Download Hormones and Reprodu..

Animal Reproduction
HORMONES AND REPRODUCTION.
The main purpose of the animal
industry is to develop an animal, which
will grow and reproduce at a fast and
economical rate. Since the growth
processes are for the most part under
endocrine control and the reproductive
processes
are
primarily
under
endocrine control, it follows that these
two areas of physiology are becoming
increasingly
important
to
those
concerned
with
growth
and
reproduction of domestic animals.
Most glands of the body deliver their
secretions by means of ducts. These
are exocrine glands. Other glands
manufacture chemical substance and
secrete directly into the blood stream
without any ducts for transmission to
various "target" tissues. These are the
endocrine glands. Their secretions, the
hormones, catalyse and control diverse
metabolic processes. They differ from
enzymes in the following ways:
1.
They are produced in an organ
other than that in which they
ultimately perform their action,
2.
They are secreted into the blood
prior to use.
3.
Hormones are not always
proteins; the known hormones include
proteins of molecular weights 30,000 or
less, smaller polypeptides, single amino
acids and steroid.
Finally a hormone may be defined as: a
chemical substance produced in one
part of the body (restricted area), that
diffuses or is transported to another
area, where it influences activity and
tends to integrate component parts
of the organism. It should be pointed
out that hormones regulate (decrease
or increase) the rates of specific
processes but do not contribute energy
to the process or initiate metabolic
reactions. Instead, hormones influence
Prepared by Tshewang Dorji
an existing reaction which is usually
one involving enzymes, thus an excess
of hormones may be as detrimental as
a deficiency since an existing reaction
could be stimulated to excess.
THE PITUITARY GLAND:
The pituitary gland is located at the
base of the brain. In man the average
weight of the gland is about 0.6 grams.
The gland consists of three parts,
anterior lobe, (pars distalis), an
intermediate lobe (pars intermedia),
and a posterior lobe (pars nervosa)
THE ANTERIOR PITUITARY.
The gland produces 6 hormones, two
of them act in their own right (growth
hormone and prolactin) and four trophic
hormones
(Adrenal
corticotrophic
hormone ACTH, Thyroid stimulating
hormone TSH, Follicular stimulating
hormone FSH and Luteinizing hormone
LH), which act by controlling the
behaviour of other hormonal glands.
Growth Hormone:
Also known as somatotrophin. (STH),
stimulates growth of all body cells but
particularly effective with bone and
muscle tissue. The action of STH is
most noticeable when either an excess
or deficiency of the hormone exists. An
excess of STH in the immature animal
results in overall excess growth,
including longer limbs, that produces a
giant individual producing a condition
called acromegaly. Deficiency of STH is
seen typically in hypophysectomised
(pituitary gland removed) young
animals. A lack of STH in the adult
usually is associated with under
secretion of all anterior -lobe hormones,
a condition called"Simmond's disease".
Prolactin:
1
Animal Reproduction
The
hormone also known as
luteotrophic
hormone
(LTH),
is
associated with the initiation and
maintenance of milk secretion in all
mammals. The release of LTH in the
mammals is reflex stimulated by
suckling (nursing). The sensory nerves
in the nipples send nerve impulses up
to the hypothalamus, which stimulates
the output of prolactin from the anterior
pituitary.
Adrenal
(ACTH):
Corticotrophic
Luteinizing Hormone (LH):
This is the hormone, which stimulates
the output of hormones from the ovary
and from the testis. The ovary secretes
primary estrogens and progesterone,
while the testis secretes testosterone.
Again LH is identical in males and
females: in males it is sometimes called
interstitial cells stimulating hormone or
ICSH.
Hormone
As the name implies, ACTH has its
greatest effect in stimulating the
adrenal cortex (but not the adrenal
medulla). Even in adrenal cortex which
has got three types of cells - ACTH
appears to be much more sensitive in
two types of cells- zona glomerulosa,
which produce mineral corticoids has
got no proven effect.
POSTERIOR PITUITARY
This produces two important hormones,
antidiuretic
hormone
(ADH
or
vasopressin) and oxytocin. Both the
hormones are made by nerve cells in
the hypothalamus. They then travel
down the nerve fibres, which pass
along the pituitary stalk to the posterior
pituitary. The hormones are actually
released into the posterior pituitary
gland.
Thyroid Stimulating Hormone (TSH)
Oxytocin:
This stimulates the thyroid gland to
secrete thyroid hormone. The output of
TSH itself is in turn controlled by the
level of thyroid hormone in the blood
and also by the level of the TSH
releasing factor (TRF) secreted by the
hypothalamus.
Follicular Stimulating Hormone
(FSH):
This is the hormone, which is essential
for the development of the egg-bearing
follicles in the ovary. It is also required
for the manufacture of sperm by the
testis in the male. Even though it is
called FSH after its action in the
female, it is now known that the FSH in
male is identical to that in females: it is
ovaries and testes, which differ in their
response to it.
