Embryonic Development of Birds

Early Development of Birds

  • Key Stages of Early Development:
    • Cleavage: The division of the fertilized egg.
    • Gastrulation: The process where the three germ layers (ectoderm, mesoderm, endoderm) form.
    • Differentiation of Germ Layers: The specialization of cells into distinct tissues.
    • Extraembryonic Membranes: Structures outside the embryo that support development.

Amniotic Egg

  • Definition: An egg with a shell and extraembryonic membranes.
  • Amniotes: Group that includes birds, reptiles, and mammals.
  • Functions of Extraembryonic Membranes:
    • Provide fluid and nourishment.
    • Protection from desiccation and predation.
    • Cushioning for the developing embryo.

Oogenesis in Birds

  • Telolecithal Secondary Oocyte: Characterized by dense yolk at one end.
  • Ovulation Frequency: Occurs once a day for each clutch.
  • Fertilization: Takes place approximately 30 minutes after ovulation.
    • Albumen Addition: Occurs within hours in the magnum.
    • Shell Formation: Takes about 20 hours in the uterus.
  • Embryo Stage at Laying: At 24 hours post-fertilization, embryo is at the blastula stage, beginning gastrulation.

Cleavage and Blastoderm Formation

  • Meroblastic Cleavage: Occurs within the reproductive tract, forming a blastodisc.
    • Cleavage begins hours after fertilization.
    • A flat disc of blastomeres develops over the subgerminal cavity.
    • The blastoderm gains polarity and differentiates into epiblast and hypoblast.
  • Koller's Sickle: A signaling center at the posterior of the blastoderm, important for establishing body axes and determining embryo polarity.

Delamination and Gastrulation

  • Delamination Process: A layer of cells separates, with migration of cells anteriorly from the posterior marginal zone where Koller’s sickle is located.
  • Formation of Germ Layers:
    • Epiblast: Becomes the embryo.
    • Hypoblast: Contributes to extraembryonic membranes.
  • Blastocoel: The cavity formed between epiblast and hypoblast; no gray crescent or archenteron is formed during this process.

Primitive Streak Formation

  • Initiation: Gastrulation begins with primitive streak formation governed by multiple gene families including WNT, BMP, and FGF.
  • Cell Movement Types During Gastrulation:
    • Invagination: Cells move inward.
    • Epiboly: Cells spread to cover the surface.
    • Involution: Cells roll inward.
    • Ingression: Cells detach and move into the interior.
  • Hensen’s Node: Functions as the organizer in the embryo similar to the Spemann organizer in frogs.

Trilaminar Embryo Development

  • Three Germ Layers Created:
    • Ectoderm: Dorsal layer (including neurectoderm).
    • Mesoderm: Middle layer.
    • Endoderm: Ventral layer, located just dorsal to the hypoblast.
  • Notochord Formation: Guides axial skeleton development and induces neurectoderm formation.
  • Mesoderm Differentiation: Leads to formation of somites, lateral plate mesoderm, and intermediate mesoderm responsible for various body systems.

Extraembryonic Membranes

  • Development of Extraembryonic Membranes: Include amnion, yolk sac, allantois, and chorion.
    • Amnion: Protective sac enclosing the embryo.
    • Yolk Sac: Provides early nutrition and later incorporated into gut.
    • Allantois: Involved in waste storage and gas exchange, contributes to the placenta in mammals.
    • Chorion: Functions in gas exchange, surrounds the yolk sac.
  • Role of Extraembryonic Membranes in Amniotic Egg: Support and protect developing embryo, facilitate nutrition, and remove waste.