invaginations

Invagination is the process by which a sheet or layer of cells or tissue folds inward to form a pouch, cavity, or vesicle. The term comes from Latin invaginare, meaning to bend inward. It is used across biological disciplines to describe inward folding as opposed to evagination, an outward protrusion.

In embryology, invagination is central to early development. During gastrulation, certain cell layers sink inward to form the primitive gut and germ layers. The neural plate also invaginates to create the neural groove and, ultimately, the neural tube, shaping the nervous system. Invagination can occur in various organisms and tissues to compartmentalize functions.

In cell and molecular biology, invagination refers to inward bending of cellular membranes. Endocytosis involves membrane invagination to internalize extracellular material, while vesicle formation relies on cytoskeletal and membrane remodeling. In mitochondria, the inner membrane forms invaginations that increase surface area, giving rise to cristae that host respiratory complexes.

Clinically, invagination has pathological usage. Intussusception describes telescoping of a segment of intestine into an adjacent segment, a condition requiring medical evaluation. Invaginations can also refer to glandular or mucosal indentations seen in histology or pathological states, where tissue folds create pockets or crypts.