Cytoarchitecture of Caudiverbera caudiverbera stage VI oocytes: a light and electron microscope study

The general characteristics and salient features of the full-grown stage VI Caudiverbera caudiverbera oocyte at the light and electron microscopy leve l are described. The oocyte is a huge cell with radial symmetry and distinc t polarity. A black animal hemisphere, rich in pigment granules and contain ing the nucleus, is clearly distinguished from the unpigmented white-yellow ish vegetal hemisphere. The cell is surrounded by a highly invaginated plas ma membrane, with numerous microvilli. The cortex underlying the plasma mem brane contains cortical and pigment granules, mitochondria, rough endoplasm ic reticulum and coated vesicles. Cytoskeletal components, such as actin fi laments and microtubules, are also found in this region. The predominant st ructures, distributed throughout the cell, are the yolk platelets, which sh ow a gradient in size with small platelets in the animal half and very larg e ones in the vegetal zone. Mitochondria are also very abundant in both hem ispheres and clouds of these organelles are found in the perinuclear region , frequently associated with microtubules. Developed Golgi complexes are pr esent in the cytoplasm and occasionally, annulate lamellae appear towards t he inner zones. The nucleus is a large structure containing numerous nucleo li. The nuclear envelope is highly invaginated, especially at the side faci ng the vegetal pole. It is regularly perforated by large nuclear pores. Our results show that the structural organization of Caudiverbera oocytes, alt hough similar to that of other amphibian oocytes. differs from them especia lly concerning the spatial distribution of several structural components.