Anatomy and Physiology of
the stallion...
by MaryAnne Leighton
Knowledge of how mature spermatozoa are
formed and the effect of stress, injury and illness
on the production of sperm is important in managing the stallion in order to optimise his fertility.
In the stallion the testes produce germ cells
(spermatozoa) and hormones that are necessary for development of male characteristics.
The testes are normally oval-shaped, approximately nine centimetres long and six and a half
centimetres wide (the larger the testis the more
sperm produced) and are both contained within
the scrotum where they move freely and can be
drawn up towards the abdomen by muscular
contraction.
In the scrotum the arrangement of veins and
arteries and the sweat glands in the skin allow
the testes to be kept two to three degrees (Celsius) cooler than the stallion’s body temperature.
This lowered temperature is essential for sperm
production and maturation. If the testes are kept
warm, for example in the case of the abdominal
(cryptorchid) testes of the rig, they are infertile.
Each testis contains millions of fine tubes, the
walls of which consist of primitive cells that
produce sperm-forming cells. These cells are
released into the centre of the tube and pass
along it to be released from the testis forty-nine
days later. However, even when released from
the testis the sperm are not fertile. The final
maturation occurs in a single, very long (about
eighty metres), coiled tube called the epididymis,
into which all the testicular tubules drain and
within which the sperm remain for a further
eleven days where they develop the ability to
move (motility) and undergo other important
changes.
Thus, in total, it takes approximately sixty days
for mature spermatozoa to be formed, transit the
epididymis and be ready for ejaculation. This is
important to know in situations in which sperm
production has been temporarily stopped, for
example by injury, infection or stress. If a stallion
continues to breed mares after an interruption
in the production of sperm, the sperm formed
before the injury or stress would soon be used
up and there would be none to replace them for
at least sixty days.
On ejaculation, the spermatozoa pass rapidly
up the vas deferens and are mixed with the fluid
part of the semen called seminal plasma which
is derived mostly from accessory gland secretions. Teasing increases the volume of semen
by stimulating accessory gland secretion, but
does not increase the total number of sperm
ejaculated. Of approximately eight successive
jets of semen expelled from the penis during
ejaculation, the first three jets have been shown
to contain approximately eighty percent of the
spermatozoa in that ejaculation.
In addition to sperm-forming tubes, the testes
contain clusters of Leydig cells that play an essential role in fertility by producing the male sex
hormone, testosterone. Testosterone is needed
for development of the penis and accessory
glands as well as other masculine characteristics, including sexual desire. The Leydig cells
function very well at deep body temperatures
which is why testosterone production is not suppressed in the rig.
Production of both sperm and testosterone
are stimulated by Follicle Stimulating Hormone
(FSH) and Luteinising Hormone (LH), produced
by the pituitary gland which is, in turn, controlled
by the brain.
Anatomy and Physiology
of the Mare...
by MaryAnne Leighton
In the mare the ovaries are bean-shaped
organs that are suspended from the roof of
the abdomen by a ligament. Like the testes,
the ovaries produce both germ cells (eggs,
or ova) and hormones under the influence of
Follicle Stimulating Hormone and Luteinising
Hormone. Unlike the stallion, the mare is born
with her full quota of germ cells (about half
a million) and produces no more during her
lifetime.
At three-week intervals during the breeding
season, Follicle Stimulating Hormone causes
the cells surrounding several of the primitive
germ cells to begin to multiply and eventually
to grow into fluid-filled sacs called follicles, in
which the germ cells can mature. At this stage,
the follicle cells produce the hormone, oestrogen, which causes the mare to come into
oestrus (to cycle or come into season). Usually
only one of these follicles, each oestrus cycle,
attains a completely mature size of four to five
centimetres in diameter.
Under the influence of oestrogen carried to it
in the bloodstream, the pituitary gland switches
from predominantly Follicle Stimulating Hor-
mone to Luteinising Hormone production causing the final maturation and eventual rupture of
the follicle and release of the egg – ovulation.
The cavity left after the follicle ruptures is filled
with a blood clot which the surrounding cells
invade and in which they proliferate to form
a corpus luteum (yellow body). The corpus
luteum produces the hormone, progesterone,
which takes the mare off heat and keeps her
off as long as it is being produced. In a normal
cycle, if a mare does not become pregnant the
corpus luteum is destroyed by prostaglandin
which is released by the lining of the uterus
fourteen to fifteen days after ovulation. When
the corpus luteum is destroyed progesterone
levels fall and the mare comes back into oestrus in about two days.
When the egg is released, it travels into the
oviduct where it remains until fertilisation which
must occur within twelve hours of ovulation
otherwise the egg degenerates. The oviduct
is a rather wavy tube about twelve millimetres
in diameter which leads into the tip of the horn
of the uterus. The uterus is a T-shaped organ
with two horns and a short body terminating in
a narrow muscular neck – the cervix.
In the average-sized mare, the cervix is about
thirty centimetres from the vulva. It is about ten
centimetres long and has a narrow, straight
bore which readily allows passage of semen
during oestrus when the muscle is relaxed and
the glands produce a lot of lubricating mucus.
At natural service or artificial insemination
the semen is passed through the cervix and
deposited in the uterus. From there it is transported to the site of fertilisation by rhythmic
contractions of the wall of the uterus and
oviduct. Although sperm can swim, this activity
is solely for the purpose of penetrating the egg
and not for transport.