Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


School of Animal Science

First Advisor

Donald L. Thompson, Jr


The present research was performed to characterize immunoreactive (ir) pituitary cells in the equine species. Two experiments were performed to measure the prevalence of single- and double-hormone labeled gonadotropes, lactotropes, somatotropes, and mammosomatotropes. In Experiments 1 and 2, pituitaries from three intact pony mares were processed for ultrastructural evaluation of cells stained for luteinizing hormone (LH), follicle-stimulating hormone (FSH), prolactin (PRL), and growth hormone (GH). Primary antibodies and gold-conjugated antibodies were used to double-stain ir-cells. The percentages of pituitary cells stained for each hormone in intact mares were: total gonadotropes, 15 to 32%; LH-only, 2 to 16%; FSH-only, 1 to 5%; LH/FSH, 6 to 24%; GH-only, 11 to 26%; PRL-only, 5 to 16%; and GH/PRL, 7 to 17%. In Experiments 3 and 4, long-term ovariectomized (OVX) mares (n = 4/group) were injected with vehicle, estradiol (35 $\mu$g/kg), or dihydrotestosterone (DHT; 150 $\mu$g/kg) for 21 days. Immunoreactive-cells were identified immunocytochemically and pituitary content of LH, FSH, GH, and PRL and plasma concentrations of LH, FSH, and PRL were measured. DHT reduced (P $$.1) on plasma PRL. Estradiol reduced (P $$.1) on plasma LH. The percentages of pituitary cells which stained for each hormone in control OVX mares were: total gonadotropes, 12%; LH/FSH, 9%; LH- or FSH-only, 2%; total lactotropes, somatotropes, and mammosomatotropes, 50%; GH-only, 22%; PRL-only, 23%; and GH/PRL, 6%. Estradiol treatment increased (P $<$.05) pituitary concentrations of LH, FSH and PRL. Estradiol also reduced (P $<$.05) the number of cells stained for LH/FSH and increased (P $<$.05) the number of lactotropes and mammosomatotropes. DHT reduced (P $<$.05) pituitary concentrations of LH and FSH but did not significantly affect tissue concentrations of PRL or GH. DHT reduced (P $<$.05) the number of cells that stained for LH/FSH and it increased (P $<$.05) the number of FSH-only cells. It is concluded that equine pituitary gonadotropes are subdivided into cells which contain one or both gonadotropins; and PRL and GH also, can be found in separate cells or in same secretory granules within one cell. Furthermore, OVX and steroid treatment preferentially regulate hormone secretion from gonadotropes that produce both LH and FSH as well as the number of ir-lactotropes and mammosomatotropes.