BiologyISC Class 12 · Human Reproduction

Gametogenesis, Spermatogenesis and Oogenesis: ISC Class 12 Biology

Updated 15 October 2026 · Fact-checked

Gametogenesis is gamete formation in the testis and ovary. In spermatogenesis one diploid primary spermatocyte gives four haploid spermatids, which become sperms by spermiogenesis. In oogenesis one primary oocyte gives one ovum and two or three polar bodies; it begins before birth and pauses at prophase I.

Understand Gametogenesis: Spermatogenesis and Oogenesis

Gametogenesis is the formation of gametes from diploid germ cells in the primary sex organs. Both processes pass through multiplication (mitosis), growth and maturation (meiosis) phases.

Spermatogenesis (in the seminiferous tubules; begins at puberty and continues even in old age).
1. Spermatogonia (2n = 46) on the inner wall of the tubule multiply by mitosis.
2. Some spermatogonia, called primary spermatocytes (2n = 46), periodically undergo meiosis.
3. Meiosis I (reduction division) gives two equal secondary spermatocytes (n = 23 chromosomes each).
4. Meiosis II gives four equal haploid spermatids (n = 23).
5. Spermiogenesis: transformation of a round spermatid into a motile spermatozoon with no further division (nucleus condenses, acrosome forms, mitochondria gather in the middle piece, tail grows).
6. Sperm heads become embedded in Sertoli cells; spermiation is the release of sperms from the seminiferous tubules.

Flow chart: spermatogonium (2n) → primary spermatocyte (2n) → [meiosis I] → 2 secondary spermatocytes (n) → [meiosis II] → 4 spermatids (n) → [spermiogenesis] → 4 sperms → [spermiation] → lumen.

Hormonal control of spermatogenesis. At puberty secretion of GnRH (hypothalamus) rises sharply. GnRH acts on the anterior pituitary, which secretes two gonadotropins:

LH acts on Leydig cells → synthesis and secretion of androgens → androgens stimulate spermatogenesis.
FSH acts on Sertoli cells → secretion of factors that help spermiogenesis.

Structure of a sperm. Head, neck, middle piece and tail, all covered by plasma membrane. The head has an elongated haploid nucleus capped by the acrosome, filled with enzymes that help fertilisation. The middle piece has numerous mitochondria that supply energy for tail movement. About 200-300 million sperms are ejaculated in one coitus; for normal fertility at least 60% must have normal shape and size and at least 40% must show vigorous motility.

Oogenesis (in the ovary; begins in the embryo).
1. Before birth: a couple of million oogonia form in each fetal ovary; no new oogonia are added after birth.
2. Oogonia enter prophase I of meiosis and are temporarily arrested there as primary oocytes; each is surrounded by a layer of granulosa cells → primary follicle.
3. Many follicles degenerate between birth and puberty, so at puberty only 60,000-80,000 primary follicles remain in each ovary.
4. From puberty: primary follicle → secondary follicle (more granulosa layers, new theca) → tertiary follicle (fluid-filled antrum; theca interna and externa).
5. In the tertiary follicle the primary oocyte grows and completes meiosis I *unequally*: a large haploid secondary oocyte keeps most of the nutrient-rich cytoplasm, and a tiny first polar body is cut off. (Whether the first polar body divides again is not certain.)
6. The tertiary follicle becomes the Graafian follicle; the secondary oocyte forms a new membrane, the zona pellucida. The follicle ruptures and releases the secondary oocyte: ovulation.
7. Meiosis II is completed only after a sperm enters, giving the ovum and a second polar body.

Result: one primary oocyte → one ovum + two (or three) polar bodies, compared with four sperms from one primary spermatocyte.

Hormonal control of oogenesis. FSH (and LH) stimulate follicle growth and oestrogen secretion; the mid-cycle LH surge induces rupture of the Graafian follicle; LH then converts the remnant into the corpus luteum.

Structure of the mature ovum (secondary oocyte at ovulation). Large cell with abundant cytoplasm and a haploid nucleus, enclosed by the zona pellucida and, outside it, the follicle cells of the corona radiata.

Spermatogenesis versus oogenesis.

	Spermatogenesis	Oogenesis
Starts	puberty	before birth
Continues	throughout adult life	until menopause (about 50 years)
Products of one mother cell	4 sperms	1 ovum + 2 or 3 polar bodies
Division	equal	unequal (cytoplasm to the oocyte)
Arrest	none	prophase I (till puberty), metaphase II (till sperm entry)
Final shaping	spermiogenesis	none

Draw and label: spermatogenesis and oogenesis schematic. Checklist: two columns; spermatogonium → primary spermatocyte → two secondary spermatocytes → four spermatids → four sperms, with 46/23 written beside each and 'mitosis', 'meiosis I', 'meiosis II', 'spermiogenesis' on the arrows; oogonium → primary oocyte → secondary oocyte + first polar body → ovum + second polar body, with chromosome numbers; mark 'before birth' and 'at puberty'.

