Regionalized Cell Division during Sea Urchin Gastrulation Contributes to Archenteron Formation and Is Correlated with the Establishment of Larval Symmetry

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Abstract
During gastrulation of the sea urchin, Lytechinus variegatus there is localized proliferation of cells in the vegetal plate region prior to its invagination. Cell counts show that during gastrulation the number of cells per embryo increases 60% from 1025 to 1640. Measurements of cell volumes suggest that some growth may follow these divisions. Feulgen staining shows that the greatest mitotic activity throughout gastrulation occurs in the vegetal plate region. Labelling embryos with 3H-thymidine reveals that incorporation in the vegetal plate is confined to cells that encircle the base of the archenteron. Pulse-chase experiments indicate that these labelled cells contribute descendants to the vegetal half of the archenteron. Additionally, 3-dimensional reconstructions of vegetal regions at different stages reveal that by the end of gastrulation two bilateral clusters of labelled cells lie at the future sites of the post-oral arms of the pluteus larva, thus marking the axes of bilateral and dorso.sbd.ventral symmetry. Our findings suggest that two of the principal events of sea urchin gastrulation.sbd.the formation of the archenteron and the establishment of symmetry in the larva.sbd.are accompanied by distinct patterns of cell division.