Human Reproduction

The One Thing

Human reproduction depends on coordinated gamete formation, fertilisation, hormonal regulation, implantation, pregnancy, childbirth, and lactation. Meiosis produces haploid gametes, while fertilisation restores the diploid chromosome number and initiates development.

Reactions, Processes and Experiments

Key Terms

• Male reproductive system: Includes the testes, ducts, accessory glands, and penis; its major functions are sperm production, sperm transport, and delivery of semen.
• Testes: Male gonads in the scrotum that produce sperm in seminiferous tubules and secrete testosterone through Leydig cells.
• Sertoli cells: Cells in seminiferous tubules that nourish developing sperm cells and help regulate spermatogenesis.
• Leydig cells: Interstitial cells of the testes that secrete androgens, mainly testosterone.
• Female reproductive system: Includes the ovaries, oviducts, uterus, cervix, vagina, and external genitalia; it produces ova, supports fertilisation, and nourishes the developing embryo.
• Ovaries: Female gonads that produce ova and secrete oestrogens and progesterone.
• Spermatogenesis: Formation of haploid spermatozoa from diploid spermatogonia in the seminiferous tubules after puberty.
• Spermiogenesis: Transformation of haploid spermatids into mature spermatozoa.
• Spermiation: Release of mature spermatozoa from Sertoli cells into the lumen of seminiferous tubules.
• Sperm structure: A sperm has a head containing the nucleus and acrosome, a mitochondria-rich middle piece, and a tail for movement.
• Oogenesis: Formation and maturation of the female gamete; it begins before birth, resumes after puberty, and usually produces one functional ovum and polar bodies from one primary oocyte.
• Primary oocyte: Diploid oocyte formed during foetal development that remains arrested in prophase I until puberty.
• Secondary oocyte: Haploid cell formed after meiosis I; released during ovulation and completing meiosis II only after sperm entry.
• Menstrual cycle: Cyclic changes in the ovaries and uterus, usually lasting about 28 days, that prepare for ovulation and possible pregnancy.
• Menstrual phase: Usually days 1–5, when the functional uterine lining is shed because ovarian hormone levels fall.
• Follicular or proliferative phase: Usually days 6–13, when follicles grow and oestrogen repairs and thickens the endometrium.
• Ovulation: Release of the secondary oocyte from a mature Graafian follicle, generally around day 14, triggered by an LH surge.
• Luteal or secretory phase: Usually days 15–28, when the corpus luteum secretes progesterone and maintains the endometrium.
• Corpus luteum: Yellow body formed from the ruptured follicle after ovulation; secretes progesterone and some oestrogen.
• Fertilisation: Fusion of a sperm and secondary oocyte, usually in the ampullary region of the oviduct, forming a diploid zygote.
• Acrosome reaction: Release of enzymes from the sperm acrosome that assists penetration of the oocyte’s protective layers.
• Zygote: Diploid cell formed immediately after fertilisation; undergoes cleavage and develops into an embryo.
• Cleavage: Rapid mitotic divisions producing smaller blastomeres without an overall increase in embryo size.
• Morula: Solid ball of approximately 8–16 blastomeres formed during early cleavage.
• Blastocyst: Hollow embryonic stage containing an outer trophoblast and inner cell mass; it implants in the endometrium.
• Implantation: Attachment and embedding of the blastocyst into the uterine endometrium, generally 6–7 days after fertilisation.
• Placenta: Temporary organ formed from foetal and maternal tissues that exchanges nutrients, gases, and wastes and secretes hormones.
• Pregnancy hormones: hCG maintains the corpus luteum early in pregnancy; later, the placenta produces progesterone, oestrogens, human placental lactogen, and other hormones.
• Gestation: Period of embryonic and foetal development inside the uterus, approximately nine months or 40 weeks from the last menstrual period.
• Parturition: Childbirth, initiated by hormonal and nervous signals and strengthened by oxytocin-driven uterine contractions.
• Lactation: Production and release of milk by mammary glands after childbirth; prolactin promotes production and oxytocin causes ejection.

Easily Confused

• Ovulation and fertilisation: Ovulation releases the secondary oocyte; fertilisation is the later fusion of the secondary oocyte with a sperm.
• Spermatogenesis and spermiogenesis: Spermatogenesis produces sperm cells from spermatogonia; spermiogenesis specifically converts spermatids into mature spermatozoa.
• Spermiogenesis and spermiation: Spermiogenesis changes cell structure; spermiation releases mature spermatozoa into the tubule lumen.
• Primary oocyte and secondary oocyte: The primary oocyte is diploid and arrested in prophase I; the secondary oocyte is haploid, formed after meiosis I, and released at ovulation.
• Morula and blastocyst: The morula is a solid ball of approximately 8–16 blastomeres; the blastocyst is hollow and contains a trophoblast and inner cell mass.
• Trophoblast and inner cell mass: The trophoblast contributes to placenta formation; the inner cell mass develops into the embryo.
• Menstrual, follicular, and luteal phases: The menstrual phase sheds the endometrium; the follicular phase rebuilds it under oestrogen; the luteal phase maintains it under progesterone.
• FSH and LH: FSH promotes follicle growth and supports spermatogenesis through Sertoli cells; LH triggers ovulation and corpus luteum formation and stimulates testosterone secretion by Leydig cells.
• Oestrogen and progesterone: Oestrogen mainly promotes endometrial growth during the follicular phase; progesterone maintains the endometrium during the luteal phase.
• Placenta and umbilical cord: The placenta is the exchange and endocrine organ; the umbilical cord connects the foetus to the placenta and contains foetal blood vessels.
• Prolactin and oxytocin: Prolactin promotes milk production; oxytocin causes milk ejection and strengthens uterine contractions during parturition.
• Sperm and ovum sex chromosomes: The sperm contributes either X or Y; the ovum normally contributes X, so chromosomal sex is determined by the sperm at fertilisation.

What Gets Asked

• Describe male and female reproductive anatomy and functions: Questions may require the male ducts—rete testis, vasa efferentia, epididymis, vas deferens, ejaculatory ducts, and urethra—or the female structures, including the oviduct regions infundibulum, ampulla, and isthmus and the uterine layers perimetrium, myometrium, and endometrium. Marks are lost by omitting named structures or their functions.
• Compare spermatogenesis and oogenesis: State that spermatogenesis begins at puberty and continues through adult life, producing four spermatozoa from one primary spermatocyte, whereas the primary oocyte population forms before birth and usually produces one functional ovum and polar bodies.
• Explain menstrual-cycle regulation: Link GnRH to FSH and LH, then to follicle growth, ovulation, corpus luteum formation, oestrogen, and progesterone. The immediate hormonal trigger for ovulation is the LH surge.
• Distinguish ovulation from fertilisation: Ovulation releases the secondary oocyte, generally around day 14; fertilisation normally occurs at the junction of the ampulla and isthmus when a sperm fuses with it.
• Sequence early development: The expected sequence is zygote, cleavage stages, morula, blastocyst, implantation, embryo, and foetus. The blastocyst implants approximately 6–7 days after fertilisation.
• Explain pregnancy, childbirth, and lactation hormonally: Include hCG, placental progesterone and oestrogens, oxytocin in parturition and milk ejection, prolactin in milk production, and the antibody-rich composition of colostrum.