Prepared by Tshewang Dorji
It acts on the myometrium (uterine
muscles) and on myoepithelial cells in
the mammary gland. During parturition
oxytocin causes contraction of the
uterus and in this manner aids
expulsion of the foetus. Following
coitus, oxytocin is believed to stimulate
uterine contraction in a manner that
aids transport of sperm to the oviducts.
Nursing causes reflex release of
oxytocin,
which
stimulates
the
myoepithelial cells surrounding alveoli
of the mammary gland to contract. This
process, called milk let down, forces
milk out of the alveoli into the ducts of
the mammary gland.
Vasopressin (ADH):
The hormone acts on the kidney to
reduce the output of water in the urine.
If the body water content is low, the
2
Animal Reproduction
plasma becomes slightly concentrated.
This change in plasma concentration is
detected by the hypothalamus, which
increases the output of ADH. The ADH
travels in the blood to the kidney and
reduces the amount of water lost in the
urine, making the urine smaller in
volume and more concentrated. On the
other hand if an excess of fluid is drunk,
this dilutes the blood a little. This
change too is detected by the
hypothalamus, which reduces the
amount of ADH secreted, so allowing
the kidney to excrete a large volume of
dilute urine.
OVARY
Follicular Hormones:When the ovaries are stimulated by the
hormone from the pituitary gland a
graafian follicle enlarges and produces
hormones, which initiate the oestrus
cycle. These hormones are usually
referred to as oestrogenic hormones.
Three such hormones have been
isolated from ovaries and urine. These
are as follows:
1.
Estradiol,
2.
Estrone and
3.
Estriol.
Out of these three hormones, Estradiol
is most effective. All the abovementioned hormones have similar
physiological properties and are spoken
of under the collective name Estrogens.
Main Functions of Oestrogen:
1.
They
produce
psychic
symptoms of heat or sexual
receptivity.
2.
They
cause
the
vaginal
epithelium to grow and cornify.
3.
They increase the capillary
supply to the endometrium with
resulting oedema.
Prepared by Tshewang Dorji
4.
5.
6.
They also cause the uterine
muscle cells to grow and
become sensitive to stimuli.
They increase the sensitivity of
the muscle of the oviduct
including ciliary movement.
Oestrogen inhibits the growth of
bones and so female ceases to
grow in size after puberty.
The
oestrogenic
reactions
are
obviously those of proestrum and
oestrus periods in which the follicles
grow and secrete these hormones. The
oestrogenic substance additionally
have profound actions which may be
grouped into effects on (i) Calcium and
Phosphorus metabolism, (ii) Skin and
related structures, the estrogens also
have effects antagonistic to androgen
actions.
Corpus Luteum Hormone:
After the follicle discharges its ovum a
corpus
luteum
develops
which
produces another female sex hormone
called progesterone.
Its main functions appear to be as
follows:
1.
To continue the changes in the
uterus started by the oestrogenic
hormones, preparing it for the
implantation of the fertilised
ovum.
2
It also plays some part in the
retention of the embryo in the
uterus. If the corpus luteum is
destroyed
during
early
pregnancy abortion takes place
3.
Progesterone stimulates the
development of the mammary
glands at puberty and it is
believed that this hormone plays
an important role in the
development of the mammary
glands during the pregnancy.
3
Animal Reproduction
4.
Due
to
secretion
of
progesterone there will be no
maturation of follicles and thus
ovulation is inhibited.
A second hormone, Relaxin, is
secreted by the corpus luteum and it
causes the relaxation of the pelvic
ligament and partial decalcification of
the pelvic bones so that the birth canal
may be enlarged at the end of
pregnancy to facilitate the passage of
the uterine contents.
PLACENTA
When pregnancy occurs, the placenta,
which is a new organ of internal
secretion, develops. It is round, flat
organ which forms the attachment
between the mother and the foetus and
through which the mother nourishes the
foetus.
It secretes at least three hormones, viz.
(i) Anterior pituitary like hormone, (ii)
Oestrogen- as a matter of fact, during
pregnancy, oestrogen secretion takes
place chiefly, probably only in the
placenta. (iii) Placenta also secretes
progesterone at the later months of
pregnancy.
Thus by the help of its hormone,
placenta - (a) stimulates the growth of
mammary glands, (b) inhibits lactation,
(c) stimulates the growth and
persistence of corpus luteum, (d)
inhibits ovulation, (e) controls anterior
pituitary, (f) stimulates thyroid and
adrenal cortex (via anterior pituitary),
(g) stimulates growth of uterus and
placenta it self, etc. In other words all
the
important
changes
during
pregnancy are carried out with the help
of the placenta.
