Reproductive System in Humans | Essay | Humans | Reproductive Biology

In this essay we will discuss about the male and female reproductive system in humans.

Essay on the Male Reproductive System:

During the developmental stages, the gonads of the genetic male fetus are induced to differentiate into testes. The testes of the male secrete testosterone, which is responsible for differentiation and development of the urinogenital system characteristic of the male. The testes remain inactive until puberty.

They are activated by the gonadotropins produced by the pituitary gland. Two sets of genes are required for the development of the male phenotype. The first set is located on the Y-chromosome and they encode the testes determining factor. These genes must be expressed for the undifferentiated gonad to form the testis.

In male the Mullerian ducts have to degenerate so that they may not form the female reproductive tract. Somatic cells of the developing testes secrete a Mullerian inhibiting substance, which helps in the regression of the Mullerian duct. Development of the male reproductive tract and secondary sexual characters are dependent upon androgens.

The fetal testes have to synthesize androgens at a critical time during the differentiation of the testis. A second set of genes is required for the complete development of the male. These genes encode the enzymes required for the conversion of cholesterol to testosterone and dihydrotestosterone.

Testes:

Testis performs two important functions. It provides an environment for spermatogenesis and secretes the hormone testosterone, which regulates a number of reproductive activities. Testes are located within the scrotum. Each testis is oval in shape and is about 4 – 6 cm in length and 2 – 3 cm in diameter. A connective tissue sheath, the tunica albuginea, surrounds it. Spermatozoa are produced in convoluted seminiferous tubules.

These tubules converge to form the rete testis, which opens to efferential ductules and epididymis. The epididymis can be differentiated into head, body and tail. The tail continues as vas deferens. The outer layer of seminiferous tubules is made up of connective tissue and smooth muscle, while the inner layer consists of the Sertoli cells within which are embedded the spermatogonia and different stages of mature and immature spermatozoa.

Sertoli cells provide nourishment and other factors necessary for sperm maturation. The fully mature spermatozoa are released into the lumen of the seminiferous tubules and subsequently move slowly to the epididymis where they are stored in the tail region. In-between the seminiferous tubules are scattered the interstitial cells of Leydig, which produce androgens.

The parent substance from which the androgens are synthesized is cholesterol. Pregenolone is synthesized from cholesterol in the Leydig cells. Pregenenolone is converted into 17 – hydroxypregnenolone. This substance is converted to 17 – ketosteroids by side-chain cleavage and these, in turn are converted into testosterone. Testosterone is the principal steroid produced by the Leydig cells.

Hormonal Control of Testicular Function:

The anterior lobe of the pituitary gland secretes gonadotropins namely Follicle Stimulating Hormone (FSH) and Leutenizing Hormone (LH). The anterior pituitary is stimulated to release these hormones through Follicle Stimulating Hormone Releasing Hormone (FSH-RH) and Leutenizing Hormone Releasing Hormone (LH – RH) from the hypothalamus. These hormones are neurohormones.

GnRH and its Effects:

Gn-RH is a peptide containing 10 amino acid residues. It is secreted by the neurons whose cell bodies are located in the arcuate nucleus of the hypothalamus. Gonadotrophic releasing hormone is transported to the anterior pituitary gland through the portal circulation and stimulates the release of the two gonadotrophic hormones, LH and FSH.

GnRH is secreted intermittently a few minutes at a time once every 1 to 3 hours. The intensity in stimulus of this hormone is determined by the frequency of the cycle of secretion and by the GnRH released from each cycle.

Regulation of Spermatogenesis by FSH and Testosterone:

LH and FSH are secreted by the gonadotroph cells in the anterior lobe of the pituitary gland. LH and FSH are glycoproteins. The quantity of the carbohydrate bound to the protein in the hormone molecule varies considerably under different conditions, which may change the activity potential of the hormone.

FSH binds to specific FSH receptors attached to the Sertoli cells in the seminiferous tubules. This causes these cells to grow and stimulate the formation of spermatozoa in the seminiferous tubules of testes after maturity either continuously or seasonally depending upon the species. Simultaneously, testosterone also exerts a strong influence on spermatogenesis.

