primary ovarian follicle

A primary ovarian follicle is characterized by a primary oocyte surrounded by a single complete layer of cuboidal granulosa cells (distinguishing it from the flattened cells of the primordial stage). The zona pellucida — a glycoprotein coat around the oocyte — begins to form at this stage.

Primordial follicles have a primary oocyte surrounded by a single layer of flattened pre-granulosa cells. Primary follicles have one complete layer of cuboidal granulosa cells. Secondary follicles have multiple granulosa cell layers (stratified granulosa) but have not yet formed an antrum.

Primary follicle growth is driven primarily by paracrine signals from the oocyte itself and the surrounding granulosa cells, with some FSH (follicle-stimulating hormone) sensitivity. Oocyte-derived growth factors (GDF-9, BMP-15) are particularly important for granulosa cell proliferation at this stage.