| dc.description.abstract |
The intrahypophysial mechanisms involved in the control of gonadotrophinsecretion remain unclear. In the horse, a divergent pattern of gonadotrophinsis observed at different stages of the reproductive cycle in response to asingle secretagogue (gonadotrophin-releasing hormone), and dramatic changes infertility take place throughout the year in response to photoperiod. Thisspecies thus provides a useful model to investigate the regulation offertility directly at the level of the hypophysis. A series of studies wereundertaken to examine the cytological arrangements and heterogeneity ofgonadotrophin storage in the pars distalis (PD) and pars tuberalis (PT) of thehypophysis of male and female horses. Specifically, the seasonal and gonadaleffects on distribution, density and hormonal identity of gonadotrophs, theexistence of gonadotroph-lactotroph associations and the expression ofprolactin receptors (PRL-R) as possible morphological bases for thedifferential control of gonadotrophin secretion were investigated. It becameapparent that both isolated and clustered gonadotrophs are normallydistributed around the pars intermedia and surrounding capillaries in the PD,and in the caudal ventral region of the PT. In the PD, no effects of season orof reproductive state on the density or number of gonadotrophs could bedetected in either male or female animals. In contrast, a fivefold increase ingonadotroph density was observed in the PT during the sexually active stage.In males, robust gonadal effects were detected on the gonadotroph population;orchidectomy significantly reduced both the number and proportion ofgonadotrophs, in relation to other hypophysial cell types, in both the PD andPT regions. Luteinizing hormone (LH) monohormonal, follicle-stimulatinghormone (FSH) monohormonal and bihormonal gonadotrophs were identified in thePD and PT of male and female horses. Interestingly, in males, the relativeproportions of gonadotroph subtypes and the LH/FSH monohormonalgonadotroph ratio were not affected by either season or the presence of thegonads. In contrast, a larger proportion of monohormonal gonadotrophs wasclearly observed in sexually active females. Specificgonadotroph-lactotroph associations and expression of PRL-R in cellsother than gonadotrophs were detected in the PD throughout the annualreproductive cycle. In addition to a stimulatory gonadal effect on lactotrophdensity, a substantial gonadal-independent effect of season was apparent onthis variable. The findings have revealed important seasonal and gonadaleffects on the cytological configuration of the equine hypophysis, which mayprovide the morphological basis for the intrahypophysial control of fertility. |
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