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 detail in thelearning card.
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 mesenchymal stem cells
- 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.
|Collagen fibers||Reticular fibers||Elastic fibers|
|Main molecule|| || || |
|Characteristics|| || || |
|Occurrence|| || |
|Associated proteins|| || || |
fibrillin gene (FBN1)! is caused by a mutation in the
- Definition: : a family of glycoproteins synthesized by fibroblasts and secreted (as triple-stranded procollagen) into the extracellular space. Collagen is the most ubiquitous protein in humans.
- Structure: a collagen molecule is a protein with a repeating amino acid sequence (Gly-X-Y)n
- Types: Out of about 42 genes coding for collagen chains, about 28 types of collagen triple helices can be assembled. Some collagen types form fibrils, e.g., collagen types I, II, III, V, and XI, while others do not, e.g., collagen types IV, VIII, and X.
- Degradation: enzymatic via specific collagenases
- Synthesis: Collagens are synthesized at the rough ER (rER).
|Collagen types||Tissue distribution||Related conditions|
|Type 1 collagen|| |
|Type 2 collagen|| |
|Type 3 collagen|| |
|Type 4 collagen|
|Type 5 collagen|| || |
Type ONE collagen is in bONE. Type TWO collagen: carTWOlage. Type THREE collagen is deficient in the vascular type of Ehlers-Danlos syndrome (ThreE D). Type FOUR collagen is under the FLOOR (part of basement membrane).
|Stages||Process||Site||Intermediate product (precursors of collagen)|
|1. Translation|| ||IC (rER)||Preprocollagen|
|2. Hydroxylation||IC (rER)||Procollagen (pro-α chain)|
|3. Glycosylation||IC (rER)|
|4. Formation of a triple helix|| ||IC (rER)||Procollagen (triple helix)|
|5. Exocytosis|| ||IC → EC|
|7. Fibrillogenesis (cross-linking)||EC||Collagen fibrils|
|8. Formation of fibers|| ||EC||Collagen fiber (end product)|
|Abbreviations: IC = intracellular; EC = extracellular; ECM = extracellular matrix; rER = rough endoplasmic reticulum|
hydroxylation of procollagen chains. leads to because it impairs
Impaired triple helix formation during collagen synthesis is the pathophysiological mechanism of .
is caused by defective cleavage of procollagen molecules.
Menkes disease occurs as a result of defective cross-linking of tropocollagen.
- Definition: elastic protein that is a major component of elastic fibers
Synthesis: several elastin molecules are cross-linked (polymerization) and form bundles as elastic fibers
- Fibrillin forms the scaffold for laying down tropoelastin
- Cross-linking between tropoelastin takes place extracellularly with the aid of transglutaminase and
- 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 = a derivative of uronic acid (e.g., glucuronic acid), second sugar = a 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
- Fibronectin: glycoprotein important for cell-matrix interactions
- Laminin: major component and organizer of the basal lamina (besides type IV collagen)
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!
|Loose connective tissue|| || || |
|Dense connective tissue|| || |
|Reticular connective tissue|| |
|Elastic ligaments|| || |
|Mucous connective tissue|| || |
|Stroma of ovary|| |