Hormones that stimulate spermatogenesis are:

a. Leutenizing Hormone:

It is secreted by the luteotrophs present in the anterior pituitary lobe. This hormone stimulates the interstitial cells of Leydig to secrete testosterone. Pituitary gonadotropin secretion is under tonic regulatory control. They undergo wide fluctuation in their circulating concentrations over short periods of time.

In human males LH is released after every 90 minutes. LH is released on exposure of a male to female, but successive presentation of the same female to a male mammal leads to habituation. LH specifically binds to Leydig cells while FSH binds to only Sertoli cells. This clearly establishes the individual roles of pituitary gonadotropins in regulating testicular function.

LH increases the cAMP levels in the interstitial cells of the testes but not the seminiferous tubules. FSH on the other hand, does not increase the cAMP levels in the Leydig cells, but stimulates the production of cAMP in seminiferous tubules in enriched fractions of the Sertoli cells.

b. Inhibin:

Inhibin is a peptide hormone produced by the Sertoli cells. It controls the secretion of pituitary FSH. Inhibin secretion by the Sertoli cells is regulated by influences deriving from maturing sperms. Pituitary FSH secretion normally is regulated by negative feedback by an inhibitory factor of Sertoli cell origin.

Granulosa cells from the ovarian follicle also secrete inhibin that acts directly on pituitary cells from males or females to supress FSH secretion. Administration of anti-inhibin antisera to rats of either sex causes increase in FSH levels. LH levels are not affected by anti inhibin antisera. The role of FSH and inhibin in the control of testicular and ovarian function is shown in Figure 1.

c. Prolactin:

Testicular receptors are maintained by one or more pituitary hormones other than the pituitary gonadotropins. Decrease in pituitary prolactin secretion decreases the testicular LH receptors, whereas prolactin treatment prevents loss of LH receptors in hypophysectomized animals. Thus, prolactin plays a role in the control of testicular Leydig cell LH receptor numbers.

d. Androgens:

Androgens control the differentiation and development of the male urinogenital system, the accessory sex organs and the external genital organs. Androgens produced by the testis are responsible for the growth and development of all those tissues that characterize the male.

Increased gonadotropins level during puberty increases the circulating levels of androgens. This increase in the level of androgens is responsible for initiating spermatogenesis and for the growth and development of the secondary sexual characteristics.

All androgens are steroid compounds. They can be synthesized from cholesterol or directly from acetyl coenzyme A. After secretion by the testes, about 97% of the hormone becomes either loosely bound to plasma albumin or tightly bound to a beta globulin called sex hormone binding globulin and circulates in blood in these states for 30 minutes to 1 hour.

By that time much of the testosterone becomes fixed to the tissues and is converted to a more active hormone known as dihydrotestosterone especially in some target organs like the prostate gland in adult and in external genitalia of the male fetus.

Intracellular Action of Testosterone:

Testosterone increases the rate of formation of proteins in target cells. In prostate gland testosterone enters the cells within a few minutes after secretion and is converted into dihydrotestosterone by an intracellular enzyme 5, α-reductase, Dihydrotestosterone binds to an intracelular receptor protein and this complex migrates to the nuclei and binds to a nuclear protein to induce the transcription process within 30 minutes.

RNA polymerase is activated and ultimately, the concentration of the cellular protein progressively increases. Thus, testosterone stimulates the production of protein and specifically those proteins in target organs responsible for the development of secondary sexual characteristics.

Some important target tissues do not have the reductase enzyme in their cells to convert testosterone into dihydrotestosterone. Therefore, in these tissues testosterone functions directly but only with its half potency. This direct action of testosterone is essential in male fetal tissues for the development of epididymis, vas deferens and seminal vesicles.

Spermatogenesis:

During sexual maturation, FSH and testosterone initiate spermatogenesis. FSH increases the size of the testis but does not increase the appearance of mature spermatozoa and secretory activity of the Leydig cells. For completion of spermatogenesis, testosterone is required. FSH initiates tie process of spermatogenesis while testosterone maintains it.

