germlayer

Germ layers are the primary cell layers formed during early embryonic development in animals, from which all tissues and organs arise. In triploblastic animals, three distinct germ layers develop: the ectoderm, mesoderm, and endoderm. Some primitive animals, such as cnidarians and ctenophores, are diploblastic, possessing only an outer ectoderm and an inner endoderm.

During gastrulation, cells reorganize and differentiate to form these layers. The ectoderm typically gives rise to the epidermis and nervous system; the endoderm lines the gut and forms associated organs; the mesoderm develops into muscles, skeleton, circulatory and urogenital systems, and various connective tissues. The three layers interact to form the body's organ systems.

Neural crest cells in vertebrates are a notable cell population that originates from the ectoderm and contributes to diverse structures, and some scientists describe this population as an additional, fourth germ layer, though it is not universally recognized as a primary germ layer.

Understanding germ layers is essential in developmental biology and medicine because disruptions in layer formation can lead to congenital abnormalities and provide clues about evolutionary biology.

See also: embryology, gastrulation, neurulation.