Lesson 6- The Reproductive System
Assignment:
• Read Chapter 8 in the textbook.
• Read and study the lesson discussion.
• Complete the Check Your Understanding activity.
Objectives: After you have completed this lesson, you will be able to:
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Identify male anatomy and relate associated hormonal function.
Discuss female anatomy and the estrous cycle.
List the steps in establishing pregnancy and identify the stages of parturition.
Discuss the clinical significance of the academic material in this chapter.
Identify common reproductive system disorders.
The Female Reproductive Tract
The female tract consists of paired ovaries, ova ducts, and uterine horns. The uterine
horns merge to form the body of the
uterus which continues on as the cervix
and then the vagina. It ends at the
external opening of the vulva. The broad
ligament suspends the female
reproductive tract from the body wall.
The ovaries produce the ova (eggs) and
hormones. They are located caudal to
the kidneys and are suspended from the
body wall by part of the broad ligament.
The ovarian pedicle contains the
ovarian artery and vein. That part of the
broad ligament that suspends the
ovaries is termed the suspensory
ligament.
The oviduct is a tube which extends
from the ovary to the uterine horns. The
oviduct carries the ova into the uterine
horn. At the end nearest the ovary, a
funnel-like structure called the
infendibulum catches the ovum when it "drops" from the ovary.
The uterine horns are lined with a vascular and glandular mucosa and contain
smooth muscle. There is species variation in the relative size of the uterine horns;
species having "litters" often have larger and longer horns. The embryo is implanted
in the uterine horn. The body of the uterus is formed by the joining of the two
uterine horns.
The cervix contains connective tissue and muscle which forms a firm tube-like
sphincter. The cervix is usually "closed" because the muscular sphincter contracts
and closes the cervix. The cervix acts as a barrier to foreign substances except
during fertilization and birth when the sphincter is relaxed or opened.
The vagina extends caudally from the uterus and is located within the pelvic canal.
This is the area where sperm are deposited during breeding. In many species, this is
also the area where the urinary tract opens via the urethra.
The vulva is the external opening of the female reproductive tract.
Hormones Involved in the Female Reproductive Cycle
The article continues and indicates that several hormones regulate estrous, the
reproductive cycle in female animals. The time of hormone secretion and their
actions vary slightly among species. For example, the female cat is a "reflex or
induced ovulator" meaning she does not drop an egg (ovulate) until after breeding,
thus her hormonal timetable is somewhat different than other species.
Follicle stimulating hormones (FSH) initiate the growth of the follicle (the ova
and its supportive cells).
It is this hormone that expels the eggs for fertilization. It is secreted from the
anterior pituitary gland.
The article goes on to explain that
[The] luteinizing hormone (LH) also stimulates the growth of the follicle. In
addition, it stimulates the rupture of the ova from the ovary. It assists in the
conversion of the area where the ova ruptured (on the ovary) into the corpus
luteum. This process is termed luteinization. LH is also secreted by the anterior
pituitary.
Estrogen is secreted by the ovary. It functions to prepare the uterus for pregnancy
and causes the female secondary sexual characteristics.
Progesterone is secreted from the corpus luteum … early in pregnancy. Later in
pregnancy, progesterone is secreted by the placenta. This hormone is responsible
for maintaining pregnancy (preventing abortion).
Prostaglandins are essential in the reproductive cycle at specific points. The
destruction of the corpus luteum, rupture of the follicle, and uterine contractions at
birth are all stimulated by the reproductive prostaglandins ….
Control of the Female Reproductive Cycle
A complex sequence of secretions regulates the estrous cycle in the female. The FSH
and LH levels are controlled by a feedback loop involving estrogen and
progesterone. Basically, FSH and LH production is decreased when progesterone
levels rise. This is logical because during pregnancy, progesterone is high and there
is no need for estrous cycling or the production of ova.
Although there is considerable difference between species regarding the levels and
times that the various hormones are secreted, all reproductive hormones and cycles
serve the same purpose.
The Phases of the Estrous Cycle
The estrous cycle is divided into four different phases and the length of the various
phases is different between species.
• Proestrus: This phase, involving the development of the reproductive tract and
follicle and the secretion of estrogen, is the "get ready" phase, regulated by
FSH and LH.
• Estrus: This phase, also termed "heat", is the period of sexual receptivity.
Ovulation occurs here, when the egg is dropped from the ovary and
fertilization also occurs. At ovulation FSH drops and LH surges or increases.
• Metestrus: In this phase progesterone is secreted from the corpus luteum. If
fertilization and implantation occur then the normal reproductive cycle is
suspended during pregnancy.
• Diestrus: This is a period of hormonal inactivity between the next estrous cycle.
Let's assume that the female's egg has been fertilized, and she has conceived.
