REPRODUCTIVE HORMONES AND THEIR FUNCTIONS
MAIN REPRODUCTIVE HORMONES AND THEIR FUNCTIONS
Hormones are chemical substances which coordinate the activities of the body. They are secreted by ductless glands in the body and transferred through the blood to the target organs on which they exert their effects. Hormones concerned with reproduction in farm animals can be grouped into male and female reproductive hormones.
MALE REPRODUCTIVE HORMONES
(i) It imitates spermatogenesis (sperm formation)
(ii) It is responsible for the imitation of male secondary sex characteristics
(iii) Maintains the sex drive (libido)
(iv) Enhances muscular and skeletal growth
(v) Reduces fat deposition
(vi) Promotes the growth of accessory sex glands
(vii) Sustains the life of sperms in the epididymis
FEMALE REPRODUCTIVE HORMONES OESTROGEN
(i) Stimulates the development of female secondary sex characteristics e.g. heart behaviour
(ii) Promotes the production of eggs through oogenesis
(iii) It is concerned with the preparation of the uterus lining for reception of the fertilized ovum
(iv) It increases blood supply as well as the water content of the uterus
(v) It stimulates the growth of the duct system in the mammary glands (udder)
(vi) In the oviduct, it increases cilliary activities and mucous secretion
(vii) Induces the rapid multiplication of epithelium in the vagina
(2) Follicle Stimulating Hormone (FSH)
(i) It stimulates the growth of the ovarian follicle
(3) Luteinizing Hormone(LH)
(i) It causes the rupture of the follicle and subsequent release of ava (i.e ovulation)
(ii) It stimulates the secretion or ovarian hormones i.e oestrogen and progesterone
(4) Progesterone (Pregnancy hormone)
(i) It ensures the development of uterus and implantation of the fertilized ovum
(ii) It inhibits oestrus (i.e it prevents the ripening of more follicles)
(iii) It causes the development of alveoli in mammary gland
(iv) It ensures the continuance of pregnancy
(i) It aids in the contraction of the female uterine muscles during pregnancy
(ii) It affects mammary gland after birth by causing milk let-down or milk production
(iii) It promotes the transport of spermatozoa in the female genital tract.
(i) It causes the relaxation of the pelvic ligament during parturition for easy passage of the young ones.
There’s a lot that goes into the making of an egg! To understand exactly how an egg is made, we first need to know what its parts are.
Parts of an Egg
Yolk. This is the most obvious part of the egg contents—the yellow part near the center.
Albumen. This is the clear part we call the egg white. It’s called this because it turns white when cooked. There are two layers of albumen: thick (near the yolk) and thin.
Chalaza. Located in the thick albumen, chalaza is simply albumen that is twisted tightly. It keeps the yolk in the middle of the egg and prevents it from sticking to the inside of the shell.
Shell membranes. The egg contents are surrounded by two thin membranes called the inner and outer shell membranes.
Shell. This is the outer covering of the egg holding everything together.
How a Hen Makes an Egg
The egg is formed in the reproductive tract of a female chicken, called a hen. The reproductive tract is divided into two major parts: the ovary and the oviduct. The ovary is where the yolk is added. When the yolk reaches the right size, it is released from the ovary by a process called ovulation. The released yolk is then picked up by the infundibulum. It is here that fertilization must take place.The yolk then passes to the magnum, where the albumen is added. It then goes on to the isthmus for the addition of the shell membranes. The developing egg spends most of its time in the shell gland, where the shell and any shell pigments are added. As the egg is being assembled it travels down the oviduct small end first. In the vagina it is pushed out the large end first. This prevents the egg from being contaminated by fecal material when it is laid.
Formation of the egg
Figure 1: Reproductive organs of the hen
Reproductive organs of the hen
The egg is formed gradually over a period of about 25 hours. Many organs and systems help to convert raw materials from the food eaten by the hen into the various substances that become part of the egg.
