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ORIGIN: United States

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
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 ++++MISSING DATA++++

ORIGIN;United States


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