FSH interacts with the receptors located in the plasmalemma of the Sertoli ceils resulting in increased production of cAMP and the synthesis of an androgen binding protein (ABP). The ABP is subsequently secreted into the lumen of the seminiferous tubules. Leydig cells contain specific receptors for LH. In response to LH, testosterone released by the Leydig cells enters the seminiferous tubules through blood circulation.

Testosterone is actively taken up by the Sertoli cells. Within the Sertoli cells testosterone is bound to the androgen binding protein. This binding brings testosterone in close contact to spermatozoa on which maturation of the spermatozoa is dependent. Thus, the hormonal effects of testosterone on spermatogenesis are mediated through the Sertoli cells.

Androgen binding protein provides a mechanism for the accumulation of androgens within the Sertoli cells and its release into the lumen of the seminiferous tubules. From the lumen, ABP transports testosterone to the epididymis, where they mature and develop the potential for fertilization and motility.

Physiological Functions of Estrogens:

Estrogens and androgens are necessary for normal reproductive function in male. Information on estrogen functions in the male is obtained by the use of gene knockout technique involving a mutant mouse line without a functional estrogen receptor.

Fertility:

In mice without functional estrogen receptor, the testes atrophy progressively with decrease in the number of spermatozoa in epididymis, their viability and motility. Estrogen regulates the reabsorption of luminal fluid in the head of epididymis. Disruption of estrogen function causes spermatozoa to enter the epididymis in diluted rather than concentrated, resulting in infertility.

Essay on the Female Reproductive System:

As in the male, in female the ovary also performs the functions of gametogenesis and as an endocrine gland. Ova released from the ovary enter the oviducts and Fallopian tube. Fertilization of the ovum usually occurs in the fallopian tubes by the spermatozoa released by the male during copulation. The fertilized ovum descends into the uterus and is embedded in its wall for further development and embryogenesis.

The ovary consists of both epithelial and mesenchymal elements. The mesenchyme differentiates into interstitial tissue, which produces estrogen. The epithelial tissue is closely associated with the germinal elements of the ovary. It provides nutrition to the oocytes and is an important source of hormones required for different stages of the ovarian cycle.

The ovary is a solid structure covered by a visceral peritoneum of flattened cells. Inside this is a layer of cuboidal cells, the germinal epithelium. The stroma is divided into outer ovarian cortex and inner ovarian medulla. At birth, each oocyte is surrounded by a single layer of flattened granulosa cells. The combined structure is known as primordial follicle.

The primordial follicles are located near the periphery or cortex of the ovary, and are separated from each other by the stromal connective tissue and interstitial tissue. The primordial follicles remain inactive until puberty. The follicular epithelial cells are transformed into a single layer of cuboidal cells surrounding the oocyte.

The entire structure is known as primary follicle. During puberty, under the influence of hormones 6 to 12 primary follicles develop into secondary follicles. The granulosa cells secrete a mucoid material that forms the zona pellucida around the oocyte.

The granulosa cells develop protoplasmic processes that penetrate the zona pellucida and touch the plasmalemma of the oocyte. Out of the 6 to 12 primary follicles, during each menstrual cycle only one develops into a mature follicle while others become atretic and disappear. The granulosa cells continue to increase in number.

The interstitial tissue adjacent to the follicle becomes arranged concentrically around it to form the theca. Thecal cells adjacent to the follicle, the theca interna, are surrounded by an outer layer of interstitial cells forming the theca externa. Granulosa cells continue to proliferate and surrounding interstitial cells are incorporated into the theca. These changes are accompanied by accumulation of a fluid in the spaces between the granulosa cells.

A large vesicle or antrum is formed due to the enlargement of the follicle. Graulosa cells adhering to the surface of the oocyte form the coronal granulosa cells while the remaining cells in contact with the surrounding theca form the membrana granulosa. A streak of granulosa cells connects the cells around the ovum with the membrana granulosa.

The layer of granulosa cells around the ovum form the corona radiata. The oocyte acquires two membranes, the inner thin vitelline membrane and outer thick zona pellucida. The zona pellucida is surrounded by the corona radiata composed of enlarged follicular cells with fine canals between them. The fully formed mature follicle is known as Graffian follicle.

