Cellular basis of gastrulation in the sand dollar Scaphechinus mirabilis

Abstract

The processes of gastrulation in the sand dollar Scaphechinus mirabilis are quite different from those in regular echinoids. In this study, we explored the cellular basis of gastrulation in this species with several methods. Cell-tracing experiments revealed that the prospective endodermal cells were convoluted throughout the invagination processes. Histological observation showed that the ectodermal layer remained thickened, and the vegetal cells retained an elongated shape until the last step of invagination. Further, most of the vegetal ectodermal cells were skewed or distorted. Wedge-shaped cells were common in the vegetal ectoderm, especially at the subequatorial region. In these embryos, unlike the embryos of regular echinoids, secondary mesenchyme cells did not seem to exert the force to pull up the archenteron toward the inner surface of the apical plate. In fact, the archenteron cells were not stretched along the axis of elongation and were in close contact with each other. Here we found that gastrulation was completely blocked when the embryos were attached to a glass dish coated with poly-L-lysine, in which the movement of the ectodermal layer was inhibited. These results suggest that a force generated by the thickened ectoderm, rather than rearrangement of the archenteron cells, may play a key role in the archenteron elongation in S. mirabilis embryos.

Behavior of Pigment Cells Closely Correlates the Manner of Gastrulation in Sea Urchin Embryos

“…After primary invagination, SMCs appear at the archenteron tip and extend filopodia toward the apical plate. During secondary invagination, archenteron cells are stretched along the axis of the archenteron, showing the existence of a tension exerted by SMCs (Kominami and Takata, 2000). Number of cells observed in a cross section of the archenteron becomes smaller as invagination proceeds, indicating that the archenteron cells are rearranged during gastrulation (Kominami and Masui, 1996).…”

“…The archenteron elongates continuously at a constant rate throughout gastrulation, hence the distinction between primary and secondary invagination cannot be noticed (Kominami and Masui, 1996;Kominami and Takata, 2000). Bottle cells are scarcely observed in the vegetal plate .…”

“…Bottle cells are scarcely observed in the vegetal plate . Although SMCs appear at the archenteron tip during invagination, they do not extend filopodia toward the apical plate and do not look to exert pulling force (Kominami and Takata, 2000). In fact, the constituent cells are not stretched along the axis of the archenteron.…”

“…In fact, the constituent cells are not stretched along the axis of the archenteron. Further, it was shown that blastoderm is continuously involuted into the base of the archenteron throughout gastrulation, and that the force exerted by ectoderm is indispensable for the progress of invagination (Kominami and Takata, 2000;Takata and Kominami, 2001). Thus, the manner of gastrulation is quite different between regular and irregular echinoids so far examined.…”

Desenvolvimento e ciclo reprodutivo da bolacha-do-mar Clypeaster subdepressus (Echinodermata: Echinoidea) de São Sebastião, SP

“…Durante a gastrulação da bolacha-do-mar Scaphechinus mirabilis, as CMS não se projetam no eixo de alongamento do arquêntero e permanecem agregadas umas às outras (Kominami e Takata, 2000). Em comparação, as CMS de C. subdepressus apresentam inúmeras projeções e filopódios ao longo da extensão do arquêntero.…”