The Questions of Developmental Biology

11. Differential cell adhesion is important in regulating sea urchin gastrulation. The micromeres
delaminate first from the vegetal plate. They form the primary mesenchyme which becomes the
skeletal rods of the pluteus larva. The vegetal plate invaginates to form the endodermal
archenteron, with a tip of secondary mesenchyme cells. The archenteron elongates by convergent
extension and is guided to the future mouth region by the secondary mesenchyme.
12. Snails exhibit spiral cleavage and form stereoblastulae, having no blastocoels. The direction
of the spiral cleavage is regulated by a factor encoded by the mother and placed into the oocyte.
Spiral cleavage can be modified by evolution, and adaptations of spiral cleavage have allowed
some molluscs to survive in otherwise harsh conditions.
13. The polar lobe of certain molluscs contains the determinants for mesoderm and endoderm.
These will enter the D blastomere.
14. The tunicate fate map is identical on its right and left sides. The yellow cytoplasm contains
muscle-forming determinants; these act autonomously. The nervous system of tunicates is formed
conditionally, by interactions between blastomeres.
15. The soil nematode Caenorhabditis elegans was chosen as a model organism because it has a
small number of cells, a small genome, is easily bred and maintained, has a short lifespan, can be
genetically manipulated, and has a cuticle through which one can see cell movements.
16. In the early divisions of the C. elegans zygote, one daughter cell becomes a founder cell
(producing differentiated descendants) and the other becomes a stem cell (producing other
founder cells and the germ line).
17. Blastomere identity in C. elegans is regulated by both autonomous and conditional
specification.
*The GLP-1 protein is localized in the ABa and ABp blastomeres, but the maternally encoded glp-1 mRNA is found
throughout the embryo. Evans and colleagues (1994) have postulated that there might be some translational
determinant in the AB blastomere that enables the glp-1 message to be translated in its descendants. The glp-1 gene is
also active in regulating postembryonic cell-cell interactions. It is used later by the distal tip cell of the gonad to control
the number of germ cells entering meiosis; hence the name GLP, "germ line proliferation."