Generation of multi-oocyte follicles in the peripubertal rat ovary: link to the invasive capacity of granulosa cells?

Abstract

Objective: To assess the presence and numbers of multi-oocyte follicles (MOFs) in the rat ovary at different stages of postnatal maturation.

Design: Animal (rat) study.

Setting: Research laboratory.

Animal(s): Female Wistar rats.

Intervention(s): Histologic/morphometric analyses in ovaries from infantile, juvenile, pubertal, and adult female rats.

Main outcome measure(s): Numbers and characteristics of MOFs in rat ovaries at different stages of postnatal maturation.

Result(s): Female rats displayed low numbers (<5/ovary) of MOFs in the infantile period (postnatal day [PND] 15). The occurrence of MOFs increased sharply by PND-21 and remained at high values (>15/ovary) up to PND-60, to decline thereafter by PND-90. The presence of irregularly shaped and connected adjacent follicles, together with the identification of ruptures at the follicle surface and the occasional invasion of the ovarian stroma by granulosa cells, strongly suggests that the majority of MOFs in peripubertal rats are generated by fusion of adjacent growing follicles.

Conclusion(s): A new mechanism for the generation of MOFs linked to the potential invasive capacity of granulosa cells is proposed. The basis for the upsurge in the generation of MOFs during the peripubertal period and whether, as predictable, this phenomenon is applicable to other mammalian species warrant further investigation.

Keywords: Multi-oocyte follicles; follicle fusion; granulosa cells; ovary.