Fertilization envelope in diapause eggs of Pontella mediterranea (Crustacea, Copepoda).

The sequence and timing of morphological changes during envelope formation was followed in diapause eggs of Pontella mediterranea (Crustacea, Copepoda). The multilayer coat enveloping these eggs resulted from the exocytosis of 4 types of cortical vesicles that sequentially released their contents in the perivitelline space. These included small high-density vesicles (hDV) with electron-dense material, vesicles (V) with dense ring granules and a uniform matrix contained within the same compartment, large high-density (HDV) vesicles, and large moderately dense (MDV) vesicles. All of these cortical vesicles were present in newly spawned, fertilized eggs. Their exocytosis resulted from egg activation. One of these cortical vesicles (V) was similar in morphology to the intracisternal granules precursors of endogenous yolk. Intracisternal granules, characteristic of previtellogenic oocytes of many crustaceans, were present in previtellogenic oocytes of P. mediterranea but disappeared in later stages of oocyte development once yolk formation was completed. We discuss the role of cortical vesicles in the formation of the complex extracellular coat enveloping copepod diapause eggs.