Notes on Fertilization and Early Embryonic Development

Sperm Journey to Fertilization

• Ejaculation deposits approximately 200 million sperm into the vagina.
• Sperm swim through the cervix aided by flagella.
• Uterine contractions direct sperm to uterine tubes (30 mins to 2 hours).
• Only about 200 sperm reach the secondary oocyte; one typically fertilizes it.

Key Processes Before Fertilization

Capacitation

• Not fully understood.
• Uterine secretions destabilize sperm plasma membrane (acrosome).
• Leads to:
  • Increased sperm activity (hyperactivity).
  • Enhanced flagella beating.
• Sperm penetrate the corona radiata and bind to the zona pellucida.

Acrosomal Reaction

• Zona pellucida has receptor proteins (ZP3) that bind sperm proteins.
• Triggering leads to:
  • Enzymes released to digest zona pellucida.
  • Sperm entering the perivitelline space.

Prevention of Polyspermy

• Fast Block: Oocyte membrane depolarizes upon sperm fusion.
• Slow Block: Intracellular calcium release triggers cortical granules to make zona pellucida impermeable.

Post-Fertilization Events

1. Formation of Zygote: Fusion of sperm and oocyte pronuclei.
2. Cleavage: Rapid mitotic division; first cleavage at 30 hours post-fertilization.
   • Produces blastomeres; by Day 3, forms morula (16 blastomeres).
3. Blastocyst Formation:
   • Uterine milk penetrates morula, forming blastocyst cavity (Day 5).
   • Zona pellucida degenerates, and blastocyst hatches.

Implantation Process

• Occurs 6-7 days post-fertilization in the uterus.
• Blastocyst attaches to endometrium:
  • Initially loose, becomes stronger as it burrows in.
• Endometrium becomes decidua after implantation:
  • Decidua Basalis: Area between embryo and stratum basalis.
  • Decidua Capsularis: Area between embryo and uterine cavity.
  • Decidua Parietalis: Remaining area.

Development of the Embryo

• By end of Day 8, blastocyst embeds in endometrium:
  • Composed of trophoblast (outer) and embryoblast (inner).
• Trophoblast differentiates into:
  • Cytotrophoblast (inner layer)
  • Syncytiotrophoblast (outer layer).
• Embryoblast forms bilaminar embryonic disc (hypoblast and epiblast).
• By Day 9, blastocyst fully embedded; lacunae form in syncytiotrophoblast.

Vascularization and Nutrient Exchange

• Lacunar networks formed by Day 12.
• Maternal blood and secretions flow into lacunae, establishing uteroplacental circulation.
• Appearance of extra embryonic mesoderm and chorionic cavity by Day 13.

Gastrulation

• Begins around Day 15:
  • Formation of primitive streak, defining body axes.
  • Cells migrate inwards (invagination) to form:
    • Definitive Endoderm (replaces hypoblast)
    • Mesoderm (between ectoderm and endoderm).
• Ectoderm remains as outer layer.

Neurulation

• Formation of neural plate at cranial end of embryo.
• Neural plate folds to become neural tube (precursor to CNS).
• Neural crest cells form peripheral nervous system.

Embryonic Folding

• Occurs in the fourth week, changing embryo shape from flat to cylindrical.
• Establishes foregut, midgut, and hindgut:
  • Foregut: Closes temporarily but opens to form mouth.
  • Midgut: Connects to yolk sac via vitelline duct.
  • Hindgut: Temporarily closed but opens for urogenital and anal openings.
• Major body plan established; germ layers differentiate into tissues and organ systems.