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COLLECTION: GOAT HANDBOOK
ORIGIN: United States
DATE INCLUDED: June 1992
Extension Goat Handbook
This material was contributed from
collections at the National Agricultural Library.
However, users should direct all inquires about the
contents to authors or originating agencies.
DOCN 000000051
NO F-8
HORMONES
G. F. W. Haenlein R. Caccese; U. of Delaware, Newark
P. H. Sammelwitz; U. of Delaware, Newark
Anatomy and Physiology
1 Hormones are endocrine gland secretions that are
transported by the vascular system to aid in the integration
of body processes by their stimulatory or inhibitory
effects on target organs. The time lapse between release
and effect is longer than for the other major response
system of the body, the nervous system. The complementary
function of the two systems provides for full coordination
of body responses of goats. The ultimate purpose of
hormones is to provide a means of adaptation between
the body and its external or internal environment.
2 Hormones may be classified into
two categories by their chemical composition. Steriod
hormones are secreted by the adrenal cortex and the
gonads. Protein or protein-like hormones are secreted
from the pituitary gland, thyroid, pancreas and adrenal
medulla.
3 Hormones regulate bodily reactions
through their effects on target organs, but they merely
modify the rate at which target organs perform functions.
They do not cause a reaction or event to occur per
se that could not otherwise occur. Hormones also function
at extremely small levels in the body, with the rate
of secretion varying according to the level of stimulation.
4 Hormonal output is often controlled
through a feedback system from the target organ. This
is most evident through the interaction of the anterior
pituitary gland, whose hormonal release controls the
level of activity of several other endocrine glands
(adrenal cortex, thyroid, gonads). The increased secretory
products of these glands serve as a negative feedback
on the pituitary, causing a reduced rate of secretion
of the stimulatory hormone.
5 The pituitary and the hypothalamus
work together as a functional unit to coordinate the
endocrine and nervous systems in their actions, with
the hypothalamus being the ''center'' of the autonomic
nervous system and master of the pituitary.
6 Posterior Pituitary The hormones
of the posterior pituitary (neurohypophysis) differ
from the other pituitary hormones in that they do
not originate from the pituitary, but are only stored
there until needed. The two hormones, oxytocin (milk
let-down hormone) and vasopressin (antidiuretic hormone
or ADH) are actually produced in the hypothalamus.
Their method of transfer from the hypothalamus to
the pituitary is unique because it is not through
the vascular system, but along the axons of the nervous
system.
7 Vasopressin, is a small polypeptide.
Its amino acid structure varies slightly. In the goat,
arginine is the main base. ADH does not always function
under every day events; but hermorrhaging, trauma,
pain, anxiety and some drugs will trigger its release
and low environmental temperatures will inhibit it.
ADH exerts its effects upon the distal tubules and
collecting ducts of the loops of Henle of the kidney,
resulting in increased water absorption.
8 Oxytocin, the other hormone of
the posterior pituitary, is one of the important and
''practical'' hormones from a dairy goat owner's point
of view, as oxytocin controls lactation and reproductive
phases of the goat. A neural stimulus, such as suckling
or washing of the udder causes the hypothalamus to
stimulate the posterior pituitary into releasing oxytocin,
which is circulated through the blood until it comes
into contact with the myoepithelial cells surrounding
the alveoli of the mammary gland. The oxytocin causes
the myoepithelial cells to contract, effectively squeezing
the milk out of the secreting alveoli and releasing
it into the milk ducts, cystern and teats of the mammary
gland. The release of oxytocin can also occur through
a conditioned reflex response, such as the banging
of milk buckets, or even just putting the goat on
the milk stand. Anything that the goat learns to associate
with being milked may serve as a stimulus of oxytocin
milk let-down through the process of conditioning.
9 Oxytocin also plays a role in reproductive
processes. During the follicular phase of the ovarian
cycle and during the late stages of gestation, oxytocin
stimulates uterine contractions, which facilitate
the transport of sperm to the oviduct at estrus and
which aid parturition. It has been demonstrated that
stretching of the cervix stimulates the release of
oxytocin.
