What is the characteristic feature differentiating secondary follicles from primary follicles?

Primary follicle: It is an immature ovarian follicle surrounded by single layer of cuboidal cells. They are known as granulosa cells.

Secondary follicles: The primary follicle develops into a secondary follicle. It consists of many layers of cuboidal cells known as membrana granulosa cells. It secretes follicular fluid.

Primary follicles	Secondary follicles
A primary follicle is a follicle that grows when a primordial follicle is stimulated.	The third stage of follicle development is known as a secondary follicle. It develops from the main follicle.
2. It is a preantral follicle without a cavity filled with fluid.	2. Similar to the primary follicle, the secondary follicle is devoid of a hollow that is filled with fluid. It is therefore a preantral follicle.
3. The primary follicle is a tiny follicle with only one layer of follicular cells.	3. Secondary follicles are structurally bigger than main follicles.
4. Additionally, it is an immature ovarian follicle with a few specialized epithelial cells around an immature ovum.	4. The secondary follicle's main oocyte is encircled by granulosa cells in many layers, a basal lamina, and a theca layer.