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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