Concise Notes on Fertilization and Embryonic Development

Development Overview

  • Two-celled stage or 16 days old: primary germ layers are ectoderm, mesoderm, endoderm.

Fertilization Process

  • Ejaculation: 300 million sperm, 2,000-3,000 reach egg vicinity.
  • Egg Journey: Takes 72 hours to reach uterus; must be fertilized within 12-24 hours.
  • Sperm Viability: Remain viable for up to 6 days after ejaculation.
  • Optimal Conception Window: Few days before ovulation to 14 hours after.

Capacitation

  • Dilution of inhibitory factors in semen by female fluids enables sperm to penetrate the egg.

Acrosomal Reaction

  • Sperm penetration involves acrosomal enzymes (hyaluronidase, acrosin).

Fertilization Mechanics

  • Fertilization results in diploid (2n) set from haploid (n) sperm and egg.
  • Polyspermy: Fertilization by two or more sperm.

Prevention of Polyspermy

  • Mechanism:
    • Sperm binding opens Na+ channels, leading to membrane depolarization, inhibiting other sperm.
    • Ca²+ inflow causes cortical reaction, releasing secretions that form an impenetrable fertilization membrane.

Conceptus Migration

  • Stages:
    • Fertilization → Pronuclei → Zygote (0 hours)
    • 2-celled stage (30 hours) → 4-celled stage → 8-celled stage → Morula (72 hours) → Blastocyst (6 days).

Embryonic Development

  • Embryonic Stem Cells (ESCs): Pluripotent cells derived from the inner cell mass of human embryo, can develop into over 200 cell types.

Blastocyst and Implantation

  • Blastocyst: Will develop into embryo and form part of the placenta for nourishment.
  • Implantation: 6 days after ovulation, blastocyst attaches to endometrium, leading to the secretion of HCG.

HCG Role

  • Stimulates corpus luteum to secrete estrogen and progesterone, suppressing menstruation.

Gastrulation (Formation of Germ Layers)

  • Migration leads to three layers:
    • Ectoderm: Epidermis, hair, nervous system.
    • Mesoderm: Skeleton, muscles, blood, lymph.
    • Endoderm: Digestive, respiratory tracts, urinary bladder.

Extraembryonic Membranes

  • Amnion: Contains amniotic fluid; protects embryo.
  • Chorion: Forms part of the placenta, involved in gas exchange, nutrition, waste removal, and hormone secretion.