10 Anterior Pituitary The hormones
of the anterior pituitary (adenohypophysis) are produced
within the pituitary itself. They consist of the follicle
stimulating hormone (FSH), luteinizing hormone (LH),
prolactin, adrenocorticotropic hormone (ACTH), thyroid-stimulating
hormone (TSH), growth hormone, and intermedin from
the pars intermedia, the segment between the anterior
and posterior pituitary.
11 The two pituitary gonadotropins,
FSH and LH, are necessary for the maintenance of gonadal
functioning. FSH, while not actually initiating the
formation of a follicle in the female goat does stimulate
overall follicular growth after several layers of
cells have already enveloped an oocyte. Follicle maturation
is achieved through the combined actions of FSH, LH
and the female sex hormones.
12 The action of LH on a follicle,
that has been subjected to FSH, is to increase the
growth rate and stimulate the secretion of estrogen
from thecal cells. Ovulation is triggered by this
process. The conversion of the follicle to a corpus
luteum is the result of LH activity; and the continued
secretion of progesterone from the corpus luteum is
also believed to be controlled by LH.
13 Prolactin, the lactogenic or luteotropic
hormone (LTH), is vital for the proper development
of lactation in goats, although it cannot initiate
the secretory process, and requires estrogen and progesterone
to ''prime'' the mammary system. Prolactin does not
seem to be as necessary for the continuation of lactation
as it is for its initial development, and for stimulating
the corpus luteum.
14 The action of these hormones in
bucks are analogous to those in female goats. FSH
in the male stimulates spermatogenesis by exerting
its effect on the seminiferous tubules. Full spermatogenesis
cannot be accomplished without the conjunctive effort
of LH, known as interstitial cell stimulating hormone
(ICSH) in the male, and certain levels of testosterone.
ICSH facilitates the production of testosterone from
the interstitial cells of the testes. Prolactin has
not been demonstrated to have specific effects in
male reproduction so far, but in general is known
to lower blood pressure.
15 Adrenocorticotropic hormone (ACTH)
secreted from the anterior pituitary causes several
events to occur, but of primary importance is the
release of adrenocorticoid steriods from the adrenal
cortex into the bloodstream. Other effects include
a reduction of lipid levels from the adrenocortical
cells, a lowered concentration of adrenal cholesterol
and ascorbic acid, a general increase in adrenal cell
size and number, along with an increase in adrenal
blood flow. ACTH promotes the secretion of aldosterone,
especially following body stress, such as loss of
blood. ACTH also influences processes not related
to adrenal function, including movement of fatty acids
and neutral fats from fat depots, ketogenesis, muscle
glycogen levels, hypoglycemia, and amino acid levels
of the blood.
16 The thyroid stimulating hormone
(TSH) promotes the release of thyroxin from the thyroid
gland. It also increases the rate of binding of iodine
within the thyroid. The release of thyroxin serves
as a general metabolic control, with higher levels
of thyroxin producing an increased metabolic rate.
17 The basic function of the growth
or somatotropic hormone (STH) is to stimulate an increase
in body size. Growth hormone, along with other pituitary
hormones, is important in protein synthesis providing
high intracellular concentrations of amino acids.
It exerts its effects on bone, muscle, kidney, liver
and adipose tissues in bones in particular, the epiphyseal
plates are sensitive to it. Growth hormone causes
hypertrophy of the kidney after one was removed. Growth
hormones regulates along with the thyroid hormone,
the glomerular filtration rate and renal blood flow
through the kidney; and growth hormone is synergistic
to ACTH and antagonistic to insulin.
18 Growth hormone mobilizes fat from
adipose tissue, resulting in increased blood levels
of ketone bodies, together with stimulation of the
alpha cells of the pancreatic islets, causing glucagon
secretion. Growth hormone also exerts a stimulating
influence on milk production in lactating goats, either
partly or entirely due to an increased amount of mammary
gland tissue.
19 It has been suggested that intermedin,
the melanophore stimulating hormone (MSH), regulates
excitatory states of the central nervous system. MSH
is involved in pigmentation patterns in mammals, although
not to the same extent as known in reptiles, amphibians
and fish where large variations in color due to changes
in temperatures, humidity and illumination can be
produced. All secretions of the anterior pituitary
require a releasing factor from the hypothalamus,
referred to as MRF, TRF, SRF, LRF, FRF, CRF and PIF,
depending on whether the MSH, TSH, STH, LH, FSH, ACTH
or LTH are involved.
