15.2 - Gametogenesis in Humans

The Necessity of Gametogenesis

• Gametogenesis is the production of reproductive cells (gametes) in the gonads: testes in males and ovaries in females.
• It produces haploid gametes ($n$).
• During fertilisation, the sperm nucleus fuses with the ovum nucleus in the Fallopian tube to form a diploid zygote ($2n$).

Spermatogenesis

• Spermatogenesis is sperm production in the seminiferous tubules.
• Sertoli cells provide nutrients during sperm production.
• Sperms move from the seminiferous tubules to the epididymis and leave through the sperm duct.
• FSH and testosterone stimulate spermatogenesis; LH stimulates testosterone secretion in the testes.

Stages of Spermatogenesis

1. Primordial germ cells divide by mitosis to form diploid spermatogonia ($2n$).
2. Spermatogonia grow into diploid primary spermatocytes ($2n$).
3. Each primary spermatocyte undergoes meiosis I to form two haploid secondary spermatocytes ($n$).
4. Each secondary spermatocyte undergoes meiosis II to form two haploid spermatids ($n$); four spermatids form in total.
5. Spermatids differentiate and specialise to form sperms.

Oogenesis

• Oogenesis is the production of secondary oocytes or female gametes in the ovaries.
• It begins before birth.

Stages of Oogenesis

1. Before birth, primordial germ cells divide by mitosis to form diploid oogonia ($2n$).
2. Each oogonium develops into a primary oocyte enclosed by follicular cells, forming a primary follicle. The primary oocyte starts meiosis I but stops at prophase I.
3. From puberty to menopause, FSH stimulates follicle development every month.
4. The primary oocyte completes meiosis I, forming a large secondary oocyte ($n$) and a small first polar body ($n$).
5. The secondary oocyte starts meiosis II and stops at metaphase II; the mature Graafian follicle releases it during ovulation.
6. If a sperm penetrates it, the secondary oocyte completes meiosis II to form an ovum and a polar body. The remaining polar bodies disintegrate.
7. After ovulation, the remaining follicle forms the corpus luteum. With fertilisation, it secretes oestrogen and progesterone; without fertilisation, it and the secondary oocyte degenerate and are removed through menstruation.

Structure of Sperm and Graafian Follicle

Structure of a Sperm

• Head: Contains the nucleus.
• Midpiece: Packed with mitochondria that generate energy for swimming.
• Tail: Enables the sperm to swim to the Fallopian tube for fertilisation.

Structure of a Graafian Follicle

• A large secondary oocyte is surrounded by a jelly coat and follicular cells.
• The secondary oocyte and follicular cells form the Graafian follicle.

Comparison between Spermatogenesis and Oogenesis

Similarities

• Both are gametogenesis processes in reproductive organs.
• Both produce haploid gametes involved in fertilisation.

Differences

Spermatogenesis	Oogenesis
Occurs in the testes.	Occurs in the ovaries.
One spermatogonium produces four functional sperms after meiosis.	One oogonium produces one functional secondary oocyte and three non-functional polar bodies after meiosis.
Sperms are small and have a head, midpiece and tail.	Secondary oocytes are large and spherical.
Meiosis I produces two secondary spermatocytes.	Meiosis I produces one secondary oocyte and one polar body.
Meiosis is completed.	Meiosis II is completed only after fertilisation.
Spermatids differentiate into sperms.	The secondary oocyte does not differentiate.
Continuous from puberty to old age; millions form daily.	Begins in the female foetus, resumes at puberty, stops at menopause; one secondary oocyte is released per menstrual cycle.