Expression of a novel matrix metalloproteinase gene during Cynops early embryogenesis

Matrix metalloproteinases (MMPs) are thought to play important roles in the gastrulation of Cynops pyrrhogaster embryos. MMP cDNAs were cloned from Cynops pyrrhogaster and we report here a novel MMP called CyMMP, which has strong similarity to MMP-21 (XMMP) in Xenopus. Reverse transcription-polymerase chain reaction (RT-PCR) analysis demonstrated that CyMMP mRNA was already present in cleavage stage embryos. The amount of the mRNA then gradually decreased, but increased again starting in late gastrula. There were regional differences in the level of CyMMP mRNA expression at late gastrula: the involved archenteron roof was the predominant site of expression of the gene, while there was weak expression in the neuroectoderm and epidermal ectoderm. We also found that the gene was activated in artificially mesodermalized ectoderm. The present findings indicate that CyMMP mRNA expression is activated in differentiating mesoderm during gastrulation, suggesting that CyMMP plays a role in gastrulation-related cell movement.

Comparative study of sequential expression of the organizer-related genes in normal Cynops pyrrhogaster embryos and mesodermalized ectoderm

An artificially mesodermalized ectoderm (mE) of early Cynops pyrrhogaster gastrula acquires the organizer property; the mE is able to induce the secondary axis. The expression of organizer-related genes was investigated during the mesodermalizing process of the mE. The expression of C. pyrrhogaster organizer-related genes, such as bra, gsc, lim-1, chd and noggin, were analyzed. Cynops pyrrhogaster shh expression was also investigated. The organizer-related genes were activated by 12 h after the mesoderm-inducing stimulus. It was noted that there was a temporal gap in the expression of each gene. The expression of bra and gsc seemed to be more quickly activated during the mesodermalizing process. While expression of lim-1 and noggin was activated later than that of bra and gsc, lim-1 expression was earlier than chd and noggin expression. Shh expression was activated later than lim-1/noggin. The present study suggests the possibility that the bra/gsc, lim-1, chd, noggin and shh genes are expressed one by one in that order during the mesodermalizing of the presumptive ectoderm. It also indicates that the sequence is not always consistent with that of the whole embryo during normal embryogenesis. The meaning of the discrepancy will be discussed in connection with the cascade of certain genes expressed during the mesodermalizing process.

Timing and mechanisms of mesodermal and neural determination revealed by secondary embryo formation in Cynops and Xenopus

We examined the timing and mechanisms of mesodermal and neural determination in Cynops, using the secondary embryo induced by transplantation of the prechordal endomesoderm. Two unique approaches were used: one was to observe gastrulation movements induced by the graft, and the other to measure the volumes of formed tissues. Transplanted graft pulled host animal cap cells inside to form a new notochord and other mesoderm of the secondary embryo, showing determination of mesoderm during gastrulation. The graft attained a certain width beneath the host ectoderm and moved near to the animal pole of the host by late gastrula, and a neural plate, which had a similar width to the graft, was formed covering the graft. The volume of neural tissues of the secondary embryo at tail-bud stages was about half that of the normal embryo, while the volumes of notochord were comparable in each case. These data suggest that prechordal endomesoderm, rather than notochord, determines the limit of neural plate in the overlying ectoderm. Similar dorsal grafts were transplanted at early gastrula in Xenopus but did not form well developed secondary embryos, demonstrating that the timing and mechanisms of mesoderm formation in Xenopus are different from those in Cynops.

Direct evidence of an essential role for extended involution in the specification of a dorsal marginal mesoderm during Cynops gastrulation

It has been indicated that specification of the dorsal marginal mesoderm of the Cynops gastrula is established by vertical interactions with other layers, which occur during its extended involution. In the present study, when the prospective notochordal area of the early gastrula was almost completely removed together with the dorsal mesoderm-inducing endoderm and most of the bottle cells, the D-less gastrulas still formed the dorsal axis with a well-differentiated notochord; in half of them, where the involution occurred bi-laterally, twin axes were observed. On the other hand, when the wound of a D-less gastrula was repaired by transplanting the ventral marginal zone and ectoderm, the formation of the dorsal axis was inhibited if the involution of the lateral marginal zone was prevented by the transplanted piece. The present study suggests that: (i) cells having dorsal mesoderm-forming potency distribute farther laterally than the fate map; and (ii) the extended involution plays an essential role in the specification of the dorsal marginal mesoderm, especially in notochordal differentiation in normal Cynops embryogenesis.

Cell-to-cell contact in primary embryonic induction: effects of lectin on electrical coupling and neural induction

The effects of lectin (concanavalin A; ConA) on the electrical coupling between inducing chorda-mesoderm and reacting ectoderm cells, and the realization of neural induction were investigated. The electrical coupling between cells of the chorda-mesoderm of the late gastrula (stage 13b) and the competent ectoderm or Con-A-treated ectoderm of the early gastrula (stage 12a) was measured. Neural induction was tested with ectoderm explants which had been combined with the inducing chorda-mesoderm for 1, 3 and 6 h. Electrical coupling was observed after 3 h. By 6 h, the coupling ratio had recovered to the same level as that between the homogeneous germ-layer cells. However, the electrical coupling did not recover in the combinant with Con-A-treated ectoderm. This suggests that Con-A disturbs close cell contact between the ectoderm and chorda-mesoderm cells. Neural induction was realized in the ectoderm which was combined with chorda-mesoderm for more than 3 h; this occurred parallel to the recovery of electrical coupling. In contrast, Con-A treatment (50 micrograms/ml) of the competent ectoderm for 30 min prevented neural induction. After 3 h of contact, the neural induction of Con-A-treated ectoderm was only one-third of that of the control ectoderm. The present study suggests that cellular contact between the inducing mesoderm and the ectoderm target cells plays an important role in the realization of neural induction.