Characterization and distribution of gonadotrophs in the pars distalis and pars tuberalis of the equine pituitary gland during the estrous cycle and seasonal anestrus.

Little is known about the neuroendocrine control of fertility in the horse. In this species, unusual features characterize the normal estrous cycle such as a prolonged preovulatory LH surge during the follicular phase and a distinctive FSH surge during the midluteal phase. This study investigated the distribution and hormonal identity of gonadotrophs in the pars distalis (PD) and pars tuberalis (PT) of the equine pituitary gland as possible morphological bases for the referred unusual endocrine characteristics. In addition, the proportion of gonadotrophs in relation to other pituitary cell types during both the estrous cycle and anestrus were investigated. Pituitary glands were collected from sexually active (n = 5) and seasonally anestrous (n = 5) mares in November, and single or double immunofluorescent staining was carried out on 6-microm sections using monoclonal antibodies to the LHbeta or FSHbeta subunits and a polyclonal antibody to ovine LHbeta. Gonadotrophs were densely distributed around the pars intermedia in the PD and in the caudal ventral region of the PT. In addition to isolated cells, clusters of gonadotrophs were found surrounding the capillaries. No significant differences were detected in the number of gonadotrophs between sexually active and anestrous mares in either the PD or PT. In the PD, gonadotrophs represented 22.7 +/- 5. 8% and 19.1 +/- 2.1% of the total cell density in sexually active and anestrous animals, respectively (P: > 0.05). However, in the PT, gonadotrophs accounted for a higher proportion of the total cell population in sexually active (6 +/- 0.1%) than in anestrous (1.2 +/- 0.05%) mares (P: < 0.02). Double immunofluorescence revealed that the majority of gonadotrophs were bihormonal (i.e., positive for LH and FSH); however, in the sexually active mare, a larger proportion of gonadotrophs (22.5 +/- 3.6%) were monohormonal for either LH or FSH, when compared to anestrous animals (9.7 +/- 1.2%; P: < 0.02). Based on these findings we conclude that: 1) although the relative distribution of gonadotrophs is similar to those reported for other species, a significantly larger proportion of gonadotroph cells is present in the equine pituitary gland; 2) gonadotroph density does not appear to differ between sexually active and anestrous mares in the PD; 3) a larger proportion of gonadotrophs is apparent in the PT of sexually active animals; and 4) although a large incidence of bihormonal gonadotrophs is present in the horse, specific LH or FSH cells differentiate predominantly during the sexually active phase.