Spermatogenesis occurs in seminiferous tubules of testes and takes approximately 74 days. Complete sequence: Spermatogonia (2n) undergo MITOSIS — both for self-renewal (maintain stem cell pool) and to produce primary spermatocytes. Primary spermatocytes (2n) undergo MEIOSIS I (reductional division) — longest phase (~22 days). Each primary spermatocyte produces 2 secondary spermatocytes (n, haploid). Secondary spermatocytes (n) undergo MEIOSIS II (equational, like mitosis) — produces 2 spermatids per secondary spermatocyte. So 1 primary spermatocyte → 4 spermatids total. Spermatids (n) undergo SPERMIOGENESIS — transformation to mature spermatozoa. No cell division — only differentiation. Spermiation: release of mature spermatozoa from Sertoli cells into tubular lumen.

Spermatogonia must maintain the stem cell pool while producing cells for spermatogenesis. If all spermatogonia entered meiosis at once, the stem cell population would be exhausted — no more sperm production. Mitosis allows asymmetric division: one daughter cell remains as spermatogonium (stem cell, self-renewal); the other daughter cell differentiates into a primary spermatocyte that enters meiosis. Two types: Type A spermatogonia (dark and pale subtypes) — stem cells. Type B spermatogonia — committed to differentiation, produce primary spermatocytes. This careful regulation of mitosis vs meiosis ensures continuous sperm production throughout male reproductive life (puberty to old age).

Meiosis I (reductional division): Primary spermatocyte (2n, 4C DNA content) → 2 secondary spermatocytes (n, 2C each). HOMOLOGOUS CHROMOSOMES separate (not sister chromatids). Chromosome number halved (2n → n). DNA amount halved. Crossing over occurs in prophase I (pachytene stage) — creates genetic variation. This is why statement B is wrong — primary spermatocytes use MEIOSIS I, not mitosis. Meiosis II (equational division): Secondary spermatocyte (n, 2C) → 2 spermatids (n, 1C each). SISTER CHROMATIDS separate. Like mitosis. Chromosome number unchanged, DNA amount halved. No DNA replication between meiosis I and II. Total: 1 primary spermatocyte → 4 genetically different haploid spermatids.

Spermiogenesis transforms round spermatids into elongated mature spermatozoa with no cell division involved. Major events: Nuclear condensation: histones replaced by protamines (arginine/cysteine-rich proteins) → DNA packed ~6× more tightly than somatic cells → highly streamlined head. Acrosome formation: Golgi apparatus produces proacrosomal granules → fuse into acrosomal vesicle → caps anterior 2/3 of nucleus. Contains enzymes: hyaluronidase, acrosin (needed to penetrate zona pellucida during fertilisation). Flagellum development: centriole migrates to posterior of nucleus → axoneme (9+2 microtubule pattern) extends → forms the tail for motility. Mitochondrial sheath: mitochondria coil around the midpiece providing ATP for flagellar movement. Cytoplasm shedding: most cytoplasm removed as residual body → phagocytosed by Sertoli cells. Final product: mature spermatozoon with head (nucleus + acrosome), midpiece (mitochondria), tail (axoneme).

Human sperm: total length ~60 micrometres. Head (4-5 microns): contains highly condensed nucleus (protamine-packed DNA). Acrosome covers anterior 2/3 of head — contains lytic enzymes for zona pellucida penetration. Neck: short connecting piece, contains proximal centriole (the origin of the axoneme). Midpiece (5-7 microns): mitochondria arranged in a tight helix around the central axoneme. Provides ATP for sperm motility. Principal piece (~40 microns): longest segment. Contains axoneme (9 outer doublets + 2 central singlets = 9+2 arrangement) surrounded by fibrous sheath. Dynein arms between doublets use ATP → sliding mechanism → flagellar beating. End piece: terminal segment (~5 microns). Sperm motility: progressive forward motility at 25-50 micrometres per second. Normal parameters: concentration >15 million/mL, motility >40%, morphology >4% normal forms.

Sertoli cells (sustentacular cells) line the seminiferous tubules and provide essential support for spermatogenesis. Functions: Nutritional support: provide glucose, lactate, amino acids to developing germ cells (which have no direct blood supply). Physical scaffolding: germ cells nestle in cytoplasmic recesses of Sertoli cells. Blood-testis barrier (BTB): tight junctions between adjacent Sertoli cells divide the tubule into basal compartment (spermatogonia, outside BTB) and adluminal compartment (meiotic and post-meiotic cells, inside BTB). The BTB protects haploid cells from immune attack — haploid antigens are foreign to the immune system. Phagocytosis: engulf residual bodies shed during spermiogenesis. Hormone production: inhibin B (suppresses FSH — feedback), androgen-binding protein ABP (concentrates testosterone in tubule), anti-Mullerian hormone AMH (causes regression of Mullerian ducts in foetal males).

Key differences: Spermatogenesis begins at puberty and continues throughout life. Oogenesis begins in foetal life — oogonia multiply only during 5th-6th month of foetal development. After birth: no new oogonia formed. Primary oocytes formed during foetal life, arrested in prophase I (dictyotene stage) until puberty. After puberty: one primary oocyte per cycle resumes meiosis. Meiosis I completed just before ovulation → secondary oocyte + 1st polar body. Meiosis II completed ONLY if fertilisation occurs. Products: spermatogenesis → 4 equal functional spermatozoa per primary spermatocyte. Oogenesis → 1 large ovum + 3 small polar bodies (unequal cytokinesis conserves cytoplasm in ovum for early embryo nutrition). Duration: spermatogenesis ~74 days. Oogenesis: primary oocyte arrested for years/decades → completes in ~14 days per cycle from follicle growth to ovulation.

Spermatogenesis regulated by HPG (hypothalamus-pituitary-gonadal) axis. GnRH (Gonadotropin-Releasing Hormone): from hypothalamus, pulsatile release every 90-120 minutes. Acts on anterior pituitary. FSH: acts on Sertoli cells → promotes spermatogenesis, stimulates ABP and inhibin production. LH: acts on Leydig cells (interstitial cells between seminiferous tubules) → stimulates testosterone synthesis and secretion. Testosterone: intratesticular concentration 200-300× higher than blood (due to ABP concentrating it) → essential for meiosis and spermiogenesis. Also: secondary sex characters, negative feedback on GnRH and LH. Inhibin B: from Sertoli cells → selectively inhibits FSH (negative feedback loop). Temperature: testes must be 2-3°C below body temperature for efficient spermatogenesis — this is why testes are in the scrotum outside the body cavity. Cryptorchidism (undescended testes) → infertility if untreated.