Gonadotroph Heterogeneity, Density and Distribution, and Gonadotroph–Lactotroph Associations in the Pars Distalis of the Male Equine Pituitary Gland

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The intrapituitary mechanisms regulating gonadotrophin secretion in the horse remain unclear. Here, we examined seasonal and gonadal effects on the gonadotroph and lactotroph populations of male horses with the aim of defining a possible morphological basis for the differential release of gonadotrophins. Pituitaries were collected from: (i) gonadal-intact horses in the breeding season (GBS); (ii) orchidectomized horses in the breeding season (OBS); and (iii) orchidectomized horses in the nonbreeding season (ONBS). Immunohistochemistry was performed using antibodies to the luteinizing hormone (LH) β subunit, follicle-stimulating hormone (FSH) β subunit and prolactin. In all groups, gonadotrophs were distributed throughout the pars distalis, with dense populations detected near the borders with the pars tuberalis and pars intermedia. The numbers of LH-monohormonal, FSH-monohormonal and bihormonal cells/field were greater in GBS than in OBS and ONBS horses. Similarly, the proportion of gonadotrophs in relation to all pituitary cells was larger in gonadal-intact than orchidectomized horses. In the absence of the gonads, no effects of season were observed on these variables. Interestingly, the relative proportions of gonadotroph subtypes and the LH/FSH gonadotroph ratio were similar among groups. Furthermore, while specific gonadotroph–lactotroph associations were identified in all groups, significant gonadal effects within the breeding season and direct effects of season within orchidectomized horses were detected for the number of lactotrophs. This study reveals a gonadal-independent effect of season on the lactotroph, but not the gonadotroph population of the equine pituitary, and a clear gonadal stimulation of both cell types within the breeding season. We suggest that alterations in intercellular arrangements, rather than changes in the incidence of gonadotroph subtypes, may contribute to the differential release of gonadotrophins and, accordingly, to the intrapituitary control of fertility throughout the male equine annual reproductive cycle.

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