The present study describes in detail the morphological characteristics of the process of ovarian follicular atresia in Redbelly tilapia
(Coptodon zillii) during the nonbreeding season using light and electron microscopy and immunohistochemistry. The follicular regression
process was initiated with shrinkage and disintegration of the nuclear membrane of oocytes resulting in dispersing of chromatin within the
ooplasm, followed by marked hyperplasia and hypertrophy of follicular and granulosa cells, which exhibited a strong phagocytic activity to
engulf the liquefied yolk particles. Rodlet cells and granulocytes were recorded on the follicular wall and invaded the regressed follicles.
Rodlet cells expressed a strong immunoreactivity to matrix metalloperoxidase (MMP-9) and α-smooth muscle actin, while neutrophils
expressed a strong reactivity to Myeloperoxidase-3 (MPO). In the advanced stage of follicular atresia, the yolk was almost phagocytized
and resorbed and the regressed follicle lost its integrity and appeared to be formed of a cellular mass of phagocytic cells. Transmission electron
microscopy revealed the presence of neutrophils, eosinophils, and dendritic cells within the atretic follicle in between these phagocytic
cells. Moreover, numerous lysosomes, granules, and phagosomes were observed within the cytoplasm of both phagocytic cells and granulocytes.
Telocytes were also demonstrated within the highly thickened richly vascularized theca layer during the late stages of follicular atresia.
Immunohistochemical staining for caspase-3 established the participation of apoptosis in the advanced stages of follicular regression.
Immune cells, rodlet cells, and telocytes in combination with follicular cells play an essential role in follicular atresia. In conclusion, the
present study provides a new evidence on the role of both somatic and immune cells in the phenomenon of ovarian follicular atresia in
Redbelly tilapia (Coptodon zillii) during the nonbreeding season.