Accumulation, organization and deployment of oogenetically derived Xenopus yolk/nonyolk proteins.

The animal/vegetal (A/V) polarity of the typical amphibian egg is immediately recognizable by the distinct pigmentation differences between the darkly pigmented (animal) and lightly pigmented (vegetal) hemispheres. That polarity arises early in oogenesis. Prior to the vitellogenic period during which yolk platelets accumulate and the original (uniform) pigmentation pattern changes, a polarized distribution of several internal components can be detected. Ribosomal DNA accumulates within a localized area in the nucleus (Brachet, 1977). The Balbiani body (containing large numbers of mitochondria) is located outside and to one side of the nucleus, while the nucleoli are later segregated within the nucleus on the opposite side (Billett & Adam, 1976). By the end of the previtellogenic period the Balbiani body (mitochondrial cloud) has moved to the periphery, marking the future vegetal pole (Heasman, Quarmby & Wylie, 1984). During early vitellogenesis the distribution of yolk in an animal/vegetal gradient pattern may be related to the 'slightly off center' position of the germinal vesicle (Nieuwkoop, Johnen & Rzehak, 1985). The uneven accumulation along the A/V axis of mRNA (Capco & Jeffery, 1982; Phillips, 1982; Carpenter & Klein, 1982; King & Barklis, 1985; Rebagliati, Weeks, Harvey & Melton, 1985; Smith, 1986) could conceivably also be initiated by those early asymmetries in oocyte organelle distribution. Nieuwkoop (1977) has speculated that those anuran egg A/V asymmetries in organelle distribution may be vertically transferred from generation to generation through the cytoplasmic continuity of the germplasm. The anuran germplasm is localized in the vegetal pole (Bounoure, 1934) and is easily visualized with contemporary histological techniques (e.g. Smith & Neff, 1985). It can be tracked through embryogenesis, and the cells that contain it (primordial germ cells) develop into the gametes of the next generation. The absence of a localized