Lecture 5a: Egg Types and Cleavage Patterns

Amniotic vs. Anamniotic Eggs

• Amniotes: Reptiles, birds, and mammals.
  • Characterized by an amniotic membrane surrounding the embryo, providing a fluid-filled sac that prevents desiccation.
  • Enables transition from aquatic to terrestrial habitats.
• Anamniotes: Lack an amniotic membrane.
  • Includes many invertebrates, fish, and amphibians.
  • Requires moist or aquatic habitats to prevent embryo desiccation.

Amniotic Egg Structure

• Four Extra-Embryonic Membranes:
  1. Amnion: Encases embryo in a fluid-filled sac.
  2. Chorion: Outermost membrane, allows gas exchange; becomes part of the placenta in mammals.
  3. Allantois: Attached to the embryo, stores nitrogenous waste; becomes umbilical cord in mammals.
  4. Yolk Sac: Provides nutrients; initial site of blood cell formation in mammals.

Egg Types Based on Yolk

• Isolécithal Eggs:
  • Small amount of yolk, evenly distributed.
  • Example: Echinoderms like starfish and sea urchins.
• Mesolecithal Eggs:
  • Moderate yolk, concentrated at the vegetal pole.
  • Example: Amphibians like frogs.
• Telolecithal Eggs:
  • Large yolk, concentrated at the vegetal pole.
  • Example: Birds and reptiles.
• Centrolecithal Eggs:
  • Large yolk, centralized.
  • Example: Insects.

Cleavage Patterns

• Cleavage: Division of cells in the early embryo.
• Holoblastic Cleavage:
  • Complete division of the egg.
  • Applicable to eggs with little or moderate yolk (isolecithal and mesolecithal).
  • Types: Radial and Spiral.
• Meroblastic Cleavage:
  • Incomplete division due to large yolk.
  • Applicable to eggs with large yolk (telolecithal).
  • Cleavage only in the animal pole.

Radial vs. Spiral Cleavage

• Radial Cleavage:
  • Cells divide at right angles, forming neat rows; common in deuterostomes (e.g., vertebrates).
  • Indeterminate, allows for cells to separate and form twins.
• Spiral Cleavage:
  • Precise angle division, cells rotate; common in protostomes.
  • Determinate, cell fate is pre-determined.

Specific Examples

• Sea Urchin: Isolecithal egg; holoblastic and radial cleavage.
• Frog: Mesolecithal egg; holoblastic and radial cleavage.
• Chicken: Telolecithal egg; meroblastic and radial cleavage.

Mammalian Cleavage

• Rotational Cleavage:
  • A modification of radial cleavage found in mammals.
  • Involves divisions that rotate to form stacks and columns.

This summary covers the types and structures of eggs based on yolk distribution and cleavage patterns, highlighting differences between holoblastic and meroblastic cleavage as well as radial and spiral cleavage.