Page 537 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

512 CHAPTER 2

VetBooks.ir 2.2 The male reproductive tract

It is essential that the veterinary clinician pregnancy rates, this may be far from the case.
possesses a thorough knowledge of the normal Stallions are frequently affected by psychological
anatomy and physiology of the breeding stallion and behavioural problems, which significantly
in order to monitor the health and reproductive affect their performance and usefulness. Medical
status of their patients, as well as helping to and surgical conditions affecting reproductive
detect variations from normal when disease ability may be completely unrelated to the
conditions arise. The breeding stallion is like a reproductive system. A wide array of disease
high-maintenance piece of delicate machinery. conditions affect the reproductive system, many
While breeders may expect him to ‘perform’ of which can be career ending for the breeding
without hesitation and achieve excellent stallion if not appropriately managed.

ANATOMY AND PHYSIOLOGY

Neuroendocrine control of stallion reproduction gonadotropic hormones, LH and follicle stimulat-
involves the hypothalamus, pituitary gland, pineal ing hormone (FSH). These act on the cells of the
gland, vomeronasal organ and testes. The impor- testis, regulating spermatogenesis and steroidogen-
tance of the pineal gland, and the resulting influence esis. FSH regulates production by Sertoli cells of a
of season, is less obvious in stallions than in mares, variety of compounds important in sperm produc-
because the stallion continues to produce sperm tion, including androgen-binding protein, oestro-
throughout the year, regardless of season. Testicular gen, growth factors, inhibin and activin. The latter
size, semen production, libido and hormone concen- two protein hormones appear to feed back to the
trations vary by season in the stallion, with maximal anterior pituitary to regulate FSH release. Sertoli
values obtained in the spring and summer months cells function to regulate seminiferous tubular fluid,
and the lowest in the winter. maintain the blood–testis barrier and support the
Gonadotropin-releasing hormone (GnRH) is developing germ cells. LH regulates the Leydig
released in a pulsatile manner by the hypothala- cells of the testis, stimulating the production of the
mus in response to both neural and hormonal con- steroid hormones testosterone, dihydrotestoster-
trol. Visual, tactile, auditory and olfactory inputs one and oestrogen. Testosterone attains high local
are important regulators of stallion reproductive concentrations within the testis, which are essential
physiology. Exposure of stallions to mares increases for normal spermatogenesis. The steroid hormones
GnRH and luteinising hormone (LH) concentra- also regulate accessory gland function and main-
tions in the pituitary. The Flehmen response, or tain libido by systemic actions via the bloodstream.
lip curl, exhibited by stallions investigating mares Testosterone and oestrogen feed back on the hypo-
directs air across the openings of the vomeronasal thalamus and anterior pituitary gland to regulate
glands, which in turn convey olfactory information LH release.
from pheromones to the hypothalamus. Although
its function is poorly understood, the vomeronasal TESTICULAR DESCENT
organ and Flehmen response are important compo-
nents of the social interactions of horses with one The testicles normally descend into a scrotal posi-
another. tion between the last 30 days of gestation and the
GnRH is transported via the hypothalamic– first 10 days post partum. In some colts, the testes
pituitary portal vessels to the anterior pituitary may descend into the inguinal region and remain
where it controls production and release of the two there for some time before fully descending into the