Oogenesis in the anuran Xenopus laevis can be divided into six stages based on the anatomy of the developing oocyte. Stage

Available evidence indicates that protein incorporation by amphibian oocytes is mediated through a process of micropinocytosis. Oocytes incubated under our standard conditions sequester protein at a constant rate for a t least 24 hours and appear to maintain approximately the same level of micropinocytosis as that observed in situ. Protein uptake is markedly sensitive to temperature and the ionic composition of the medium, with calcium being a particularly important component. Magnesium, phosphate, and additional energy sources are of only minor significance. Based on these observations, several slightly improved reformulations of the standard incubation medium are provided

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The relationship of the cells and tissues which comprise the developing ovarian follicle in Xenopus laevis has been studied with scanning and transmission electron microscopy. The saclike ovary is covered on its coelomic side by a squamous epithelium. The cells of this epithelium are extensively interdigitated, and each bears a short, centrally positioned cilium. The lumenal surface of the ovary is covered with a layer of nonciliated squamous cells. The areas of cell-cell contact are characterized by desmosomes in both epithelia, and between the epithelia lies a connective tissue layer-the theca-which contains collagen fibers, blood vessels, nerves, smooth muscle cells and oogonia. Beneath the theca in each follicle lies a single layer of flat stellate follicle cells. Associations between adjacent follicle cells are intermittent, leaving wide spaces or channels. Junctional contacts between neighboring follicle cells are characterized by desmosomes. From the basal surface of each follicle cell extend long, broad macrovilli which penetrate the underlying acellular vitelline envelope and contact the surface of the oocyte. Evidence is presented which suggests that follicle cells may produce and release components which participate in the formation of the vitelline envelope which consists of a 3-dimensional lattice of ropey fibers. Passageways through the vitelline envelope allow the maintenance of contact between oocyte and follicle cells and also allow ready penetration of materials both to the oocyte (e.g., vitellogenin) and from it (e.g., cortical granule material) at different stages of its development.The morphological and functional relationships between developing amphibian oocytes and their investing layers have been the subject of numerous investigations (see reviews by Wischnitzer "661 and Lofts "741). Developmental aspects of Xenopus oocytes, in particular, have been studied by transmission electron microscopy (TEM) and reported in detail This study of the Xenopus ovary confirms certain earlier observations made with light and TEM and, in addition, adds a new dimension and perspective t o our understanding of the organization and probable interactions of various cellular elements of the ovarian follicle. It is the intent of this report, then, to supplement, amplify, and clarify certain aspects of earlier studies of the developing ovarian follicle in Xenopus by examining new data obtained primarily with SEM and correlating it with data previously reported.