Anteroposterior specification in amphibian embryogenesis: The regulative roles of positive and negative controls of mitogenesis

Post‐nodal mesoblast caudalizes the host axis and inhibits cell population growth, and induces new primitive streaks in chick embryos

“…Consistent with the theory that caudalization occurs in the form of a postero-anterior gradient (Yamada 1950a(Yamada ,b, 1994(Yamada , 1995, we suggest that PN cells, positioned near the host axis, induce new primitive streaks and prevent the normal gradient borderline from occurring by adding posterior mesoderm and caudalizing the host axis. This is probably why PN do not shorten the tail region but do cause shortening of the head region.…”

“…Cell cycle regulation may be a crucial step in the cascade of expression of genes involved in establishing axial patterns (Modak et al 1993). Yamada (1994, 1995) has hypothesized that the antero–posterior specification in the mesoderm and the ectoderm is a combined effect of a positive cell cycle control during dorsalization and a positive cell cycle regulation during caudalization. Thus, caudalization is indeed regulated by a negative cell cycle control (Ghatpande et al 1991, 1993; Modak et al 1993; Joshi 1999; present data).…”

“…Lithium chloride (LiCl), Trypan blue, isonicotinic acid hydrazide and retinoic acid (RA) affect both processes adversely by caudalizing the host axis, although these reagents target different biochemical processes (Nicolet 1965; Joshi et al 1990; Ghatpande et al 1991, 1993; Modak et al 1993). Therefore, caudalization has been suggested to be under a negative cell cycle control (Ghatpande et al 1991, 1993; Modak et al 1993; Yamada 1994, 1995). The purpose of this study was to test this hypothesis by grafting posterior mesoderm near the host axis.…”

Cell proliferation and protein synthesis as initial factors in determination of axial polarity

“…1). Previously, evidence that metabolic gradients (Child 1941) and cell proliferation (Yamada 1995) play a role in axial patterning has been provided. In the present paper these ideas have been extended by consideration of the rate of protein synthesis in relation to that of cell proliferation.…”

“…Yamada (1995) has provided evidence that ectodermal dorsalization depends on stimulation of cell proliferation in amphibian embryos, while caudalization may depend on a negative mitogenic signal. The present paper will examine the roles of proliferation and protein synthesis in the initial stages of establishment of anterior–posterior and dorsal–ventral axial polarities in developing animals.…”