BiologyCBSE Class 12 · Human Reproduction

Gametogenesis: Spermatogenesis and Oogenesis

Updated 15 October 2026 · Fact-checked

Gametogenesis is the formation of gametes. In spermatogenesis, diploid spermatogonia in the seminiferous tubules form primary spermatocytes, which undergo meiosis to give four haploid spermatids that mature into sperms. In oogenesis, which starts before birth, one primary oocyte gives only one ovum and polar bodies, with meiosis completed only after sperm entry.

Understand Gametogenesis: Spermatogenesis and Oogenesis

Spermatogenesis (in the testis, from puberty). The inner wall of each seminiferous tubule is lined by spermatogonia (male germ cells) and Sertoli cells (nourish the germ cells). Leydig (interstitial) cells lie outside the tubules and secrete androgens.
1. Spermatogonia (2n = 46) multiply by mitosis.
2. Some become primary spermatocytes (2n = 46), which periodically undergo meiosis.
3. Meiosis I (reduction division) → two equal secondary spermatocytes (n = 23 each).
4. Meiosis II → four equal haploid spermatids (n = 23).
5. Spermiogenesis: spermatids are transformed into spermatozoa (sperms); their heads stay embedded in Sertoli cells.
6. Spermiation: sperms are released from the seminiferous tubules.
Flow chart: spermatogonium (46) → primary spermatocyte (46) → [meiosis I] → 2 secondary spermatocytes (23) → [meiosis II] → 4 spermatids (23) → [spermiogenesis] → 4 sperms (23) → [spermiation] → lumen.

Hormonal control. Spermatogenesis starts at puberty because of a significant rise in GnRH from the hypothalamus. GnRH acts on the anterior pituitary to release LH and FSH.

LH acts on Leydig cells → androgens (testosterone) → stimulate spermatogenesis.
FSH acts on Sertoli cells → factors that help spermiogenesis.

Structure of a sperm. A plasma membrane covers the whole sperm, which has a head, neck, middle piece and tail.

Head: elongated haploid nucleus, capped in front by the acrosome, which is filled with enzymes that help the sperm enter the ovum.
Middle piece: many mitochondria, which supply energy for tail movement.
Tail: gives motility to swim towards the ovum.

A man ejaculates about 200-300 million sperms at a time. For normal fertility, at least 60 per cent must have normal shape and size and at least 40 per cent must show vigorous motility. Sperms with seminal plasma form semen.

Oogenesis (in the ovary, starts before birth).
1. In the fetal ovary, a couple of million oogonia (2n) form; no more are added after birth.
2. They enter meiosis I and are arrested at prophase I: these are primary oocytes.
3. Each primary oocyte is surrounded by a layer of granulosa cells → primary follicle. Many follicles degenerate between birth and puberty; only 60,000-80,000 primary follicles remain in each ovary at puberty.
4. More granulosa layers and a new theca → secondary follicle.
5. A fluid-filled cavity, the antrum, appears → tertiary follicle; the theca becomes theca interna and theca externa.
6. Inside the tertiary follicle the primary oocyte completes meiosis I: an *unequal* division gives a large haploid secondary oocyte (keeps most of the nutrient-rich cytoplasm) and a tiny first polar body.
7. The tertiary follicle becomes the mature Graafian follicle; the secondary oocyte forms a new membrane, the zona pellucida.
8. Ovulation: the Graafian follicle ruptures and releases the secondary oocyte (ovum).
9. Meiosis II is completed only when a sperm enters: another unequal division gives the haploid ovum (ootid) and a second polar body.
Flow chart: oogonium (46) → primary oocyte (46, arrested in prophase I) → [meiosis I in tertiary follicle] → secondary oocyte (23) + first polar body → [ovulation] → [meiosis II at sperm entry] → ovum (23) + second polar body.

Spermatogenesis vs oogenesis.

