An Analysis of the Postgastrula Differentiation of the Hypomere

Abstract

ABSTRACT An experimental analysis of the competence of the Urodele hypomere for different kinds of mesodermal histogenesis was continued by heteroplastic and homoioplastic transplantations of portions of neurula and tail-bud trunk hypomere to a position overlying the somitic mesoderm in Ambystoma punctatum and Taricha torosa. The major portion of the transplanted hypomere differentiated, in association with the overlying ectoderm (either host or donor), into a loose connectivetissue dermis. The specific characteristics of this connective tissue differed with the age of the donor tissue (older hypomere more rapidly developed the associated homogeneous ground substance) and with the species of the host (all graft tissue on A. punctatum hosts developed more rapidly the loose connective-tissue dermis and then, often, a fibrous connective tissue was formed). The association of donor tissue and host tissue in the ventral somite tip region resulted in host-donor responses which involved both tissues and which gave evidence of further histogenetic competence on the part of the hypomere. Apparently in response to influences of the ventral somite tip, the immediately superficial donor cells increased in number (by mitotic activity and aggregation) and developed an outgrowth from this ‘limb-bud’ mass. Younger ventral hypomere demonstrated greater histogenetic competence in responding to this host influence than did the older hypomere and, in T. torosa, the dorsal hypomere was more competent in this respect than was the ventral hypomere. It was also shown that the development of the outgrowth would not occur in the absence of the superficial somite tissue of the ventral myotome even though an initial mass of donor cells might be produced. Possibly in response to stimulation by the grafted hypomere, the host’s pronephric duct in the graft region increased in size (number of cells constituting the duct wall) and branched (regenerated) with the supernumerary nephric duct passing posteriad along the experimentally created hypomere-somite boundary. Both host and donor cells seemed to be incorporated into this new duct. Responding to tubule induction by the supernumerary pronephric duct, the closely associated ventral somite cells attempted development of mesonephric tubule rudiments. These supernumerary rudiments were more anterior than the normal host rudiments and were noticeably less well differentiated. Such experimentally produced developmental associations deprived the host trunk myotomes of their ventral tips, which resulted in a lack of ventrolateral myotome extension and thus in the subsequent failure of ventro-lateral musculature to appear in the graft region of the host trunk.