In the 12th State Board Biology curriculum (specifically for Tamil Nadu, which closely follows the
NCERT syllabus), Section 2.2 is dedicated to Gametogenesis. This is a crucial topic as it
explains how haploid gametes (sperm and egg) are formed from diploid germ cells, a
fundamental process for sexual reproduction.
2.2 Gametogenesis
Definition: Gametogenesis is the biological process by which diploid (2n) germ cells undergo
meiosis and differentiation to produce haploid (n) gametes (sex cells). In males, this process is
called spermatogenesis (formation of spermatozoa or sperm), and in females, it's called
oogenesis (formation of ova or eggs).
I. Spermatogenesis (Formation of Sperm)
● Location: Occurs in the seminiferous tubules of the testes in males.
● Onset: Initiates at puberty and continues throughout the reproductive life of a male.
● Phases of Spermatogenesis:
1. Multiplication Phase:
■ The immature male germ cells, called spermatogonia (diploid, 2n=46
chromosomes), present on the inside wall of the seminiferous tubules,
multiply by repeated mitotic divisions.
■ This increases their number. Some spermatogonia act as stem cells, while
others differentiate.
2. Growth Phase:
■ Some spermatogonia grow in size to become primary spermatocytes
(diploid, 2n=46 chromosomes). They accumulate nutrients and prepare for
meiotic division.
3. Maturation Phase (Meiosis):
■ Meiosis I: Each diploid primary spermatocyte undergoes Meiosis I
(reductional division) to form two equal, haploid cells called secondary
spermatocytes (n=23 chromosomes).
■ Meiosis II: Each secondary spermatocyte then rapidly undergoes Meiosis II
(equational division) to produce two equal, haploid cells called spermatids
(n=23 chromosomes).
■ Therefore, from one primary spermatocyte, four haploid spermatids are
produced.
4. Spermiogenesis (Differentiation/Spermateliosis):
■ This is the final stage where the non-motile, round spermatids undergo
morphological transformation to become mature, motile spermatozoa
(sperms).
■ During spermiogenesis, the nucleus condenses, a tail (flagellum) develops,
and the acrosome (a cap-like structure containing enzymes) forms over the
head.
■ After spermiogenesis, sperm heads become embedded in the Sertoli cells
(nurse cells) for nourishment and support.
■ Spermiation: The process of release of mature sperms from the
seminiferous tubules into the lumen.
● Hormonal Control of Spermatogenesis:
○ Gonadotropin-Releasing Hormone (GnRH): Secreted by the hypothalamus, acts
on the anterior pituitary gland.
○ Luteinizing Hormone (LH): Secreted by the anterior pituitary, acts on Leydig cells
to stimulate the synthesis and secretion of androgens (testosterone). Androgens
are essential for spermatogenesis.
○ Follicle-Stimulating Hormone (FSH): Secreted by the anterior pituitary, acts on
Sertoli cells and stimulates the secretion of factors like Androgen-Binding Protein
(ABP), which concentrate testosterone in the seminiferous tubules, and inhibin,
which regulates FSH secretion.
II. Oogenesis (Formation of Ovum)
● Location: Occurs in the ovaries of females.
● Onset: Oogenesis is a complex and discontinuous process that begins during fetal
development and is completed only upon fertilization.
● Phases of Oogenesis:
1. Multiplication Phase (Fetal Life):
■ In the fetal ovary, millions of gamete mother cells called oogonia (diploid,
2n=46 chromosomes) are formed by mitotic divisions.
■ No more oogonia are formed or added after birth in a female.
2. Growth Phase (Fetal Life):
■ Oogonia grow in size and enter into Meiosis I, pausing at Prophase I. These
are now called primary oocytes (diploid, 2n=46 chromosomes).
■ Each primary oocyte gets surrounded by a layer of granulosa cells, forming a
primary follicle.
■ A large number of these primary follicles degenerate from birth to puberty. At
puberty, only 60,000-80,000 primary follicles are left in each ovary.
3. Maturation Phase (Post-Puberty & Fertilization):
■ At Puberty: The primary follicles mature into secondary follicles, then into
tertiary follicles, and finally into a mature Graafian follicle.
■ The tertiary follicle is characterized by a fluid-filled cavity called the antrum.
■ Within the tertiary follicle, the primary oocyte completes its Meiosis I. This is
an unequal division, resulting in:
■ A large, haploid secondary oocyte (n=23 chromosomes)
■ A tiny, haploid first polar body (which may or may not complete
Meiosis II).
■ The secondary oocyte retains most of the cytoplasm and nutrients.
■ The secondary oocyte begins Meiosis II but arrests at Metaphase II. It is this
secondary oocyte, enclosed within the Graafian follicle, that is released
during ovulation.
■ After Ovulation/Upon Fertilization: Meiosis II is completed only if a sperm
penetrates the secondary oocyte. This again is an unequal division,
resulting in:
■ A large, haploid ovum (egg) (n=23 chromosomes)
■ A tiny, haploid second polar body.
■ If fertilization does not occur, the secondary oocyte degenerates.
● Hormonal Control of Oogenesis:
○ GnRH: From the hypothalamus, stimulates the anterior pituitary.
○ FSH: From the anterior pituitary, stimulates the growth and development of ovarian
follicles (containing primary oocytes) and secretion of estrogen by granulosa cells.
○ LH: From the anterior pituitary, triggers ovulation (LH surge) and stimulates the
formation of the corpus luteum (after ovulation) and its secretion of progesterone
(essential for maintaining pregnancy) and some estrogen.
III. Differences Between Spermatogenesis and Oogenesis
Feature Spermatogenesis Oogenesis
Location Testes (seminiferous tubules) Ovaries
Onset At puberty Initiates during fetal life
Continuity Continuous process from Discontinuous; pauses at
puberty throughout life various stages
Number of gametes 4 functional spermatozoa from 1 functional ovum and 2-3 polar
one primary spermatocyte bodies from one primary oocyte
Size of gametes Small, motile (sperm) Large, non-motile (ovum),
contains abundant cytoplasm
and nutrients
Cytokinesis Equal cytokinesis Unequal cytokinesis (produces
polar bodies)
Polar body formation No polar bodies formed Polar bodies are formed (first
and second)
Duration Relatively shorter (approx. 74 Much longer, spread over years
days in humans) (from fetal life to fertilization)
Timing of Meiosis II Occurs immediately after Arrested at Metaphase II,
Meiosis I completed only upon
fertilization
Gametogenesis is a fundamental process ensuring the formation of haploid gametes with
genetic variation, crucial for the continuity and diversity of the human species.