Start: puberty vs fetal life.
Continuity: continuous through adult life vs discontinuous (arrests in prophase I and metaphase II), monthly release, ends at menopause.
Output per meiosis: 4 functional sperms vs 1 ovum + polar bodies.
Cytoplasm: divided equally vs unequally (to stock the ovum with food).
Place of completion: entirely in testis vs meiosis II completes in the oviduct after sperm entry.

Draw and label 1: human sperm. Checklist: oval head with a cap in front, short neck, middle piece with spiral mitochondria, long tail. Labels: plasma membrane, acrosome, nucleus, neck, middle piece, mitochondria, tail.

Draw and label 2: sectional view of a seminiferous tubule. Checklist: ring of cells around a lumen; large Sertoli cells reaching the lumen; cells getting smaller and more mature towards the lumen; interstitial cells outside. Labels: spermatogonium, primary spermatocyte, secondary spermatocyte, spermatid, sperms, Sertoli cell, interstitial (Leydig) cells.

Draw and label 3: sectional view of an ovary. Checklist: oval ovary with follicles at different stages around the edge. Labels: primary follicle, secondary follicle, tertiary follicle (antrum), Graafian follicle, ovum (secondary oocyte), corpus luteum, blood vessels.

Gametogenesis: Spermatogenesis and Oogenesis: key facts

Sperms from spermatocytes: $1 primary spermatocyte \to 2 secondary \to 4 spermatids \to 4 sperms$; Each secondary spermatocyte gives 2 sperms; each spermatid gives 1 sperm.
Ova from oocytes: $1 primary oocyte \to 1 secondary oocyte \to 1 ovum + polar bodies$; Polar bodies are not functional gametes (NCERT notes it is uncertain whether the first polar body divides again or degenerates); only one ovum per primary oocyte.
Chromosome numbers: $spermatogonium, primary spermatocyte, oogonium, primary oocyte = 46; all later stages = 23$; Meiosis I halves the number; meiosis II separates chromatids.
Hormone axis: $GnRH \to LH \to Leydig cells (androgens); FSH \to Sertoli cells$; Androgens stimulate spermatogenesis; Sertoli factors help spermiogenesis.
Fertility thresholds: $\geq 60% normal shape and size; \geq 40% vigorous motility$; Out of about 200-300 million sperms per ejaculate.

How to answer Gametogenesis: Spermatogenesis and Oogenesis questions

Most questions ask for a stage, a chromosome number, a count, a hormone or a comparison. Anchor every answer to the flow charts.

1Write the stage sequence first, marking where meiosis I and II happen; then read off chromosome numbers (46 before meiosis I is over, 23 after).
2For counts, multiply by what each stage yields: primary spermatocyte × 4, secondary spermatocyte × 2, spermatid × 1; any oocyte × 1.
3For hormonal control, name the gland, the hormone and the target cell for each step: hypothalamus (GnRH) → pituitary (LH, FSH) → Leydig/Sertoli.
4For oogenesis 'where/when', link each event to the follicle stage: arrest in prophase I (primary follicle, before birth), completion of meiosis I (tertiary follicle), meiosis II (at sperm entry).
5For differences, give matching pairs on the same point (when, where, number of gametes, cytoplasm division).
6In diagrams, draw the sperm in proportion (tail longest) and label the acrosome and mitochondria; they carry the function marks.

Gametogenesis: Spermatogenesis and Oogenesis in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Semen analysis and fertility thresholds

Suppose a fertility clinic report shows that a man's ejaculate has 240 million sperms, of which 132 million have normal shape and size and 84 million show vigorous motility.

Does each value meet the thresholds for normal fertility? What could the doctor investigate next, linking to the hormones of spermatogenesis?

Try it first, then show the solutionSolution

Normal shape: 132 ÷ 240 = 55%, below the required 60%.
Vigorous motility: 84 ÷ 240 = 35%, below the required 40%.
Both fall short. Motility depends on energy from the mitochondria of the middle piece; numbers and shape depend on spermatogenesis and spermiogenesis, which need LH (via Leydig cell androgens) and FSH (via Sertoli cells). The doctor could check LH, FSH and testosterone levels.

