Molluscan Oogenesis

ARRIGHETTI, F.

Spawning: Biology, Sexual Strategies and Ecological Effects

Nova Publishers

Año: 2014; p. 27 - 48

General morphological aspects of molluscan female reproductive tract and oogenesis, in particular vitellogenesis, are reviewed in this chapter. Most molluscan are dioecious with the gonad located in the posterior part of the animal. In general, oogenesis is localized in a well-defined ovary, in where the growing oocyte is often in association with accessories cells that play an important role in its growth. Even though, in a number of species the transfer of nutrients from storage or digestive sites to the gonad has been proved. The disposition and number of accessory cells associated with single oocytes differ among molluscs and several functions have been attributed: synthesis and transfer of yolk precursor; synthesis and transfer of cytoplasmic organelles; the formation of egg envelopes; phagocytosis; hormone production; Transportation of the oocyte. Oogenesis comprises a proliferative phase (premeiotic stage) followed by a growth phase (previtellogenic and vitellogenic stage). The vitellogenic stage is highly variable since the composition and organization of yolk differs amongspecies and since different types of specialized cells may participate in the synthesis and transport of yolk or yolk precursors. The processes of yolk accumulation could be: autosynthetic, when yolk is synthesized by the oocyte itself; heterosynthetic when yolk is synthesized outside the oocyte ant then transported to it; or both. Mature oocytes display a wide range of egg envelope morphologies: primary envelopes are formed within the ovary, secondary envelopes are produced by accessory cells and tertiary envelopes formed by the accessory sex glands.