The presence and distribution of gap junctions in the oocyte-follicle cell complex of the zebrafish, Brachydanio rerio.

Membranes of teleost female gametes and investing layer of follicle cells characteristically exhibit a prolonged, close spatial relationship. This relationship is manifested, in part, by numerous folds of the plasma membrane of both oocyte and follicle cells that extend through channels called pore canals within a thick, laminated extracellular coat, the vitelline membrane. During oogenesis the folded oocyte processes, or microvilli, increase in number and length, but decrease in width. Follicle cell processes are frequently wider and less numerous than oocyte microvilli, but do not usually touch the unfolded portion of the oocyte surface. Throughout development, distal portions of oocyte microvilli contact either the proximal portion of follicle cell processes or unfolded parts of the follicle cell plasma membrane where many membranous specializations characteristic of gap junctions are present, especially during intermediate and later stages of oogenesis. Freeze-fracture replicas of the oocyte-follicle complexes revealed numerous localized aggregations of discrete intramembranous particles approximately 9-15 nm diameter that are characteristic of gap junctions. Gap junctions were located in the region of contact between the terminals of oocyte microvilli in relation to the different areas of the follicle cell plasma membrane. In ultrathin sections, the apposed plasma membranes of oocyte microvilli and follicle cells, in places, take the form of a seven-layered membrane approximately 18 nm thick which is bisected by a 2 nm gap that appears somewhat electron dense due to uranyl acetate stain. The location of gap junctions in the zebrafish oocyte-follicle is different from the situation in mammals where gap junctions are associated with cumulus cell processes as they make contact with the oocyte. The entire oocyte-follicle cell complex in the zebrafish contains the structural pathways for the transfer of small molecules between these cells.