Evagination

Definition

The outward folding, protrusion, or ballooning of a tissue layer to form a pouch, sac, or diverticulum; the morphological opposite of invagination. In embryology and organogenesis, evagination describes how an epithelial sheet extends outward from a body wall or cavity to generate new structures without breaking continuity with the original layer.

Etymology

From Latin evaginare, to unsheathe or turn outward (ex- 'out' + vagina 'sheath')

Example

In insect development, the imaginal discs of holometabolous larvae arise as epithelial evaginations that later invaginate and develop into adult appendages and body wall; similarly, certain pheromone-producing glands in moths form by evagination of the integument.

Synonyms

• protrusion
• outpocketing

Related Terms

• invagination
• Diverticulum
• Imaginal disc
• organogenesis
• epithelium
• Morphogenesis

Usage Notes

Distinguished from invagination (inward folding) and evagination is not synonymous with eversion (turning inside out of an existing structure). In arthropod morphology, evagination often refers specifically to the initial outward growth of ectodermal or endodermal tissue that will later differentiate into specialized organs or appendages.