Evagination

Evagination is the outward protrusion or budding of a tissue or organ surface, producing a hollow projection such as a duct, gland, or vesicle. In developmental biology, it refers to the outward expansion of an epithelial sheet or lining, in contrast to invagination, which forms by inward folding. Evagination contributes to the formation of various hollow structures and luminal spaces during organogenesis.

The process is driven by coordinated cellular behaviors, including localized cell proliferation, changes in cell shape and polarity, cell migration, and remodeling of the extracellular matrix. Signaling pathways that regulate tissue growth and patterning—such as those involved in fibroblast growth factor, Wnt, and Sonic hedgehog signaling—often guide where and when evagination occurs. Mechanical forces generated by the cytoskeleton and by interactions with surrounding tissues also play a key role in shaping outward protrusions.

Contexts and examples vary by organ system. Evagination can generate ducts or vesicles during embryonic development, contributing to the architecture of glands and other hollow structures. In postembryonic or pathological contexts, evagination can manifest as outpouchings of mucosa through muscular layers, known as diverticula, as seen in Meckel’s diverticulum of the ileum or colonic diverticula.

See also: invagination, diverticulum, morphogenesis.