Connective tissue is the most abundant type of tissue in the body. It serves to connect and support other tissues and also has regulatory and immunologic functions. Connective tissue consists of cells, mainly fibroblasts, and an extracellular matrix (ECM). The specific composition of the ECM determines the biochemical properties of the connective tissue. There are many different types of connective tissue, with loose and dense connective tissue being the most common.
Disorders of connective tissue are discussed in “”
Connective tissue consists of specialized cells that are embedded in the extracellular matrix (ECM). Connective tissue is classified as loose or dense connective tissue depending on the ratio and structure of its components.
Fibroblasts (the most common cell type in connective tissue)
- Origin: derived from
- Function: synthesis and organization of the ECM
- Histological features: spindle-shaped cells arranged in a branching pattern
- Fibrocytes: fibroblast with low metabolic activity
- Myofibroblasts: contractile hybrid cells with features of both fibroblasts and smooth muscle cells
Transient immune cells
The extracellular matrix (ECM) is composed of various macromolecules arranged in a three-dimensional structure. Its specific composition determines the biochemical properties of the connective tissue.
Collagen molecules are the basis of collagen fibers and reticular fibers. They account for the majority of proteins in the ECM, which makes them the most abundant proteins in the human body. Elastic fibers are composed of elastin molecules and can be found together with collagen fibers in tissues that require elasticity in addition to tensile strength, e.g., the lung.
|Overview of extracellular matrix fibers|
|Collagen fibers||Reticular fibers||Elastic fibers|
|Main molecule|| || || |
|Characteristics|| || || |
|Occurrence|| || |
|Associated proteins|| || || |
- Structure: A collagen molecule is a protein with a repeating amino acid sequence (Gly-X-Y)n.
- Function: organize, strengthen, and provide elasticity to the extracellular matrix
|Types of collagen|
|Tissue distribution||Related conditions|
|Type I collagen (90% of body collagen)|| |
|Type II collagen|| |
|Type III collagen (reticulin)|| |
|Type IV collagen|
|Type V collagen|| |
- Degradation: enzymatic via specific collagenases (zinc-dependent proteinases that belong to the matrix metalloproteinases family)
- Synthesis: Collagens are synthesized at the rough ER (rER) and undergo extensive post-translational modifications.
|Overview of collagen synthesis|
|Stages||Process||Site||Intermediate product (precursors of collagen)|
|1. Translation|| || |
|2. Hydroxylation|| || || |
|3. Glycosylation|| |
|4. Formation of a triple helix|| || |
|5. Exocytosis|| || |
|6.|| || |
|7. Fibrillogenesis (cross-linking)|| || |
|8. Formation of fibers|| || || |
|Abbreviations: IC = intracellular; EC = extracellular; ECM = extracellular matrix; rER =|
Type ONE collagen is in bONE.
Type TWO collagen: carTWOlage.
Type THREE collagen is deficient in the vascular type of Ehlers-Danlos syndrome (3D = ThreE D).
Type FOUR collagen is under the FLOOR (part of basement membrane).
Impaired triple helix formation during collagen synthesis is the pathophysiological mechanism of .
is caused by defective cleavage of procollagen molecules.
- Definition: elastic protein that is a major component of elastic fibers
- Rich in the nonhydroxylated amino acids glycine, proline, and lysine, in contrast with the hydroxylated proline and lysine residues (hydroxyproline, hydroxylysine) present in collagen.
- Provides tissue with elasticity (via alternating α-helices and hydrophobic domains)
- Elastin has a spontaneous, disorganized spatial structure (relaxed conformation) that temporarily adopts an orderly arrangement only when stretched.
- Occurrence: tissues requiring high elasticity and the ability to recoil after a transient stretch
- Synthesis: Several elastin molecules are crosslinked (polymerization) and form bundles as elastic fibers.
- Degradation: enzymatically via elastase (elastase inhibition by )
- Definition: a family of unbranched polysaccharide chains of repeating disaccharide units with multiple negative charges that constitute a large volume fraction of the ECM
Structure: repeating units of
- First sugar: derivative of uronic acid (e.g., glucuronic acid)
- Second sugar: hexosamine (e.g., the amino sugar N-acetylglucosamine)
Four main groups
- Hyaluronic acid
- Chondroitin sulfate and dermatan sulfate
- Heparan sulfate
- Keratan sulfate
- Bind H2O in connective tissue due to its negative charges: act as a cushion
- Component of
- Definition: proteins with numerous covalently linked GAG side chains
Glycoproteins of the ECM
- Definition: proteins with short carbohydrate side chains that contribute to the organization of the extracellular matrix by offering specific binding sites for cells and other matrix molecules
Proteoglycans are primarily composed of carbohydrates that are attached to the side of a small core protein. In contrast, glycoproteins are mainly composed of a protein that is attached to the side chain of a short carbohydrate.
Connective tissue types
|Types of connective tissue|
|Loose connective tissue|| || |
|Dense connective tissue|| || |
|Reticular connective tissue|| |
|Elastic ligaments|| || |
|Mucous connective tissue|| |
|Stroma of ovary|| |
Specialized connective tissue
| || |
- Fibrosis: proliferation of fibroblasts and deposition of collagen in tissues