Gametogenesis = formation of gametes (sperm + ova) from primordial germ cells (PGCs).

Spermatogenesis (in seminiferous tubules of testis):
• Spermatogonium (2n) → Primary spermatocyte (2n) → Meiosis I → Secondary spermatocyte (n) → Meiosis II → 4 Spermatids (n) → Spermiogenesis → 4 Spermatozoa.
• Continuous from puberty onwards.
• Duration: ~60 days in bull, ~57 in stallion, ~62 in dog.

Oogenesis (in ovary):
• PGCs → Oogonia (2n) (mitosis stops in fetal life).
• Each oogonium becomes a Primary oocyte (2n) — arrested in prophase of Meiosis I until puberty.
• At ovulation: completes Meiosis I → Secondary oocyte (n) + 1st polar body.
• Meiosis II completes only at fertilization → Ovum (n) + 2nd polar body.
• One primary oocyte → 1 ovum + 3 polar bodies (vs sperm = 4 viable cells).

GAMETOGENESIS — FORMATION OF GAMETES

Both gametes share the same goal — to halve the chromosome number from diploid (2n) to haploid (n) through meiosis — but the two pathways differ markedly in timing and yield.

I. SPERMATOGENESIS:

Site: Seminiferous tubules of the testis. The basal compartment houses spermatogonia; the adluminal compartment houses developing spermatocytes/spermatids — separated by tight junctions of Sertoli cells = the blood-testis barrier.

Stages:
1. Spermatogonia (2n) — stem cells along basement membrane. Type A (self-renewing) and Type B (committed to differentiation).
2. Primary spermatocyte (2n) — undergoes Meiosis I (DNA recombination + chromosome reduction).
3. Secondary spermatocyte (n) — short-lived; undergoes Meiosis II.
4. Spermatid (n) — round cell; differentiates by spermiogenesis:
• Acrosome formation (from Golgi).
• Nuclear condensation.
• Flagellum (tail) formation.
• Mitochondrial midpiece assembly.
• Cytoplasm shed as 'residual body' (eaten by Sertoli cell).
5. Spermatozoon — released into tubule lumen → travels via rete testis → efferent ducts → epididymis (where it acquires motility + matures).

Total duration: ~60 days in bull, ~57 in stallion, ~62 in dog.

Sperm structure: Head (nucleus + acrosome) + Neck + Midpiece (mitochondria) + Principal piece + End piece.

II. OOGENESIS:

Site: Cortex of ovary.

Stages:
1. PGCs, specified from epiblast-derived progenitors, migrate from a temporary location near the yolk sac to genital ridge in early fetus.
2. Oogonia (2n) proliferate in fetal ovary; mitosis ceases before birth.
3. Each oogonium becomes a Primary oocyte (2n) and enters Meiosis I but ARRESTS in prophase I (diplotene/dictyate stage) — and stays arrested for years (until ovulation).
4. At each oestrous cycle, a few primary oocytes resume meiosis under FSH/LH.
5. In most domestic mammals, the primary oocyte completes meiosis I before ovulation and is released as a secondary oocyte, usually arrested in metaphase II; fertilization triggers completion of meiosis II. Canine exception: the bitch normally ovulates immature primary (germinal-vesicle-stage) oocytes, which complete maturation in the oviduct about 48–72 hours after ovulation.

YIELD: One primary oocyte → 1 ovum + 3 (small, non-functional) polar bodies. Asymmetric division concentrates cytoplasm + nutrients in the ovum.

Follicular development parallels oogenesis: primordial → primary → secondary → tertiary (antral) → mature (Graafian) follicle. The follicle wall provides hormonal support (granulosa + theca cells).

III. KEY DIFFERENCES (Sperm vs Egg):
• Number per cycle: sperm millions; egg 1-many (multiparous species like sow have many).
• Size: sperm tiny + motile; ovum large + non-motile.
• Cytoplasm: minimal in sperm; maximum in ovum.
• Production timing: continuous in male from puberty; interrupted in female (cyclic).