An electron microscopic study of oogenesis in Marenzelleria viridis (Verrill 1873) (Polychaeta; Spionidae) with special reference to large cortical alveoli

Summary

Oogenesis in Marenzelleria viridis was studied by ultrastructural investigation. The ovaries are formed on the greatly ramified nephridial blood vessels and are enveloped by a thin layer of follicle cells which contain glycogen and a few lipid droplets. The oogonia vary in shape, are about 10 μm in diameter and are not linked by intercellular bridges. Developing previtellogenic and vitellogenic oocytes remain inside the ovary until a late stage of oogenesis. The vitellogenic oocytes contain abundant proteosynthetic rough endoplasmic reticulum and active Golgi complexes. Early developing oocytes are intimately associated with the perivascular cells of blood vessels. There are two possible pathways of uptake of exogenous precursors by the growing oocytes. Low molecular weight precursors may enter the oocytes via the abundant spherical surface granules of the microvilli tips. Endocytotic vesicles along the oolemma indicate the probable uptake of large molecular weight precursors via perivascular cells of blood vessels and the egg envelope. Mature oocytes contain 10 to 18 cortical alveoli each up ta 26 μm in size. The ultrastructure of developing eggs in M. viridis is compared with those in other polychaete species. The importance of slow oogenesis for this species, which lives in brackish shallow habitats, is discussed.

Key words:

• Cortical alveoli
• cortical granules
• endocytosis
• intraovarian oogenesis
• Marenzelleria viridis
• polychaete