In some types of cells, the cell membrane present differentiations that are essential for the specific functions of the cells. The major differentiations are the microvilli and the structures for reinforcement of adhesion or union among cells (cell junctions).
Microvilli are multiple external projections of the membrane resembling glove fingers. This differentiation is found in cells of tissues where it is advantageous to enhance the size of the surface in contact with the exterior, for example, in the enteric (intestinal) epithelium for absorption of nutrients.
Membrane differentiations for reinforcement of adhesion among cells happen mainly in epithelial tissues where the require for coverage and impermeability requires cells to be "glued" to neighbouring cells. These differentiations can be interdigitations, desmosomes, tight junctions (zonula occludens), zonula adherens (adherens junctions) and gap junctions.