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 mesoderminducing 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.

Gastrulation and pre-gastrulation morphogenesis, inductions, and gene expression: Similarities and dissimilarities between urodelean and anuran embryos

“…(K) When the LDMZ is combined with a small piece of animal cap ectoderm, the animal cap ectoderm is induced to notochord (Nt) and muscle cells (Mus), while LDMZ itself differentiates into a mass of endodermal cells. Suzuki et al, 2002), while the notochord-inducing activity of the LDMZ remains active . Suzuki et al (2002) treated Cynops embryos with anti-morphogenesis reagents such as suramin (aspecific FGF-inhibitor, Gerhart et al, 1991;Grunz, 1992;Oschwald et al, 1993;Cardellini et al, 1994;Wallingford et al, 1997;Kaneda et al, 2002;Sakaguchi et al, 2002) or nocodazole (inhibitor for microtubule polymerization, Lane and Keller, 1997).…”

“…In Xenopus, bottle cells are progressively formed by cells of the lower half of the suprablastoporal endoderm layer around the entire circumference of the marginal zone, but neither bottle cells nor the blastopore forms on the ventral side in Cynops (Doi et al, 2000). Cell lineage analyses have suggested that blastopore (bottle cell)-forming activity is not always defined in a particular blastomere at the cleavage stage in either urodeles or Xenopus (Holtfreter, 1944;Nakamura and Kishiyama, 1971;Doucet-de Bruïne, 1973;Smith and Slack, 1983;Gimlich, 1985;Takasaki, 1987;Hardin and Keller, 1988;Bauer et al, 1994;Suzuki et al, 2002), indicating that the bottle cells are induced before the onset of gastrulation. It can therefore be assumed that bottle cell induction and mesoderm induction occur in a temporally synchronized manner.…”