Ovary: primary follicles

Primordial follicles are the smallest follicles, with their follicular cells forming a simple squamous layer. One follicle (red arrow) is in transition, lined with simple squamous follicular cells to the right of the arrow (primordial follicle) and by simple cuboidal follicular cells (primary unilaminar follicle) to the left of the arrow. A primary oocyte is housed within each follicle.

Primary unilaminar follicles are larger than primordial follicles; their follicular cells have increased in height to form a simple cuboidal or columnar epithelium. The primary oocyte has also increased in size from that present in the primordial follicle.

Follicular cells continue to increase in volume and number, becoming a stratified epithelial layer and forming a primary multilaminar follicle.

Each primordial and primary follicle contains a primary oocyte. Each oocyte increases in diameter as the follicular cells increase in size and number.

Stromal cells are located in the connective tissue surrounding the follicles. These connective tissue cells are multipotential and produce not only the connective tissue stroma of the ovary, but also differentiate into the thecal layers that surround advanced follicles.