Conditions and Participants of Interaction
The formation of any junction relies on the interaction of adhesive membrane proteins from neighboring cells. The primary prerequisite for their formation is prolonged physical contact.
Junctions can form between different types of cells:
- Homotypic: forming uniform sheets in epithelial and certain muscular tissues.
- Heterotypic: forming 'nurse-pupil' interaction systems (characteristic of the testes and thymus).
- Neuronal: specialized connections between nerve cell processes, as well as between nerve fibers and muscles.
Functional Classification
All intercellular connections are conventionally divided into three main functional groups:
- Mechanical junctions: provide strong cell cohesion (including simple and anchoring junction types).
- Occluding junctions: create a barrier that completely separates environments on opposite sides of a cellular sheet.
- Communicating junctions: designed for the direct exchange of molecules or the transmission of nerve impulses.
Mechanical Junctions: Simple and Anchoring
Simple junctions lack complex specialized structures. The plasma membranes are separated by a distance of 15–20 nm, and adhesion is mediated by glycoprotein macromolecules (cadherins, integrins). A specific variant of this contact is interdigitations. These are characteristic 'finger-like' evaginations where the membrane of one cell invaginates into the cytoplasm of another to maximize the contact surface area.
Complex anchoring junctions include:
- Desmosome (Macula adherens): a small, circular spot of adhesion. Intracellularly, the membrane is thickened by a layer of desmoplakins, to which bundles of intermediate filaments (e.g., keratin in epithelia) attach. In the intercellular cleft, the two halves of the junction are linked by the adhesive protein desmoglein.
- Hemidesmosome: located on the basal surface of the cell. It is structurally similar to half of a desmosome, but anchors the cell not to neighboring cells, but to an extracellular structure—the basement membrane.
- Adherens junction (Zonula adherens): shaped like a belt. Unlike the punctate desmosome, the protein vinculin accumulates beneath the membrane here, serving as an attachment site for thin actin filaments.
Barrier and Exchange Connections
Tight junction (Zonula occludens) belongs to the occluding type. Cell membranes are tightly 'stitched' together by specialized proteins, forming a reticular meshwork on the surface. This junction completely seals the intercellular space and prevents the diffusion of substances.
For the exchange of substances, communicating junctions exist:
- Gap junctions (nexuses, gap junctions): regions with a diameter of 0.5–3 µm where membranes are apposed as closely as 2 nm. They are pierced by hollow protein channels composed of two docked halves—connexons (each projecting 1–1.5 nm into the intercellular gap). Gap junctions provide metabolic and electrical coupling by allowing ions, sugars, and amino acids to pass through.
- Synapses: chemical or electrical junctions for signal transmission. A classic chemical synapse consists of a presynaptic membrane, a synaptic cleft, and a postsynaptic membrane with receptors. The signal is transmitted using a specialized agent—a neurotransmitter.