Human Reproduction notes

Human reproduction is sexual reproduction in which male and female gametes fuse to form a diploid zygote. NCERT focuses on reproductive organs, gamete formation, hormonal control, menstrual cycle, fertilisation, implantation, pregnancy, parturition and lactation. The male system produces and transfers sperms, while the female system produces ova, supports fertilisation, implantation and foetal development. Gametogenesis creates haploid gametes through meiosis. The menstrual cycle prepares the uterus every month for possible pregnancy. Fertilisation usually occurs in the ampullary-isthmic junction of the fallopian tube, followed by cleavage, blastocyst formation and implantation. Pregnancy is maintained by placenta and hormones, and lactation provides nutrition and immunity through milk and colostrum.

The male reproductive system includes paired testes, accessory ducts, accessory glands and external genitalia. The testes lie outside the abdominal cavity in the scrotum because spermatogenesis requires a temperature about 2 to 2.5°C lower than body temperature. Each testis contains seminiferous tubules where sperms are formed with support from Sertoli cells. Leydig cells in the interstitial spaces secrete androgens, mainly testosterone. Sperms move through rete testis, vasa efferentia, epididymis, vas deferens, ejaculatory duct and urethra. Seminal vesicles, prostate and bulbourethral glands add secretions that form seminal plasma. The penis transfers semen into the female reproductive tract during insemination.

The female reproductive system consists of paired ovaries, paired oviducts, uterus, cervix, vagina, external genitalia and mammary glands. Ovaries are the primary female sex organs; they produce ova and ovarian hormones such as estrogen and progesterone. Each oviduct has infundibulum with fimbriae, ampulla and isthmus. The ampullary-isthmic junction is the usual site of fertilisation. The uterus is a muscular, inverted pear-shaped organ with perimetrium, myometrium and endometrium. The endometrium undergoes cyclic changes and supports implantation. The cervix opens into the vagina, forming the birth canal. Mammary glands are modified sweat glands that produce milk after childbirth.

Gametogenesis is the formation of haploid gametes from diploid germ cells. In males, spermatogenesis starts at puberty inside seminiferous tubules. Spermatogonia divide by mitosis, form primary spermatocytes, undergo meiosis I to form secondary spermatocytes and meiosis II to form spermatids. Spermiogenesis converts spermatids into spermatozoa, and spermiation releases them. In females, oogenesis begins during foetal life. Primary oocytes start meiosis but arrest in prophase I until puberty. Usually one secondary oocyte and polar body are formed per cycle. Ovulation releases a secondary oocyte arrested in metaphase II, and meiosis completes only after fertilisation. Hormones regulate both processes through GnRH, FSH, LH, testosterone, estrogen and progesterone.

The menstrual cycle is the cyclic change in ovaries and uterus of human females, repeated approximately every 28 to 29 days from menarche to menopause. It includes menstrual, follicular or proliferative, ovulatory and luteal or secretory phases. During menstruation, the endometrial lining breaks down if pregnancy has not occurred. In the follicular phase, FSH stimulates follicular growth and estrogen rebuilds the endometrium. Around mid-cycle, a rapid LH surge causes ovulation. After ovulation, the ruptured follicle becomes corpus luteum and secretes progesterone, which maintains a secretory endometrium. If fertilisation does not occur, corpus luteum degenerates, hormone levels fall and menstruation begins again.

Fertilisation is the fusion of haploid male and female gametes to form a diploid zygote. In humans, it normally occurs at the ampullary-isthmic junction of the oviduct. Sperms deposited in the vagina travel through cervix, uterus and oviduct. A sperm penetrates the corona radiata and zona pellucida using acrosomal enzymes, triggering completion of meiosis II in the secondary oocyte. After fertilisation, the zygote undergoes mitotic cleavage while moving toward the uterus, forming morula and then blastocyst. The blastocyst has trophoblast and inner cell mass. Trophoblast attaches to the endometrium, leading to implantation. Placental formation begins through interaction between foetal trophoblast and maternal uterine tissues.

Pregnancy begins after successful implantation and lasts about nine months in humans. The placenta forms a structural and functional connection between embryo and mother. It helps in nutrient supply, gas exchange, waste removal and hormone secretion. Important placental hormones include hCG, hPL, estrogens and progesterone; relaxin is also secreted later in pregnancy. During development, major organs form in early embryonic stages and foetal growth continues through the trimesters. Parturition is childbirth, triggered by neuroendocrine signals from the fully developed foetus and placenta. Oxytocin causes strong uterine contractions through positive feedback. After delivery, mammary glands secrete milk. The first milk, colostrum, is rich in antibodies and provides passive immunity.