Cell movements during the initial phase of gastrulation in the sea urchin embryo.

The morphogenetic processes responsible for the initial phase of gastrulation in sea urchin embryos are not known. Here we report observations of the size and position of clones of cells derived from horseradish peroxidase (HRP)-injected mesomeres and macromeres. The displacement of these clones during the initial phase of gastrulation suggests that involution is a mechanism involved in primary invagination. Experiments with embryos marked with vital dyes indicate that movements occur only during a brief phase coincident with the invagination of the vegetal plate. Counts of cells derived from HRP-injected mesomeres and macromeres suggest it unlikely that localized growth in the vegetal plate is involved in gastrulation. An analysis of changes in cell shape during the initial phase of gastrulation indicates that there is a stage-dependent shift from cells being columnar to having their apices skewed toward the vegetal plate and an increase in the proportion of cells having basal processes during gastrulation. When embryos are grown in the presence of monoclonal antibodies to the apical lamina or monovalent fragments of these antibodies, the initial phase of gastrulation is delayed and they form partial exogastrulae. Analysis of embryos marked with HRP indicate that the antibody treatments interfere with the cellular movements observed in untreated embryos. We conclude that directed movements of cells within the blastoderm, probably employing tractoring on components of the hyaline layer, cause the buckling of the vegetal plate and displacement of presumptive endoderm cells seen during the initial phase of gastrulation.