Morphological Changes in the Thecal Layer during the Maturation of the Preovulatory Ovarian Follicle of the Domestic Fowl (Gallus domesticus)

Abstract

The aim of the present study was to evaluate changes in the steroidogenic tissue, the blood vessels and the undifferentiated cell islets observed in the theca of developing follicles in the hen ovary. Samples of preovulatory follicles (F1 to F5 in order of decreasing size) and small yellow follicles (SYF) were obtained from White Leghorn hens for light microscopy studies. Cell suspensions of the whole theca obtained from follicles at the same stage of development were analyzed for 17β-estradiol and testosterone secretion.Typical steroidogenic cells with abundant cytoplasmic lipid droplets were identified in the theca interna beside blood vessels and nerve fibers. Islets of undifferentiated cells were observed in the theca externa. The steroidogenic tissue of the theca interna was evident in the small yellow follicle. However, considering the growth of the developing follicle, the total volume of steroidogenic cells increased from the SYF up to F1 follicle. The blood vessels reached the maximum development in the largest follicle (F1). On the other hand, undifferentiated cell islets were markedly reduced in the F1 follicle. The structural changes herein described could be correlated to modifications in the steroid hormone secretion during the maturation of the ovarian follicle.