invagination

Invagination is the inward folding or infolding of a sheet of cells or tissue, forming a pocket, pouch, or tube. It is a common morphogenetic mechanism in embryonic development and a mode of membrane remodeling in cells. The term derives from Latin invaginare.

In embryology, invagination is a key step of gastrulation. For example, in amphibians the dorsal lip of the blastopore invaginates to form the archenteron, the primitive gut. Similar inward movements of epithelial sheets occur in many species, contributing to the formation of the gut tube, neural structures, and organ primordia. Mechanistically, invagination is often driven by apical constriction and coordinated cell rearrangements regulated by signaling pathways.

In cell biology, invagination describes membrane infolding that generates vesicles or compartments. This includes endocytosis, such as clathrin-coated pits; caveolar endocytosis; and related processes that internalize extracellular material or reorganize membrane-bound compartments within the cell.

In medicine, intestinal invagination, or intussusception, is a condition in which a portion of intestine telescopes into an adjacent segment, potentially causing obstruction and ischemia. It most commonly affects children. Symptoms include intermittent abdominal pain, vomiting, and, in some cases, bloody stools. Diagnosis is typically via ultrasound, which may show a target or donut sign, and radiographs. Management ranges from nonoperative enema reduction in stable patients to surgical intervention when reduction fails or there are signs of perforation or necrosis.

Invagination is distinct from evagination, which refers to outward folding of tissue. The concept also appears in other organisms and contexts, with details varying by system.