Mesoderm induction and mesoderm-inducing factors in early amphibian development

Abstract

Recently, significant advances have been made in the analysis of mesoderm induction, one of the first inductive interactions in vertebrate development. In this article, I review these advances in the context of earlier work on the subject and go on to discuss what problems remain and how they might be approached. I shall concentrate on cell biological and embryological aspects of mesoderm induction rather than on the transcriptional control of genes which are activated in response to induction. This work has recently been reviewed elsewhere (Gurdon et al. 1989).Mesoderm induction probably occurs during the development of all vertebrates (Nieuwkoop et al. 1985; Smith, 1988) but the phenomenon has been most extensively studied in the Amphibia. This is for several reasons: amphibian embryos are large, making micro-dissection relatively easy; they are accessible to experimental manipulation at all developmental stages, from oogenesis to adulthood; development is rapid: the body plan is established and tissue-specific gene activation occurs within 24 h; amphibian embryos may be obtained in large numbers, which, together with their large size, makes it possible to extract enough material for biochemical analysis; and finally, the embryos do not grow - their early development consists of a series of cleavage divisions (see Fig. 1). This last point means, firstly, that it is possible to construct fate maps by injecting inert lineage tracers into selected blastomeres; because there is no growth the markers do not become diluted during development (see, for example, Jacobson & Hirose, 1978, 1981; Gimlich & Cooke, 1983; Gimlich & Gerhart, 1984; Heasman et al. 1984; Cooke & Webber, 1985; Dale & Slack, 1987a; Moody, 1987a,b; see also Fig. 2). Furthermore, since embryonic blastomeres survive on their yolk reserves, they will divide and even differentiate in a simple buffered salts solution. Thus it is possible to test defined molecules for their effects on differentiation without interference by poorly characterized serum components. In view of these advantages, this review deals exclusively with mesoderm induction in Amphibia.Mesoderm induction was discovered using the techniques of experimental embryology. At morula stages, the amphibian embryo seems to consist of only two cell types: prospective ectoderm in the animal hemisphere and prospective endoderm in the vegetal hemisphere (Fig. 3A; see Jones & Woodland, 1986). The evidence for this comes from experiments in which different regions of the embryo are dissected and cultured in isolation: even blastomeres of the equatorial ‘marginal zone’, which give rise to mesoderm in normal development, form epidermis if they are isolated before the 64-cell stage.If, however, they are dissected later than this they form mesoderm as well as epidermis (Nakamura & Matsuzawa, 1967; Nakamura et al. 1970b).One interpretation of this result is that mesoderm formation depends on an interaction between animal and vegetal blastomeres. This conclusion was confirmed by Ogi (1%7, 1%9) and by Nieuwkoop (1969), both of whom juxtaposed cells from the animal cap of the blastula-staged embryo with cells of the vegetal pole (Fig. 3B). When animal pole cells are cultured alone they form an atypical type of epidermis, while vegetal pole cells alone form poorly differentiated endoderm. Neither make mesoderm. Combinations of animal and vegetal pole cells, however, form a variety of mesodermal cell types.Neither Ogi nor Nieuwkoop had access to cell lineage markers and they differed in the interpretation of their experiment. Ogi, with tentative support from Nakamura et al. (1970a), was reminded of work on the seaurchin embryo indicating the existence of an animalvegetal double-gradient system (Horstadius, 1935). According to this view, apposition of animal and vegetal regions should result in regulation of the gradients, with both components giving rise to mesoderm. Nieuwkoop, however, regarded the mesoderm as being formed entirely from the ectodermal component, as a result of induction by prospective endoderm. He subsequently demonstrated that this view was correct by a quantitative analysis of the structures formed from animalvegetal combinations of blastomeres from Xenopus embryos (Sudarwati & Nieuwkoop, 1971) and by using [3H]thymidine to mark the animal pole component of combinations made from axolotl embryos (Nieuwkoop & Ubbels, 1972).Nieuwkoop made a second major contribution to the understanding of mesoderm induction by demonstrating that the type of mesoderm that forms in animal-vegetal combinations depends on the origin of the vegetal inducing cells (Boterenbrood & Nieuwkoop, 1973). Vegetal pole cells from the dorsal side of the axolotl blastula tended to induce dorsal cell types such as notochord and muscle while lateral and ventral vegetal blastomeres induced blood, a characteristic ventral cell type, along with mesenchyme and meso-thelium. Nieuwkoop concluded from this that the pattern of c