Abstract
Follicle waves represent the synchronous and rapid growth of a cohort of early antral follicles in response to increasing follicle-stimulating hormone (FSH) levels. During the menstrual/estrous cycle, a major follicular wave produces one dominant follicle in mono-ovulatory species or multiple in polyovulatory species, while the other growing follicles regress (atresia). This process is called follicle selection, but the underlying mechanisms are poorly understood. Previous studies hinted that follicle growing-speed and FSH-receptivity are possible predictors for the fate of follicles. The dominant follicle is hypothesised to acquire its characteristics since its conception, through oocyte control.
RT-qPCR was used to assess the correlation between follicle growing-speed determined by expression of the proliferation marker, (Pcna) and FSH receptor (Fshr) expression in early antral (150-200m) granulosa cells. Fshr was positively correlated with the Pcna, supporting the concept that fast-growing follicles were more receptive to FSH. However, Fshr was slightly variable among the fast-growing follicles, suggesting that Fshr was not solely responsible for follicle growing speed.
Pcna expression was used for classifying non-atretic follicles into fast-growing, and slow-growing groups. RNA-sequencing was performed to identify differentially expressed genes (DEGs) between oocytes from fast- and slow-growing follicles. The transcriptome of fast-growing follicle oocytes were variable, but ~30% presented distinct DEGs, heavily contrasting with the more homogeneous slow-growing follicle oocytes. Metabolic processes were upregulated in fast-growing follicle oocytes, indicating competency of keeping up with maturation-related energy demands. These oocytes also had upregulated antioxidant genes, suggesting that they were capable of maintaining redox homeostasis in response to the metabolic oxidative stress. Furthermore, growth hormone response was upregulated in the fast-growing follicle oocytes. Negative regulation in TGF signalling was also enriched, which may be a negative feedback for increased signalling. Further studies should be performed to test whether the follicles with oocytes presenting distinct DEGs were indeed ovulatory.
RNA-sequencing was also performed on superovulated and naturally-ovulated oocytes. Compared with the early antral oocytes, the superovulated oocyte transcriptomes were similar to naturally-ovulated oocytes, but a large number of DEGs were identified between these groups. DNA methylation-related genes were altered in the superovulated oocytes, which may relate to imprinting errors observed in assisted reproduction embryos. Meiotic cellular reorganisation processes also seemed to be dysregulated in superovulated oocytes. Furthermore, metabolic processes were upregulated in superovulated oocytes, which could indicate poor quality as metabolic activity is usually low in mature oocytes. These findings suggested that fast-tracking follicle development with gonadotropin stimulation may dysregulate the processes of oocyte maturation.
The findings in this study partially supported the hypothesis that fast-growing follicles contain high quality oocytes. Further studies should be performed to test whether the fast-growing follicles with the distinct DEGs were ovulatory. The quality of an oocyte may be used to predict follicle selection, but the precisely what makes an oocyte “good” quality is still unknown.