The Questions of Developmental Biology

Fibropellins are stored in secretory granules within the oocyte, and are secreted from
those granules after cortical granule exocytosis releases the hyalin protein. By the blastula stage,
the fibropellins have formed a meshlike network over the embryo surface. At the time of
invagination, the vegetal plate cells (and only those cells) secrete a chondroitin sulfate
proteoglycan into the inner lamina of the hyaline layer directly beneath them. This hygroscopic
(water-absorbing) molecule swells the inner lamina, but not the outer lamina. This causes the
vegetal region of the hyaline layer to buckle (Figure 8.21B,C). Slightly later, a second force
arising from the movements of epithelial cells adjacent to the vegetal plate may facilitate this
invagination by drawing the buckled layer inward (Burke et al. 1991).
At the stage when the skeletogenic mesenchyme cells begin ingressing into the
blastocoel, the fates of the vegetal plate cells have already been specified (Figure 8.22; Ruffins
and Ettensohn 1996). The endodermal cells adjacent to the micromere-derived mesenchyme
become foregut, migrating the farthest distance into the blastocoel. The next layer of endodermal
cells becomes midgut, and the last circumferential row to invaginate forms the hindgut and anus.
Second and third stages of archenteron invagination
The invagination of the vegetal cells occurs in three discrete stages. After a brief pause,
the second phase of archenteron formation begins. During this time, the archenteron extends
dramatically, sometimes tripling its length. In this process of extension, the wide, short gut
rudiment is transformed into a long, thin tube (see Figure 8.17, 12 hours; Figure 8.23). To
accomplish this extension, the cells of the archenteron rearrange themselves by migrating over
one another and by flattening themselves (Ettensohn 1985; Hardin and Cheng 1986). This
phenomenon, wherein cells intercalate to narrow the tissue and at the same time move it forward,
is called convergent extension. Moreover, cell division continues, producing more endodermal
and secondary mesenchyme cells as the archenteron extends (Figure 8.24; Martins et al. 1998).
In at least some species of sea urchins, a third stage of archenteron elongation occurs.
This last phase is initiated by the tension provided by secondary mesenchyme cells, which form at
the tip of the archenteron and remain there (see Figure 8.17, 13 hours; Figure 8.25). Filopodia are
extended from these cells through the blastocoel fluid to contact the inner surface of the