New Insights into Human Oocytes

The study, led by Pauliina Damdimopoulou, Senior Lecturer and Research Group Leader at the Department of Clinical Science, Intervention and Technology, provides a detailed comparison of immature follicles from both children and adults. The findings reveal significant previously unknown variations in immature oocytes and identify key markers involved in the early growth activation of follicles.

Understanding Ovarian Follicles

Ovarian follicles are essential for female fertility, forming during fetal development and establishing a finite reserve of approximately one million primordial follicles. Each follicle contains one immature oocyte that is arrested in development and stays dormant until it is activated to grow. The activation and growth of follicles to ovulation is a process that spans about a year in humans. While full growth to ovulation is only possible after puberty, the early stages of growth occur even in children. This study focuses on the early arrested follicles and their first stages of growth, providing the first ever comparison between adult and child follicles. 

Key Findings

The research team analyzed the transcriptomes of 120 immature follicles from both children and adults. They discovered two main types of follicles in both age groups: one with a gene expression profile typical of immature oocytes, and another with very low oocyte markers but a predicted role in signaling. This suggests that a subset of follicles may not be focused on maturing their oocytes but instead may play a role in secreting growth factors, potentially supporting the growth of other follicles.

While transcriptional changes during early follicle growth were similar overall in both children and adults, variations related to the extracellular matrix, theca cells, and miRNA profiles were observed. This suggests that the immature follicles formed during fetal development undergo changes as girls reach sexual maturity. The study also found that follicle morphology alone is insufficient for reliably categorizing follicles, highlighting the need for additional markers. The identification of marker genes for follicle subtypes and early growth could help refine methods for fertility assessment in women. 

Implications for Fertility Preservation

Children and women undergoing cancer treatments are at a higher risk of infertility because chemotherapy is toxic to follicles. Some of the samples in the study were derived from children undergoing fertility preservation, a technique in which a piece of an ovary containing immature follicles is removed and cryopreserved before the damaging treatments begin. The authors found that follicles from treated children displayed increased inflammation. It is not known how inflammation in follicles might impact the quality of oocytes.