An electron microscope study of differentiation and growth in oocytes of Ophioderma panamensis

Abstract

Electron microscope studies on oocytes of the ophiuroid Ophioderma panamensis indicate the numerous nucleoli to be active throughout much of oogenesis. The nucleoli appear to “spin out” coarse strands of granules and fibers from their periphery (nucleolonema), and become attached at certain points to the inner layer of the nuclear envelope. The nucleolonema appear to fragment into small granular clusters or granule-filament complexes, both of which are also observed in the cytoplasm. During the period of nucleolar activity, mitochondria illustrate a special relationship to the nuclear surface, being preferentially aligned adjacent to the outer layer of the nuclear envelope. Proteid yolk granules first become apparent within certain of the Golgi cisternae. The origin of cortical granules also appears to be associated with the Golgi complexes. During oocyte differentiation an extensive membrane system of vesicles and lamellae, part rough- and part smooth-surfaced, forms in the peripheral portion of the ooplasm. The morphological variations observed during oocyte growth indicate that this membrane system differentiates into (a) annulate lamellae, (b) endoplasmic reticulum which becomes organized into concentric whorls of membranes (within which lipid droplets appear), and (c) single lamellae of endoplasmic reticulum which partially surround each of the Golgi complexes in the cell. The specific arrangement and association between the Golgi complexes and endoplasmic reticulum strongly suggest that the two organelles play a role in the elaboration of proteid yolk granules and cortical granules.