Ultrastructural study of oogenesis in Marphysa sanguinea (Annelida: Polychaeta: Eunicida) from the Lagoon of Tunis

Abstract

Ultrastructural features of oogenesis of the iteroparous and long lived eunicid polychaete Marphysa sanguinea (Montagu, 1815) from the Lagoon of Tunis were studied using both light and transmission electron microscopy methods. The ovaries consist of coelomic germ-cell clusters surrounded by a thin envelope of follicle cells derived from the peritoneum. They are attached to the genital blood vessels. Oogenesis is asynchronous; therefore, germ cells in premeiotic and previtellogenic phases are observed in one cluster. The oogenesis is of the extraovarian type. In each cluster the more differentiated oocytes detach and float free in the coelomic cavity where they undergo vitellogenesis as solitary cells. Inside the ovary, follicle cells are connected to young oocytes by intercellular bridges and data suggest that in the early stages of vitellogenesis, ribosomes, multivesicular bodies and dense bodies are transferred from the follicle cells to the oocytes through the intercellular bridges. Different morphological evidence supports the heterosynthetic (presence of endocytotic vesicles, abundant microvilli adorning the surface of the egg) and autosynthetic (proteosynthetic organelles well developed, formation of blebs by the external membrane of the nuclear envelope) origin of yolk in M. sanguinea. The cytoplasmic material of the mature oocytes is asymmetrically distributed; large lipid droplets (1.5 μm in size) and large yolk spheres (6 μm in size) occupy the vegetal pole of the oocyte while small yolk spheres (2-3 μm in size) and numerous membranous organelles (mitochondria, cisternae of endoplasmic reticulum) occupy the animal hemisphere. Spherical to elongated cortical granules up to 1.8 μm in size are located in the cortex of the mature oocytes. Finally, fibrogranular aggregates (“nuage”) similar to that within the nucleus are observed in the cortical ooplasm located in the animal pole of mature oocytes. These aggregates probably come from the nucleus and represent maternal cytoplasmic determinants of embryonic cell fate. Oocytes that have completed vitellogenesis measure 250–300 μm in diameter.

Keywords

• marphysa sanguinea
• annelid
• polychaete
• oogenesis
• vitellogenesis
• ultrastructure