20 Pineal The pineal gland or epiphysis
in goats and most other mammals is responsible for
melatonin synsthesis. It functions on a photo-receptive
basis, causing different levels of melatonin production
depending on light intensity. The pineal also affects
the development and function of the gonads.
21 Thyroid The thyroid gland is filled
in its spherical follicles with a colloid consisting
of thyroxin bound to a protein which is essential
to life. Thyroxin is secreted into the blood and lymph
system to control the rate of oxidative metabolism.
Another hormone, calcitonin, is also produced by the
thyroid tissue, aids in the metabolism of calcium,
and is essential for general bone development.
22 The thyroid is interrelated to
other endocrine glands, the adrenals and the gonads
through the pituitary; but there is little interrelationship
between the thyroid and the parathyroid. The structure
of the thyroid hormone, thyroxin, is unique because
the element iodine is essential for biological activity
and release from the gland itself. Thyroxin is necessary
for the maturing of animals. While growth hormone
is responsible for physical growth, thyroxin is necessary
for the proper differentiation of body structures.
Growth and eruption of the teeth of goats is under
thyroid control, as well as the growth of horns. Even
the skin and hair are affected by thyroid changes.
A lack of thyroxin will cause a thinner coat of hair,
with individual hairs being more coarse and brittle.
23 Reproductive failures and deficiencies
in both sexes may be at least partly attributed to
a lack of thyroxin, causing a variety of problems
from abortions and stillbirths in does to impaired
spermatogenesis and lowered libido in bucks. Perhaps
the most sensitive reproductive gland of all is the
mammary gland. Thyroxin has been shown to be a powerful
galactopoietic agent, increasing milk production 10
to 30when feeding iodinated casein. The feeding of
throproteins, however, has the disadvantage of increasing
the susceptibility of the goats to heat stress during
periods of high temperatures.
24 The thyroid hormone has an impact
on thermoregulatory processes. By increasing the general
rate of oxygen consumption at the cellular level,
heat production is increased. Thyroxin stimulates
general nervous functions at all levels, decreases
the threshold of sensitivity to many stimuli, shortens
reflex time and increases neuromuscular irritability.
There is also an interrelationship between thyroxin,
epinephrine and norepinephrine, the hormones of the
adrenal medulla. Thyroxin inhibits the breakdown of
epinephrine and norepinephrine.
25 Low levels of thyroxin during
developmental stages have detrimental effects on the
nervous system, the cerebral cortex, and general myelin
development.
26 Goiter, the enlargement of the
thyroid area, can be brought about by either hyper
or hypothyroid conditions. The most common cause in
animals is a deficiency of iodine making the animal
hypothyroid. Many feedstuffs have goitrogenic effects,
and inhibit the activity of the thyroid. Vegetables
such as cabbage, soybeans, lentils, linseed, peas,
peanuts and all of the cruciferous (mustard-like)
plants possess goitrogens such as thiocyanate and
goitrin which is especially prevalent in the Brassica
family. They interfere with the process of trapping
iodine by the thyroid, and their effects can be counteracted
by feeding increased levels of iodine in the ration.
27 Parathyroid This gland, located
dorsal to the thyroid in goats, is responsible for
the maintenance of proper calcium levels in the blood
and extracellular fluids. Parathormone, the secretion
of the parathyroid increases calcium levels in the
blood and affects calcium and phosphate levels of
the bones and kidneys. Thyrocalcitonin from the thyroid
has the opposite function to parathormone, causing
a decrease in blood serum levels of calcium during
events of hypercalcemia. Parathormone effects bones
directly by mobilizing calcium from the bones into
the bloodstream. Parathormone also lowers the ability
of the kidney to excrete calcium, thereby increasing
calcium retention. Parathormone and vitamin D work
together on calcium release from bone and in increased
absorption of calcium from the intestine.
28 Pancreas The pancreas is primarily
an organ of digestive secretions, although there are
functionally different groups of cells mixed throughout
the pancreas, known as the Isles of Langerhans. They
have rich blood supplies and consist of so-called
alpha and beta cells. The alpha cells are responsible
for the production of glucagon. Low b ++++MISSING
DATA++++
29 Other sources of steriod hormones
besides the adrenal cortex are the ovaries, testicles
and placenta. Steroids are inactivated by their target
organs and in the liver and kidney. These inactivated
hormonal substances are water soluble and are readily
eliminated through the urine.
