Early Embryogenesis

Zygote Formation

• Zona Pellucida: thick outer glycoprotein coat around the egg cell.
• Plasma Membrane: just inside the zona pellucida.
• Sperm Penetration: Sperm merges its genetic material with the egg's, forming a zygote.

Cleavage

• Definition: Splitting without growth.
• Process: Divides zygote into 2, 4, 8, 16, and 32 cells within the zona pellucida.
• New Name: At 32 cells, the zygote is now called a morula (Greek for mulberry).

Compaction and Differentiation

• Compaction: Cells in the morula get closer together.
• Differentiation: Cells start to become different from one another.
  • Trophoblasts: Outer cells.
  • Embryoblasts: Inner cells.

Blastulation

• Blastocyst Formation: Embryoblasts cluster, forming a cavity called the blastocoel.
• Zona Pellucida Disintegration: Zona pellucida starts to fall away.
• Amniotic Cavity Formation: New cavity forms within the embryoblasts.
  • Hypoblasts: Lower layer of inner cell mass.
  • Epiblasts: Upper layer of inner cell mass.
• Bilaminar Disk: Formed by epiblasts and hypoblasts.

Gastrulation

• Primitive Streak: Formation on the bilaminar disk, marking the start of gastrulation.
• Cell Migration: Epiblast cells migrate and differentiate into three layers:
  • Ectoderm: Outer layer.
  • Mesoderm: Middle layer.
  • Endoderm: Lower layer.

Neurulation

• Notochord Formation: Differentiation of mesoderm cells to form the notochord.
• Neural Plate: Thickening of ectoderm above the notochord.
• Neural Tube: Neural plate cells dive into the mesoderm, forming the neural tube.
• Neural Crest Cells: Cells break off from the ectoderm during neural tube formation, going into the mesoderm to differentiate.

Summary

• From zygote -> Cleavage -> Morula -> Blastocyst -> Bilaminar Disk -> Gastrulation -> Three Germ Layers -> Neurulation.
• Key structures: Zona pellucida, trophoblasts, embryoblasts, blastocoel, amniotic cavity, hypoblasts, epiblasts, primitive streak, ectoderm, mesoderm, endoderm, notochord, neural plate, neural tube, neural crest cells.