The first stage of growth or further development of the follicle, in which the egg is enclosed by a single layer of round-shaped follicle cells, which are multiplying. The majority of primary follicles undergo atresia; a minority develop into secondary follicles.

The process by which a primary follicle or a tertiary follicle stops growing, leading to disappearance (apoptosis) of its follicle cells and the oocyte, or egg, they contain. Such a follicle is called an atretic follicle.

In medicine the word primary can denote primacy in one of 3 ways: (1) event-wise (e.g. primary amenorrhea is when there has been no precedent -- no prior period -- whereas secondary amenorrhea) follows prior periods, and likewise the distinction between primary infertility and secondary infertility, a distinction based on prior successful pregnancy); (2) developmentally or time-wise (e.g. a primary follicle gives rise to a secondary follicle, then to a tertiary follicle); or (3) causally (e.g. primary ovarian failure is based within the ovary itself, whereas secondary ovarian failure is secondary to failure of gonadotropins). These sort of distinctions is one reason why medical school takes a long time.

The resting, unstimulated stage of the follicle, in which the egg (as a primary oocyte) is enclosed by just a few thinly stretched follicle cells. Primordial follicles persist in the ovary from fetal life to the time of menopause, declining in number every day during this time, as some start to develop into primary follicles, most of which then are lost through the process of follicular atresia. What the stimulus or signal is for a particular primordial follicle to start growing remains completely unknown (it is independent of FSH).

The word is used medically in one of three ways: (1) part of a sequence, as in primary follicle, secondary follicle, tertiary follicle; (2) when a person has a precedent for a contrary state (e.g. secondary amenorrhea, when a woman has had at least one spontaneous menstrual period, but then menstruation stops; secondary dysmenorrhea means periods that have painful after having been not so -- or not-so-much; secondary infertility means having trouble getting pregnant despite having become pregnant successfully in the past); or (3) when the source of dysfunction lies elsewhere (e.g. secondary ovarian failure means ovaries that do not function because, in this example, the pituitary gland on which the ovaries depend is not functioning).