30 Deficiences in glucocorticoid
levels have detrimental effects on the general body
metabolism. A primary function of the glucocorticoids
is as a catalyst in the gluconeogenic process, i.e.
formation of glucose from proteins and fats. They
also help regulate water metabolism together with
the mineral corticoids and aldosterone.
31 The secretion of the glucocorticoids
from the adrenal cortex is stimulated by ACTH. Due
to the negative feedback system involved between glucocorticoid
levels and ACTH secretion from the anterior pituitary,
a condition of adrenal atrophy will develop if glucocorticoid
injections are administered for any length of time.
32 Structural changes, in the form
of size increases of the adrenals can be observed
in animals that are involved in stress situations.
The stress of crowding is a major factor in adrenal
enlargement, and adrenal weights of wild animals are
used as a measure of population density. Over-activity
of the adrenals produces androgens that inhibit the
production of gonadotropins, which lower the reproductive
performance of the population.
33 Gonads Sex hormones are primarily
secreted by the ovaries and testes, also by non-gonadal
organs such as the adrenals and the placenta, to some
extent. They are of four types: androgens, estrogens,
progestins and relaxin. The first three types are
steroids while the fourth is a protein.
34 The strongest and most predominant
of the androgens is testosterone, which is produced
by the interstitial or Leydig cells of the testicles.
Testosterone and its by-products androsterone and
dehydroandrosterone, circulate throughout the bloodstream,
and are bound to plasma proteins. They are rapidly
used by target organs or degraded by the liver and
kidneys. Testosterone and related hormones are responsible
for male secondary sex characteristics of bucks, body
conformation, muscular devevlopment and libido. They
are also responsible for the growth and development
of secondary sex glands of the males, as well as maintaining
the viability of the spermatozoa and stimulating penile
growth.
35 The ovaries produce two steroid
hormones, estradiol and progesterone; plus another
protein hormone, relaxin. Estradiol, a specific estrogen,
comes from the Graffian follicles; progesterone from
the corpus luteum. A ruptured follicle (corpus luteum),
developes into a second endocrine structure, and its
primary production shifts from estrogen to progesterone.
The function of progesterone is in the areas of preparation
of the uterus for implantation and maintenance of
pregnancy. Also involved is the suppression of formation
of new follicles, new estrus and preparation of the
female goat for lactation through increased mammary
development.
36 Relaxin is a hormone related specifically
to the birth process, and does not appear until late
in pregnancy, just before parturition. It acts upon
the ligaments and musculature of the pelvis, cervix
and vagina. The precise site of formation of this
hormone is not known, yet it is speculated that production
may occur in the cells that are located in the boundary
region of the cortex and medulla of the ovaries.
37 During pregnancy, the uterus itself
takes on hormonal functions through the production
of placental hormones: chorionic gonadotropin, estrogens
and progestrone. These hormones serve to maintain
the uterus in a way that is favorable for the continued
growth and development of the mammary gland.
38 Gastrointestinal Tract All hormones
secreted by the gastrointestinal mucosa and small
intestine are related to the digestive process. Five
of these have been chemically identified, with the
possibility of more existing, making the small intestine
a major site of hormonal production, second only to
the pituitary.
39 One hormone, secretin is responsible
for stimulating pancreatic, bile and duodenal secretions.
While causing an increase in fluid levels of the intestine,
it has no effect on actual enzymatic increases. It
also seems to have negative effects on the activity
of the stomach.
40 A second hormone, enterokinin,
causes an increased rate of secretion of digestive
juices and enzymes of the small intestine.
41 Enterogastrone and cholecystokinin
are two hormones that are related to fat levels in
the diet. Enterogastrone inhibits rates of gastric
secretion; in response to feed fat in the intestine,
it slows down rate of feed passage so that more time
can be spent in the digestion of feed.
42 Cholecystokinin causes the gallbladder
to contract, thus emptying bile into the small intestine.
This emulsifies fat to enable normal digestion by
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HORMONES
COLLECTION;GOAT HANDBOOK
ORIGIN;United States
DATE_INCLUDED;June 1992
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