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Embryonic Development

In animals, one can usually distinguish 4 stages of embryonic development.

  • Cleavage
  • Patterning
  • Differentiation
  • Growth


Cleavage

Mitosis and cytokinesis of the zygote, an unusually large cell, produces an increasing number of smaller cells, each with an exact copy of the genome present in the zygote. However, the genes of the zygote are not expressed at first. The activities of cleavage are controlled by the mother's genome; that is, by mRNAs and proteins she deposited in the unfertilized egg. Cleavage ends with the formation of a blastula.



Patterning

During this phase, the cells produced by cleavage organize themselves in layers and masses, a process called gastrulation. The pattern of the future animal appears:

  • front and rear (the anterior-posterior axis)
  • back side and belly side (its dorsal-ventral axis)
  • left and right sides.


There is little visible differentiation of the cells in the various layers, but probes for cell-specific proteins reveal that different groups of cells have already started on specific paths of future development.


Gastrulation forms three major "germ layers": ectoderm, mesoderm, and endoderm. By gastrulation, the genes of the zygote genome are being expressed



Differentiation

In time, the cells of the embryo differentiate to form the specialized structures and functions that they will have in the adult. They form neurons, blood cells, skin cells, muscle cells, etc., etc. These are organized into tissues, the tissues into organs, the organs into systems.



Growth

After all the systems are formed, most animals go through a period of growth. Growth occurs by the formation of new cells and more extracellular matrix.
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