Prepared by Tshewang Dorji
ROLE OF VARIOUS HORMONES IN
FEMALE REPRODUCTIVE
MECHANISM.
A number of hormones related to
reproduction have been isolated and in
some cases, synthesised.
At birth, or soon afterward, the ovaries
contain many ova surrounded by
granulosa cells. On reaching puberty,
the anterior pituitary secretes the follicle
stimulating
hormone
(FSH)
in
increasing quantities. This causes the
multiplication of granulosa cells and
also stimulates the ovaries to produce
graafian follicles. Eventually some
follicles become so large that they start
secreting
oestrogen
hormone.
Oestrogen thus secreted, acts as
below.
1.
The reproductive organ attains
full growth.
2.
Female characters other than
reproductive
organs
also
develop.
3.
The heat period starts.
According to the general view, (based
on experiments on the rat), the
oestrogen secreted by the follicle,
reacts upon the anterior pituitary
causing a reduction in FSH secretion
simultaneously produces luteinizing
hormone (LH). However, evidence
obtained (from the cow) indicates that
oestrogen does not cause a cessation
of FSH secretion, nor does it stimulate
the secretion of LH and cessation of
oestrogen.
The resulting cessation of oestrogen
secretion causes heat to cease and
about 10-12 hours after the end of
oestrus, ovulation takes place in the
cow by the action of LH.
LH produces contraction of muscle
fibres in the ovary, which in
combination with the degenerative
4
Animal Reproduction
action of an enzyme weakens the
follicle wall, causing ovulation.
The second function of LH is to convert
the empty graafian follicle into corpus
luteum, (CL) by stimulating the
granulosa cells of the follicle.
The anterior pituitary gland now
secretes prolactin, which stimulates the
corpus luteum (intermittent endocrine
gland) to produce progesterone. The
progesterone then totally suppresses
the anterior pituitary for FSH and LH
activity and stimulates it for the
production of L.T.H. (Luteotrophic
hormone) also called prolactin, which in
turn
maintains
the
C.L.
for
progesterone production. Progesterone
plays an important role, which are as
follows:
6.
1.
Male hormone is called an androgen.
The specific androgen synthesised in
the testes is testosterone by the cells of
Leydig (also called as interstitial cells)
from
cholesterol.
This hormone
testosterone after its release from the
testis probably remains in the blood as
testosterone propionate bound to the
blood proteins for not over 15 to 34
minutes. It is then converted by the liver
into
Androsterone
and
Dehydroepiandrosterone (DHEA), the
primary urinary product. In general the
degradation
products
show
a
decreased or no activity. Testosterone
and other two degraded products are
all steroid in nature and commonly
referred to as androgens. FSH
stimulates the germinal epithelium to
produce spermatozoa while LH or ICSH
(interstitial cell stimulating hormone)
stimulates the interstitial cells to
produce the male hormone. It has been
shown that testosterone acts directly on
the pituitary to inhibit FSH release and
2.
3.
4.
5.
To continue the change in the
uterus,
started
by
the
oestrogenic
hormones,
preparing for the implantation of
the fertilised egg.
For retention of the embryo in
the uterus. If the corpus luteum
is destroyed during early
pregnancy, abortion takes place.
Stimulates the development of
the mammary glands during
pregnancy period.
Essential for the formation of
placenta. If C.L. is removed after
implantation of the foetus in the
uterus, placenta fails to develop,
the embryo dies and abortion
takes place.
Desensitises the uterine muscle
to the action of oxytocin. Thus
uterine contraction absolutely
ceases and the growth of uterus
is facilitated. At full term C.L.
regresses and placenta also
degenerates to some extent.
Hence, progesterone secretion
falls. Oxytocin acts unopposed
and parturition starts.
Prepared by Tshewang Dorji
7.
Due
to
secretion
of
progesterone further maturation
of follicles and ovulation are
inhibited.
It induces the uterine glands of
the endometrium to secrete the
uterine
milk essential for
embryonic nutrition.
If there is no conception the C.L. starts
regressing after about 10 days and falls
deficient in progesterone secretion.
This frees the anterior pituitary
temporarily, which starts producing
F.S.H. and L.H. and the cycle is
repeated, the cow then comes into heat
after 21 days.
TESTES
5
Animal Reproduction
by way of a centre in hypothalamus inhibits LH release and hence androgen
synthesis.
Functions of testosterone:
1.
2.
3.
4.
5.
Growth of accessory male sex organs.
Development of male secondary sex characters.
Increasing the span of life and fertilizing power of the spermatozoa.
Stimulates the number and thickness of muscle fibres and the working ability of
the muscle.
Stimulates the formation of RBC and haemoglobin.
Prepared by Tshewang Dorji
1