Page 538 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Reproductive system: 2.2 The male reproductive tr act 513

VetBooks.ir scrotum. The hormonal and physical events lead- is the outer layer of the testis. The vaginal cavity is
the space between the parietal and visceral layers of
ing to testicular descent are poorly understood, and
many theories have been proposed. Recently, homo-
amount of viscous fluid, allowing some free move-
zygous deletion of the AKR1C gene was found in the vaginal tunic. Normally it contains a very small
6/67 (9%) of cryptorchidism cases examined. This ment of the testis within it. The vaginal cavity is a
is the first report of a genetic mutation responsible potential space within which considerable fluid may
for cryptorchidism in horses. Androgen produc- accumulate as a result of a variety of causes.
tion by the developing fetal gonads probably plays The scrotum of the normal stallion should appear
an important role, as do müllerian inhibiting factor slightly pendulous, globular and generally sym-
and epidermal growth factor. Traction of the guber- metrical. Normal variations may be observed in the
naculum, which attaches the caudal pole of the testes positioning of the testes if one is relatively anterior
to the inguinal region, is believed to draw the devel- to, or ventral to, the other. The skin should have no
oping epididymides into the inguinal ring, aided by evidence of trauma, scarring or skin lesions. Scrotal
intra-abdominal pressure, elongation of the vaginal skin lesions can cause significant alterations in tes-
process and expansion of the inguinal ring. tis temperature and affect fertility. Palpation of the
scrotum of a normal stallion reveals a thin and pli-
SCROTUM able covering, which slides loosely and easily over
the testicles and epididymides within.
The scrotum of the stallion is located high in the
inguinal region and is much less pendulous than in TESTICLES
the ruminant species. The scrotum consists of two
distinct pouches that contain, protect and thermo- The testicles of a normal stallion are palpable as two
regulate the testes, epididymides, spermatic cords oval structures lying horizontally within the scrotal
and cremaster muscles. The testes are located in the pouches. Normal orientation of the testicle is ascer-
scrotum in order to maintain testicular temperature tained by palpation of the tail of the epididymis and
at several degrees below core body temperature, a the ligament of the tail of the epididymis (or caudal
necessity for normal spermatogenesis. The wall of ligament of the epididymis) at the caudal pole of the
the scrotum consists of four layers: skin; tunica dar- testicle (Fig. 2.121). The ligament is palpable as a
tos; scrotal fascia; and parietal vaginal tunic.
The scrotal skin is thin, generally hairless and
slightly oily. It contains numerous sebaceous and 2.121 A B
sweat glands, which assist in testis thermoregulation.
The tunica dartos layer lines both scrotal pouches
and extends into the median septum, seen externally
as the median raphae of the scrotum. The degree of C
contraction or relaxation of this layer allows altera- E
tions in the size, shape and position of the scrotum
in relation to the body wall, thereby aiding testis
thermoregulation. The scrotal fascia is a loose con-
nective tissue layer between the tunica dartos and D
the parietal vaginal tunic that allows the testes and Fig. 2.121 Gross appearance of the right testis and
associated parietal tunic layer to move freely within epididymis of the stallion. The head of the epididymis
the scrotum. The parietal vaginal tunic, which lies at the cranial pole of the testis. The body of the
forms during testicular descent, is an evagination of epididymis courses dorsolaterally to the testis, and
the parietal peritoneum through the inguinal rings. the tail of the epididymis lies at the caudal pole. A =
This layer forms a sac that lines the scrotum and is deferent duct; B = body of epididymis; C = epididymis
closely apposed to the visceral vaginal tunic, which head; D = right testis; E = epididymis tail.