Cellular mechanisms of epiboly in leech embryos

Gastrulation in leech embryos is dominated by the epibolic movements of two tissues: germinal bands, composed of segmental precursor cells, and an overlying epithelium that is part of a provisional integument. During gastrulation, the germinal bands move over the surface of the embryo and coalesce along the prospective ventral midline. Concurrently, the epithelium spreads to cover the embryo. We have begun to analyze the mechanisms involved in gastrulation in the leech by assessing the independent contributions of the epithelium and the germinal bands to these cell movements. Here we describe cellular events during epiboly in normal embryos and in embryos perturbed by either reducing the number of cells in the epithelium, or by preventing the formation of the germinal bands, or both. These experiments indicate that both the germinal bands and the epithelium are able to undergo epibolic movements independently, although each is required for the other to behave as in control embryos.