Part I.  30 points

Fill-in-the-blank questions.  Choose from the list of possible answers listed on the next page.


a)                  _________________________ is the structure at the anterior end of the primitive streak of the chick gastrula, and it has Organizer activity.


b)                  During the __________________________, cell cycle times lengthen, and embryonic transcription increases dramatically.


c)                  The mesoderm of the neurula stage embryo can be divided into five regions.  The ________________________ forms the notochord. 


d)                  Some regions of the limb bud have been defined as having important roles during limb development.  The _________________________ consists of rapidly dividing mesenchymal cells at the distal end of the limb bud.  These cells stop proliferating if the __________________________ is surgically removed from the limb bud.


e)                  Mice that carry a deletion in the _________________________ gene have foot pads (ventral structures) on the dorsal surface of their feet.


f)                    In the peripheral nervous system, axons are often bundled in nerve fibers, and neuronal cell bodies are clustered in __________________________.


g)                  ______________________________ are secreted by target cells.  They are chemoattractants for migrating cells or growth cones, and they suppress apoptosis.


h)                  During gastrulation, the ectodermal cells expand to cover the outside of the embryo via a process called ___________________________.


i)                    During mammalian gastrulation, individual cells migrate into the blastocoel.  This type of cell movement is called ________________________.




List of Possible Answers for Questions on page 1



Apical Ectodermal Ridge



Convergent Extension


Desert Hedgehog




Fibroblast Growth Factor




Growth cones

Henson's Node





Involuting Mesoderm

Lateral plate mesoderm


Midblastula Transition


Neural ectoderm



Posterior Marginal Zone

Progress Zone



Somitic mesoderm




Tyrosine kinase


Zone of Polarizing Activity



Part 2.  70 points. 


1.  (12 points)

Describe two cells that migrate to their final location during mammalian embryogenesis.  For each, describe (a) where the cell initiates its migration; (b) where it migrates to; and (c) its function after it completes migration and differentiates.


Cell 1








Cell 2










2)  10 points

Dickkopf (Dkk1) protein is produced by Spemann's Organizer.  Describe

a)      It's developmental function (What is the loss-of-function phenotype; What aspect of Organizer function does it mediate?)









b)      It's biochemical function (What does it bind, inhibit, or activate?).







3.  (20 points)

The cells of the Zone of Polarizing Activity secrete Sonic Hedgehog.  When researchers stain a limb bud with a Shh-specific antibody, Shh expression appears to be restricted to the Zone of Polarizing Activity.  However, it is possible that Shh is present beyond the ZPA at levels below the level of antibody detection. 

(a)    Design an experiment to test the hypothesis that Shh acts as a morphogen to pattern the limb bud.  (10 pts)

(b)    Describe two possible results of this experiment.  For each, explain how you would interpret the result. (10 pts)






4. (15 points)

The flat bones in the skull are formed by a mechanism that is fundamentally different from endochondral ossification, the process by which the rest of our bones are formed.


a)      Describe skull bone formation.  Describe the role of at least one cell type.












b)      Describe endochondral ossification, including a description of at least two cell types involved in this process.


















c)       Describe "growth plates".  Why do our long bones eventually stop growing?










5.  (13 points) 

Some of the cells in the neural tube will form neurons, while others will form glial cells.  This cell fate decision is regulated by Notch signaling.  If Notch signaling is inhibited, then excess neurons form. 


(a)    Does Notch function as a ligand or as a receptor (2 pts)?


(b)  Assume that the cells below belong to an equivalence group in the neural tube.  Mark the cells that will form neurons with an "N". Note that one cell has already been specified. (5 points)

















(c)  For the equivalence group diagramed below, assume that the cells in row 4 have a mutation in Notch.  Mark the cells that will form neurons with an "N".  You may choose to explain your reasoning, but it is not required.  (6 points)