The extracellular matrix (on top):
From How do cells connect with one another?:
Animal cells are held together by several different things – mostly involving proteins and other molecules forming bridges from one cell to another.
There is something called the ‘extracellular matrix’ between most cells. This can take the form of a flat layer, called the basement membrane, or a thicker mass in which individual cells can float or move around. This matrix is made up of all sorts of things, mainly collagen, fibronectin, laminin, hyaluronic acid and others (you can look all these things up if you are unfamiliar with them).
Cells can stick to the extracellular matrix through various different types of ‘adhesion molecule’. Integrins are an important type of these. integrins are made up of 2 subunits, an alpha and a beta subunit, and there are several different forms of each subunit, so different combinations result in different types of integrins that are specific to different molecules within the extracellular matrix. For example, alpha5-beta1 binds to fibronectin, alpha2-beta1 binds to collagen, and alpha7-beta1 binds to laminin-1.
Different types of cells can have greater expression of specific types of integrins, which means they are able to bind more strongly to extracellular matrices that are particularly rich in a certain molecule, e.g. laminin-1 for muscle cells.
Other cell adhesion molecules include cadherins, the extracellular portions of which bind to other cadherins on adjacent cells, and the intracellular portions bind to the actin cytoskeleton inside a cell via other proteins that are involved in cell signalling (catenins). These can mediate transmission of signals from one cell to another.
Gap junctions can be visualised as rivets with a hole in the middle, clamping two adjacent cell membranes together while forming a gap or channel between teh cells that connects their cytoplasm. These junctions are made up of a protein, called a connexon, on each membrane, each of which consists of 6 subunits, or connexins. The connexins can be in ‘open’ or ‘closed’ formations, which opens or closes the channel.
Of course, adhesion between cells is very important – as you say, it stops us falling apart! it also allows for structure of tissues and organs, which is vital for their function. The junctions that mediate cell-cell adhesion often mediate communication, which again, is vital for the coordinated function of tissues and the body as a whole. Also, regulation of adhesion is important – think of wound healing. For a wound to close up and heal, cells need to be able to migrate to the wound edge. The immune system is dependent on cells moving round the body in order that they can detect foreign bodies. Inappropriate regulation of cell adhesion can cause disease – reduced adhesion in cancer cells allow them to migrate (metastasise) to distant sites in teh body.