Differentiation of the Cortical Cytoplasm and Inclusions in Oocytes of the Frog

Studies on differentiation of endodermal derivatives in amphibian embryos (Kemp, 1946, 1951) brought to my attention the well known fact that endodermal cells generally retain their stored yolk much longer than ectodermal or mesodermal cells. This observation aroused my curiosity concerning the general problems of synthesis and utilization of yolk in oocytes and embryos. The opportunity to apply electron microscopy to these problems came while I was working with Dr. T. N. Tahmisian at the Argonne National Laboratory during the summers of 1954 and 1955 and has been continued at the University of Michigan, where Dr. R. E. Hartman has kindly permitted me to ~use the electron microscope at the School of Public Health. This short report is based on work to be published in detail elsewhere (Kemp, 1956). Before the young oocyte starts to synthesize lipide or protein inclusions, it has already started to grow (Kemp, 1953). Lampbrush chromosomes (Dodson, 1948; Gall, 1954) produce many nucleoli, which move to the periphery of the nucleus as they grow (Figs. I and 2). I have not observed the direct passage of nucleoli or nucleolar fragments through the porous nuclear membrane, but small granules and rods appear in the perinuclear cytoplasm after the nuclear membrane starts to fold (Fig. 3). Lipide droplets called lipochondria by Holtfreter (1946) soon appear throughout the cytoplasm either independently or clustered about yolk nuclei (Fig. 4). At the same time, the follicular epithelial'cells adjacent to the oocyte pull away partially, thus leaving spaces into which grow microvilli erupting from the surface of the oocyte. Yolk platelets first appear in the peripheral endoplasm. In an oocyte at stage YI, measuring about 350 ~z in diameter, the cortical cytoplasm has differentiated into three distinct regions (Fig. 5). Bordering on the peripheral endoplasm is a layer containing distinctive cortical granules, first described for the frog by Motomura (1952). External to these granules is a basal cortical layer from which emerge the microvilli comprising the outermost cortical zone. Processes from follicular cells extend inward toward the microvillous layer and sometimes appear to interdigitate with microvilli (Fig. 6).