Ultrastructural aspects of mouth formation in the starfish Pisaster ochraceus

Early events during mouth formation in embryos of the starfish Pisaster ochraceus have been studied with scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Embryos examined by TEM were fixed in glutaraldehyde with Alcian blue, a dye which preserves extracellular materials. Initially, mesenchyme cells migrate off the tip of the archenteron, leaving a defect in the cell layer that is covered by the basal lamina. This region of "naked" basal lamina bulges into the blastocoele and forms a hemispherical blister. At the same time that this is occurring, filamentous and conical processes extend from the inner surface of the presumptive stomodeal ectoderm cells which are located directly opposite the bulge of basal lamina. These processes penetrate the ectodermal basal lamina and project "naked" plasmalemma into the blastocoele. Shortly after this, the blister of endodermal basal lamina becomes irregular in shape, and scattered cells are found both within the blister and between it and the presumptive stomodeal ectoderm cells. Processes of endodermal cells appear to make contact with the filamentous processes of the stomodeal ectoderm cells. In other embryos which appear to be at a slightly later stage, the free basal lamina is conical in shape and is associated with at least one conical ectodermal process. In yet other embryos, the free endodermal basal lamina is either in contact with several other large processes forming a circular region of contact, or the free endodermal and ectodermal basal laminae are fused at the edge of the circle. Degeneration of both the ectodermal and endodermal basal laminae located within the circle, and subsequent invagination of the stomodeal ectoderm, appear to complete this process. The pulsations of stomodeal ectodermal cells seen throughout early stages in mouth formation may be involved in these events.