The hen, unlike most animals, has only one functional ovary – the left one – situated in the body cavity near the backbone. At the time of hatching, the female chick has up to 4000 tiny ova (reproductive cells), from some of which full-sized yolks may develop when the hen matures. Each yolk (ovum) is enclosed in a thin-walled sac, or follicle, attached to the ovary. This sac is richly supplied with blood.
The mature yolk is released when the sac ruptures, and is received by the funnel of the left oviduct (the right oviduct is not functional). The left oviduct is a coiled or folded tube about 80 cm in length. It is divided into five distinct sections, each with a specific function, as summarised in table 1.
Table 1: Functions of various different sections of the hen’s oviduct
Section of oviduct Approximate time egg spends in this section Functions of section of oviduct
1 Funnel (infundibulum) 15 minutes Receives yolk from ovary. If live sperm present, fertilisation occurs here (commercially produced table eggs are not fertilised)
2 Magnum 3 hours Albumen (white) is secreted and layered around
3 Isthmus 1 hour Inner and outer shell membranes are added, as are some water and mineral salts
4 Shell gland (uterus) 21 hours Initially some water is added, making the outer
white thinner. Then the shell material (mainly
calcium carbonate) is added. Pigments may also
be added to make the shell brown
5 Vagina/cloaca less than 1 minute The egg passes through this section before
laying. It has no other known function in the
HERE YOU WILL FIND EVERY AVAILABLE TOPICS ABOUT AGRICULTURAL SCIENCE AND BIOLOGY. AND THE LINKS TO THEIR VARIOUS SOURCES.
118. FARMING PRACTICES
119. BUSH BURNING
121. FERTILIZER APPLICATION
122. ORGANIC MANURING
123. FARM YARD MANURE
126. CROP ROTATION
133. FARM POWER AND MACHINERY
134. SOURCES OF FARM POWER
135. HUMAN SOURCE
142. FIELD MACHINES
164. SIMPLE FARM TOOLS
165. AGRICULTURAL MECHANIZATION
LH is a type of glycoprotein that is produced in the anterior pituitary via gonadotroph cells and serves to regulate the function of the gonads. In males LH stimulates the production and secretion of testosterone from the testes via leydig cells. In females LH stimulates the production of oestrogens and progesterone from the ovary via theca interna cells and luteal cells. Concentrations of LH increase during ovulation and with the formation of the corpora lutea with progesterone secretion. The secretion of LH is regulated via the secretion of GnRH (see earlier section).
As shown previously, in males there are between 4 to 12 GnRH pulses per day and this therefore means that LH also peaks throughout the day. During these peaks, the production and secretion of testosterone increases. Testosterone secretion also is pulsatile.
Follicle Stimulating Hormone (FSH)
FSH is a type of glycoprotein that is produced in the anterior pituitary via gonadotroph cells. FSH secretion is regulated by GnRH from the hypothalamus. The target tissue of FSH in males are the sertoli cells within the testes and in the female the granulosa cells of the ovary. FSH stimulates the maturation of germ cells within the testes and ovaries. In the female it also stimulates follicular development and oestradiol synthesis.
In the male FSH also stimulates the secretion of inhibin which has a negative feedback directly to the anterior pituitary. Although GnRH is released in a pulsatile fashion and the other gonadotropic hormone LH is therefore also pulsatile, FSH concentrations do not fluctuate as much as that of LH. This is because of the added regulatory feedback mechanism of inhibin within the regulatory pathways for FSH secretion.
Prolactin is a protein that is produced from by the anterior pituitary via lactotroph cells. This hormone exerts a stimulatory effect on milk synthesis within the mammary glands. It has also been shown to have some degree of gonadal function in some domestic species and rodents. In birds increased concentrations of prolactin have been linked with brooding behaviours and the associated metabolic changes that birds undergo during brooding.