Ovulation:

The solid follicle develops in it a small cavity called as the antrum. The antrum gradually becomes wider. Release of the oocyte is known as ovulation. A single ovum is expelled from a ovarian follicle into the abdominal cavity in the middle of each monthly menstrual cycle.

Ovulation is caused due to increased turgidity and contraction of smooth muscle fibers around the follicles. The ovum passes through one of the Fallopian tubes into the uterus, if it is fertilized by the sperm.

In rabbit ovulation occurs after copulation. Mammalian ovulation is a unique biological process as it involves the physical disruption of healthy tissue at the surface of the ovary. Ovulation requires a surge of pituitary gonadotropins. The capillaries in the follicular wall dilate after about 4-6 hours of the starting of the ovulatory process.

As the time of rupture approaches, the apex of a mature follicle protrudes more and more above the surface of the ovary and the follicle wall gradually becomes thinner. The apical most part of the follicle becomes translucent and rapidly protrudes above the normal wall of the follicle wall to form a stigma.

The follicle ruptures within several minutes after the formation of the stigma. The final rupture of the follicle is dependent upon the degradation of the collagenous connective tissue in the thecal layer of the follicle wall. After the rupture of the wall of the follicle, the oocyte and surrounding cells are extruded within a couple of minutes.

The purposes of the ovarian follicle are to:

(i) Preserve the resident oocyte,

(ii) Mature the oocyte at the optimal time,

(iii) Produce hormones to develop a proliferating endometrium,

(iv) Release of the oocyte at the proper time,

(v) Formation of a high quality corpus luteum for implantation, and

(vi) Secrete hormones required for gestation.

Corpus Luteum:

After rupture and release of the ovum from the Graffian follicle, the granulosa and thecal cells increase in number and the blood clot is absorbed. The granulosa cells start to accumulate large quantity of cholesterol and this process of leutenization forms the corpus luteum. Cells derived from the theca interna migrate into the luteal tissue to give rise to small luteal cells, theca lutein cells and fibrobasts.

Hormonal Influence on the Ovary:

The female hormonal system consists of three hormones:

(i) A hypothalamic releasing hormone, the gonadotropin releasing hormone (GnRH),

(ii) The anterior pituitary hormones, follicle stimulating hormone (FSH) and leutenizing hormone (LH) both of which are secreted on stimulation by the GnRH, and

(iii) The ovarian hormones, estrogen and progesterone secreted by the ovaries in response to the two pituitary hormones. These different hormones are not secreted in constant amounts and continuously throughout the menstrual cycle but are secreted at different rates during different parts of the cycle.

Estrogen Biosynthesis:

The two cell theory of estrogen secretion states that the thecal cells produce carbon 19 androgens and these are delivered to the granulose cells where they are converted into aromatic compounds, the estrogens (Fig. 3). This theory is supported by the observation that granulose cells from several species secrete estrogens if provided with an androgen substrate.

This theory is supported by the observation that granulosa cells of several species secrete estrogens if given an androgen substrate. Thecal cells also produce large amount of androgens. The CYP 17 gene that codes for 17-hydroxylase and C17-20 lyase is actively regulated by LH. Granulosa cells have an aromatase system.

As follicular maturation progresses, the ability of the granulosa cells to aromatize androgens increases. Estrogen production increases within the follicles during the preovulatory phase and is highest at the time of the LH and FSH surge.

Before exposure to a high level of LH, androgen and estrogen levels predominate; after the LH surge and during the luteal phase of the cycle, progesterone is the major steroid produced. A complex number of interactions involving LH, FSH, androgens, progesterone and estrogens are involved in the shift from estrogens to progesterone synthesis.

Modified version of the two cell theory states that LH stimulates androgen production within the thecal cells. Androgens are then aromatized within the thecal cells but are also made available to the granulosa cells for aromatization to estrogens.

The estrogens produced by the thecal cells are the major source of circulating levels of the steroid, whereas estrogens synthesized by the granulosa cells are regulated through FSH stimulation of cAMP production and later activation of aromatase activity.

