GAMETOGENESIS

 GAMETOGENESIS

Prepared By Dr Bhabesh Nath

Assistant Professor

Department of Zoology

B N College (Autonomous) Dhubri

1. Introduction

Gametogenesis is the process by which specialised diploid germ cells in the gonads produce haploid gametes (sperms and ova) through mitosis, meiosis and cell differentiation. It is the first event of sexual reproduction and makes fertilisation, restoration of the diploid number and genetic recombination possible.

• Spermatogenesis: formation of male gametes (spermatozoa) in the testes.

• Oogenesis (ovogenesis): formation of female gametes (ova) in the ovaries.

Key point: Gametogenesis halves the chromosome number (2n → n) so that fertilisation restores it. Without meiosis the chromosome number would double in every generation.

1.1 Origin of germ cells

The primordial germ cells (PGCs) are set aside very early in development, separate from the somatic cells. In humans they are first seen in the wall of the yolk sac (derived from the epiblast) and migrate by amoeboid movement along the hindgut and dorsal mesentery to the genital (gonadal) ridges at about the 5th to 6th week. In the gonad they divide mitotically and differentiate into spermatogonia in a male or oogonia in a female. The gonadal environment, not the PGC itself, decides the path taken.

1.2 General phases of gametogenesis

Phase	What happens	Cell division
1. Multiplication (proliferation)	Gonial cells (spermatogonia or oogonia) increase in number	Repeated mitosis
2. Growth	Gonial cell enlarges, accumulates nutrients and becomes a primary spermatocyte or primary oocyte. Much greater in oogenesis (yolk, RNA, organelles)	None; DNA replication in interphase
3. Maturation	Meiosis I and II reduce the chromosome number to haploid	Meiosis
4. Differentiation	Spermatids transform into spermatozoa (spermiogenesis). In oogenesis it is minor	None

 

2. Meiosis: the core of gametogenesis

Meiosis consists of two successive divisions (I and II) with only one round of DNA replication. Meiosis I is reductional (homologous chromosomes separate); meiosis II is equational (sister chromatids separate, like mitosis).

Prophase I (long and complex)

• Leptotene: chromosomes condense and appear as long thin threads.

• Zygotene: homologous chromosomes pair (synapsis) and the synaptonemal complex forms, giving bivalents.

• Pachytene: thick chromosomes; crossing over between non-sister chromatids, mediated by recombination nodules.

• Diplotene: synaptonemal complex dissolves; homologues begin to separate but remain joined at chiasmata. In human oocytes this stage is prolonged for years (dictyotene).

• Diakinesis: maximum condensation; chiasmata terminalise; nucleolus and nuclear envelope disappear.

Metaphase I (bivalents align at the equator), Anaphase I (homologues move to opposite poles; centromeres do not divide) and Telophase I produce two haploid cells, each with chromosomes still made of two chromatids. After a short interkinesis with no DNA replication, meiosis II proceeds as in mitosis.

Genetic significance

• Crossing over exchanges segments between homologues, creating new gene combinations.

• Independent assortment of homologues gives 2ⁿ possible combinations (2²³ ≈ 8.4 million in humans).

• Together with random fertilisation these are the main sources of variation, the raw material for evolution.

3. Spermatogenesis

Spermatogenesis takes place in the seminiferous tubules of the testis, starting at puberty and continuing through life. In humans, one cycle from spermatogonium to sperm takes about 64 to 74 days, and a further period of about 10 to 14 days is spent in the epididymis.

3.1 Structure supporting spermatogenesis

• Seminiferous tubule wall: germ cells in all stages lie in layers, the earliest (spermatogonia) near the basement membrane and the latest (spermatids and sperm) near the lumen.

• Sertoli cells (sustentacular or nurse cells): tall cells extending from basement membrane to lumen. They nourish and support developing germ cells, phagocytose residual bodies and secrete androgen-binding protein (ABP), inhibin and fluid. Tight junctions between them form the blood-testis barrier, protecting haploid cells from the immune system.

• Leydig (interstitial) cells: lie between tubules and secrete testosterone under LH.