Gametogenesis: Spermatogenesis and Oogenesis: key facts

Products of one mother cell: $1 primary spermatocyte \to 4 sperms; 1 primary oocyte \to 1 ovum + 2 or 3 polar bodies$; Three polar bodies only if the first polar body also divides.
Counting back from sperms: $primary spermatocytes = \frac{sperms}{4}, secondary spermatocytes = \frac{sperms}{2}$; Spermatids = sperms (spermiogenesis has no division).
Oocytes ovulated over a period: $ovulations \approx (years from menarche) \times (cycles per year)$; Count from menarche, not from birth; one secondary oocyte per cycle.
Chromosome numbers in humans: $spermatogonium, primary spermatocyte: 46; secondary spermatocyte, spermatid, sperm: 23$; Same pattern for oogonium (46), primary oocyte (46), secondary oocyte and ovum (23).

How to answer Gametogenesis: Spermatogenesis and Oogenesis questions

ISC asks labelled stages from a diagram, differences between spermatogenesis and oogenesis, hormonal control, and counting problems on gametes, polar bodies and oocytes.

1For a diagram, name each stage and write the chromosome number beside it; the stage after meiosis I is a secondary spermatocyte, not a spermatogonium.
2For counting sperms, work in factors of 2 per meiotic division: primary → 2 secondary → 4 spermatids → 4 sperms.
3For counting ova, use 1 ovum per primary oocyte and say 2 or 3 polar bodies.
4For oocytes over years, count from menarche (puberty) to the date asked, multiply by cycles per year, and state the assumption.
5For hormones, link each to its target cell: GnRH → pituitary; LH → Leydig cells; FSH → Sertoli cells.
6Use spermiogenesis for spermatid → sperm and spermiation for release; do not write spermatogenesis for either.

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.

Why the oocyte pool is limited but sperm production is not

Suppose two colleagues, a man and a woman, both aged 38, are discussing whether to start a family later. A friend claims, 'Both of you can keep making new gametes for life, so there is no hurry for either of you.'

Using gametogenesis, explain why the friend is wrong for one of them.

Try it first, then show the solutionSolution

In the man, spermatogonia keep multiplying by mitosis from puberty onwards, so new sperms are produced continuously, even in old age.
In the woman, all oogonia are formed before birth; no new oogonia are added afterwards.
Primary follicles are lost steadily: from about 60,000-80,000 per ovary at puberty, they keep degenerating, and one oocyte is ovulated each cycle.
Menstrual cycles and ovum formation stop at about 50 years (menopause), so her fertile period is limited.

Answer: Sperm formation continues for life because spermatogonia keep dividing, but a woman's oocytes all originate before birth and run down until menopause at about 50, so the claim is wrong for her.

Memory hook: Men run a factory; women draw from a savings account opened before birth.

Tricks to remember Gametogenesis: Spermatogenesis and Oogenesis

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

Gonia, cyte, cyte, tid, zoa
Spermato-GONIA → primary CYTE → secondary CYTE → spermaTID → spermatoZOA: 'Go See See Tiny Zoo'.
Use it when: Naming stages in order in diagram questions.
LH to Leydig, FSH to Sertoli
L goes with L (LH-Leydig); the other one (FSH) goes to Sertoli; both letters S.
Use it when: Hormonal control of spermatogenesis.
Spermio-GENESIS shapes, spermi-ATION sails
Genesis makes the shape; -ation is the sailing out (release).
Use it when: Avoiding the spermiogenesis/spermiation swap.

Gametogenesis: Spermatogenesis and Oogenesis: solved examples

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

Example 1: Spermiogenesis versus spermiation · 2 marks · Short answerBoard level

Define spermiogenesis and spermiation. In which structure does each occur?

Try it first, then show the solutionSolution

Spermiogenesis: transformation of spermatids into spermatozoa (sperms) without division, in the seminiferous tubules while attached to Sertoli cells (1 mark).
Spermiation: release of the sperms from the Sertoli cells into the lumen of the seminiferous tubule (1 mark).

Answer: Spermiogenesis shapes spermatids into sperms; spermiation releases them; both in the seminiferous tubule.

Example 2: Hormonal control in the male · 3 marks · Short answerBoard level

Explain how hormones start and maintain spermatogenesis at puberty.

Try it first, then show the solutionSolution

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

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

Example 3: Counting back from an ejaculate · 2 marks · Short answerHarder board level

Suppose one ejaculate contains 250 million sperms. What is the minimum number of (a) spermatids, (b) secondary spermatocytes, (c) primary spermatocytes needed to produce them? How many meiotic divisions (counting meiosis I and II of one cell as one meiosis) is that?