Estradiol is oxidized to estrone in the liver (Fig. 4). Estrone is hydrated to form estriol. During pregnancy, the placenta is an additional source of androgens. Aromatization of androstenedione and testosterone is a major source of estrogens in the male and old females.

Biosynthesis of Progesterone:

The growth of the ovarian follicles before ovulation is dependent on FSH and LH, both acting together FSH promotes the growth of the follicle by acting through receptors on the granulosa cells and inducing the aromatase enzyme required for the conversion of androgens to estrogens.

The action of FSH is enhanced by androgens. Androgens arise from the theca under the control of LH. FSH receptors occur only on granulosa cells, while LH receptors present initially only in the theca. appear in the granulosa cells and are coupled to the cAMP as FSH receptors.

During the preovulatory surge, LH acting on the granulosa cells initiates luteinization, resulting in a reduction of aromatase activity and the enhancement of progesterone synthesis and secretion as the luteinized granulosa cells transform to become the corpus luteum. Synthesis of progesterone is an early step in the biosynthesis of androgens and estrogens within the thecal cell. Carpus luteum is the major source of circulating progesterone.

Effect of Gonadotrophic Hormones:

The ovarian changes during the sexual cycle depend completely on the gonadotrophic hormones, FSH and LH secreted by the anterior pituitary gland. Ovaries remain inactive until stimulated by these hormones. At the age of 11 to 16 years pituitary begins to secrete more FSH and LH, which initiates the menstrual cycle.

During the life cycle of a normal female, the reproductive phase is characterized by monthly rhythmical changes in the rate of secretion of female hormones and corresponding changes in the ovaries and sex organs. This rhythmical pattern is called as the female sexual cycle or menstrual cycle. The duration of the cycle usually is 28 days.

The menstrual cycle can be divided into three phases:

(i) Proliferation of the endometrium or proliferative phase,

(ii) Development of secretory changes in the endometrium or secretory phase, and

(iii) Degeneration of the endometrium of the uterus or degenerative phase. The menstrual cycle includes a period of bleeding called menstruation and the days are counted from the first day of bleeding in the menstrual period.

Proliferative Phase:

In the beginning of the menstrual cycle, the endometrium of the uterine wall is in a state of degeneration. After menstruation, only a thin layer of endometrial stroma and epithelial cells are left and these are located in the deeper portion of glands and crypts of endometrium.

Therefore, to prepare the uterine wall for implantation after ovulation, following changes occur:

(1) The adenohypophysis secretes FSH under the influence of FSH-RH from the hypothalamus. The hypothalamus secretes FSH-RH when decrease in the level of estrogen and progesterone secreted in the proceeding menstrual cycle remove its inhibitory effect.

(2) FSH stimulates the growth of selected few primary follicles and maturation of primary follicles.

(3) FSH stimulates the follicular cells to secrete estrogen, which inhibits the secretion of FSH and stimulates hypothalamic secretion of LH-RH. This in turn stimulates the secretion of leutenizing hormone by the pituitary.

(4) LH induces the mature Graffian follicle to burst and release the ovum. This process occurs after 14 days and is known as ovulation. The ovum at this stage is in secondary oocyte stage with the second meiotic division in progress.

(5) Estrogen prepares the wall of the uterus for implantation by proliferating the endometrial epithelial cells. Blood vessels in the uterine wall become elongated and coiled, and vascularization increases. Uterine glands secrete a nutritive fluid into the cavity. Glycogen and fat accumulate.

(6) Under the influence of estradiol, the lining of Fallopian tube is thickened and the ciliary movements are increased. These changes help in conveying the ovum to the uterus.

Secretory Phase (Progesterone Phase):

The stage lasting for about ten days can be further differentiated into the following steps:

(1) LH and prolactin are at their peak of secretion from the anterior lobe of the pituitary. They stimulate the follicular cells of the empty Graffian follicles after the release of the ovum to grow rapidly and become filled with a yellow substance called luteum.

The cavity of follicle becomes filled with blood and broken down thecal cells. The follicle is known as corpus luteum. The yellow corpus luteum secretes the ovarian hormones, progesterone and .small quantities of estradiol.

