© 1984 by The Society for Integrative and Comparative Biology
Primary Invagination of the Vegetal Plate During Sea Urchin Gastrulation1
Department of Biology, Yale University New Haven, Connecticut 06511
The initial phase of echinoid gastrulation, primary invagination, involves an inpocketing of a monolayered epithelium. To gain information about the nature of the mechanical forces that are responsible for primary invagination, several experimental approaches have been taken, using the transparent embryos of the sea urchin, Lytechinus pictus, as the principal material. Vegetal plates isolated microsurgically well before the onset of gastrulation will invaginate normally, demonstrating that the forces responsible for primary invagination are generated by the cells in the vegetal 
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of the embryo. As shown by serial reconstructions of L. pictus embryos, relatively few cells (about 100) take part in primary invagination. Both the number of cells and the total volume of tissue in the wall of the archenteron increase with time. Even so, it can be shown that very little movement of cells over the lip of the blastopore takes place during primary invagination, and this process is best viewed as a simple inpocketing of the vegetal epithelium. The cells in the wall of the archenteron have a distinctive shape; they are elongated along their apico-basal axes and frequently have enlarged, rounded, basal ends. However, they do not undergo any dramatic changes in shape during primary invagination. In particular, there is only a slight decrease in the height of the cells (length along the apico-basal axis), a result that is inconsistent with the hypothesis that invagination is due to cell rounding (Gustafson and Wolpert, 1967). Examination of L. pictus and Strongylocentrotus purpuratus gastrulae by transmission electron microscopy reveals that cells in the wall of the archenteron continue to be joined by typical junctional complexes during primary invagination. In addition, the morphology of the junctional complex at the gastrula stage is more elaborate than previously described. Sparse bands of micronlaments are associated with the plasma membrane at the level of the junctional complexes in both endodermal and ectodermal cells. These and other relevant data on early echinoid gastrulation are discussed in relation to several possible mechanisms of epithelial morphogenesis.