Expression and Immunolocalisation of Follicle-stimulating Hormone Receptors in Gonads of Newborn and Adult Female Horses

In mares, FSH and its receptor (FSHR) are essential for ovarian function. The objective of the present study was to analyse FSHR gene expression at the mRNA and protein levels in ovarian tissue from newborn and adult horses. Expression of mRNA was analysed by reverse transcription polymerase chain reaction, whereas FSHR protein was visualised by immunohistochemistry (IHC), immunofluorescence labelling (IF) and western blot. FSHR mRNA was detected in ovarian follicles and luteal tissue from adult mares, as well as in the ovaries of neonates. Follicular growth up to 4mm in diameter was already present in neonates. Using IHC and IF, FSHR protein was detected in granulosa cells, cumulus cells and inconsistently in oocytes, independent of the animal's age or the stage of folliculogenesis. A lower FSHR expression was observed in theca cells in comparison to granulosa cells. FSHR was abundant in the ovarian stroma cells of neonates but not of adults. Luteal cells stained positive for FSHR independent of the stage of corpus luteum development. The presence of FSHR protein in various cell populations of the ovary was confirmed by western blot. In conclusion, FSHR is present in horse ovaries consistently from birth onwards and expression remains constant during the oestrous cycle.