Try it first, then show the solutionSolution

Each spermatid becomes one sperm: 250 million spermatids (½ mark).
Each secondary spermatocyte gives 2 spermatids: 125 million; each primary spermatocyte gives 4: 62.5 million (1 mark).
One meiosis per primary spermatocyte: 62.5 million meioses (½ mark).

Answer: (a) 250 million (b) 125 million (c) 62.5 million; 62.5 million meioses.

Example 4: Fifty mother cells each · 3 marks · Short answerHarder board level

Fifty primary spermatocytes and fifty primary oocytes complete gametogenesis (assume every oocyte is fertilised). How many functional gametes and how many polar bodies are formed? Explain the difference.

Try it first, then show the solutionSolution

Spermatocytes: 50 × 4 = 200 sperms (1 mark).
Oocytes: 50 ova; polar bodies 50 × 2 = 100, or 150 if every first polar body also divides (1 mark).
Meiosis in the oocyte is unequal: the cytoplasm stays with one cell so that the ovum is large and food-rich, while the polar bodies are tiny; in males both divisions are equal (1 mark).

Answer: 200 sperms; 50 ova with 100 (or 150) polar bodies; unequal cytokinesis gives one large ovum.

Example 5: Two pauses in oogenesis · 3 marks · Short answerHarder board level

State the two points at which human oogenesis is arrested, and the event that releases each arrest. Why is oogenesis said to begin before birth?

Try it first, then show the solutionSolution

First arrest: prophase I in the primary oocyte, from fetal life; meiosis I is completed in the tertiary follicle after puberty, during a menstrual cycle (1 mark).
Second arrest: the secondary oocyte at ovulation has not completed meiosis II; it is completed only when a sperm enters (1 mark).
Oogonia form and enter meiosis in the fetal ovary; no new oogonia are formed after birth (1 mark).

Answer: Arrest in prophase I (until a follicle matures after puberty) and before completion of meiosis II (until sperm entry); oogonia and primary oocytes form in the fetus.

Example 6: A lifetime of ovulations · 3 marks · Short answerHOTS / competency

Suppose a woman has menarche at 12 and menopause at 50, ovulates once in each cycle, has about 13 cycles a year and is never pregnant. (a) About how many secondary oocytes does she release? (b) At puberty her two ovaries held 120,000-160,000 primary follicles. What percentage of these is ever ovulated? (c) What happens to the rest?

Try it first, then show the solutionSolution

(a) 50 − 12 = 38 years; 38 × 13 = 494 ovulations (1 mark).
(b) 494 ÷ 160,000 × 100 = 0.31%; 494 ÷ 120,000 × 100 = 0.41%: well under 1% (1 mark).
(c) The rest degenerate (follicular atresia), as many follicles do between birth and puberty too (1 mark).

Answer: (a) About 494. (b) About 0.3-0.4%. (c) The remaining follicles degenerate.

Where students lose marks in Gametogenesis: Spermatogenesis and Oogenesis

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

Mistake: Naming a spermatocyte as a spermatogonium
In 2024 examiners noted that many candidates wrote spermatogonia for the labelled spermatocytes in a spermatogenesis diagram.
Do this: Write the chromosome number beside each stage; the cell entering meiosis I is the primary spermatocyte.
Usually costs: ½ to 1 mark.
Mistake: Writing spermatogenesis for spermiogenesis
Examiners (2024) recorded 'spermatogenesis' as the process changing spermatid to sperm.
Wrong working
Spermatid → sperm by spermatogenesis.
Right working
Spermatid → sperm by spermiogenesis.
Do this: Spermatogenesis is the whole process; spermiogenesis is its last, shaping step.
Usually costs: 1 mark.
Mistake: Counting oocytes from birth
In 2025 examiners noted that candidates counted cycles from birth instead of from puberty to conception.
Wrong working
30 years × 12 = 360 oocytes.
Right working
(30 − 14) years × 12 = 192 oocytes.
Do this: Ovulation begins at menarche; subtract the age at menarche first.
Usually costs: The whole 1-mark item.
Mistake: Saying the ovum is released at ovulation as a fully mature ovum
Students forget the second arrest.
Do this: At ovulation the secondary oocyte is released; meiosis II is completed only when a sperm enters.
Usually costs: ½ to 1 mark.

Exam tips

Write chromosome numbers (46 or 23) beside every stage in diagrams and flow charts.
Use the terms spermiogenesis and spermiation separately; ISC lists both.
In counting problems, state assumptions (one ovulation per cycle, 12 or 13 cycles a year).
For oogenesis, mention 'before birth', '60,000-80,000 follicles per ovary at puberty' and 'unequal division'.
Link hormones to target cells: LH-Leydig, FSH-Sertoli.

