Cell adhesion molecules
Cell-to-cell binding largely depends on transmembrane glycoproteins. These molecules are responsible for aggregation and are divided into several basic classes:
- Cadherins. These are integral fibrillar membrane proteins. Their main function is to ensure the adhesion of homogeneous cells, and this process is calcium-dependent (requires Ca2+ ions). Cadherins are localized in large quantities in the epidermis, trophoblast and placental cells, as well as in muscle fibers, cardiomyocytes, and neurons.
- Selectins. These are also integral plasma membrane proteins. They mediate the adhesion of endothelial cells and the binding of circulating blood elements (platelets and leukocytes), which is crucial for the regulation of inflammatory responses and hemostasis.
- Integrins. This class of molecules primarily mediates the connection of cellular structures with the extracellular matrix (various substrates), but at the same time they can also participate in direct intercellular adhesion.
- Neural cell adhesion molecules (N-CAM). They belong to the extensive immunoglobulin superfamily. Their main role is the formation of contacts between neurons and direct participation in the formation of synaptic connections.
Simple intercellular junctions
In addition to complex protein complexes, cells are held together by simple junctions.
The main characteristic of such a junction is the presence of a gap between the plasmalemmas about 20 nm wide. This space is not empty; it is densely filled with glycocalyx. The main function of a simple junction is to maintain the membranes at a strictly defined distance from each other. This allows preserving a free intercellular space through which the transport of various substances can be carried out.
The following morphological variants of simple junctions are distinguished:
- Interdigitated junction. Cell membranes form multiple protrusions resembling teeth that wedge into each other.
- Finger-like junction (interdigitation). It appears as deeply interlocking outgrowths of the membranes of adjacent cells, which significantly increases their contact area.
Complex specialized junctions
In vertebrates, specialized intercellular junctions are divided into three key functional groups:
- Anchoring (adherens) junctions. The main task of these structures is to provide high mechanical strength to tissues. This group includes adhesion junctions (intermediate junctions) and desmosomes. A desmosome looks like a structure with distinct protein plaques, to which cytoskeletal filaments approach from the inside of the cell, securely fixing the contact.
- Tight (occluding or zonula occludens) junctions. They create a reliable permeability barrier. As a rule, such structures are located in the upper part of the cell boundary and literally "stitch" the plasmalemmas of adjacent cells together, leaving no space for the free flow of fluid.
- Gap (communicating) junctions, or nexuses. They provide direct ionic and metabolic exchange. The structural units of a gap junction are specialized protein complexes — connexons. They form microscopic channels between the cytoplasms of adjacent cells, usually located in the lower part of the intercellular boundary.
Special signaling junctions
Certain types of intercellular junctions are grouped exclusively by their functional feature—the ability to carry out targeted signal transmission. These include:
- Chemical synapses. Characteristic of the nervous system and ensure the transmission of a nerve impulse with the participation of neurotransmitters.
- Immunological synapses. Formed during the immune response, in particular between antigen-presenting cells and T lymphocytes, for the recognition of foreign agents.
Briefly: answers to topical questions
Which specific proteins make up a desmosome?
A desmosome consists of specific proteins that form its intracellular and intercellular parts.
- Desmoplakins — form the cytoplasmic plaque (thickening) on the inner side of the membrane.
- Desmogleins — adhesion molecules located in the expanded intercellular space.
- Cadherins — transmembrane proteins that adhere to each other in the intercellular cleft.
- Anchoring proteins — connect elements of the cytoskeleton to the intracellular part of cadherins.
Which cytoskeletal filaments approach desmosomes, and which approach adhesion junctions?
Different types of cellular cytoskeleton elements approach desmosomes and adhesion junctions.
| Junction type | Cytoskeletal filaments | Features |
|---|---|---|
| Desmosomes | Intermediate filaments | In the epithelium, they have a tissue-specific composition and are represented by keratin |
| Adhesion junctions | Thin filaments (microfilaments) | Formed by the protein actin and attached to vinculin |
What individual proteins make up connexons?
Connexons are protein complexes that serve as the structural units of gap junctions.
- Connexins — transmembrane integral proteins from which this complex is built.
Each connexon is formed from six connexin globules penetrating the cell membrane. These proteins surround a central hydrophilic pore with a diameter of 1.5 nm, creating a channel for the exchange of ions and small water-soluble molecules between adjacent cells.
Tight junctions are usually located at the very top of cells, as if "sewing" their membranes together; anchor junctions (such as desmosomes) are slightly lower, and gap communicating junctions with their connexon channels are located even lower.
Class and exam questions
What is the purpose of a simple intercellular junction?
It is necessary to keep the cell membranes at a distance of about 20 nm. This preserves a free intercellular space filled with glycocalyx, which allows for the transport of substances.
Which structure is responsible for attaching the cell to the extracellular matrix?
The connection of cells with the extracellular matrix (substrates) is carried out by specialized transmembrane glycoproteins called integrins.
What are connexons and where are they located?
Connexons are protein complexes that are the main structural units of gap junctions (nexuses). They form channels for ionic and metabolic exchange between cells.
What else to study on this topic
- Structural organization and protein composition of desmosomes
- Mechanisms of Ca2+-dependent cell aggregation by cadherins
- The role of selectins in inflammation and hemostasis
- Structure and functioning of chemical synapses
- Differences between T-lymphocyte immunological synapses and neural contacts
Master the entire topic in the "Biology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Intercellular junctions" and 3,793 other pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 129 video lessons on the subject
Other topics in the section "The Cell — The Elementary Unit of Life"
Free video lessons: BiologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.