Conditions and participants of interaction
The basis for the formation of any junction is the interaction of adhesive membrane proteins of neighboring cells. The main condition for their formation is prolonged physical contact.
Junctions can occur between different cells:
- Identical cells: forming unified sheets in epithelial and certain muscle tissues.
- Dissimilar cells: forming "feeder-nurse" interaction systems (typical of the testes and thymus).
- Neuronal cells: specialized connections between the processes of nerve cells, as well as between nerve fibers and muscles.
Functional classification
All intercellular connections are conventionally divided into three main functional groups:
- Mechanical junctions: provide firm cell adhesion (include simple and anchoring types of connections).
- Occluding junctions: create a barrier that completely separates environments on opposite sides of the cell sheet.
- Communicating junctions: designed for direct molecular exchange or transmission of nerve impulses.
Mechanical junctions: simple and anchoring
Simple junctions lack complex specialized structures. The plasmalemmas are located at a distance of 15–20 nm, and adhesion is provided by glycoprotein macromolecules (cadherins, integrins). A special case of such a junction is interdigitations. These are peculiar "finger-like" protrusions where the membrane of one cell invaginates into the cytoplasm of another to maximize the contact area.
Complex anchoring junctions include:
- Desmosome (Macula adherens): a small, rounded adhesion plaque. Inside the cell, the membrane is thickened by a layer of desmoplakin proteins, which are approached by bundles of intermediate filaments (e.g., keratin in epithelium). 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 neighbors, but to a non-cellular structure — the basal lamina.
- Adherens junction (Zonula adherens): ribbon-shaped. Unlike the spot desmosome, here the protein vinculin accumulates beneath the membrane, to which thin actin filaments attach.
Barrier and exchange junctions
Tight junction (Zonula occludens) belongs to the occluding type. Cell membranes are tightly "stitched" together by specialized proteins, forming a meshwork on the surface. This junction completely seals the intercellular space and prevents substances from diffusing.
For substance exchange, there are communicating junctions:
- Gap junctions (nexuses, gap-junctions): areas with a diameter of 0.5–3 µm, where membranes are apposed down to 2 nm. They are pierced by hollow protein channels consisting of two docking halves — connexons (each protruding into the cleft by 1–1.5 nm). Nexuses 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 via a special agent — a neurotransmitter.
Briefly: answers to topical questions
Which structural proteins form tight junctions (Zonula occludens)?
Tight junctions (zonula occludens) are formed by specialized proteins that ensure the tight apposition of the plasmalemmas of neighboring cells and seal the intercellular space.
The molecular composition includes:
- Occludins.
- Claudins.
- Other proteins.
In specialized tight junctions between Sertoli cells, at least 50 rows of protein molecules participate in forming the junction, ensuring high impermeability.
Which proteins make up the molecular composition of a desmosome?
The molecular composition of a desmosome (macula adherens) includes intracellular, cytoskeletal, and intercellular proteins.
Main structural components:
- Desmoplakins — form the plasmalemma thickening (cytoplasmic plaque) on the intracellular side.
- Keratin — a tissue-specific intermediate filament protein (in epithelial tissue), bundles of which approach the desmoplakin layer to distribute mechanical stress.
- Desmogleins — adhesive anchoring proteins located in the expanded intercellular space, connecting the two halves of the junction.
How not to mix up the proteins of anchoring junctions: Desmoplakins work in the Desmosome (letter "D"), while Actin works in the Adherens junction (letter "A").
Class and exam questions
What is the main difference between a desmosome and an adherens junction?
The differences lie in their shape and molecular composition. A desmosome is a punctate "spot" with desmoplakins and intermediate filaments, whereas an adherens junction is a continuous ribbon around the cell with vinculin and actin filaments.
How is the function of gap junctions (nexuses) regulated?
The permeability of connexon channels depends on the concentration of calcium ions. When Ca²⁺ levels rise, the conformation of the proteins changes, and the channel lumen is securely closed.
Where in the body can hemidesmosomes be found?
Hemidesmosomes are localized exclusively on the basal surface of cells. Their main task is anchoring the cell sheet to the basal lamina.
What else to study on this topic
- Molecular structure of the connexon in a gap junction
- Specifics of tissue intermediate filaments (keratins, etc.)
- Mechanism of neurotransmitter release at the synapse
- Adhesive glycoproteins of simple junctions: integrins and cadherins
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