 

3.2 Stages of spermatogenesis

A. Spermatocytogenesis (formation of spermatocytes)

1. Multiplication phase: spermatogonia (2n) divide mitotically. Type A spermatogonia act as stem cells (some renew the stock) while others give type B spermatogonia.

2. Growth phase: a type B spermatogonium grows and replicates its DNA to become a primary spermatocyte (2n, 4C). Growth is modest.

3. Maturation phase: the primary spermatocyte completes meiosis I to form two secondary spermatocytes (n, 2C). Each completes meiosis II to form two spermatids (n, C). So one primary spermatocyte gives four haploid spermatids.

Daughter cells of one spermatogonium remain linked by cytoplasmic bridges (incomplete cytokinesis), so a clone develops synchronously.

Cell	Chromosome number	DNA content	Division
Spermatogonium	2n (46)	2C	Mitosis
Primary spermatocyte	2n (46)	4C	Meiosis I
Secondary spermatocyte	n (23)	2C	Meiosis II
Spermatid	n (23)	C	None; differentiates
Spermatozoon	n (23)	C	Mature, motile

 

 

Fig. 2: Stages of spermatogenesis, with the four phases and chromosome number (n) and DNA content (C) at each stage.

B. Spermiogenesis

Transformation of a round, non-motile spermatid into a streamlined, motile spermatozoon. No cell division occurs.

• Nucleus: shrinks and condenses; histones are replaced by protamines, making the DNA compact and protected.

• Golgi complex: forms proacrosomal granules that fuse into the acrosome (a cap on the nucleus) containing hydrolytic enzymes such as hyaluronidase and acrosin.

• Centrioles: the proximal centriole lies at the base of the nucleus; the distal centriole gives rise to the axial filament (axoneme, 9+2 arrangement) of the tail.

• Mitochondria: aggregate in a spiral around the axoneme in the middle piece (nebenkern, mitochondrial sheath) to supply ATP for movement.

• Cytoplasm: most is shed and phagocytosed by Sertoli cells as the residual body.

C. Spermiation

Release of mature sperm from the Sertoli cells into the lumen of the tubule. They are carried to the epididymis, where they gain motility and fertilising ability (capacitation is completed later in the female tract).

3.3 Structure of a mature spermatozoon

Part	Contents	Function
Head	Haploid nucleus with acrosome cap	Carries genetic material; acrosome helps penetrate the egg coats
Neck	Proximal and distal centrioles	Connects head to tail; basal body of the axoneme
Middle piece	Axoneme surrounded by spiral mitochondria	Energy (ATP) production
Tail (flagellum)	Axoneme, principal piece and end piece	Locomotion (whip-like movement)

Most animal sperm are flagellated; some (for example nematodes) have amoeboid, aflagellate sperm. Human sperm is about 50 to 60 µm long.

 

Fig. 3: Structure of a mature mammalian spermatozoon.

3.4 Hormonal control

• GnRH from the hypothalamus stimulates the anterior pituitary to release FSH and LH (ICSH in males).

• LH acts on Leydig cells, which secrete testosterone; testosterone is essential for meiosis and spermiogenesis.

• FSH acts on Sertoli cells, promoting ABP secretion and sperm maturation.

• Inhibin from Sertoli cells exerts negative feedback on FSH; testosterone inhibits GnRH and LH.

4. Oogenesis

Oogenesis is the formation of a mature ovum in the ovary. Unlike spermatogenesis, it begins before birth, is discontinuous (with long arrests), and one primary oocyte produces only one functional ovum and two or three small non-functional polar bodies.

4.1 Stages of oogenesis

A. Multiplication phase (fetal life)

PGCs in the developing ovary become oogonia (2n), which divide repeatedly by mitosis. In humans this ends before birth. Oogonia number about 6 to 7 million by the fifth month of gestation.

