A cyto‐embryological study of gastrulation in the sand dollar, Scaphechinus mirabilis

Abstract

Processes of gastrulation in the sand dollar Scaphechinus mirabilis were compared with those in the sea urchin Hemicentrotus pulcherrimus, which seemed to show a typical pattern of gastrulation. Measurement of the archenteron length clearly demonstrated that invagination processes in H. pulcherrimus are divided into two phases, the primary and secondary invagination. On the other hand, invagination in S. mirabilis was revealed to continue at a constant rate. To see the movement of cells during gastrulation, embryos were labeled with Nile blue. In H. pulcherrimus embryos, labeled cells were observed along the full length of the archenteron, if the embryos had been labeled before and during the primary invagination. Labeled cells were never observed in the embryos stained after the primary invagination. In contrast, labeled cells were always discerned at the basal part of the archenteron in S. mirabilis, even if the embryos were stained after invagination had undergone considerable progress. The number of cells in the archenteron of S. mirabilis embryos increased with the advancement of gastrulation, while the numbers were almost constant in H. pulcherrimus. These results suggest that the cellular basis of gastrulation in S. mirabilis is quite different from that in well‐known species of sea urchins.

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

“…As described, the archenteron cells are rearranged during gastrulation in popular sea urchins, such as Strongylocentrotus purpuratus, Lytechinus variegatus, or Hp (Ettensohn, 1985;Hardin, 1988;Kominami and Masui, 1996). As clearly shown in Figs.…”

“…Adults of Hp , Sm and Am were collected at the seashore near Matsuyama city. Adults and embryos of Hp , Em and Sm were handled as described before (for Hp and Sm , Kominami and Masui, 1996;for Em , Takata and Kominami, 2003). Animals of other species were kept in aquaria with aeration at 24 ° C. Gametes were handled with a standard method, and embryos were cultured at 24 ° C. Millipore-filtered seawater (MFSW) was supplemented with 100 units /ml penicillin and 50 µ g/ml streptomycin.…”

“…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). Further, the invagination proceeds autonomously, i.e., independently of the force exerted by the ectodermal layer surrounding the vegetal plate .…”

“…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 .…”

Body Plan of Sea Urchin Embryo: An Ancestral Type Animal

“…Animals and gametes were handled as described before (Kominami and Masui, 1996). Millipore-filtered seawater (MFSW) supplemented with100 units/ml penicillin (Meiji Seika, Tokyo, Japan) and 50 µ g/ml streptomycin (Meiji Seika) was used throughout experiments.…”