Prolactin secretion is regulated by the hypothalamus which produces several neurohormones that affect prolactin concentrations. The most important within this is dopamine (or prolactin inhibitory hormone, PRL-IH) which exerts a totally dominant inhibitory action on prolactin synthesis. The hypothalamic regulation of prolactin secretion is via signals from the central nervous system. Prolactin synthesis is increased when the mother is suckling via a reflex stimulation of the teats. This stimulation reflex reduces the secretion of dopamine and increases the hormone prolactin releasing hormone (PRL-RH). Once prolactin binds to it’s target receptors within the mammary gland cells, it activates an intracellular tyrosine kinase. When this occurs in the developing animal this binding can also cause the differentiation of mammary epithelial cells during pregnancy. The half-life of prolactin is approximately 20mins.
Estradiol can also have an effect on the prolactin producing cells within the anterior pituitary and is responsible for increased concentrations of prolactin in females undergoing puberty and may also contribute to the increased concentrations during late pregnancy.
OT is a neuropeptide (a octapeptide) which is synthesised in the hypothalamus and stored in the posterior pituitary. OT is primarily involved in upregulating the activity of smooth muscle cells in the uterus and the smooth muscles surrounding the alveoli ducts of the mammary glands. At parturition, OT causes strong contractions from the myometrium. OT is also essential for ‘milk let-down’ in most domestic species.
OT binds to receptors in the membrane of target cells which activates phospholipase C. OT facilitates the generation of the driving pressure behind pushing the milk towards the large excretory ducts and the teats.
Estradiol (E2) is a steroid hormone and is part of the oestrogens group of hormones and is the principle oestrogen in females. Estrone and estriol are chemically similar to estradiol but are found in lower concentrations and have a lower estrogenic activity. Production of oestrogens occurs in the ovary via granulosa cells, the placenta and the Zona reticularis of the adrenal cortex. In males in it is produced in sertoli cells found in the testes. Estradiol is synthesised from cholestrol.
Oestrogens have a number of functions related to reproduction and other areas of physiology. In relation to the reproductive role of oestrogens, they stimulate follicular growth and maturation, induce the female to begin displaying oestrous behaviour to facilitate mating, prepare the external genitalia for copulation and create favourable conditions for the development of fertilised egg cells. Oestrogens also contribute to the growth and development of mammary tissue and prepare the uterus for parturition.
Effects on reproductive organs:
Vagina: slight mucous secretion, hyperaemia, oedema
Cervix: relaxation, liquification of mucous plug (causing the bull string)
Uterus: stimulates uterine gland development, sensitization of the endometrium to oxytocin, immune activation (local), leucocyte infiltration, secretion of PGF2a and PGE2
Fallopian tube: increased motility and cilia activity
Mammary gland: stimulates mammary duct development
Corpus luteum: Luteolytic (bovine and ovine) but luteotrophic (equine and porcine)
Where oestrogens stimulate growth of follicles in the ovaries, oestrogens secreted from the ovary in the follicular phase (proestrous and oestrous) lead to hypertrophy of the epithelium and the endometrium. Secretory glands within the uterus enlarge and secretion is initiated leading to thickening of tissues. The blood vessels supplying the uterus and external genitalia dilate and blood flow to these areas increases significantly. Oedema occurs within the uterus and surrounding connective tissues. Oestrogen also causes increased uterine muscle tone. In the cervix oestrogens stimulate increased mucus secretion and the vaginal epithelium becomes keratinised.
In males the target tissue is the brain where it causes maturation of the brain during development. This maturation process ensures the appropriate development of male sexual behaviours. E2 in the male also inhibits long bone growth.
Progesterone is a steroid hormone that along with oestrogens is based on a cholesterol molecule produced by the corpus luteum and the placenta using cholesterol as the base molecule. Progesterone is produced by the corpus luteum as well as by the feto-placental unit and in the zona reticularis of the adrenal cortex (to a lesser extent). More detailed information regarding corpus luteum formation and regression please use the links. Progesterone prepares the uterus for reception of fertilized oocytes and is transported via the blood bound to plasma proteins. Progesterone also prepares the mammary tissues for milk production as well as inhibiting female reproductive behaviours associated with oestrous.
