By definition this signals the end of the primary follicle. FSH, LH, and estrogen must be present for follicle to grow beyond primary follicle stage. Books are vague on exactly when FSH and LH are required. Estrogen, mostly estradiol, secretion starts sometime during the primary follicle stage. Some books say very early antral formation.
Antral cavity filled with liquor folliculi is evident but still forming.
As it gets larger, it causes the follicle to become larger. Follicular fluid is a transudate of plasma and granulosa cell secretions. Conversion of cholesterol to testosterone under influence of LH Granulosa cells pushed into multiple rows around antral cavity as they proliferate. Oocyte is pushed to one side on cavity and is surrounded by a few rows of granulosa cells called The corona radiata. Oocyte and corona together are called the cumulus oophorous.