B. Growth phase

Oogonia enlarge to become primary oocytes (2n, 4C) and begin meiosis I, but stop in diplotene of prophase I (dictyotene stage), surrounded by a layer of follicle cells. The oocyte then grows greatly by accumulating yolk (vitellogenesis), ribosomes, mRNA, lipids and cortical granules. In birds and amphibians the lampbrush chromosomes of this stage are active in RNA synthesis. In humans the primary oocyte stays arrested from fetal life until puberty and beyond (up to about 50 years).

C. Maturation phase

• Meiosis I is completed shortly before ovulation, stimulated by the LH surge. Division is highly unequal: one large secondary oocyte (n, 2C) and a tiny first polar body.

• The secondary oocyte starts meiosis II but is arrested at metaphase II, and is ovulated in this state.

• Meiosis II is completed only if a sperm penetrates. It yields a large ovum (n, C) and a second polar body. The first polar body may also divide, giving three polar bodies in total, which later degenerate.

Key point: In humans, the oocyte is arrested twice: at prophase I (diplotene) from fetal life to puberty, and at metaphase II until fertilisation. Unequal cytokinesis keeps nearly all the cytoplasm and yolk in one cell.

Cell	Chromosome number	DNA content	Stage
Oogonium	2n (46)	2C	Fetal ovary, mitosis
Primary oocyte	2n (46)	4C	Arrested in prophase I (diplotene)
Secondary oocyte + first polar body	n (23)	2C	Arrested in metaphase II; ovulated
Ovum + second polar body	n (23)	C	After sperm entry

 

 

Fig. 4: Stages of oogenesis in humans, showing the two arrest points and the polar bodies.

4.2 Follicle development (mammals)

Stage	Features
Primordial follicle	Primary oocyte surrounded by one layer of flat follicle cells; formed in fetal life
Primary follicle	Cuboidal follicle cells (granulosa); zona pellucida begins to form
Secondary follicle	Multi-layered granulosa; theca interna and externa differentiate
Tertiary (antral) follicle	Fluid-filled antrum appears; oocyte on a cumulus oophorus
Graafian (mature) follicle	Large follicle bulging from the ovary surface; primary oocyte completes meiosis I; ovulation follows
Corpus luteum	After ovulation the ruptured follicle becomes the corpus luteum, secreting progesterone and some oestrogen

 

4.3 Numbers in the human ovary

• About 6 to 7 million oogonia and oocytes at about 20 weeks of gestation.

• About 1 to 2 million at birth (the rest lost by atresia).

• About 300,000 to 400,000 at puberty.

• Only about 400 to 500 are ovulated during the reproductive life (one per menstrual cycle).

4.4 Egg types and egg envelopes

Classification by yolk

Type	Yolk	Examples
Microlecithal (isolecithal)	Little yolk, evenly distributed	Amphioxus, eutherian mammals, echinoderms
Mesolecithal (moderately telolecithal)	Moderate yolk, concentrated at the vegetal pole	Frogs and other amphibians
Macrolecithal (megalecithal)	Large amount of yolk, with the cytoplasm at the animal pole	Reptiles, birds, egg-laying mammals
Centrolecithal	Yolk in the centre, surrounded by cytoplasm	Insects

 

Egg envelopes

• Primary: secreted by the oocyte itself (vitelline membrane; zona pellucida in mammals is largely oocyte-derived glycoproteins ZP1 to ZP3).

• Secondary: secreted by follicle cells (for example the chorion of insects).

• Tertiary: secreted by the oviduct or accessory glands (albumen, shell membranes and shell in birds; jelly coat in amphibians).

4.5 Hormonal control

• FSH stimulates follicle growth; the growing follicle secretes oestrogen.

• A mid-cycle LH surge triggers completion of meiosis I and ovulation.

• After ovulation LH maintains the corpus luteum, which secretes progesterone to prepare the uterus.

