Chapter 2 — Human Reproduction (Part 3): Gametogenesis, Fertilisation & Implantation

3.4 Gametogenesis

The primary sex organs — testis in males and ovary in females — produce gametes. Gametogenesis includes spermatogenesis in males and oogenesis in females.

3.4.1 Spermatogenesis

In testes, the immature male germ cells (spermatogonia) produce sperms by spermatogenesis that begins at puberty.

1. Spermatogonia (2n) multiply by mitotic division.
2. Some spermatogonia (called primary spermatocytes) periodically undergo meiosis.
3. Primary spermatocyte (2n, 46 chromosomes) → Meiosis I → two haploid secondary spermatocytes (n, 23 chromosomes).
4. Secondary spermatocytes → Meiosis II → four equal-sized spermatids (n).
5. Spermatids transform into spermatozoa (sperms) by the process of spermiogenesis.
6. Sperm heads embed in Sertoli cells; release of mature sperm into lumen is called spermiation.

Spermatogenesis is initiated by an increase in GnRH at puberty. GnRH stimulates the anterior pituitary → secretion of FSH and LH. LH acts on Leydig cells → testosterone → spermatogenesis. FSH acts on Sertoli cells → secretion of factors that aid spermiogenesis.

3.4.2 Structure of a Sperm

A sperm is composed of a head, neck, middle piece and tail. The whole body of the sperm is enveloped by a plasma membrane. The head contains an elongated haploid nucleus, the anterior part of which is covered by a cap-like acrosome. The acrosome is filled with enzymes that help fertilisation. The middle piece contains numerous mitochondria that produce energy for the movement of the tail.

3.4.3 Oogenesis

The process of formation of mature female gamete is called oogenesis. It is initiated during embryonic development when about 2 million gamete mother cells (oogonia) are formed in each foetal ovary; no more oogonia are formed and added after birth.

1. Oogonia enter into prophase-I of meiosis and get temporarily arrested as primary oocytes.
2. Each primary oocyte is surrounded by a layer of granulosa cells → primary follicle.
3. Many primary follicles degenerate during phases from birth to puberty; only 60,000-80,000 primary follicles remain in each ovary at puberty.
4. A primary follicle gets surrounded by more layers of granulosa cells and a theca → secondary follicle → tertiary follicle with a fluid-filled cavity called antrum.
5. The primary oocyte completes its first meiotic division — an unequal division producing a large secondary oocyte (n) and a tiny first polar body.
6. Tertiary follicle changes into the mature Graafian follicle.
7. Secondary oocyte is surrounded by a new membrane zona pellucida. The Graafian follicle ruptures → ovulation releases the secondary oocyte (ovum).
8. Meiosis-II is completed only at fertilisation — yielding the ootid + second polar body; ootid matures into the ovum.

3.4.4 Spermatogenesis vs Oogenesis

3.5 Fertilisation

During copulation (coitus), semen is released by the penis into the vagina (insemination). The motile sperms swim rapidly, pass through the cervix, enter into the uterus and finally reach the junction of the isthmus and ampulla (ampullary-isthmic junction) of the fallopian tube. The ovum released by the ovary is also transported to the ampullary-isthmic junction where fertilisation takes place.

Fertilisation can only occur if the ovum and sperms are transported simultaneously to the ampullary-isthmic junction. During fertilisation, a sperm comes in contact with the zona pellucida layer of the ovum and induces changes in the membrane that block the entry of additional sperms. Thus, it ensures that only one sperm fertilises an ovum.

The secretions of the acrosome help the sperm enter into the cytoplasm of the ovum through the zona pellucida and the plasma membrane. This induces the completion of meiotic division of the secondary oocyte. The second meiotic division is also unequal and results in the formation of a second polar body and a haploid ovum (ootid). The chromosomes from the haploid sperm and the haploid ovum fuse together to form a diploid zygote.

3.5.1 Sex Determination

Sex of the baby is determined by the father, not by the mother. All ova carry an X chromosome. Sperms carry either an X or a Y chromosome. After fertilisation:

• X (sperm) + X (ovum) → XX zygote → female (girl)
• Y (sperm) + X (ovum) → XY zygote → male (boy)

3.6 Cleavage and Implantation

The mitotic division (cleavage) starts as the zygote moves through the isthmus of the oviduct (mesoderm) towards the uterus and forms 2, 4, 8, 16 daughter cells called blastomeres.

• The embryo with 8 to 16 blastomeres is called a morula.
• The morula continues to divide and transforms into the blastocyst.
• The blastomeres in the blastocyst are arranged into an outer layer called the trophoblast and an inner cell mass.
• The trophoblast layer attaches to the endometrium and the inner cell mass differentiates into the embryo. After attachment, the uterine cells divide rapidly and cover the blastocyst.
• As a result, the blastocyst becomes embedded in the endometrium of the uterus. This is called implantation — and it leads to pregnancy.

3.6.1 Worked Examples