Identification of a C1q family member associated with cortical granules and follicular cell apoptosis in Carassius auratus gibelio

C1q family proteins with C1q domain have been reported in vertebrates, but their biological roles are currently unknown. In this study, a C1q-like factor, designated Carassius auratus gibelio ovary-specific C1q-like factor (CagOC1q-like), was identified as a cortical granules component. Immunofluorescence localization revealed that the C1q family member was specifically expressed in follicular epithelial cells, and associated with cortical granules in fully grown oocytes. Moreover, it was discharged to the perivitelline space and egg envelope upon fertilization. As it is the first identified C1q family member that is expressed in follicular cells that surround oocyte, CagOC1q-like was applied to detection of follicular cell apoptosis and deletion. The entire cytological process of follicular cell apoptosis and deletion was clearly seen from double visualizations of follicular cells with CagOC1q-like immunofluorescence and apoptotic follicular cells labeled by terminal deoxynucleotidyl transferase (TdT)-mediated dUTP nick end labeling (TUNEL) during oocyte maturation and ovulation.