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Animal Development

Overview of Animal Development in Deuterostomes

  • Organisms Examined: Seastar (Starfish), Frog, Chicken

  • Common Reproductive Process: Sexual reproduction following a generalized sexual life cycle.

Gamete Production

  • Process: Gametes are produced through meiosis.

    • Male Gamete: Microgamete (sperm)

    • Female Gamete: Macrogamete (ovum)

  • Fertilization: Fusion of sperm and ovum forming a zygote (diploid cell).

Cleavage Stage

  • Definition: A series of mitotic divisions of the zygote.

  • Process:

    • Zygote divides into 2 cells, then 4, then 8, continuing this pattern.

    • Eventually forms a morula (32 or 64 cells).

  • Blastula Formation: The morula develops into a ball (blastula) that often has a fluid-filled cavity called the blastocoel.

Gastrulation

  • Definition: Complex cell movements post-blastula formation that shape the developing organism.

  • Blastopore: The opening where cells migrate, leading to the formation of the archenteron (primitive gut).

    • In deuterostomes, this opening will ultimately develop into the anus.

  • Germ Layers Formation:

    • Endoderm: Inner layer lining the archenteron.

    • Ectoderm: Outer protective layer covering the embryo.

    • Mesoderm (in non-Porifera and non-Cnidaria phyla): Formed by:

    • Proliferation of the mesentoblast cell (in protostomes)

    • Outpocketing of the archenteron (in deuterostomes)

Tissue Formation by Germ Layers

  • Ectoderm: Brain, spinal cord, connective tissue, muscle in head, special sense organs.

  • Mesoderm: Skeletal muscle, skeleton, kidneys.

  • Endoderm: Digestive glands, epithelia of the respiratory tract, thyroid gland.

Neurulation

  • Definition: A subsequent process following gastrulation where the nervous system begins to form.

  • Process:

    • The ectodermal cell sheet above archenteron thickens and folds into the neural groove.

    • Closure of the neural groove forms the neural tube, which will develop into the brain and spinal cord.