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Hormonal Control of Gametogenesis, [Academic script], , Course, , :, , Zoology, , Name, , :, , B. Sc. 2nd Semester, , Paper No., , :, , & Title, , :, , Comparative Anatomy and Developmental Biology of, , Vertebrates, , Topic No., , :, , & Title, , :, , Hormonal Control of Gametogenesis, , Lecture Title, , :, , Hormonal Control of Gametogenesis

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Hormonal Control of Gametogenesis, , Introduction, As animals became more complex, specific organs and organ systems developed to support, specific functions for the organism. The reproductive structures that evolved in land animals, allow males and females to mate, fertilize internally, and support the growth and development, of offspring. Processes developed to produce reproductive cells that had exactly half the, number of chromosomes of each parent so that new combinations would have the appropriate, amount of genetic material., Gametogenesis, the production of sperm and eggs, takes place through the process of meiosis., During meiosis, two cell divisions separate the paired chromosomes in the nucleus and then, separate the chromatids that were made during an earlier stage of the cell’s life cycle. Meiosis, produces haploid cells with half of each pair of chromosomes normally found in diploid cells., The production of sperm is called spermatogenesis and the production of eggs is, called oogenesis., Reproduction and Hormonal Influence, Sex Steroids from the Gonads Influence Most Aspects of Reproduction in all Vertebrates, and probably most of the Invertebrates because reproduction is closely linked with endocrine, function., Hormones produced by the gonads of animals have vital functions in nearly all aspects of, reproduction, from reproductive behaviours to the ability to produce offspring., In males, the gonads are called testes. They produce and house the gametes, called sperm cells., In addition, the testes produce several related steroid hormones collectively termed androgens., In many vertebrates, including humans, the primary androgen is testosterone., The gonads of females are called the ovaries, which contain the female gametes, or egg cells., Other cells within the ovaries secrete two major steroid hormones, progesterone and a family, of related hormones called estrogens. The major estrogen in many animals, including humans,, is estradiol., Collectively, the male and female gonadal steroid hormones are referred to as the sex steroids., The sex steroids are responsible for male- or female-specific reproductive changes associated, with courtship and mating., In vertebrates, androgens from the developing testes are required for development of the male, phenotype. Exposure of foetuses to abnormal concentrations of male or female hormones can, lead to ambiguous phenotypes and sexual behaviour after birth., Sex steroids are also chiefly responsible for development of secondary sex characteristics for, each sex, which in humans include growth of the appropriate external genitals, growth of the, breasts in women, development of facial hair in men, distribution and amount of fat & muscle, in the body., Finally, sex steroids are required for maturation of gametes, the transition of young animals, to reproductive maturity, and the ability of females to produce young ones.

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The ability of the testes and ovaries to produce sex steroids depends on the presence of the, gonadotropins secreted by the anterior pituitary, which are the same in both sexes and, include follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Therefore,, identical anterior pituitary gland hormones control the gonads in both male and female animals., , HORMONAL CONTROL OF SPERMATOGENESIS., Hormones play an organizational role, priming the body to behave in a certain way once, puberty begins, and an activational role, referring to changes in hormones during adolescence, that trigger behavioural and physical changes., MAJOR CELLS RESPONSIBLE FOR TESTICULAR FUNCTION (Fig. 1), •, , GERM CELLS: spermatogonia to spermatozoa, , •, , SERTOLI CELLS: regulate germ cell development to spermatozoa, , •, , INTERSTITIAL LEYDIG CELLS: that secrete testosterone, the major androgen, , At the onset of puberty, the hypothalamus begins secreting high pulses of GnRH, or, gonadotropin-releasing hormone. In response, the pituitary gland releases follicle stimulating, hormone (FSH) and luteinizing hormone (LH) into the male system for the first time. (Fig., 2) FSH enters the testes, stimulating the Sertoli cells, which help to nourish the sperm cells, that the testes produce, to begin facilitating spermatogenesis., LH also enters the testes, stimulating the interstitial cells, called Leydig cells, to make and, release testosterone into the testes and the blood. (Fig 3), Testosterone, the hormone responsible for the secondary sexual characteristics that develop, in the male during adolescence, stimulates spermatogenesis, or the process of sperm, production in the testes.(Fig 3) Secondary sex characteristics include a deepening of the voice,, the growth of facial, axillary, and pubic hair, and the beginnings of the sex drive., A negative feedback system occurs in the male with rising levels of testosterone acting on the, hypothalamus and anterior pituitary to inhibit the release of GnRH, FSH, and LH., The Sertoli cells produce the hormone inhibin, which is released into the blood when the sperm, count is too high. This inhibits the release of GnRH and FSH, which will cause spermatogenesis, to slow down. If the sperm count reaches 20 million/ml, the Sertoli cells cease to release, inhibin, allowing the sperm count to increase. (Fig. 4), Hormonal Control of Oogenesis, The control of reproduction in females is more complex than that of the male. As with the male,, the hypothalamic hormone GnRH (gonadotropin-releasing hormone) causes the release of the, hormones FSH (follicle stimulating hormone) and LH (luteinizing hormone) from the anterior, pituitary. Gonadotropins, i.e. LH and FSH stimulate follicular development and secretion of, oestrogens by the growing follicles. Both LH and FSH attain a peak level in the middle of, the cycle (14th day) and induces the mature Grafian Follicle to burst and eject its egg into the, fallopian tube. This is called ovulation (Fig.5). LH also stimulates the formation of corpus, luteum from the ruptured follicle and secretion of progesterone from corpus luteum. (Fig.6)

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As the level of progesterone increases in the blood it decreases the secretion of FSH and LH, from the anterior pituitary. So this inhibits future maturation of follicle and ovulation till, pregnancy is over. (Fig. 6)., The granulose cells of the developing ovarian follicles synthesize estrogen. Estradiol and, progesterone are steroid hormones that prepare the body for pregnancy. Estradiol is the, reproductive hormone in females that assists in endometrial regrowth, ovulation, and calcium, absorption; it is also responsible for the secondary sexual characteristics of females. These, include breast development, flaring of the hips, and a shorter period necessary for bone, maturation. Progesterone assists in endometrial re-growth and inhibition of FSH and LH, release., The levels of progesterone and estrogen has a negative feedback control on the release of, GnRH from the hypothalamus. As the level of progesterone increases in the blood it decreases, the secretion of FSH and LH from the anterior pituitary. So this inhibits future maturation of, follicle and ovulation till pregnancy is over. (Fig. 7).