5. Spermatogenesis vs Oogenesis

Feature	Spermatogenesis	Oogenesis
Site	Seminiferous tubules of testis	Ovary (follicles)
Start	Puberty	Fetal life (meiosis I begins before birth)
Duration	Continuous throughout adult life	Discontinuous; ends at menopause
Multiplication phase	Long, continues in adult	Only in fetal life
Growth phase	Short; little growth	Long; large increase in size, yolk stored
Gametes per primary gamete cell	4 functional sperm	1 ovum + 2 or 3 polar bodies
Cytokinesis	Equal	Unequal
Polar bodies	Absent	Present
Differentiation	Spermiogenesis needed	Not needed
Size and motility	Small, motile	Large, non-motile
Arrest points	None	Prophase I and metaphase II
Number produced	Hundreds of millions per day	About 400 to 500 in a lifetime
Hormones	FSH, LH, testosterone, inhibin	FSH, LH, oestrogen, progesterone

 

6. Significance and abnormalities

6.1 Significance

• Maintains a constant chromosome number across generations.

• Generates genetic variation through crossing over and independent assortment.

• Produces gametes specialised for their roles: sperm for delivery of DNA, egg for nutrients and early development.

6.2 Errors in gametogenesis

Nondisjunction is the failure of homologues (meiosis I) or sister chromatids (meiosis II) to separate, producing gametes with an extra or missing chromosome (aneuploidy). Its incidence in oocytes rises with maternal age.

Condition	Karyotype	Cause
Down syndrome	47, trisomy 21	Nondisjunction of chromosome 21
Klinefelter syndrome	47, XXY	Extra X chromosome in a male
Turner syndrome	45, X	Absence of one sex chromosome in a female

 

7. Quick revision points

• One primary spermatocyte gives 4 sperm; one primary oocyte gives 1 ovum.

• Spermatogonia and oogonia are 2n; spermatids and ova are n.

• Spermiogenesis is the conversion of spermatids to sperm and involves no division.

• Acrosome from the Golgi; mitochondrial sheath in the middle piece; tail from the distal centriole.

• Human primary oocyte arrests at diplotene; the secondary oocyte at metaphase II.

• Sertoli cells nourish germ cells and form the blood-testis barrier; Leydig cells secrete testosterone.

Probable exam questions

1. Describe spermatogenesis with suitable diagrams.

2. Describe the process of oogenesis in mammals and explain its significance.

3. Differentiate between spermatogenesis and oogenesis.

4. Write short notes on spermiogenesis, polar bodies, structure of sperm and egg envelopes.

5. Explain the role of hormones in gametogenesis.

Tip: practise redrawing Figs. 1 to 5 from memory with correct labels; neat labelled diagrams carry good marks in practical and theory papers.

8. Multiple Choice Questions

Choose the correct option. The answer key with short explanations follows the questions.

Q1. Which of the following cells is haploid?

(a)  Spermatogonium

(b)  Primary spermatocyte

(c)  Oogonium

(d)  Secondary spermatocyte

Q2. How many functional spermatozoa are formed from one primary spermatocyte?

(a)  1

(b)  2

(c)  4

(d)  8

Q3. The blood-testis barrier is formed by tight junctions between:

(a)  Leydig cells

(b)  Sertoli cells

(c)  Spermatogonia

(d)  Epididymal cells

Q4. The acrosome of a sperm is derived from the:

(a)  Mitochondria

(b)  Centriole

(c)  Golgi complex

(d)  Endoplasmic reticulum

Q5. Testosterone is secreted by:

(a)  Spermatogonia

(b)  Sertoli cells

(c)  Epididymal cells

(d)  Leydig cells

Q6. Inhibin secreted by Sertoli cells exerts negative feedback on:

(a)  FSH

(b)  LH

(c)  Testosterone

(d)  Progesterone

Q7. Spermiogenesis is the:

(a)  Mitotic division of spermatogonia

(b)  Meiotic division of spermatocytes

(c)  Transformation of spermatids into spermatozoa

(d)  Release of sperm into the lumen

Q8. In the human female, the primary oocyte is arrested at:

(a)  Metaphase II

(b)  Leptotene

(c)  Pachytene

(d)  Diplotene (dictyotene) of prophase I

Q9. A human oocyte at ovulation is a:

(a)  Primary oocyte in prophase I

(b)  Secondary oocyte arrested at metaphase II

(c)  Ovum at telophase II

(d)  Oogonium

Q10. Meiosis II of the oocyte is completed:

