Embryology: Types of Eggs

Embryology: Types of Eggs

• Animal eggs contain nutrients (protein, lipid, glycogen) to nourish the embryo, collectively called yolk.
• Yolk proteins are produced in the liver (vertebrates) or fat bodies (insects).
• Eggs are classified by yolk presence/absence and quantity:

Classification by Yolk Amount

• Alecithal: No yolk, found in placental mammals.
• Oligolecithal: Small amount of yolk, found in Echinoderms and Amphioxus.
• Mesolecithal: Moderate amount of yolk, found in toads.
• Macrolecithal: Large amount of yolk, found in many species.

Classification by Yolk Distribution

• Isolecithal: Yolk distributed evenly throughout the cytoplasm, found in fishes.
• Centrolecithal: Yolk concentrated in the center of the egg, found in insects and arthropods.
• Telolecithal: Yolk concentrated at one end (pole) of the egg, found in some fishes and toads.

Zygote Formation

• A fertilized egg (zygote) develops into an embryo after the first cleavage (division).
• The zygote forms a diploid multicellular organism via repeated mitotic divisions and cellular differentiation.

Cleavage

• Cleavage is the initial mitotic divisions in a fertilized egg.
• The pole with most yolk is called the vegetal pole, the opposite the animal pole.
• Cleavage initiates by furrows (grooves) that start at a single point on the egg.
• A cleavage furrow forms and deepens, eventually separating the egg into blastomeres.

Yolk Influence on Cleavage

• Yolk affects cleavage rate and pattern.
• Determinate cleavage: The fate of cells is determined early in development; each blastomere may not form a whole embryo.
• Indeterminate cleavage: Each blastomere can develop into a complete organism.

Planes of Cleavage

• Cleavage occurs in specific planes:
  • Meridional: Passes through the animal-vegetal axis, bisecting both poles.
  • Vertical: Passes from the animal to vegetal pole, not through the egg's center.
  • Equatorial: Passes at right angles to the main axis, at the midpoint between animal and vegetal poles.
  • Latitudinal/horizontal: Similar to equatorial but courses through the cytoplasm on either side.

Types of Cleavage Patterns

• Holoblastic: Complete divisions of the egg.
  • Equal holoblastic: Blastomeres are roughly equal in size.
  • Unequal holoblastic: Blastomeres uneven in size due to yolk concentration.
• Meroblastic: Partial divisions, where only part of the egg divides.
  • Discoidal meroblastic: A disc-shaped region of the egg (blastodisc) divides, while the yolk remains undivided. Found in reptiles and birds.
  • Superficial meroblastic: Nuclei migrate to the periphery, and cleavage occurs only in the peripheral cytoplasm. Found in insects.

Explore the fascinating world of embryology with this quiz on the different types of eggs based on yolk presence, amount, and distribution. Learn about alecithal, oligolecithal, mesolecithal, and macrolecithal eggs, as well as their developmental significance in various species.