Relationship among follicular apoptosis, integrin β1 and collagen type IV during early ovarian regression in the teleost Prochilodus argenteus after induced spawning

Early ovarian regression was analyzed in the neotropical freshwater teleost, curimata-pacu (Prochilodus argenteus), in order to evaluate follicular apoptosis, basement membrane morphology, and integrin beta(1) and collagen type IV immunostainning in postovulatory follicles. Mature females were induced to spawn by using carp pituitary extract for study of ovarian regression up to 5 days post-spawning. Morphometric analyses showed that the postovulatory follicle area decreased progressively after spawning and was coupled to the gonadosomatic index (r=0.92). During ovarian regression, follicular cells detached from the neighboring cells and basement membrane and then died by apoptosis. The follicular basement membrane became thicker and diffuse and was breached during regression of the postovulatory follicles. Follicular apoptosis was detected by TUNEL, histology, and electron microscopy. The ladder pattern of apoptotic DNA was revealed by agarose gel electrophoresis. The apoptotic index for the follicular cells increased until 3 days post-spawning and decreased thereafter. Immunohistochemistry reactions detected caspase 3, integrin beta(1), and collagen type IV in the follicular layer of the postovulatory follicles. Labeling for integrin beta(1) and collagen type IV decreased significantly, whereas a peak in cell death occurred 3 days post-spawning. At 4-5 days post-spawning, the connective theca was more thickened and vascularized. Simultaneously, granulocytes migrated toward the follicular lumen. Thus, follicular apoptosis contributes to early ovarian regression in P. argenteus. Additionally, our findings suggest integrin beta(1) and collagen type IV as possible survival factors for follicular cells in teleost ovary.