Blastopore formation and dorsal mesoderm induction are independent events in early Cynops embryogenesis

Abstract

The independent roles of blastopore formation and dorsal mesoderm induction in dorsal axis formation of the Cynops pyrrhogaster embryo were attempted to be clarified. The blastopore-forming (bottle) cells originated mainly from the progeny of the mid-dorsal C and/or D blastomeres of the 32-cell embryo, but were not defined to a fixed blastomere. It was confirmed that the isolated dorsal C and D blastomeres autonomously formed a blastopore. Ultraviolet-irradiated eggs formed an abnormal blastopore and then did not form a dorsal axis, although the lower dorsal marginal zone (LDMZ) still had dorsal mesoderm-inducing activity. Involution of the dorsal marginal zone was disturbed by the abnormal blastopore. These embryos were rescued by artificially facilitating involution of the dorsal marginal zone. Suramin-injected and nocodazole-treated blastulae did not have involution of the dorsal marginal zone, although the blastopore was formed. Neither embryos formed the dorsal axis. The dorsal mesoderm-inducing activity of the LDMZ in the nocodazole-treated gastrulae was still active. In contrast, the LDMZ of the suramin-injected embryos lost its dorsal mesoderm-inducing activity. bra expression was activated in the nocodazole-treated embryos but not in the suramin-injected embryos. The present study suggested that (i) the dorsal determinants consist of blastopore-forming and dorsal mesoderm-inducing factors, which are not always mutually dependent; (ii) both factors are activated during the late blastula stage; (iii) the dorsal marginal zone cannot specify to an organized notochord and muscle without the involution that blastopore formation leads to; and (iv) the localization of both factors in the same place is prerequisite for dorsal axis formation.