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How do cell movements during gastrulation establish the three primary germ layers?
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Animal development
The embryos of many animals appear similar to one another in the earliest stages of development and progress into their specialized forms in later stages.
Gastrulation is a critical phase in animal development during which a single-layered blastula is reorganized into a structure containing three primary germ layers: the ectoderm, mesoderm, and endoderm.1
These layers are established through complex cell movements that vary by species:
- Invagination and Epiboly: In many animals, cells move inward (invagination) to form a cup-shaped gastrula. In amphibians, this process is assisted by epiboly, where the ectoderm thins and expands to cover the interior layers.2
- Migration via the Primitive Streak: In higher vertebrates like birds and mammals, cells of the epiblast migrate through an elongated structure called the primitive streak. Cells moving through this seam turn downward to form the mesoderm and endoderm, while those remaining on the surface become the ectoderm.34
- Mesoderm Formation: The mesoderm may form through the splitting of cells from an endomesodermal mass or via outpocketings of the archenteron (primitive gut), eventually positioning itself between the ectoderm and endoderm.12
By the end of these movements, the germ layers are positioned to serve as the foundation for organogenesis.14