Pre-Fertilisation Events

These are all the processes that prepare the sperm and egg for fusion.

Gametogenesis (Formation of Gametes)

• Location: Seminiferous tubules of the testes

• Starts at puberty and continues throughout life.

• Process:

  • Spermatogonia (2n) → undergo mitosis

  • Become primary spermatocytes (2n) → undergo meiosis I → secondary spermatocytes (n)

  • Secondary spermatocytes → meiosis II → spermatids (n)

  • Spermatids mature into spermatozoa (n) (sperm cells)

Structure of sperm:

• Head (contains nucleus and enzymes to penetrate egg)

• Midpiece (packed with mitochondria for energy)

• Tail (for movement)

• Location: Ovaries

• Starts before birth, paused at prophase I until puberty.

• Process:

  • Oogonium (2n) → primary oocyte (2n)

  • At puberty, 1 oocyte per month resumes meiosis I → forms:

    • One secondary oocyte (n) (large)

    • One polar body (n) (small, degenerates)

  • Secondary oocyte is released during ovulation and arrested at metaphase II

  • Meiosis II is completed only if fertilisation occurs

Ovulation

• Around day 14 of the menstrual cycle, the Graafian follicle bursts and releases the secondary oocyte into the fallopian tube (oviduct).

Ejaculation and Sperm Transport

• During ejaculation:

  • About 200–300 million sperm are released.

  • They are mixed with secretions from:

    • Seminal vesicles (fructose)

    • Prostate gland (alkaline fluid)

    • Bulbourethral glands (lubrication)

• Path of sperm:

  • Vagina → Cervix → Uterus → Fallopian tube

Note: Only ~100-1000 sperm reach the oocyte; just one fertilises it.

Fertilisation (Syngamy)

This is the fusion of male and female gametes in the ampulla region of the fallopian tube.

Steps of Fertilisation:

1. Sperm reaches the oocyte

   • Guided by chemical signals (called chemotaxis).

2. Acrosome Reaction:

   • The acrosome (enzyme-filled cap) of sperm releases enzymes (like hyaluronidase) to break the protective layers of the oocyte:

     • Corona radiata

     • Zona pellucida

3. Sperm Penetration:

   • The sperm enters the oocyte cytoplasm.

   • Meiosis II in the oocyte completes, forming ovum (n) and second polar body (n).

4. Nuclear Fusion:

   • Male and female pronuclei fuse → forms a zygote (2n) with 46 chromosomes.

Fertilisation is complete within 24 hours of ovulation.

Post-Fertilisation Events

Everything that happens after zygote formation, leading to embryo development and birth.

Zygote Cleavage

• Cleavage: A rapid series of mitotic divisions.

• No growth, just division.

• Forms a solid ball of cells called the morula (~16 cells).

Blastocyst Formation

• Morula develops into a blastocyst (hollow ball with fluid-filled cavity).

• Blastocyst has:

  • Trophoblast (outer layer – becomes placenta)

  • Inner cell mass (ICM) – becomes the embryo

Implantation

• Day 6–7 after fertilisation

• Blastocyst attaches to endometrium (uterine lining)

• Trophoblast secretes enzymes to embed it into the wall.

Successful implantation = pregnancy begins

Placenta Formation

• A temporary organ formed by maternal tissue + fetal trophoblast.

• Functions:

  • Nutrient and oxygen transfer

  • Waste removal

  • Hormone production (e.g., hCG, progesterone, estrogen)

  • Protection (immune barrier)

Embryonic Development

• Inner cell mass differentiates into:

  • Ectoderm → skin, brain, nerves

  • Mesoderm → muscles, bones, heart

  • Endoderm → lungs, liver, gut

• Week 3–8: Major organs form → called embryo

• After 8 weeks → called a foetus

Pregnancy & Birth

• Total duration: About 9 months (38–40 weeks)

• Labour begins with:

  • Uterine contractions

  • Cervical dilation

  • Amniotic sac may rupture ("water breaking")

• Baby is delivered through the vagina (birth canal).