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
Which cell has 23 chromosomes?
(a) Spermatogonium (b) Primary spermatocyte (c) Secondary spermatocyte (d) Primary oocyte
Hint
After meiosis I.
Answer
(c) Secondary spermatocyte.
Q2 · 3 marks · Short answerBoard level
Sketch a human spermatozoon with five labelled parts. What do its acrosome and the mitochondria of its middle piece do?
Hint
Head, neck, middle piece, tail.
Answer
Labels: plasma membrane, acrosome, nucleus (head), neck, middle piece with mitochondria, tail. Acrosome: contains enzymes that help the sperm fertilise the ovum. Middle piece: its mitochondria produce energy for tail movement.
Q3 · 1 mark · Very short answerHarder board level
Suppose a woman had her first period at 13 and became pregnant for the first time at 34. If one primary oocyte completes meiosis I and is ovulated in each of 12 cycles a year, how many secondary oocytes were released before that pregnancy?
Hint
Count years from menarche.
Answer
(34 − 13) × 12 = 21 × 12 = 252.
Q4 · 1 mark · Assertion-reasonHarder board level
Assertion (A): The secondary oocyte is much larger than the first polar body.
Reason (R): Meiosis I in the primary oocyte is an unequal division in which the secondary oocyte keeps most of the 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).
Q5 · 2 marks · Short answerHarder board level
Describe the human secondary oocyte released at ovulation and name, in order, the coverings a sperm must cross to reach its plasma membrane. Why is this cell not yet called an ovum?
Hint
Two coverings; the second meiotic arrest.
Answer
It is a large cell with abundant cytoplasm and a haploid nucleus. The sperm must first pass the follicle cells of the corona radiata and then the zona pellucida. It is not yet an ovum because meiosis II is incomplete; meiosis II is completed only after a sperm enters, giving the ovum and the second polar body.
Q6 · 2 marks · Short answerHarder board level
Name the follicle stage in which the primary oocyte completes meiosis I, and describe two features of that follicle.
Hint
Fluid-filled cavity.
Answer
The tertiary follicle. It has a fluid-filled cavity, the antrum, and its theca is organised into an inner theca interna and an outer theca externa.
Q7 · 3 marks · Short answerHOTS / competency
Suppose a drug blocks FSH receptors on Sertoli cells in an adult man but leaves LH action normal. Predict the effect on (a) androgen levels, (b) formation of spermatids, (c) formation of mature sperms. Explain.
Hint
Which step needs Sertoli factors?
Answer
(a) Normal: LH still acts on Leydig cells, which secrete androgens. (b) Spermatids can still form, since androgens stimulate spermatogenesis (meiosis). (c) Fewer normal sperms: FSH-stimulated Sertoli factors help spermiogenesis, the transformation of spermatids into sperms, so this step would be impaired.
Q8 · 3 marks · Case-basedHOTS / competency
Read and answer.
Suppose a diagram of the seminiferous tubule shows cells P (on the basement membrane), Q (large cells just inside P), R (smaller cells nearer the lumen), S (round cells next to the lumen) and T (tailed cells in the lumen).
(a) Name P, Q, R and S and give the chromosome number of each in humans. (2)
(b) Name the process by which S becomes T, and the process by which T leaves the tubule wall. (1)
Hint
Stages arranged from outside inwards.
Answer
(a) P spermatogonium (46), Q primary spermatocyte (46), R secondary spermatocyte (23), S spermatid (23). (b) Spermiogenesis turns spermatids into sperms; spermiation releases them into the lumen.

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.How many spermatids form from one primary spermatocyte?Show answer
Four.
2.At which stage are primary oocytes arrested before birth?Show answer
Prophase I of meiosis.
3.What forms around the secondary oocyte in the Graafian follicle?Show answer
The zona pellucida.
4.Which hypothalamic hormone rises at puberty?Show answer
GnRH (gonadotropin releasing hormone).
5.What is the minimum percentage of sperms with vigorous motility for normal fertility?Show answer
40 per cent.
6.What triggers completion of meiosis II in the secondary oocyte?Show answer
Entry of a sperm.

Gametogenesis: Spermatogenesis and Oogenesis: frequently asked questions

What is the difference between spermatogenesis and spermiogenesis?

Spermatogenesis is the whole formation of sperms from spermatogonia; spermiogenesis is its final step, in which spermatids are transformed into sperms without division.

How many ova are formed from one primary oocyte?

One ovum, with two or three polar bodies.

When does oogenesis start?

In the fetal ovary before birth; oogonia form and enter meiosis there and arrest as primary oocytes.

What is spermiation?

The release of sperms from the Sertoli cells into the lumen of the seminiferous tubule.

Which hormones control spermatogenesis?

GnRH from the hypothalamus; LH (acting on Leydig cells to release androgens) and FSH (acting on Sertoli cells) from the anterior pituitary.