Follicle Maturation; During reproductive years, primordial follicles develop into primary follicles. Early follicle maturation involves enlargement of the follicle, proliferation of the follicular cells, and formation of a fluid-filled cavity (antrum). Then the cell grows to about 10 mm, and the primary oocyte is now surrounded by a clear layer, the zona pellucida and layers of follicular cells, the corona radiata