THE ORIGIN OF CORTICAL VESICLES AND THEIR ROLE IN EGG ENVELOPE FORMATION IN THE SPINY EGGS OF A CALANOID COPEPOD, CENTROPAGES-VELIFICATUS

The mature oocytes of the marine calanoid copepod, Centropages velificatus, contain two morphologically distinct populations of cortical vesicles that undergo sequential exocytoses at the time of spawning. The contents of the primary cortical vesicles are released first and form the primary egg envelope. This is followed by the exocytosis of the secondary cortical vesicles. These contain numerous intracisternal granules that, upon release into the perivitelline space, transform into a mass of fine fibers. The continual accumulation of fibers constitutes an extracellular matrix between the primary envelope and the egg's plasmalemma. Further amassment of the fibers beneath the primary egg envelope results in the formation of long, spiny projections. The evolution of the cortical vesicles was traced to the early vitellogenic oocytes and appears to be unique. The two populations of cortical vesicles are synthesized together within the same cisternal elements of rough endoplasmic reticulum (RER). The RER originates from membranous blebs off both the nuclear membrane and stacks of annulate lamellae in the early vitellogenic oocytes. Numerous intracisternal granules are present within the RER. Some of these granules fuse, forming a dense, ring-like structure in the extremities of the cisternae. These bud off from the RER to become the primary cortical vesicles. The unfused intracisternal granules remain as discrete bodies within irregular profiles of vesicular ER and comprise the secondary cortical vesicles.