Polysaccharide Complexes in Full‐Grown Oocytes of the Newt, Pleurodeles, and Changes in their Distribution During Progesterone‐Induced Maturation

Abstract

Immature full-grown oocytes of Pleurodeles waltlii contain large amounts of small electron-dense polysaccharidic granules. These granules lack a limiting membrane, and have a dense but heterogeneous matrix and an apparent diameter of 24-36 nm. Their structure, organization and distribution strongly suggest that they are glycogen granules. On the other hand, mature oocytes both after oviposition or 22-24 hr after in vitro progesterone stimulation contain no polysaccharide granules or complexes. During the first 9-10 hr after hormonal stimulation, granules were abundant and present both individually and as large strands occupying most of the space between the organelles. Granules were frequeqtly found packed together and arranged in regularly arrayed stacks within large subcortical ant cortical vacuoles. Between 4 and 6 hr after progesterone addition, oocytes released the contents of vacuoles to the outside. Between about 11 and 14 hr after progesterone addition, oocytes still contained large amounts of polysaccharide complexes, but the vacuoles were empty. From about 15 hr after progesterone treatment until the end of maturation, the complexes progressively disappeared from the cytoplasm, coincident with the detachment of the follicle cell layer from the oocytes and a reduction in the number and size of microvilli.Oocyte maturation is a complex process by which meiosis of full-grown oocytes is reinitiated starting from the stage of the first meiotic prophase and ending at the stage at which fertilization takes place, that is the second meiotic metaphase in amphibians. Full-grown urodele oocytes at stage VI of oogenesis can be induced to undergo maturation in vitro by progesterone (1-3). The earliest response to progesterone so far reported is intracellular release of calcium ions (4). Among the other most obvious early changes occurring during urodele maturation are the movement of the germinal vesicle toward the surface of the animal pole and its subsequent breakdown (3) and increase in the number of annulate lamellae until the stage of germinal vesicle breakdown (5). However, there is to date very little information available on urodele oocytes, and in fact there are onIy two reports describing and classifying the different steps of oogenesis, in Pleurodeles waltlii (6) and Triturus viridescens (7). Some particular features have been addressed, such as yolk formation (8, 9) and follicle-oocyte relations (7, 10) in developing oocytes of Triturus, annulate lamellae in developing oocytes (1 1) and in maturing oocytes (9, the cytoskeleton of full-grown oocytes (12) and the organization of the germinal vesicle and of the nucleoli of maturing oocytes of Notophthalmus (3) and Cynops (13). In the present study, a * This work on the extracellular release of polysaccharides during urodele oocyte maturation was carried out in collaboration with Bertrand Picheral, who died in 1983. This paper is dedicated to his memory. 763