Connective Tissue Fibres: Providing Strength and Flexibility
Connective tissues are characterised by an abundant extracellular matrix (ECM) produced by its cells. The ECM includes different combinations of protein fibres, such as collagens and elastic fibres, and ground substance.
Collagen Fibres: The Strong and Abundant Scaffold
Collagen is the most abundant protein in the human body and a key element of all connective tissues, as well as epithelial basement membranes. It forms various structures like fibres, sheets, and networks, all extremely strong and resistant to shearing and tearing forces.
Collagen is produced primarily by fibroblasts through a complex multi-step process. The basic subunit of collagen fibres is the procollagen molecule, a triple helix formed by three intertwined α chains. Different combinations of these α chains produce the various types of collagen.
Collagen can be categorised based on the structures formed by their interacting subunits:
Fibrillar collagens:
Subunits aggregate to form large fibrils, visible by light and electron microscopy.
Type I collagen: the most abundant and widely distributed type, forms large eosinophilic bundles called collagen fibres. These fibres are often densely packed, forming structures like tendons, organ capsules, and dermis.
Type II collagen: found in cartilage, forms fibrils but does not form fibres or bundles.
Type III collagen: forms thin, argyrophilic fibres often found with Type I collagen, providing structural maintenance in expansible organs.
Sheet-forming collagens:
Type IV collagen: produced by epithelial cells, forming the major structural protein of external laminae and basal lamina in all epithelia.
Type I collagen fibres have a characteristic structure, with adjacent collagen subunits staggered by 67 nm, creating a pattern of transverse striations visible in electron micrographs. These striations, along with the highly regular orientation of subunits, make collagen fibres birefringent under polarised light.