Around the middle of pregnancy, the primary oocytes become surrounded by follicular cells (epithelial granulosa cells) originating from the mesonephros, and perhaps also from the coelomic epithelium. These cells correspond to the Sertoli cells in the male gonad. Initially, the envelope of follicular cells is flat and has one layer (primordial follicle). Some of the primordial follicles develop into primary follicles before birth. The primary oocyte increases in volume. The follicular cells have a variety of shapes; some are cuboidal and some look columnar. A glycoprotein layer, believed to be secreted by the oocyte and the follicular cells, develops between them. During the postnatal period and under the influence of the gonadotropic hypophyseal hormones FSH (follicle-stimulating hormone) and LH (luteinizing hormone), the oocyte expands. The follicular epithelium becomes multilayered, and the glycoprotein layer becomes the zona pellucida: in this way, the secondary follicle develops. Stroma cells are arranged in a circular pattern in the vicinity of the secondary follicle and form the theca folliculi, which is separated from the follicular cells by a delicate basal lamina.