Effects on reproductive organs:
Vagina: slight mucous secretion, paleness, exfoliation
Cervix: closure, formation of the mucous plug
Uterus: stimulates uterine gland secretions, sensitization of the endometrium to oxytocin, decreases uterine motility, immunosuppression, inhibition of PGF2a and PGE2
Fallopian tube: increased secretion, decreased motility
Mammary gland: stimulates lobulo-alveolar development
The concentration of progesterone increases after ovulation increasing the growth of glands found in the endometrium resulting in increased secretion. These secretions include mucin, carbohydrates and specific proteins that are designed for nourishment of the embryo prior to implantation. Progesterone also stimulates the growth of the endometrium and stabilises smooth muscle cells to ensure that they do not contract during foetal development. Once near term, the concentration of progesterone decreases, altering the ratio between progesterone and oestrogen. This stimulates myometrial activity and prepares the uterus for parturition.
Progesterone During Pregnancy
During pregnancy the plasma concentration of progesterone is maintained at an elevated level. Progesterone also inhibits secretion of FSH and LH (negative feedback at hypothalamic level by inhibiting GnRH) and thus also prevents the ovulation of follicles during the luteal phase and during pregnancy. In most domestic species the corpus luteum persists for the entire length of gestation.
The exception to this rule is the mare in which the progesterone concentration falls during the later stages of pregnancy. This is due to the regression of the corpus luteum around day 180 of the 330-340 day gestation period.
It is possible to use the relative concentration of progesterone as an aid to pregnancy diagnosis, for example in cattle. However, for a definitive diagnosis a high level of progesterone is required on two separate samples due to the overlap between the luteal phase and pregnancy.
The male sex hormone is called testosterone and this hormone is required for spermatogenesis. Testosterone is a steroid hormone that is produced in the leydig cells within the testes. A relatively high concentration of testosterone is maintained within the testicular tissue and testosterone is circulated around the body by diffusion of the hormone from the spermatic cord into the testicular veins and arteries. The primary action of testosterone is anabolic growth, spermatogenesis promotion and promotion of secretion from the accessory sex glands.
Male sex hormones are regulated by negative feedback systems that operate at various levels within the male sex hormone system. The starting point for the production of testosterone (and therefore the production of spermatozoa)is the hypothalamus. The hypothalamus contains neuroendocrine cells that are capable of secreting a substance called Gonadotropin-releasing hormone or GnRH. GnRH stimulates basophilic cells in the adenohypophysis, via the “portal system” to secrete two intermediate hormones within the male sex hormone cycle; Luteinizing hormone (LH) and Follicle-Stimulating Hormone (FSH).
The secretion of GnRH is pulsatile and can vary greatly throughout the day and/or year, and therefore the secretion of LH and FSH are also pulsatile (although the plasma concentration of FSH does not fluctuate as much as LH due to the effect of Inhibin, see below). The activity of GnRH neuroendocrine cells is determined by spontaneous rhythms and by sensory impulses. Cycles such as seasonal sexual activity are controlled by this pulsatile system. In male animals there are generally 4 to 12 GnRH pulses per day.
When LH binds to the Leydig cells, it stimulates the cellular messenger cAMP to activate protein kinase A. Protein kinase A undergoes a series of phosphorylations that in turn activate a series of enzymes that synthesis testosterone from the cholesterol base molecule. A portion of the testosterone produced in the Leydig cells diffuses into the Sertoli cells that are positioned adjacent to the Leydig cells in the testes but seperated by a basal lamina. This secreted testosterone is converted to to the female sex hormone estradiol in the Sertoli cell and as with the testosterone, a proportion diffuses into the blood, becoming part of the negative feedback system for LH.
Testosterone inhibits the secretion of GnRH from the hypothalamus and therefore secretion of LH from the pituitary gland. If the testes are removed via castration, blood concentrations of LH and FSH will increase as there is only limited negative feedback.
