Follicular development and expression of nuclear respiratory factor-1 and peroxisome proliferator-activated receptor γ coactivator-1 alpha in ovaries of fetal and neonatal doelings1

Abstract

In livestock, the ovarian reserve of follicles is established during the fetal stage. However, at least two-thirds of the oocytes present in the reserve die because of apoptosis before birth. Notably, mitochondria have been reported to play a crucial role in the fate (life/death) of oocytes. In this study, mitochondrial regulators nuclear respiratory factor-1 (NRF-1) and PPAR γ coactivator-1 alpha (PGC-1α) were examined during this period of follicle development to investigate their effects on follicular development and apoptosis. Fetal and neonatal Capra haimen were used, ranging in age from 60 d postcoitum (dpc) to 30 d postpartum (dpp). Our data demonstrated that egg nests were the earliest recognizable gamete cells in ovaries of fetal and neonatal doelings. Proportions of egg nests decreased from 92.68 to 25.08% whereas single follicles increased from 7.32 to 74.92% between 60 and 120 dpc. Subsequently, between 90 and 120 dpc, the proportion of primordial follicles increased from 9.98 to 61.56% (P < 0.01). However, it did not change between 1 and 30 dpp (P = 0.12). The proportion of primary follicles increased from 1.23 to 37.93% between 90 dpc to 1 dpp (P = 0.01) but did not change between 1 and 30 dpp (P = 0.11). Meanwhile, proportions of secondary and tertiary follicles increased in an age-dependent manner. In addition, results of this study suggested that NRF-1 and PGC-1α proteins are mainly localized in germ cells of egg nests, cytoplasm of oocytes, and granulosa cells of follicles ranging from primordial to tertiary follicles. The transcript abundance of NRF-1 mRNA was up-regulated in 60-dpc-old ovaries compared with 1-dpp-old ovaries (P < 0.05), but the PGC-1α mRNA expression pattern did not change (P = 0.05). Nevertheless, the number of terminal deoxynucleotidyltransferase UTP nick-end labeling (TUNEL) positive cells and caspase-3 activity in 60-dpc-old ovaries was less than those in 1-dpp-old ovaries (P < 0.01, P = 0.01). In conclusion, our results demonstrate that the key stage of primordial follicle formation is between 90 and 120 dpc in Capra haimen. Also, this study suggests that NRF-1 and PGC-1α might have roles in cell apoptosis during ovarian development of fetal and neonatal Capra haimen. These results improve our understanding of apoptotic mechanisms in oogenesis and folliculogenesis.