(2) Estrogen increases cellular proliferation in the uterine endometrium while progesterone causes swelling and secretory activity of the endometrium. This hormone stimulates the endometrial glands to secrete a nutrient fluid for the fetus.

Cytoplasm of the stromal cells, lipid and glycogen deposits and blood supply to the endometrium increase tremendously. At the peak of the secretory phase, usually about one week after ovulation, the endometrium becomes 5-6 mm thick. Progesterone is also required for proper implantation of the fetus.

(3) Progesterone inhibits the release of FSH to prevent the development of additional follicles and ova.

Menstrual or Bleeding Phase:

This phase lasting for 4-5 days is characterized by:

(1) In case of failure of fertilization, the secondary oocyte undergoes degeneration and autolysis. High levels of progesterone in blood inhibit the release of pituitary LH.

(2) Absence of LH brings about autolysis of the corpus luteum and subsequent decrease in the level of progesterone. Regression of the corpus luteum begins about a week after ovulation and after ten days it is replaced by a small, non-functional whitish structure called as the corpus albicans.

(3) The uterine wall degenerates due to the deficiency of progesterone and is sloughed off. Blood vessels rupture causing bleeding. The lining of the Fallopian tubes also degenerates and breaks down. The cast off uterine and Fallopian tissue, and blood from the ruptured blood vessels passes out through the vaginal opening.

This process is called as menstruation or menstrual flow. This occurs after about 25 days and continues for 3-5 days. During normal menstruation, about 50 to 100 ml of blood and 30 ml of serous fluid are lost. The menstrual flow does not clot because fibrinolysin is released along with endometrial breakage.

In case if excessive bleeding occurs, fibrinolysin cannot prevent clotting. The presence of clot during menstrual flow is a serious problem that needs clinical treatment. The basal part of the endometrium remains in tact during menstruation and is the source of new lining wall to the uterus during the next cycle.

(4) Degeneration of the carpus luteum decreases the levels of progesterone and estradiol, which in turn removes their inhibitory effect on hypothalamus. Therefore, the hypothalamus starts to secrete FSH-RH and this stimulates the pituitary to produce FSH.

Functions of Ovarian Hormones:

The ovarian follicles are sources of three types of steroid hormones; progestins, androgen, and estrogens. Their levels vary in the menstrual cycle and drastically change in the pregnancy. In the follicular stage of the menstrual cycle, estradiol is dominant, whereas during the luteal phase and during pregnancy, progesterone prevails over others.

Estrogens:

Estrogens are mainly secreted by the ovary but minute quantities are also produced by adrenal cortex. During pregnancy, the placenta also contributes to the secretion of estrogens. Estrogens produced during the puberty in female are responsible for growth and development of the vagina, uterus, and oviducts, organs essential for the transportation of ovum, maturation of the zygote and implantation of the embryo.

Estrogens exert their effect on deposition of fat, growth and development of the mammary glands. In human females, three estrogens occur in significant quantities in the plasma. These are estradiol, estrone and estriol. The principal estrogen is estradiol. Small amount of estrone is secreted by the ovaries but most of it is formed from androgens secreted by the adrenal cortex.

Estriol a weak estrogen, is an oxidative product derived from both estradiol and estrone in the liver. The estrogenic potency of estradiol is 12 times higher than that of estrone and 80 times greater than that of estriol.

Functions:

(i) Estrogens promote proliferation and growth of specific cells in the body and are mainly responsible for development of secondary sexual characteristics of the female.

(ii) During puberty secretion of the estrogens increases by 20 times or more. The female sex organs increase in size due to their influence. Ovaries, fallopian tubes, uterus and vagina are all enlarged. Fat deposits in the external genitalia, pubis, labia majora and labia minora. The vaginal epithelium becomes converted from cuboidal into a stratifed type which is more resistant to infection and trauma.

(iii) Estrogens cause glandular tissue to proliferate and number of ciliated epithelial cells increases. The activity of cilia is also increased to promote the transport of fertilized ovum towards uterus.

(iv) Estrogens promote the development of the stromal tissue of breasts, growth of duct system and deposition of fat in breasts.

