Q1 LONG ANSWER 10M ★ High-yield

Describe gametogenesis. Compare spermatogenesis with oogenesis and add a note on the significance of meiosis.

Anatomy · 1st Professional — Pre-Clinical

Gametogenesis is the process by which diploid primordial germ cells are converted into haploid gametes, by meiosis and cytodifferentiation.

Primordial germ cells appear in the wall of the yolk sac near the allantois in the 4th week and migrate to the genital ridge by the 6th week
They are not formed in the gonad — their failure to migrate causes gonadal agenesis, and their stray remnants give rise to teratomas in the midline

Two purposes — it halves the chromosome number, so the diploid number is restored at fertilisation; and it generates genetic variety by crossing over and by independent assortment
Prophase I is long and has five stages — leptotene, zygotene, pachytene (crossing over occurs here), diplotene, diakinesis

Stage	Chromosomes	Event
Spermatogonium	46, 2C	Type A (stem) and type B; mitosis
Primary spermatocyte	46, 4C	The largest germ cell; enters meiosis I
Secondary spermatocyte	23, 2C	Short-lived; rarely seen in a section
Spermatid	23, 1C	Undergoes spermiogenesis
Spermatozoon	23, 1C	Released into the lumen

Golgi forms the acrosome, a cap of hydrolytic enzymes
Nucleus condenses and elongates
Centriole forms the tail; mitochondria form the middle-piece spiral
Excess cytoplasm is shed as the residual body and phagocytosed by Sertoli cells
Begins at puberty and continues throughout life
Takes about 64–74 days; about 300 million sperm are produced daily
Requires a temperature 2–3°C below body temperature, which is why the testes descend

Feature	Spermatogenesis	Oogenesis
Begins	At puberty	In fetal life
Ends	Continues into old age	At the menopause
Duration of one cycle	64–74 days	Up to 40–50 years (arrested in prophase I)
Products per primary cell	Four functional spermatozoa	One ovum and three polar bodies
Cytoplasmic division	Equal	Grossly unequal — conserves cytoplasm for the zygote
Arrest	None	Twice — prophase I and metaphase II
Number produced	About 300 million a day	About 400 in a lifetime

Part	Contents	Function
Head	Condensed haploid nucleus capped by the acrosome	Carries the genome; acrosomal enzymes penetrate the oocyte
Neck	Proximal centriole	Connects head to tail
Middle piece	Spiral sheath of mitochondria	Generates ATP for motility
Principal piece	Axoneme in the 9 + 2 arrangement, with a fibrous sheath	The main propulsive segment
End piece	Axoneme only	Terminal

Total length about 60 micrometres; the head alone is 4–5
Normal semen — volume 1.5 mL or more, concentration 15 million/mL or more, motility 40% or more, and normal forms 4% or more
The oocyte is the largest cell in the body at about 120 micrometres, and the spermatozoon among the smallest — a difference of some hundred thousand times in volume

Non-disjunction — failure of chromosomes to separate; gives trisomy 21 (Down), 18 (Edwards), 13 (Patau), and Klinefelter (47,XXY) and Turner (45,X) syndromes
Advanced paternal age raises the risk of new point mutations, such as achondroplasia, because of the many mitotic divisions of spermatogonia
Cryptorchidism — the higher temperature abolishes spermatogenesis while Leydig cells survive, so the man is infertile with normal testosterone
Chemotherapy and radiotherapy destroy the rapidly dividing spermatogonia; sperm banking should be offered before treatment
The male contributes the sex chromosome that determines sex — a point of real social importance in India, where women are still blamed for the sex of a child
Semen analysis is the first investigation in male infertility, repeated after 3 months because one cycle of spermatogenesis takes about 74 days
Intracytoplasmic sperm injection allows fertilisation with a single sperm, bypassing capacitation and the acrosome reaction entirely
Varicocele raises scrotal temperature and impairs spermatogenesis; it is commoner on the left because of the right-angled drainage of the left testicular vein
Anti-sperm antibodies form when the blood–testis barrier is breached by trauma, infection or vasectomy reversal
Ovarian reserve falls steadily from birth, and its assessment by anti-Mullerian hormone and antral follicle count guides fertility treatment

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