Human Reproduction

The One Thing

Human reproduction depends on the coordinated functioning of reproductive organs, haploid gametes, hormones, and reproductive cycles. Meiosis produces gametes with 23 chromosomes, while fertilisation restores the diploid number of 46 chromosomes and initiates embryonic development.

Reactions, Processes and Experiments

Key Terms

• Testes: Paired male reproductive organs in the scrotum that produce sperm and secrete testosterone.
• Seminiferous tubules: Highly coiled tubules in the testes where sperm formation occurs.
• Sertoli cells: Cells that nourish developing sperm cells and help regulate spermatogenesis.
• Leydig cells: Interstitial cells that secrete testosterone.
• Epididymis: A coiled duct where sperm mature and are temporarily stored.
• Accessory glands: The seminal vesicles, prostate gland, and bulbourethral glands, which add secretions to sperm to form semen.
• Ovaries: Paired female reproductive organs that produce ova and secrete oestrogen and progesterone.
• Fallopian tubes: Ducts that receive the ovum; fertilisation usually occurs at the ampullary-isthmic junction.
• Uterus: A muscular organ where implantation and development of the embryo and foetus occur.
• Endometrium: The uterine lining that thickens during the menstrual cycle and supports implantation.
• Spermatogenesis: Formation of mature sperm from diploid spermatogonia in the seminiferous tubules.
• Spermiogenesis: Transformation of spermatids into mature spermatozoa.
• Spermiation: Release of mature spermatozoa from Sertoli cells into the lumen of seminiferous tubules.
• Oogenesis: Formation and maturation of the female gamete, beginning before birth and continuing cyclically after puberty.
• Primary oocyte: A diploid female germ cell that begins meiosis during foetal development and remains arrested in prophase I until puberty.
• Secondary oocyte: The haploid cell formed after meiosis I, released during ovulation, and completing meiosis II only after fertilisation.
• Menstrual cycle: Cyclic changes in the ovaries and uterus, usually lasting about 28 days and controlled by reproductive hormones.
• Follicular phase: The first ovarian-cycle phase, during which follicles grow under FSH and oestrogen levels rise.
• Ovulation: Release of a secondary oocyte from a mature Graafian follicle, mainly triggered by an LH surge.
• Luteal phase: The post-ovulation phase in which the corpus luteum secretes progesterone and helps maintain the endometrium.
• Corpus luteum: The yellow body formed from the ruptured follicle after ovulation; it secretes progesterone and some oestrogen.
• Fertilisation: Fusion of haploid sperm and ovum nuclei to form a diploid zygote.
• Acrosome: A cap-like structure containing enzymes at the tip of a sperm head that helps penetrate the ovum coverings.
• Cleavage: Rapid mitotic divisions of the zygote without an increase in overall size.
• Morula: A solid ball of approximately 8 to 16 blastomeres formed during early cleavage.
• Blastocyst: A hollow embryonic structure containing an outer trophoblast and inner cell mass.
• Implantation: Attachment and embedding of the blastocyst into the uterine endometrium.
• Trophoblast: The blastocyst’s outer layer, which assists implantation and contributes to placenta formation.
• Inner cell mass: The blastocyst cell group that develops into the embryo.
• Placenta: A temporary organ that exchanges nutrients, gases, and wastes between maternal and foetal blood without direct mixing.
• Human chorionic gonadotropin: A hormone secreted by the developing embryo that maintains the corpus luteum during early pregnancy and is detected in pregnancy tests.
• Parturition: Childbirth, initiated by hormonal and nervous signals and strengthened by oxytocin-induced uterine contractions.
• Lactation: Production and release of milk by the mammary glands after childbirth.
• Prolactin: A hormone that stimulates milk production.
• Oxytocin: A hormone that promotes uterine contractions during childbirth and milk ejection during breastfeeding.

Easily Confused

• Spermatogenesis and spermiogenesis: Spermatogenesis is the complete formation of sperm from spermatogonia, whereas spermiogenesis is specifically the transformation of spermatids into spermatozoa.
• Spermiogenesis and spermiation: Spermiogenesis changes the structure of spermatids; 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.
• Follicular phase and luteal phase: The follicular phase precedes ovulation and involves follicle growth, whereas the luteal phase follows ovulation and involves corpus luteum activity.
• Ovulation and fertilisation: Ovulation releases the secondary oocyte from the ovary; fertilisation is the fusion of sperm and ovum nuclei, usually in the fallopian tube.
• Trophoblast and inner cell mass: The trophoblast is the outer blastocyst layer involved in implantation and placenta formation; the inner cell mass develops into the embryo.
• hCG and progesterone: hCG maintains the corpus luteum during early pregnancy, whereas progesterone maintains the uterine endometrium.
• Prolactin and oxytocin: Prolactin stimulates milk production; oxytocin promotes milk ejection and uterine contractions.
• Embryo and foetus: The developing individual is called an embryo during early development and is commonly called a foetus after major body parts become recognisable.
• Menstruation and implantation: Menstruation is breakdown of the endometrium after hormone levels fall; implantation is embedding of the blastocyst into the endometrium.

What Gets Asked

• Reproductive-organ functions: Questions may ask for the roles of the testes, seminiferous tubules, Sertoli cells, Leydig cells, epididymis, accessory glands, ovaries, fallopian tubes, uterus, or endometrium. Marks are lost by assigning testosterone secretion to Sertoli cells rather than Leydig cells, or sperm maturation to the testes rather than the epididymis.
• Chromosome number and meiosis: Questions may require the relationship diploid germ cell (2n) → meiosis → haploid gametes (n); sperm (n) + ovum (n) → zygote (2n). The key error is failing to state that human somatic cells contain 46 chromosomes and gametes contain 23.
• Comparison of spermatogenesis and oogenesis: Questions may ask why one primary spermatocyte produces four functional spermatozoa but one primary oocyte generally produces one large ovum and polar bodies. The specific mark-losing omission is unequal cytokinesis in oogenesis.
• Menstrual-cycle regulation: Questions may test the four broad phases, the approximately 28-day cycle, ovulation near day 14, and hormonal roles. Marks are lost by attributing ovulation to FSH rather than the LH surge, or by omitting corpus luteum degeneration and falling progesterone and oestrogen in menstruation.
• Fertilisation and early development: Questions may require the site of fertilisation, acrosomal reaction, prevention of polyspermy, cleavage, morula, blastocyst, and implantation. The common specific confusion is placing fertilisation in the uterus rather than the ampullary-isthmic junction of the fallopian tube.
• Pregnancy, childbirth, and lactation: Questions may ask about placental functions, hCG, parturition, colostrum, prolactin, and oxytocin. Marks are lost by confusing milk production, controlled by prolactin, with milk ejection, promoted by oxytocin.