. Based on what you know about the distribution of Eph receptors and ephrins in the retina and tectum, are these molecules distributed in a manner consistent with them playing a role in retinal-tectal guidance? Explain your answer. (3 points)

Certain Eph receptors are expressed at high levels on the surface of temporal axons, but at much lower levels on nasal axons. Ephs are expressed at high levels in the posterior tectum, but not the anterior. Cells detect the ephrins via the Eph receptors, and the signal acts as a repellant. Thus temporal axons avoid the posterior tectum.

C. The guidance of commissural neurons by the floor plate in the vertebrate spinal cord appears to operate in a different manner from the Eph/ephrin system. How do the two systems differ? (2 points)

Netrins secreted by the floor plate act as an attractive cue for the commissural neurons, as opposed to the repulsion seen with the Eph/ephrin system in the tectum.

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