Cells are often found in sheets called epithelia, in which cells seal one surface from the other via tight junctions, connect to each other via gap junctions, adhere to each other using desmosomes, and adhere to the basal lamina using hemidesmosomes.
Vertebrate body surfaces are covered by cells arranged into sheets. Epithelium (plural, epithelia) is a tissue made of a sheet of cells that covers the surface of the body as well as the outside and inside of many internal organs. An epithelium may be simple, consisting of a single layer of cells that sit next to one another, or stratified, consisting of cells packed on top of one another in a thick layer. Most epithelia have a basal surface that attaches to connective tissue via a layer of extracellular matrix called the basal lamina and have an apical surface that faces air or fluid.
Cells connect to one another at cell junctions. In epithelium, the junction is usually a tight junction. A tight junction is a protein junction that connects the plasma membranes of two cells, preventing molecules from passing through the space between cells. A tight junction seals the basal face of the epithelium from the apical face. The junction is mediated by proteins that connect the cells. In a tight junction, the proteins are called claudins and occludins. They lie in strands along the tight junction, sealing off and preventing the fluid on the apical surface from moving to the basal surface without being transported through the cells.
A gap junction is a structure that connects adjacent cells via protein channels, allowing the direct movement of small molecules and ions from the cytoplasm of one cell to the cytoplasm of the next cell. A protein complex called a connexon extends from one cell and touches the connexon of the adjacent cell. These complexes contain a channel that opens and closes based on external stimuli, allowing ions to pass freely from one cell to the next or stop as required. Gap junctions are vital in tissues containing cells that must respond as a single unit to stimuli, such as muscle cells in the heart that must beat in sync.
Junctions are also required to bind epithelial cells to one another and to the basal lamina. A desmosome is a cell junction that connects cells via specialized proteins and helps maintain tissue strength. Desmosomes attach proteins to keratin filaments inside the cell via an attachment plaque made of protein. The attachment proteins, which span the cell membrane, bind to one another in the intercellular space.
A hemidesmosome is a junction complex that binds epithelial cells to the basal lamina (border between epithelium and connective tissue). A hemidesmosome is very similar to a desmosome, except that it uses a different protein to bind and secure the epithelial tissue to the basal lamina.