Answer: 55% normal shape (needs ≥ 60%) and 35% motility (needs ≥ 40%): both below normal; check GnRH-LH-FSH-testosterone levels.

Memory hook: Sixty to look right, forty to swim right.

Tricks to remember Gametogenesis: Spermatogenesis and Oogenesis

Short memory aids. Say each one aloud once, then use it on a question the same day.

Sperm: 1 → 2 → 4 → 4
One primary, two secondary, four spermatids, four sperms.
Use it when: Counting questions.
Leydig = LH, Sertoli = Sustain (FSH)
L for LH on Leydig; Sertoli sustains and feeds, with FSH.
Use it when: Matching hormones to target cells.
Egg waits, sperm hurries
Oogenesis pauses twice (prophase I before birth, meiosis II until sperm entry); spermatogenesis runs non-stop from puberty.
Use it when: Comparison and assertion-reason items.

Gametogenesis: Spermatogenesis and Oogenesis: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Hormonal control of spermatogenesis · 2 marks · Very short answerBoard level

Explain the hormonal regulation of spermatogenesis. (Asked in the 2024 sample paper in a similar form.)

Try it first, then show the solutionSolution

At puberty the hypothalamus secretes more GnRH, which makes the anterior pituitary release LH and FSH (1 mark).
LH acts on Leydig cells to secrete androgens that stimulate spermatogenesis; FSH acts on Sertoli cells, which secrete factors that help spermiogenesis (1 mark).

Answer: GnRH → LH (Leydig cells → androgens → spermatogenesis) and FSH (Sertoli cells → spermiogenesis).

Example 2: Counting gametes · 2 marks · Very short answerBoard level

How many sperms are formed from (a) 50 primary spermatocytes (b) 50 secondary spermatocytes (c) 50 spermatids? How many ova are formed from 50 primary oocytes?

Try it first, then show the solutionSolution

(a) 50 × 4 = 200 (b) 50 × 2 = 100 (c) 50 × 1 = 50 (1 mark).
Each primary oocyte gives only one ovum; the rest of the products are polar bodies: 50 ova (1 mark).

Answer: (a) 200 (b) 100 (c) 50 sperms; 50 ova.

Example 3: Where meiosis I is completed · 3 marks · Short answerHarder board level

Name the follicle stage in which the first meiotic division of oogenesis is completed. Name its products and give the chromosome number of the cell before and the cells after this division.

Try it first, then show the solutionSolution

The primary oocyte completes meiosis I inside the tertiary follicle (1 mark).
Products: a large secondary oocyte and a small first polar body; the division is unequal (1 mark).
Primary oocyte: 46; secondary oocyte: 23; first polar body: 23 (1 mark).

Answer: Tertiary follicle; secondary oocyte (23) and first polar body (23) from a primary oocyte (46).

Example 4: Why polar bodies? · 2 marks · Very short answerHarder board level

Polar bodies are formed in oogenesis but not in spermatogenesis. Give the reason and its biological advantage.

Try it first, then show the solutionSolution

In oogenesis both meiotic divisions divide the cytoplasm unequally; the small cells that get very little cytoplasm are the polar bodies. In spermatogenesis cytoplasm divides equally, giving four similar cells (1 mark).
Advantage: one ovum keeps almost all the nutrient-rich cytoplasm to support the early embryo, while chromosome number is still halved (1 mark).

Answer: Unequal cytokinesis concentrates cytoplasm in one ovum; sperms need little cytoplasm, so division is equal.

Example 5: Sperm parts and functions · 5 marks · Long answerHarder board level

Draw a labelled diagram of a human sperm. Give the function of the acrosome, the middle piece and the tail. Why does a sperm lose most of its cytoplasm during spermiogenesis?

Try it first, then show the solutionSolution

Diagram with plasma membrane, acrosome, nucleus, neck, middle piece (mitochondria) and tail, correctly proportioned (2 marks).
Acrosome: cap of enzymes that help the sperm penetrate the layers around the ovum during fertilisation (1 mark).
Middle piece: mitochondria produce energy (ATP) for tail movement; tail: lashing movements make the sperm motile (1 mark).
A streamlined sperm with little cytoplasm is light and fast; it only has to deliver its nucleus, while the ovum provides the cytoplasm for the zygote (1 mark).

