Invagination means to fold inward or to sheath.

Invagination, in biology, can refer to a number of processes.

(1) Invagination is the morphogenetic processes by which an embryo takes form, and is the initial step of gastrulation, the massive reorganization of the embryo from a simple spherical ball of cells, the blastula, into a multi-layered organism, with differentiated germ layers: endoderm, mesoderm, and ectoderm. More localized invaginations also occur later in embryonic development, to form coelem, etc.

(2) Invagination is the formation of a cleavage furrow during cytokinesis in animal cells.

(3) Invagination in some bacteria (also called a mesosome) is a tightly-folded region of the cell membrane containing all the membrane-bound proteins required for respiration and photosynthesis. It can also be associated with the nucleoid.

Developmental biology | Cell biology