According to the article,
Oogenesis is the development of the ova or egg inside the ovary. Only a small
number of ovum mature in the female lifetime, compared to the sperm production
in the male. It is estimated that about four hundred ova mature in the lifetime of the
human female compared to approximately five million sperm that are produced in
one ejaculate of the human male. Oogenesis occurs after puberty and involves
cellular sexual reproduction.
Next, meiosis occurs. This process was discussed in Lesson One. If you recall from
that discussion, during meiosis, four cells are formed.
In females only, one viable cell, the ova, results. That particular ova has received
most of the cytoplasm during meiosis and it is able to nourish the developing
embryo until it is implanted in the uterus … .
[During pregnancy,] there is a continual secretion of progesterone from the corpus
luteum to maintain pregnancy. During later gestation (period of time the animal is
pregnant), the progesterone is produced by the placenta. The attachment of the
fetus to the uterus varies among species. In the dog and cat, the fetal placenta is
attached to the uterus only around the middle of the fetus … . In the horse and hog,
the entire placenta is attached throughout its surface … . In cattle and other
ruminants, the placenta and uterus are fused together at "buttons" which are the
fetal cotyledons.
To begin the birthing process, the fetus secretes a hormone called ACTH
which stimulates increased cortisol levels in both the fetus and the mother. The
cortisol is believed to then stimulate prostaglandin and estrogen secretion from the
uterus. This in turn stimulates the secretion
of oxytocin which causes contractions and
birth. Oxytocin also causes milk letdown
and passing of the afterbirth.
Mammary glands are modified sweat
glands. There is species variation in the
numbers, location, and anatomy of the
glands. These glands are composed of
connective tissue (to provide support and
structure), blood vessels, lymphatic vessels,
and glandular tissue. The mammary gland
contains alveoli, (somewhat like those in
the lungs) where milk is stored and
secreted. The alveoli are connected by a
duct system and empty into the teat. Milk is
composed of water, protein, fat, sugar, and
vitamins. Specialized enzyme systems in
the mammary glands transform blood
components to milk. The actual
mechanisms of this transformation are not
well understood.
You may view a number of videos on Pregnant Horse Care at
Horse Pregnancy: Foaling -- powered by eHow.com
The Male Reproductive System
Additional information obtained from Vet 111: Animal Physiology, indicates that the
male reproductive system consists of:
• The paired testicles, which are surrounded by a "skin sac," called the scrotum.
• The epidydimus, which extends from each testicle. It is a long tubule where
sperm mature and are stored.
• The ductus deferens (also called the vas deferens), which is the tubular
continuation of the epidydimus into the urethra, the common passageway
for both urine and semen.
• The accessory sex glands, which vary among species. This includes the prostate
gland, which is present in all domesticated animals; the bulbourethral
gland; and the vesicular glands.
• The penis, which carries the semen into the female reproductive tract. The
protective skin surrounding the penis is called the prepuce.
The testicles are composed of tightly coiled tubes, the semineferous tubules. There
are about nine hundred per testicle, which are separated by connective tissue. The
tubules produce spermatozoa or "sperm." Millions of spermatozoa are produced
daily in most healthy young male mammals. The spermatozoa develop from stem
cells which line the semineferous tubules. These cells first divide by mitosis. Then,
later, some cells will undergo meiosis to become spermatocytes, which finally
divide to form sperm. After the sperm are formed in the tubules, they are carried to
the epidydimus in fluid.
During development of the male, the testicles descend from inside the abdomen
through an opening in the scrotum. In livestock, most males are born with
descended testicles while in dogs the testicles descend shortly after birth. When
testicles do not descend, the animal is termed cryptorchid. If the testicles remain
inside the abdomen, the animal is usually sterile, since the high temperature inside
the body inhibits the development of the sperm. The scrotal temperature is usually
about two degrees cooler than the body temperature. Sometimes, the testicles will
descend only partially, and in this case, the animals are usually fertile.
The epidydimus is divided into three portions: the head, body, and tail. In the male
human, the coiled epidydimus is about eighteen feet in length. It is the storage and
maturation area for sperm (for about one month) before ejaculation.
The spermatic cord (which is severed and ligated during castration) contains the
testicular artery and vein, nerves, lymphatic vessels, the cremaster muscle, and the
ductus deferens.
The accessory sex
glands secrete fluid
which contain sugars
and vitamins to
nourish the
spermatozoa. About
90% of the ejaculate is
fluid from the
accessory glands.
The penis is the tubelike structure which
contains a common
passageway for the
urinary and
reproductive tract.
The distal, free part of
the penis is called the
glans. The penis
contains erectile
tissue which fills with
blood and becomes
enlarged during
breeding. There is
considerable variation
among species in the
shape, size, and even location of the penis.