Answer: Acrosome: enzymes for entry; middle piece: energy; tail: motility. Little cytoplasm keeps the sperm light and motile; the ovum supplies cytoplasm.

Example 6: Why fertility ends after menopause · 3 marks · Short answerHOTS / competency

Suppose a graph shows the number of primary follicles per ovary falling from about 2 million at birth to about 70,000 at puberty and to nearly zero around age 50. Explain both falls and why a woman generally cannot conceive after about 50.

Try it first, then show the solutionSolution

All oogonia are formed before birth and no new ones are added; primary follicles only decline (1 mark).
Many follicles degenerate between birth and puberty (leaving about 60,000-80,000 per ovary); after puberty, each cycle a group develops, one usually ovulates and the rest degenerate (1 mark).
Around 50 the stock of follicles is almost exhausted, ovulation and the menstrual cycle stop (menopause), so no ovum is released to be fertilised (1 mark).

Answer: No new oogonia after birth; follicles degenerate or are used each cycle; by about 50 they run out (menopause), so no ovulation.

Where students lose marks in Gametogenesis: Spermatogenesis and Oogenesis

The slips examiners see most often, and what to write instead.

Mistake: Writing that the ovum is released after meiosis II
Students assume the egg is fully formed at ovulation.
Wrong working
At ovulation the mature ovum (ootid) is released.
Right working
At ovulation a secondary oocyte is released; meiosis II completes only after sperm entry.
Do this: Remember: ovulation releases the secondary oocyte.
Usually costs: 1 mark.
Mistake: Swapping LH and FSH targets
Both are pituitary gonadotropins.
Wrong working
FSH acts on Leydig cells to release androgens.
Right working
LH → Leydig cells → androgens; FSH → Sertoli cells → spermiogenesis factors.
Do this: L with L: LH on Leydig.
Usually costs: 1 mark.
Mistake: Diploid secondary spermatocyte
Students think meiosis I does not halve the number.
Wrong working
Secondary spermatocyte has 46 chromosomes.
Right working
Meiosis I is the reduction division: secondary spermatocyte has 23.
Do this: Only spermatogonia and primary spermatocytes are 46.
Usually costs: 1 mark.
Mistake: Confusing spermiogenesis with spermiation
Similar words.
Wrong working
Spermiation is the change of spermatids into sperms.
Right working
Spermiogenesis = spermatid to sperm; spermiation = release of sperms from the tubule.
Do this: Spermia-TION = exiTION (exit).
Usually costs: 1 mark.

Exam tips

Draw the two flow charts side by side with chromosome numbers; they answer most 3-mark questions directly.
In oogenesis, always mention the arrest at prophase I and the 60,000-80,000 follicles left at puberty.
For 'functions of sperm parts', pair each label with its function in one line.
Use exact terms: spermatogonia, Sertoli cells, Leydig cells, spermiogenesis, spermiation, Graafian follicle, zona pellucida.
In comparison tables, write the same feature for both columns in the same row.