Effects of Male Sex Hormones
Testosterone plays a crucial role in the development of male sex organs during fetal growth where increased production of testosterone causes penis growth and development of accessory sex glands during puberty. Testosterone also affects a number of other characteristics of the male, often called the “secondary sex characteristics”. Testosterone is able to bind to receptors in the cytosol of cells in the same manner as other steroid hormones and these hormone-receptor complexes are then able to bind to DNA in the nucleus resulting in alterations in the level of transcription of specific genes.
Testosterone has a number of anabolic effects stimulating the development and growth of the skeleton and skeletal muscles. Muscle masses show a general increase and in certain body regions such as the neck of stallions or bulls there is obvious hypertrophy. Testosterone also alters behaviour in terms of increasing the degree of sex drive and as a result of the action in several areas of the brain, behaviour can become more aggressive. The larynx of males also enlarges during puberty and the vocal cords lengthen resulting in a deeper and stronger voice.
Testosterone also causes an increase in the level of pheromones to be secreted by glands in the skin which attract and evoke sexual behaviour in females. Glands use in scent marking and territorial marking are also activated by testosterone. In certain species, tusks, antlers and horns are also stimulated to develop.
Inhibin is a type of glycoprotein that is synthesised within the granulosa cells of ovarian follicles in females and in sertoli cells located in the seminiferous tubules within the testes in the male. In both males and females the target organ for inhibin is the adenohypophysis, specifically the gonadotroph cells (basophilic cells).
In the male inhibin production is stimulated via androgens. Inhibin inhibits FSH secretion, which together with decreased concentrations of LH and testosterone results in decreased spermatogenesis and therefore decreased sperm output and quality.
In females some studies have suggested that inhibin may also be produced by the placenta. In females inhibin inhibits FSH secretion. It does however not have any effect on the secretion of LH. When inhibin is secreted, a relatively higher concentration of LH is secreted from the anterior pituitary gland than FSH. Therefore during follicle development, the increased LH concentration causes cessation of the recruitment of further follicles under the effect of FSH. The hormonal changes resulting from the production of inhibin cause some of the previously recruited follicles to undergo atresia.
Inhibin in the female can also be diminished by GnRH and enhanced by insulin-like growth factor-1 (IGF-1).
Activin is a glycoprotein that is produced within granulosa cells in females and sertoli cells in the male. Activin is thought to play an almost directly opposite role to that of inhibin and is involved in many physiological functions including stimulation of FSH synthesis and other roles including cell proliferation, cell differentiation, apoptosis and homeostasis.
The target tissue for activin in the male is the epididymis where it enhances spermatogenesis via increased FSH secretion. Activin also enhances the effect of LH on the testes.
In the female activin has an effect on the anterior pituitary gland, specifically on gonadotroph cells, resulting in increased FSH secretion. The increased concentrations of activin results in increased FSH binding on the female follicle and FSH-induced aromatisation (increased synthesis of oestrogens). Activin also enhances the action of LH in the ovary.
A further non-reproductive role of activin is it’s role in skin lesions where it is thought to stimulate keratinocytes.
Prostaglanin is a C2O fatty acid and is produced within the uterine endometrium and vesicular glands. Estradiol stimulates prostaglandin synthesis while progesterone inhibits it. The target tissue in the female is the corpus luteum, uterine myometrium and ovulatory follicles. In the female PGF2α cause luteolysis and can also cause the induction of tone and contractions within the uterus. It plays an important role in partuition in ruminants.
If a pregnancy is to remain viable then luteolysis needs to be avoided and this is achieved where concentrations of PGF2α remain below a threshold level allowing the corpus luteum to continue to secrete progesterone and thus maintain pregnancy. There are two main factors involved in the regulation of uterine secretions of PGF2α; oxytocin secretions from the corpus luteum and molecules secreted by the developing embryo that facilitate the maternal recognition of pregnancy.
