title: Morphological diversity of blastula formation and gastrulation in temnopleurid sea urchins.
abstract: Embryos of temnopleurid sea urchins exhibit species-specific morphologies. While Temnopleurus toreumaticus has a wrinkled blastula and then invaginates continuously at gastrulation, others have a smooth blastula and their 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 three Temnopleurus species. 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 the following factors: change of cell shape, rearrangement, pushing up and towing of cells. We conclude that (1) many aspects of early morphogenesis differ even among very closely related sea urchins with indirect development and (2) many of these differences may be caused by the cell shape and structure of blastomeres or by differences in extracellular matrix composition.
authors: Biological Institute, Faculty of Education, Yamaguchi University, Yoshida 1677-1, Yamaguchi 753-8513, Japan chisak@yamaguchi-u.ac.jp.
authors: Chisato
authors: http://orcid.org/0000-0002-2035-555X
authors: C
authors: Kitazawa
authors: Biological Institute, Graduate School of Education, Yamaguchi University, Yoshida 1677-1, Yamaguchi 753-8513, Japan.
authors: Tsubasa
authors: T
authors: Fujii
authors: Biological Institute, Faculty of Education, Yamaguchi University, Yoshida 1677-1, Yamaguchi 753-8513, Japan.
authors: Yuji
authors: Y
authors: Egusa
authors: Department of Biology, Graduate School of Science and Engineering for Research, University of Toyama, Toyama 930-8555, Japan.
authors: Miéko
authors: M
authors: Komatsu
authors: Laboratory of Environmental Biology, Graduate School of Medicine, Yamaguchi University, Yoshida 1677-1, Yamaguchi 753-8512, Japan.
authors: Akira
authors: A
authors: Yamanaka
keywords: Blastular wall; Cell morphology; Extracellular matrix; Gut elongation; Primary mesenchyme cells ingression