Morphological diversity of blastula formation and gastrulation in temnopleurid sea urchins

Embryos of temnopleurid sea urchins exhibit species-specific morphologies. While Temnopleurus toreumaticus has a wrinkled blastula, others have a smooth blastula. Embryos of T. toreumaticus invaginate continuously at gastrulation, whereas in some others invagination is stepwise. We studied blastula and gastrula formation in four temnopleurids using light and scanning electron microscopy to clarify the mechanisms producing these differences. Unlike T. toreumaticus, blastomeres of mid-blastulae in T. reevesii, T. hardwickii and Mespilia globulus formed pseudopods. Before primary mesenchyme cells ingressed, embryos developed an area of orbicular cells in the vegetal plate. The cells surrounding the orbicular cells extended pseudopods toward the orbicular cell area in T. toreumaticus, T. reevesii and T. hardwickii. In T. toreumaticus, the extracellular matrix was well-developed and developed a hole-like structure that was not formed in others. Gastrulation of T. reevesii, T. hardwickii and M. globulus was stepwise, suggesting that differences of gastrulation are caused by all or some of factors: change of cell shape, rearrangement, pushing up and towing of cells. These species-specific morphologies may be caused by the shape and surface structure of blastomeres with cell-movement.\n\nSummary statementTemonopleurid embryology