Practice questions on Gametogenesis: Spermatogenesis and Oogenesis

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The number of chromosomes in a secondary spermatocyte of a human is:
(a) 46 (b) 23 (c) 92 (d) 44
Hint
After meiosis I.
Answer
(b) 23.
Q2 · 1 mark · Assertion-reasonHarder board level
Assertion (A): The secondary oocyte is much larger than the first polar body.
Reason (R): The first meiotic division of the primary oocyte is unequal, and the secondary oocyte retains the bulk of the nutrient-rich cytoplasm.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Unequal cytokinesis.
Answer
(a). The unequal division leaves almost all the cytoplasm in the secondary oocyte, so the polar body is tiny.
Q3 · 1 mark · Very short answerBoard level
Which cells secrete androgens, and under which hormone's influence? Where are these cells located?
Hint
Interstitial spaces.
Answer
Leydig (interstitial) cells, under LH; they lie in the interstitial spaces outside the seminiferous tubules.
Q4 · 3 marks · Short answerHOTS / competency
Suppose a boy has a condition in which the testes stay in the abdomen instead of descending into the scrotum. Explain why spermatogenesis is affected, and which two cell types of the testis would be most affected in function.
Hint
Temperature of the scrotum.
Answer
The scrotum keeps the testes about 2-2.5 °C below normal body temperature, which is needed for spermatogenesis; in the warmer abdomen spermatogenesis is impaired, reducing sperm count. Sertoli cells (which nourish germ cells and support spermiogenesis under FSH) and Leydig cells (androgen secretion under LH) do not function normally, so sperm numbers and maturation suffer.
Q5 · 3 marks · Short answerHarder board level
Differentiate between spermatogenesis and oogenesis on four points.
Hint
When, continuity, number of gametes, cytoplasm.
Answer
Spermatogenesis: begins at puberty; continuous; one primary spermatocyte gives 4 sperms; equal cytoplasmic division; completed in the testis. Oogenesis: begins in fetal life; discontinuous (arrests), ends at menopause; one primary oocyte gives 1 ovum + polar bodies; unequal division; meiosis II completed in the oviduct after sperm entry.
Q6 · 4 marks · Case-basedHarder board level
Read and answer.
Suppose a histology slide shows a seminiferous tubule with labelled cells: A (large cells extending from the base to the lumen), B (cells at the base dividing by mitosis), C (cells with half the chromosome number but still round) and D (cells with tails in the lumen). Cells E lie in the spaces between tubules.
(a) Identify A and E. (1)
(b) Give the chromosome number of B and C. (1)
(c) Name the processes that convert C into D and release D into the lumen, and the hormone that acts on A. (2)
Hint
Large supporting cells; interstitial cells.
Answer
(a) A = Sertoli cells; E = Leydig (interstitial) cells. (b) B (spermatogonia) = 46; C (spermatids) = 23. (c) Spermiogenesis converts spermatids into sperms; spermiation releases them into the lumen; FSH acts on Sertoli cells.
Q7 · 2 marks · Very short answerHarder board level
A primary oocyte is formed before birth but may complete meiosis I only 30 years later. Give two features of the follicle in which this division finally occurs, and name the structure released at ovulation.
Hint
Antrum; theca.
Answer
Tertiary follicle: has a fluid-filled antrum; theca organised into theca interna and theca externa (several granulosa layers). At ovulation the Graafian follicle releases the secondary oocyte (with zona pellucida).

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Chromosome number of a spermatid?Show answer
23.
2.Which cells nourish developing sperms?Show answer
Sertoli cells.
3.At which stage is the primary oocyte arrested before birth?Show answer
Prophase I of meiosis.
4.How many primary follicles remain per ovary at puberty?Show answer
About 60,000-80,000.
5.What does the middle piece of a sperm contain?Show answer
Numerous mitochondria.
6.When is meiosis II of oogenesis completed?Show answer
When a sperm enters the secondary oocyte.

Gametogenesis: Spermatogenesis and Oogenesis: frequently asked questions

What is the difference between spermatogenesis and spermiogenesis?

Spermatogenesis is the whole process from spermatogonia to sperms. Spermiogenesis is its last step: the transformation of spermatids into sperms.

Why does oogenesis produce only one ovum?

Both meiotic divisions divide the cytoplasm unequally, so one cell keeps most of the food-rich cytoplasm and the others become polar bodies that do not become gametes.

What is released at ovulation in humans?

The secondary oocyte, surrounded by the zona pellucida, released when the Graafian follicle ruptures.

Which hormones control spermatogenesis?

GnRH from the hypothalamus stimulates LH and FSH from the pituitary. LH makes Leydig cells secrete androgens; FSH acts on Sertoli cells to support spermiogenesis.

Why can a woman not produce new eggs after birth?

All oogonia are formed in the fetal ovary and no new ones are added after birth; the ovary only uses up its stock of follicles.