1. Gametogenesis

• Gametogenesis: The process of formation of gametes.
• Male gamete: Sperm.
• Female gamete: Ovum (female gamete).
• Primary sex organs: Testes in males and ovaries in females.

2. Spermatogenesis

Definition: The process of formation of spermatozoa (sperms) from spermatogonia in the testes.

• Begins at puberty.
• Occurs in the seminiferous tubules of the testes.

Steps of spermatogenesis

Stage	Process	Chromosome number
Spermatogonium	Divides by mitosis	46 (diploid)
Primary spermatocyte	Undergoes meiosis I	46 (diploid)
Secondary spermatocytes (2)	Formed after meiosis I	23 each (haploid)
Spermatids (4)	Formed after meiosis II	23 each (haploid)
Spermatozoa (4)	Formed from spermatids	23 each (haploid)

Important terms

• Spermiogenesis: Transformation of spermatids into spermatozoa.
• Spermiation: Release of spermatozoa from Sertoli cells into the lumen of seminiferous tubules.
• Sertoli cells: Support developing sperm cells and provide factors that help spermiogenesis.
• Leydig cells: Produce and secrete androgens.

Hormonal control of spermatogenesis

Hormone	Source	Function
GnRH	Hypothalamus	Stimulates the anterior pituitary to secrete LH and FSH
LH	Anterior pituitary	Acts on Leydig cells and stimulates androgen secretion
FSH	Anterior pituitary	Acts on Sertoli cells and stimulates secretion of factors that help spermiogenesis
Androgens	Leydig cells	Stimulate spermatogenesis

Sequence to remember:

GnRH → LH and FSH

• LH → Leydig cells → Androgens → Spermatogenesis
• FSH → Sertoli cells → Factors supporting spermiogenesis

3. Structure of a Human Sperm

A sperm is a microscopic structure consisting of four parts.

Part	Important features and functions
Head	Contains an elongated haploid nucleus
Acrosome	Cap-like structure covering the anterior portion of the head; contains enzymes that help fertilisation
Neck	Connects the head and middle piece
Middle piece	Contains numerous mitochondria that produce energy for movement
Tail	Helps in sperm motility
Plasma membrane	Encloses the entire sperm

Important facts about human sperm

• Each sperm contains 23 chromosomes.
• The human male ejaculates approximately 200–300 million sperms during coitus.
• For normal fertility, according to the NCERT text:
  • At least 60% of sperms must have normal shape and size.
  • At least 40% must show vigorous motility.

4. Male Accessory Ducts and Glands

Accessory ducts

• Rete testis
• Vasa efferentia
• Epididymis
• Vas deferens
• Ejaculatory duct
• Urethra

Functions of accessory ducts:

• Transport sperms.
• Secretions of the epididymis, vas deferens, seminal vesicles and prostate are essential for sperm maturation and motility.

Accessory glands

• Seminal vesicles
• Prostate gland
• Bulbourethral glands

Important terms

• Semen: Sperms together with seminal plasma.
• Androgens: Maintain the functions of male accessory ducts and glands.

5. Oogenesis

Definition: The process of formation of a mature female gamete from oogonia.

Important facts

• Oogenesis begins during embryonic development.
• A couple of million oogonia are formed within each fetal ovary.
• No new oogonia are formed and added after birth, according to NCERT.
• Primary oocytes remain temporarily arrested in prophase I of meiosis.
• At puberty, approximately 60,000–80,000 primary follicles remain in each ovary.

Stages of oogenesis and follicular development

Stage	Description
Oogonium	Diploid female germ cell
Primary oocyte	Enters meiosis I during embryonic development and gets arrested in prophase I
Primary follicle	Primary oocyte surrounded by a layer of granulosa cells
Secondary follicle	Primary follicle develops more layers of granulosa cells and a new theca
Tertiary follicle	Contains a fluid-filled cavity called the antrum
First meiotic division	Produces a large haploid secondary oocyte and a tiny first polar body
Graafian follicle	Mature follicle containing the secondary oocyte
Ovulation	Release of the secondary oocyte from the ovary

Important structures of the ovarian follicle

• Granulosa cells: Surround and support the developing oocyte.
• Theca interna: Inner layer of the theca.
• Theca externa: Outer layer of the theca.
• Antrum: Fluid-filled cavity of the tertiary follicle.
• Zona pellucida: Membrane that surrounds the secondary oocyte.

Meiotic division during oogenesis

• The first meiotic division is unequal.
• It produces:
  1. One large haploid secondary oocyte.
  2. One tiny first polar body.
• The secondary oocyte retains most of the nutrient-rich cytoplasm.
• The first polar body may divide further or degenerate; NCERT states that this is not certain.
• The Graafian follicle ruptures and releases the secondary oocyte during ovulation.

Why does the secondary oocyte retain most of the cytoplasm?

It ensures that the female gamete retains sufficient cytoplasm, organelles and stored nutrients to support early development after fertilisation.

6. Differences Between Spermatogenesis and Oogenesis

Feature	Spermatogenesis	Oogenesis
Site	Seminiferous tubules of testes	Ovaries
Begins	At puberty	During embryonic development
Germ cells	Spermatogonia	Oogonia
Growth phase	Comparatively short	Long and pronounced
Cytoplasmic division	Equal	Unequal
Products of meiosis	Four haploid spermatids	One large secondary oocyte and polar body after meiosis I
Functional gametes	Four sperms from one primary spermatocyte	One functional female gamete from one primary oocyte
Nature	Continuous after puberty	Cyclic release of secondary oocytes during reproductive life
Gamete size	Small	Large
Motility	Sperm is motile	Female gamete is non-motile

7. Quick Revision: Must-Remember Facts

46 → 23 chromosomes

Meiosis I reduces the chromosome number from diploid to haploid.

Spermiogenesis vs spermiation

Spermiogenesis = spermatids transform into spermatozoa.

Spermiation = spermatozoa are released into the lumen of seminiferous tubules.

LH vs FSH

LH acts on Leydig cells; FSH acts on Sertoli cells.

Oogenesis begins before birth

Primary oocytes are arrested in prophase I.

Antrum and zona pellucida

Antrum = fluid-filled cavity of the tertiary follicle.

Zona pellucida = membrane surrounding the secondary oocyte.

Ovulation

Release of the secondary oocyte from the Graafian follicle.