Oxytocin secretion via the corpus luteum stimulates endometrial production of PGF2α and by the end of the luteal phase the concentration of oxytocin and the number of oxytocin recptors within the endometrium allow the production of enough PGF2α to breach the threshold level and cause luteolysis. During pregnancy the embryonically produced pregnancy recognition molecules inhibit the secretion of PGF2α from the endometrium ensuring that luteolysis cannot occur.
Normally the concentration of PGF2α in arterial blood is relatively low due to extensive metabolism by PGF2α-dehydrogenase (in especially the lungs). These levels are below the threshold required to cause luteolysis as PGF2α production in early gestation is low.
The ovarian artery is wrapped around the uterine vein. This creates a countercurrent mechanism by which the lipid soluable prostaglandins are able to diffuse from the uterine vein into the ovarian artery. During the latter stages of the luteal phase as PGF2α production increases luteolysis will occur as PGF2α Is able to reach its target in the ovary before being metabolized in systemic circulation.
Horses and pigs do not poses this countercurrent mechanism. In these spp. the [PGF2α-dehydrogenase] in systemic circulation is much lower in order to induce luteolysis when Prostaglandin concentration rises.
PGE2 is another form of prostaglandin that is produced by the ovary, uterus and embryonic membranes. This form of prostaglandin also has other important roles including vasodilation, smooth muscle relaxation, and inhibition of the release of noradrenaline from sympathetic nerve terminals.
In females it’s target tissue is the cervix (it is a potent cervical dilator), corpus luteum and the oviduct where it helps induce ovulation and the secretion of progesterone from the corpus luteum. PGE2 also plays an important role during labour where it aids the softening of the cervix in animals with a soft-type cervix(equine and human) and aids stimulation of uterine contractions. It can thus be used to prepare the tract for parturition.
Human Chorionic Gonadotrophin (hCG)
hCG is a form of glycoprotein that is synthesised within the trophoblast cells of a blastocyst. hCG is particularly important in primate reproduction where it has a similar effect to LH in stimulating the continued production of progesterone and oestrogens. This represents part of the system involved in foetal-maternal communication and pregnancy recognition. Primate blastocysts therefore produce hCG in relatively high concentrations during the first 3 months of pregnancy. hCG has also been suggested to play a role in defence of the embryo from the maternal immune system during the initial stages of pregnancy. In males hCG increases the growth of the foetal testes.
As hCG is only produced by embryonic cells, the presence of this hormone within maternal blood can be used for pregnancy confirmation.
Equine Chorionic Gonadotrophin (eCG)
eCG is a form of glycoprotein that is produced from chorionic girdle cells. Chorionic tissues in horses as well as primates also form hormones. eCG is formed in foetal endocrine cells and is found within the maternal circulation. eCG is thought to play a similar role in horses to hCG in primates in terms of pregnancy recognition. Foetal production of eCG is highest between 30-70 days of pregnancy. The primary target of eCG are the ovaries where they faciliate the formation of the accessory corpora lutea and ensure that progesterone production is maintained.
eCG is also thought to stimulate follicular growth and ovulation in the horse. If eCG is given to other species it acts in a similar manner to FSH and therefore eCG is often used to induce super-ovulation in species where a large number of oocytes are required for embryo transfer.
Placental Lactogen (PL)
Placental lactogen is a form of protein that is produced by the placenta and is chemically close in composition to growth hormone. The primary target tissue of PL are the mammary glands where they stimulate the growth of alveoli during pregnancy.
PL is also referred to as Chorionic Somatomammotropin (CS).
Relaxin is produced mainly by the corpus luteum in most species and in the placenta(main contributor in the equine) and ovaries throughout pregnancy. During pregnancy relaxin prevents the initiation of uterine contractions, together with progesterone. Relaxin accumulates troughtout pregnancy and is released in lare amounts a few days before partus. Its target organs are the cervix, vagina, pubic symphesis and related structures. Relaxin is responsible for the softening and relaxation of connective tissues in the cervix, muscles and ligaments in the pelvis prior to parturition. Estradiol priming is required for this. This relaxation of tissues via relaxin is performed in conjunction with prostaglandin.