Progestins:

The most important progestin is progesterone. Small quantities of 17-hydroxy progesterone is also secreted and it has the same effect. Progesterone is the ovarian hormone of pregnancy and is responsible for preparing the reproductive tract for implantation of the zygote and subsequent and maintenance of the pregnant state.

Functions:

(a) Effects on Uterus:

The most important function of progesterone is to promote secretory changes in the uterine endometrium during the later half of monthly menstrual cycle thus preparing the uterus for implantation of fertilized ovum. Progesterone decreases the frequency and intensity of uterine contraction and thus helps in preventing expulsion of the implanted zygote.

(b) Effect on Fallopian Tubes:

Progesterone also promotes secretory changes in the mucosal lining of the Fallopian tubes. These secretions are required for the nutrition of the fertilized ovum.

(c) Effects on Breasts:

Progesterone promotes development of the lobules and alveoli of the breasts. This causes enlargement of the breasts. It cannot initiate the secretion of milk because milk secretion requires the hormone prolactin. It is speculated that preovulatory plasma levels of progesterone may trigger sexual behaviours in some species.

In rodents, progesterone is necessary for induction of sexual receptivity. Progesterone also plays a role in the nest- building activity and brooding behaviours in some birds. The following table summarizes the physiological actions of progesterone and estradiol.

Mechanisms of Action of Ovarian Steroid Hormones:

Estradiol and progesterone interact with cytoplasmic and nuclear protein receptors resulting in the release of the two-receptor subunits with attached steroid hormones from association with a heat shock protein. Identical subunits either singly or together interact directly with the DNA hormone responsive element to activate the transcriptional events leading to the translation of a cell specific protein.

Ovulation and Role of Luteinizing Hormone:

Ovulation is the process bf release of ovum from the ovarian Graffian follicles. This usually occurs in healthy normal women on fourteenth day after the last menstruation. Shortly before ovulation, the protruding outer wall of the follicles swells rapidly and a small area in the capsule called the stigma protrudes out.

In the next thirty minutes, fluid begins to ooze from the follicle through the stigma. Two minutes later, as a follicle becomes smaller due to loss of fluid, stigma ruptures and the ovum comes out surrounded by several thousand granulosa cells called the corona radiata.

Initiation of Ovulation:

Large quantity of luteinizing hormone is secreted by the pituitary gland. The LH in turn causes rapid secretion of follicular steroid hormone, progesterone.

Within a few hours two events occur:

(i) The theca externa (outer capsule of follicle) begins to release proteolytic enzymes from lysosomes that causes swelling of the follicle and degeneration of the stigma.

(ii) Simultaneously, there is rapid growth of new blood vessels into the wall of the follicles and at the same time prostaglandins are secreted in the follicular tissues. Finally, the combined follicle swelling and simultaneous degeneration of the stigma cause follicle rupture to discharge the ovum.

Ovulatory Surge of Leutenizing Hormone:

LH is essential for the growth and development of the follicle, and ovulation. About two days before the LH secretion increases markedly. FSH secretion also increases and the two hormones act together to produce rapid growth of the follicle during last few days before ovulation. LH secretion also has an effect on the theca cells to secrete progesterone.

The changes that occur can be summarized into:

(i) Rapid growth of the follicle,

(ii) Diminishing level of estrogen secretion, starting of the secretion of progesterone and ovulation occurs.

Menopause:

At the age of 40 to 50 years, sexual cycle in the female becomes irregular and ovulation fails to occur during many of the cycles. When secretion of female sex hormones and the process of ovulation stop completely, it is called menopause.

Throughout a womens reproductive life, about 400 follicles grow and ovulate. But at the age of about 45 years, only a few primordial follicles remain to be stimulated by FSH and LH, and production of estrogen decreases to zero when primordial follicles number becomes almost nil. Therefore, estrogen cannot inhibit the production of FSH and LH which are produced in large quantity in menopause.

The loss of estrogen causes many physiological changes in the function of the body such as:

(i) Extreme flushing of the skin,

(ii) Irritability,

(iii) Fatigue,

(iv) Anxiety and

(v) Decreased strength and calcification of bones throughout the body.

These symptoms are sometimes cured by regular administration of estrogen.