Animal reproduction, gynaecology and obstetrics

Subject classification: this is a biology resource.

Preliminary note

The area of animal reproduction, gynaecology and obstetrics is also known as Theriogenology.

While the below is relevant to animal reproduction it should eventually be moved to a separate page as the wikiU school of veterinary medicine continues to develop.

Pituitary gland and gonadotrophins

The pituitary gland is the main endocrine gland, and is considered as the control center of the organic hormonal system. It has two regions: the anterior lobe (adenohypophysis) and the posterior (neurohypophysis). In mammals the anterior lobe does not contain nerve fibers, does not direct neural contact with the hypothalamus, and communicates trough a vascular system, the porta hypothalamus-pituitary system. The posterior lobe consists in neural tissue and is connected with the hypothalamus trough the hypothalamus-pituitary pedicle. Between the anterior and posterior lobe there is a small division of the anterior lobe, the pars intermedia. The main hormones directly related with reproduction secreted by the pituitary are: Follicle stimulation hormone (FSH), Luteinizing hormone (LH) and prolactin (PRL), all of them secreted by the adonohypophysis. Oxitocin is a hormone secreted by the neurohypohysis (Capen and Martin, 1898).

Prolactin

Prolactin (PRL) is a protein hormone which consists of a single polypeptide chain of 198 amino acids in human, bovine and ovine species. Bovine prolactin is virtually identical to the ovine prolactin with small differences in the amino acid structure. Historically it was considered that there was only inhibitory regulation of the hypothalamus on the pituitary secretion of PRL, but in recent years was determined the existence of several factors, both stimulatory and inhibitory. However, the influence of the hypothalamus is mostly inhibitory.

The integrity of the hypothalamus-pituitary connection is necessary for prolactin levels to remain normal. If the connection is affected, serum prolactin levels will increase markedly. It was demonstrated that the main prolactin inhibitory factor (PIF) are catecholamines, mainly dopamine, although also the gamma amino butyric acid (GABA), somatostatin and GAP exert an inhibitory effect (Lamberts and Macleod, 1990).

Dopamine is a more potent inhibitor of prolactin secretion. Dopamine-secreting neurons have their cellular bodies in the arcuate nuclei, with small axons ends in the median eminence. After stimulating dopaminergic neurons it is possible to detect significant amounts of dopamine in the portal blood system, much higher than those found in systemic blood. Dopamine concentration is inversely proportional to prolactin concentration. There are several factors that stimulate hypothalamic secretion of prolactin: they are serotonin, tirotrophin releasing hormone (THRH) and the vasoactive intestinal peptide (VIP; Lamberts and Macleod, 1990).

Oxytocin

The neurohypophysis releases two hormones: oxytocin and vasopressin or antidiuretic hormone (ADH). The two hormones are synthesized by neurons localized in the paraventricular and supraoptic nucleus of the hypothalamus (Richard, 1993). Neurons secreting oxytocin and AHD are not same, although they are located in the same nucleus. Once the hormone is synthesized it is transported through the axoplasmatic fluid of unmyelinated fibers which are directed to the neurohypophysis and they end on fenestrated capillaries where their turn their product (Richard, 1993). The presence of oxytocin has been detected also in other areas of the brain, acting as a neurotransmitter, regulating other physiological functions. Oxitocin is also in the corpus luteum in the ovine and human ovaries.

Pituitary gonadotrophins

Gonadotrophins are hormones which stimulate gonadal functions. They are the main mediators of the nervous central system in the endocrine and gametogenic gonadal activities. LH, FSH and TSH are glycoproteins composed of two different subunits called α and β. The α subunit is identical between these three hormones in the same species (Liao and Pierce, 1970; Williams, 1980). The β subunit is specific of each hormone in each species, and is the determinant of biological activity (Pierce and Parsons, 1981). FSH promotes growth and maturation of ovarian follicles in females. In males it acts on the primary states of the germinal cells in spermatogenesis, to the secondary spermatocite. In females LH is responsible for the rupture of the follicle wall and ovulation. In males LH stimulates Leydig cells that produce androgens, mainly testosterone (Setchell, 1993).

References

• 1. Capen CC, Martin SL. 1989. The pituitary gland. In: Mc Donald LE, Pineda MH, eds. Veterinary endocrinology and reproduction. Philadelphia: Lea & Febiger.
• 2. Lamberts SWJ, Macleod RM. 1990. Regulation of prolactin secretion at the level of the lactotrph. Physiological Reviews 70:279-318.
• 3. Liao T, Pierce J. 1970. The presence of a common type of subunit in bovine Thyroid-stimulating and luteinizing hormones. Journal of Biological Chemistry 245:3275-3281.
• 4. Pierce J, Parsons T. 1981. Glycoprotein hormones: structure and function. Annual Reviews of Biochemistry 50:465-495.
• 5. Richar, P. 1993. The hypothalamo-neurohypophysial system. In: Reproducton in Mammals and Man. Thibault C, Levasseur M, Hunter R (eds). Ellipses, Paris.
• 6. Setchell BP. 1993. Male Reproduction. In: Reproduction in domestic animals. King GJ, ed. New York: Elsevier Science Publishers BV.
• 7. Williams J, davies T, Catt K, Pierce J. 1980. Receptor-binding activity of highly purified bovine luteinizing hormone and thyrotropin, and their subunits. Endocrinology 106:1353-1359.