PROLIFERATING CELL NUCLEAR ANTIGEN MARKS THE INITIATION OF FOLLICULAR-GROWTH IN THE RAT

Despite expanding knowledge on the kinetic aspects of folliculogenesis, the question of what initiates follicle growth remains unanswered, Efforts to solve this problem have been thwarted by the absence of sensitive markers to identify the onset of follicular growth. In this study, we determined whether increased proliferating cell nuclear antigen (PCNA) correlates with initiation of follicle growth and might therefore be useful for studying early events in this process. Paraffin sections of ovaries from cycling adult rats, prepubertal eCG + hCG-primed rats, and prepubertal control rats were processed for immunocytochemistry by use of a PCNA primary antibody, In primordial follicles, neither granulosa cells nor oocytes stained for PCNA. PCNA immunoreactivity coincided with the earliest sign of follicle growth, appearing in pregranulosa cells of early primary follicles just beginning to grow, In all primary follicles, some granulosa cells were PCNA-positive, PCNA immunoreactivity in oocytes first appeared in primary follicles, preceding oocyte enlargement. In preantral and antral follicles, granulosa cell PCNA staining was uniform throughout the granulosa cell layers. Oocytes were positive for PCNA in both preantral and antral follicles. PCNA expression diminished in atretic follicles, In CL, granulosa cell PCNA expression was also decreased. Theca cell PCNA expression was first evident in the transitional follicle (1-2-layer granulosa cells) and was present in all stages thereafter. The pattern of PCNA expression did not differ among adult cycling, prepubertal eCG + hCG-stimulated, and control rat ovaries. These results suggest that the expression of PCNA, an essential regulator of the cell cycle, marks the initiation of follicle growth, coinciding with and in some cases preceding the first sign of granulosa cell growth and preceding the oocyte growth.