In biology and medicine, the term ‘bridge’ is often used to refer to a structural or functional support, connection, or organisational framework. In the human body, there are a variety of bridge-like structures that play key roles at the cellular, tissue or organ level. The following are the main ‘bridge’ structures in the body and their functions: 

1. Cytoskeleton 

Functions: Maintains cellular morphology, provides mechanical support, and participates in cellular motility and transport of materials.

Components: 

Microtubules: the ‘highway’ inside the cell, supporting the cell and participating in mitosis.

Microfilaments (actin): involved in cell contraction and migration (e.g. muscle contraction).

Intermediate Filaments: increase the tensile strength of cells (e.g. keratin supports skin cells).

Analogy: similar to the reinforcement framework of a building, maintaining the stability of the cell structure.

2. Extracellular Matrix (ECM) 

Function: Provides physical support for tissues and organs and regulates cell behaviour (e.g. proliferation, differentiation).

Main components: 

Collagen (Collagen): the most abundant protein in the body, forms fibrous networks (e.g. skin, bone, tendon).

Elastin: gives tissues their elasticity (e.g. blood vessels, lungs).

Proteoglycans: fill spaces, keep tissues hydrated and resistant to compression (e.g. cartilage).

Analogy: similar to reinforcement and cement in concrete, supports tissue architecture.

3. Scaffold Proteins (Scaffold Proteins) of the neural synapse 

Function: anchors receptors and signalling molecules in the postsynaptic membrane of neurons (e.g. PSD, Postsynaptic Dense Zone) to ensure efficient transmission of nerve signals.

Key proteins: 

PSD-95: anchors NMDA receptors and maintains synaptic plasticity (associated with learning memory).

Homer protein: connects metabotropic glutamate receptors to intracellular calcium stores.

Analogues: soldering points similar to circuit boards to ensure precise signalling.

4. Chromosome Scaffold 

Function: Maintains chromosome morphology during mitosis and helps with the orderly folding and segregation of DNA.

Key components: 

Condensin, Cohesin: compresses DNA into chromosomes.

Analogy: similar to the binding threads of a book, preventing DNA from ‘falling apart’. 6. Sarcomere of Skeletal Muscle 

Function: basic unit of muscle contraction, formed by a regular arrangement of actin and myosin fibres.

Key structures: 

Z-wire (Z-disc): anchors actin and maintains stability of the sarcomere.

Titin: largest known protein, acts as a ‘spring’ connecting the Z-wire to the M-wire.

Analogy: similar to the hinges of an extension ladder, allowing for muscle extension and contraction. 7.

5. Endothelial Glycocalyx 

Function: ‘glycoprotein bridge’ covering the endothelial surface of blood vessels, regulates vascular permeability, anticoagulant and inflammatory responses.

Composition: glycosaminoglycans (e.g., acetylheparin sulfate), proteoglycans.

Analogy: similar to the ‘anti-fouling coating’ on the inner wall of blood vessels, protects vascular function.

7. Cell Junctions 

Function: ‘bridge’ between cells, maintains tissue integrity.

Types: 

Tight Junctions: close cell gaps (e.g., intestinal barriers).

Desmosomes: enhance mechanical connections between cells (e.g. skin tensile resistance).

Gap Junctions: allow direct exchange of small molecules (e.g. synchronised beating of heart muscle cells).

8. Organic matrix in biomineralisation 

Function: Guides the orderly deposition of minerals (e.g. hydroxyapatite) to form bones or teeth.

Key components: 

Collagen fibres (bone matrix) + non-collagenous proteins (e.g. osteocalcin).

Analogy: Similar to ‘scaffolding’ in construction, guides mineral crystallisation.