(a)  At ovulation

(b)  On fertilisation (sperm penetration)

(c)  At implantation

(d)  At menstruation

Q11. The first polar body is formed at the end of:

(a)  Meiosis II

(b)  Mitosis of oogonia

(c)  Fertilisation

(d)  Meiosis I

Q12. The approximate number of oocytes in a human ovary at birth is:

(a)  6 to 7 million

(b)  3 to 4 lakh

(c)  1 to 2 million

(d)  400 to 500

Q13. The egg of a frog is:

(a)  Mesolecithal

(b)  Microlecithal

(c)  Alecithal

(d)  Centrolecithal

Q14. Centrolecithal eggs are found in:

(a)  Frog

(b)  Insects

(c)  Birds

(d)  Humans

Q15. Synapsis (pairing of homologous chromosomes) occurs in:

(a)  Zygotene

(b)  Leptotene

(c)  Diplotene

(d)  Diakinesis

Q16. Crossing over takes place in:

(a)  Leptotene

(b)  Zygotene

(c)  Diplotene

(d)  Pachytene

Q17. Chiasmata first become clearly visible in:

(a)  Diplotene

(b)  Pachytene

(c)  Zygotene

(d)  Leptotene

Q18. The mitochondrial sheath of a spermatozoon lies in the:

(a)  Head

(b)  Neck

(c)  Middle piece

(d)  End piece

Q19. Completion of meiosis I in the oocyte and ovulation are triggered by:

(a)  Progesterone

(b)  FSH

(c)  Inhibin

(d)  LH surge

Q20. The karyotype of Klinefelter syndrome is:

(a)  45, X

(b)  47, XXY

(c)  47, trisomy 21

(d)  47, XYY

Answer key

Q	Ans	Explanation
1	(d)	Meiosis I produces haploid (n) secondary spermatocytes.
2	(c)	Two meiotic divisions give four spermatids, all of which become sperm.
3	(b)	Sertoli cell junctions protect developing haploid cells.
4	(c)	Proacrosomal granules from the Golgi fuse to form the acrosome.
5	(d)	Leydig (interstitial) cells respond to LH.
6	(a)	Inhibin suppresses FSH secretion.
7	(c)	It is a differentiation process with no cell division. Release is spermiation.
8	(d)	Arrest lasts from fetal life until just before ovulation.
9	(b)	Meiosis II is completed only after sperm entry.
10	(b)	Sperm entry triggers completion and the second polar body forms.
11	(d)	Unequal division of meiosis I gives a secondary oocyte and the first polar body.
12	(c)	Out of about 6 to 7 million at mid-gestation, the rest are lost by atresia.
13	(a)	Moderate yolk, concentrated at the vegetal pole.
14	(b)	Yolk lies in the centre of the egg in insects.
15	(a)	Pairing and synaptonemal complex formation mark zygotene.
16	(d)	It occurs between non-sister chromatids of bivalents at pachytene.
17	(a)	Homologues separate partly at diplotene but stay joined at chiasmata.
18	(c)	Mitochondria spiral round the axoneme in the middle piece.
19	(d)	The mid-cycle LH surge triggers both.
20	(b)	Nondisjunction gives an extra X chromosome in a male.

 

9. References

1. Gilbert, S.F. and Barresi, M.J.F. Developmental Biology. Oxford University Press.

2. Sadler, T.W. Langman's Medical Embryology. Wolters Kluwer (Lippincott Williams & Wilkins).

3. Hall, J.E. Guyton and Hall Textbook of Medical Physiology. Elsevier.

4. Tortora, G.J. and Derrickson, B. Principles of Anatomy and Physiology. Wiley.

5. Alberts, B. et al. Molecular Biology of the Cell. W.W. Norton and Company.

6. Kardong, K.V. Vertebrates: Comparative Anatomy, Function, Evolution. McGraw-Hill Education.

7. Verma, P.S. and Agarwal, V.K. Chordate Embryology. S. Chand and Company.

8. Jordan, E.L. and Verma, P.S. Chordate Zoology. S. Chand and Company.