Gametogenesis
- The process involved in the maturation of male and female gametes (spermatozoa & ova) before they unite to form a zygote.
Oogenesis (Formation of Ova)
- The process by which a mature ovum develops from the primordial germ cells.
- The primordial germ cells originate from the yolk sac and migrate to the developing ovary.
Stages of Oogenesis:
- Primordial Germ Cells → Oogonia (Mitosis) → Primary Oocyte
- Primary Oocyte (Meiosis I) → Arrested in Prophase I until puberty
- At Ovulation: Primary Oocyte completes Meiosis I → Secondary Oocyte + First Polar Body
- Secondary Oocyte (Meiosis II) → Arrested in Metaphase II until fertilization
- At Fertilization: Secondary Oocyte completes Meiosis II → Mature Ovum + Second Polar Body
Key Points:
- Oogenesis begins before birth and pauses until puberty.
- A female is born with all the primary oocytes she will ever have.
- Meiosis II is completed only upon fertilization.
Spermatogenesis (Formation of Spermatozoa)
- The process by which spermatogonia mature into spermatozoa.
- Occurs in the seminiferous tubules of the testes and continues from puberty throughout life.
Stages of Spermatogenesis:
- Spermatogonia (Mitosis) → Primary Spermatocyte
- Primary Spermatocyte (Meiosis I) → Two Secondary Spermatocytes
- Secondary Spermatocytes (Meiosis II) → Four Spermatids
- Spermatids → Mature Spermatozoa (Spermiogenesis)
Key Points:
- Spermatogenesis begins at puberty and continues throughout life.
- Each primary spermatocyte produces four spermatozoa.
- The entire process takes approximately 64–72 days.
Fertilization
- The fusion of the male (sperm) and female (ovum) gametes to form a zygote.
- Occurs in the ampulla of the fallopian tube within 24 hours of ovulation.
Mechanism of Fertilization:
- Capacitation: Sperm undergoes changes to penetrate the ovum.
- Acrosomal Reaction: Enzymes break down the zona pellucida.
- Penetration of Zona Pellucida: Single sperm enters the ovum.
- Completion of Meiosis II by Secondary Oocyte
- Fusion of Male & Female Pronuclei → Formation of Zygote
Cleavage & Blastocyst Formation
- Cleavage: Rapid mitotic divisions of the zygote forming blastomeres.
- Morula: Solid mass of blastomeres (16-cell stage, ~3 days post-fertilization).
- Blastocyst: Hollow structure formed (~5 days post-fertilization) before implantation.
Implantation (Nidation)
- Process by which the blastocyst attaches & embeds into the endometrium of the uterus (~6-7 days post-fertilization).
Stages of Implantation:
- Apposition: Loose contact with endometrium.
- Adhesion: Firm attachment.
- Invasion: Trophoblast cells penetrate the endometrium.
Placenta Formation
- The placenta is formed by maternal (decidua basalis) and fetal (chorion frondosum) components.
- Functions:
- Exchange of gases (Oâ‚‚, COâ‚‚)
- Nutrient and waste exchange
- Hormone production (hCG, estrogen, progesterone)
Fetal Membranes & Amniotic Fluid
- Chorion & Amnion: Protective membranes around the fetus.
- Amniotic Fluid Functions:
- Cushions the fetus
- Maintains temperature
- Allows fetal movement
