GAMETOGENESIS (formation of gametes)
· Formation of sperm from the germ cell in the testes is spermatogenesis.
· The process begins at puberty.
· Spermatogonia present in the lining of seminiferous tubules undergo mitotic division to increase their number.
· Each spermatogonium is diploid (2n) which contain 46 chromosomes.
· Innermost layer of spermatogonial becomes larger called primary spermatocyte.
· Primary spermatocyte undergoes meiosis-I to form two equal haploid (n) secondary spermatocytes (n).
· Each secondary spermatocyte undergoes meiosis-II to form two equal, haploid spermatids.
· Each primary spermatocyte produces four spermatids.
· Spermatids transformed into spermatozoa (sperms) by the process called spermiogenesis.
· The sperm head embedded in the Sertoli cell.
· Release of sperm from the seminiferous tubule is called spermiation.
Hormonal control of spermatogenesis:
· This process is initiated at puberty due to secretion of gonadotrophins releasing hormone (GnRH)
· GnRH secreted form hypothalamus and stimulate anterior pituitary to secrete two gonadotrophins.
o Luteinizing hormone (LH) and
o Follicle stimulating Hormone (FSH)
· LH acts on Leydig cells and stimulates synthesis of androgens.
· Androgen stimulates spermatogenesis.
· FSH acts on Sertoli cells and stimulates spermatogenesis in other ways.
Structure of sperm:
· Ultrastructure of sperm consists of a head, neck, a middle piece and a tail.
· Whole body of sperm surrounded by plasma membrane.
· The sperm head contain an elongated haploid nucleus.
· Above the nucleus a cap like structure present called acrosome.
· The acrosome contains enzymes which help in fertilization of ovum.
· The middle piece contains mitochondria, which provide energy for movement of tail that facilitate sperm motility.
· Human male ejaculates 200-300 million sperms during coitus.
· 60 percent must have normal shape and size and 40 percent of them must show vigorous motility.
· Sperm released from seminiferous tubules enters into accessory ducts.
· On their way fluids from seminal vesicle and prostate gland added which collectively called as Semen.
· The function of male accessory ducts and glands are maintained by testicular hormone androgen.
· Formation of a mature female gamete or ovum is called oogenesis.
· Oogenesis starts during embryonic stage, 25th week of the foetal age.
· Germinal epithelium of ovary divided mitotically to produce millions of gamete mother cell or oogonia.
· No oogonia formed or added after birth.
· Oogonia enters into meiosis-I and proceeds up to diakinesis of Prophase-I and get suspended, at this stage called primary Oocytes.
· Each primary oocyte surrounded by layers of granulose cells and then called primary follicle.
· At puberty only 60,000 to 80,000 primary oocytes are left in each ovary.
· After puberty primary follicles get surrounded by more layers of granulosa cells and a new theca to form secondary follicles.
· The secondary follicle transformed into tertiary follicle, characterized by a fluid filled cavity called antrum.
· The theca layers organized into an inner theca interna and outer theca externa.
· During the growth of primary follicle into tertiary follicle during puberty, the primary oocyte restarts its first meiotic division and completes it within tertiary follicle resulting two unequal haploid cells.
o Large haploid cell is called secondary oocyte.
o A tiny cell called first polar body.
· The secondary oocyte retains bulk of the nutrient rich cytoplasm of primary oocyte.
· The tertiary follicle having secondary oocyte further changes into Graafian follicle.
· The secondary oocyte surrounded by a new membrane, zona pellucida.
· The secondary oocyte undergoes second meiotic division continued upto metaphase-II and get suspended until entry of sperm.
· At this stage Graafian follicle releases secondary oocyte from the ovary by the process called ovulation.
· On entry of a sperm into the secondary oocytes stimulates it to complete meiosis-II and there is formation of a haploid ovum and a second polar body (n).
· Reproductive cycle of female primates is called menstrual cycle.
· The first menstruation begins at puberty is called Menarche.
· Menstrual cycle repeated at an average interval of 28/29 days.
· One ovum is released in the middle of each menstrual cycle.
Menstrual cycle has following phases:
· 1st phase of menstrual cycle.
· Menstrual flow occurs.
· Lasts for 3-5 days.
· Breakdown of endometrial lining and blood vessel.
· Mucus and blood comes out through vagina.
· It occurs only when ovum released but no fertilization.
· Lack of menstruation is the indication of pregnancy.
· Menstrual phase followed by follicular phase.
· Primary follicle becomes Graafian follicle.
· Regeneration and proliferation of uterine endometrium.
· LH and FSH level increases gradually in follicular phase.
· Level of estrogen increases as it is secreted from growing follicle.
· It lasts for 5-13 days.
· FSH and LH attain peak level in this period (14th day).
· This is called LH surge, which induces rupture of Graafian follicle and release of ovum from the ovary called ovulation.
· Remaining part of Graafian follicle transformed into corpus luteum.
· Corpus luteum produces large amount of progesterone.
· Progesterone maintains the uterine endometrium, and prepares it for implantation.
· Thickness of uterine endometrium increase in many folds, due to proliferation.
· If there is fertilization, corpus luteum grows further and pregnancy continued, menstrual cycle stopped.
· In the absence of fertilization corpus luteum degenerates.
· Disintegration of endometrium leading to menstruation.
· Menstrual cycle ceases around 50 years of age, called menopause.
Humans reproduce sexually by the union of male and female sex cells.Reproductive system of male and female are different, they are structured to function together to achieve internal fertilization.
Delivery of foetus is known as parturition. It is triggers with the release of oxytocin. Production of milk from mammary gland is known as lactation.