infolding

Infolding is the inward folding of a surface or sheet into its interior, producing invaginations, pockets, or enclosed compartments. The term covers processes in biology, geology, and materials science where a once external surface bends inward rather than outward.

In biology, infolding commonly refers to membrane curvature that leads to vesicle formation. In cells, the plasma membrane infolds to form endocytic vesicles during nutrient uptake and receptor internalization. The process is driven by cytoskeletal forces and membrane-bending proteins that shape curvature and help pinch off vesicles. Infoldings also occur in organelles, such as the inward folds of mitochondrial inner membranes (cristae) and thylakoid networks, which increase surface area for energy conversion and other metabolic processes.

Developmentally, infolding is a mechanism for shaping tissues. Epithelial sheets can invaginate to form glands, ducts, or neural structures, a morphogenetic process that contributes to organ formation and body plan organization.

In geology and materials science, infolding describes inward bending of layers or surfaces under stress. In rocks, compression can create trough-like folds or complex infolded structures; in engineered materials, surface infolding can influence porosity, strength, and transport properties.

Overall, infolding contrasts with outward folding or evagination and plays a key role wherever inward curvature and compartmentalization are functionally important. See also invagination, endocytosis, cristae, and morphogenesis.