Hormonal Regulation in the Male
From the hypothalamus, the gonadal releasing factor travel to the pituitary gland
which secretes FSH and LH. The FSH stimulates the development of the sperm. The
LH stimulates the production of testosterone. The leydig cells of the testicles
produce testosterone. It is secreted in small amounts in fetus and neonate, then
increase greatly at puberty. Testosterone causes the typical male sexual
characteristics including:
• increased muscle mass (up to 50% more than female);
• increased calcium (in the bone) and bone thickness;
• increased numbers of red blood cells;
• increased metabolic rate.
Reproductive System Disorders
The Merck Veterinary Manual explains several disorders of the male reproductive
system in greater detail:
Cryptorchidism is a failure of one or both testicles to descend into the scrotum and
is seen in all domestic animals … . Predisposing factors include testicular hypoplasia,
estrogen exposure in pregnancy, breech labor compromising blood supply to the
testes, and delayed closure of the umbilicus resulting in an inability to increase
abdominal pressure. Bilateral cryptorchidism results in sterility. Unilateral
cryptorchidism is more common, and the male is usually fertile due to sperm
production from the normally descended testicle.
Prolapse of the prepus is a common defect in bulls … . A long, pendulous sheath, a
large preputial orifice, and absence or poor development of the retractor prepuce
muscles are predisposing inherited anatomic abnormalities. Prolapse of the prepuce
predisposes the animal to injury which can lead to abscess formation, scarring, or
adhesions … . Surgical correction of the prolapse is possible, but as genetic
predisposition may play a role, castration should be carefully considered.
Penile deviation is a common cause of copulatory failure in bulls. Two types of
penile deviation are described—premature spiral deviation of the penis (corkscrew
penis) and ventral deviation of the penis (the free part of the penis curves
downward). Both conditions are caused by insufficiency of the dorsal apical
ligament of the penis. Trauma is rarely involved …. Deviations of the penis are
diagnosed by careful observation of bulls at the time of service or during test
mating. Surgical correction can be done, but should not be performed if inheritance
is possible.
Persistent penile frenulum is not uncommon and is regarded as an inherited defect.
Affected bulls are unable to protrude the penis from the sheath and, in most cases,
can not achieve intromission (ability to enter the vagina). Surgical correction should
not be done in bulls intended for seed-stock breeding.
Short retractor penis muscle may occur congenitally or after injury to the penis or
prepuce. Affected animals have normal libido, but during attempted service the
penis is only partially protruded from the sheath and the ejaculatory thrust does not
occur. Failure of erection in bulls may be a congenital condition but is generally a
sequela of trauma … .
Potential disorders of the female reproductive system include the following:
Double external os of the cervix is due to a failure of several ducts to fuse.
It may be present as a band of tissue caudal to or in the external os of the cervix. In
other cases, there is a true double external os opening into a single caudal part of
the cervical canal. Affected females usually conceive normally. Rarely, a true double
cervix, with a complete septum between the two cervical canals, each opening into
its respective uterine horn, occurs.
Gartner's ducts, located beneath the mucosa of the floor of the vagina, may develop
multiple cysts, which are generally of no clinical significance.
Freemartins are sterile females born twin to a male. In cattle with multiple
conceptions, the placental
blood vessels form a
common circulation
between the fetuses prior
to sexual differentiation,
allowing [hormones]
secreted by the male to
inhibit development of the
female tract. In
approximately 92 percent
of cases of mixed-sex twins,
the females are sterile.
Freemartins have a short
vagina that ends blindly
without communication
with the uterus. The cervix
is absent. The ovaries
usually fail to develop and
remain small.
Rectovaginal constriction is a simple autosomal recessive defect of Jersey cattle
resulting in a severe constriction of the anus or vestibule. In females, it is
characterized by inelastic constrictions at the junctions of the anus, rectum, and
vulva … . Affected cattle can copulate and defecate, but rectal examinations are
difficult to perform and the vaginal constriction can lead to severe dystocia
(difficulty in giving birth).
In addition, affected females tend to develop mammary problems.
Summary
As stated in your textbook, veterinarians not only need to be aware of the anatomy
and potential disorders of the reproductive system, but also how to aid producers in
effectively fertilizing animals to maximize profits.
Sources Cited:
Northern Virginia Community College. "Reproductive System."
Vet 111: Animal Physiology. 28 Dec.
2006<http://loudoun.nvcc.edu/vetonline/vet111/reproduction/ps
lreproless.HTM>.
Kahn, Cynthia, ed. "Congenial and Inherited Anomalies of the
Reproductive System: Introduction." The Merck Veterinary
Manual. Ninth Edition. New Jersey: Merck & Co. Inc. 2006. 28
Dec.
2006<http://www.merckvetmanual.com/mvm/index.jsp?cfile=ht
m/bc/110200.htm>.