Ultrastructure of oogenesis in an ovoviviparous and an oviparous freshwater snail (caenogastropoda)

The ultrastructural features of oogenesis are described in an ovoviviparous (Melanoides tuberculata) and an oviparous (Melanopsis buccinoidea), freshwater snail. Yolk formation in both species appears to be primarily autosynthetic. In M. tuberculata protein yolk, formed by the endoplasmic reticulum and Golgi bodies, appears first. As vitellogenesis progresses, the ooplasm not only accumulates two types (based on their morphological appearance) of large protein yolk granules and quantities of glycogen, but also some lipid. By contrast in M. buccinoidea early stages of vitellogenesis involve the formation of large amounts of lipid (the main storage product in these eggs). Mid- to late-stage oocytes begin to accumulate one type of protein yolk granule and glycogen. In both species follicle cells are closely associated with the developing oocytes and it is suggested that they play an essential role in oocyte nutrition.