Observations on the primordial oocyte of the bandicoot Isoodon macrourus (Peramelidae, Marsupialia).

An ultrastructural study of bandicoot primordial follicles and oocytes was undertaken, as information on this subject is lacking in marsupials. Conspiculous features of the ooplasm are a paranuclear complex (PNC), a vesicle-microtubule complex (VMC) and an aggregate of tubular cisternae (ATC). The PNCappears as one or, more rarely, several homogeneous eosinophil bodies at the light microscope level. Ultrastructurally it is particulate, consisting of five distinct types of bodies, most of which are composed of concentric fibrillar whorls, but others appear homogeneous, granular or crystalline. Embedded among the particles is a group of Golgi-like vesicles. The bandicoot PNC-unlike similar structures found in the ooplasm of a variety of vertebrates, and known variously as "Balbiani body", "yolk nucleus", etc.-totally lacks nitochondria. The VMC consists of vesicle-like organelles which may be drawn out into tubular extensions, while the bounding membrane may be decorated with granules. Bundles of microtubules ramify between the vesicles, from which they appear to originate. The vesicles contain a matrix similar to the ooplasm. The ATC contains a homogeneous substance more electron-dense than the surrounding ooplasm. 'Dense bodies' occur in the cytoplasm of both the follicle cells and the oocytes. These are elongate membrane-bound organelles, circular in cross section. An electron-dense core is separated from the membrane by a narrow, less dense zone. The genesis and morphogenetic significance of these various organelles is unknown.