Germ layers

Article By:

Shook, David R. Department of Biology, University of Virginia, Charlottesville, Virginia.

Last reviewed:January 2020

DOI:https://doi.org/10.1036/1097-8542.288000

Distinct, coherent tissue layers in the early embryo of all multicelled animals, found after the major reorganization of cells that occurs during gastrulation. The germ layers are a fundamental aspect of the structure of the embryo at the end of this period of development and reflect one of the earliest steps of differentiation toward adult tissue types. The germ layers include ectoderm, which forms the outer surface of the embryo; endoderm, which forms the inner surface lining the gut cavity; and mesoderm, which forms a layer in between the other two in most groups of animals (phyla). Remarkably, each germ layer in even distantly related species tends to form a similar set of differentiated tissue types in the adult: ectoderm gives rise to tissues including the epidermis, nervous system, and sensory organs; endoderm gives rise to tissues including the inner lining of the respiratory and digestive tracts and accessory digestive organs; and mesoderm gives rise to tissues including muscles, bone, connective tissue, vascular tissue and blood, and some internal organs such as excretory organs and somatic gonads. See also: Developmental biology; Embryogenesis; Embryonic differentiation; Gastrulation

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