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Connective tissue overview
Connective tissue is a remarkable and intricate network within our bodies, serving a multitude of vital functions. Its construction relies on a complex interplay of various elements and processes that ensure its strength, flexibility, and resilience.
At the heart of connective tissue lies the extracellular matrix (ECM), a dynamic scaffold that forms the foundation for the tissue's structural integrity. The ECM is composed of fibrous structural proteins such as collagen and elastin, proteoglycans and hyaluronic acid, a linear polysaccharide composed of repeating disaccharide units, plays a pivotal role. It's the go-to molecule for retaining water, contributing to the tissue's hydration and lubrication.
The structure of proteoglycans is characterized by a unique combination of a protein core and long chains of carbohydrates known as glycosaminoglycans (GAGs). Here's a breakdown of the structural components:
- Core Protein: Proteoglycans have a core protein at their center, which provides the structural framework for the molecule. The core protein varies between different types of proteoglycans and determines their specific functions within tissues. It can be a large, multi-domain protein.
- Glycosaminoglycan (GAG) Chains: Surrounding the core protein are one or more long chains of glycosaminoglycans (GAGs). GAGs are linear, repeating polysaccharides made up of disaccharide units. Common types of GAGs found in proteoglycans include chondroitin sulfate, keratan sulfate, and heparan sulfate. These GAG chains are highly negatively charged due to the presence of sulfate or carboxyl groups. Different proteoglycans have varying types and lengths of GAG chains, which contribute to the diversity of proteoglycan structures and functions. For example, chondroitin sulfate proteoglycans have chondroitin sulfate GAG chains, while keratan sulfate proteoglycans have keratan sulfate GAG chains. The GAG chains in proteoglycans are highly hydrophilic due to their negative charges. This property enables proteoglycans to attract and bind water molecules, contributing to tissue hydration. The hydrated proteoglycan complexes help maintain the structural integrity and elasticity of connective tissues such as cartilage and skin.
- Linkage Region: The GAG chains are covalently attached to the core protein through a specialized region called the linkage region. This linkage region contains specific amino acid residues, often serine or threonine, to which the GAG chains are attached. The attachment forms a stable bond, allowing the GAG chains to extend outward from the core protein.
Collagen exists in various types: 1, 2, 3, and 4, and provide tensile strength and support. Collagen molecules are rich in amino acids like proline, glycine, and lysine. Their unique post-translational modifications, such as lysine hydroxylation and hydroxylysine formation, are catalyzed by lysyl hydroxylase enzymes, dependent on the presence of vitamin C.
Proline hydroxylation is crucial for collagen stability, ensuring the proper folding and cross-linking of collagen molecules. Deficiencies in vitamin C can lead to scurvy, a condition characterized by weak and fragile connective tissue due to impaired collagen formation.
Elastin, another key component, endows connective tissue with elasticity. This protein relies on the enzymatic action of lysyl oxidase, which requires copper as a cofactor. Zinc also plays a role in maintaining the integrity of the ECM by regulating metalloproteinases (MMPs), enzymes responsible for ECM degradation.
Chitosan, derived from chitin, another natural polymer found in the shells of crustaceans and insects, can also be incorporated into the ECM. This reinforces the connective tissue, adding to its structural integrity.
The fibroblast, a specialized cell type, is the architect behind the assembly and maintenance of connective tissue. It synthesizes ECM components and orchestrates their organization. Interestingly, fibroblasts operate within a circadian rhythm, and are mainly active throughout the day, which influences the timing and efficacy of ECM synthesis and repair.
In conclusion, connective tissue is a masterful blend of intricate components and finely-tuned processes. From collagen's tensile strength to elastin's elasticity, and from fibroblasts' rhythmic orchestration to the micronutrient support of copper and zinc, every element plays a vital role in building and maintaining the connective tissue that keeps the body strong and functional.
conceptExtracellular matrix (ECM)
The extracellular matrix (ECM) is found in the spaces between cells, forming a large proportion of tissue volume. It is also found between organs and as such contributes to the body's shape, plasticity, and partitioning. The ECM is composed of three associated macromolecules: (1) fibrous structural proteins such as collagen and elastin, (2) glycoproteins, and (3) proteoglycans and hyaluronic acid. Typically, the ECM forms either basement membrane or interstitial matrix and, in doing so, performs several functions, including retaining water, minerals, and nutrients as well as acting as the substrate for cell-cell contact, migration, and adherence. The extracellular matrix occupies the intercellular spaces. It is most abundant in connective tissues such as the basement membrane, bone, tendon, and cartilage, where definition is given to the ECM by the proportions and organization of various components. The elastin of skin and blood cells provides resiliency, collagen provides strength to tendons, and the calcified collagen matrix of bone provides strength and incompressibility. Integrins are a family of heterodimeric proteins composed of α and β subunits that are the main cellular receptors for the ECM. Integrins have several distinctive features from other adhesion proteins. They interact with an arginine-glycine–aspartic acid (RGD) motif of ECM proteins. Integrins link the intracellular cytoskeleton with the ECM through this RGD motif. Without this attachment, cells normally undergo apoptosis. Integrins can bind to more than one ligand and many ligands can bind to more than one integrin. Examples of integrins include fibronectin receptors and laminin receptors.
Ref:
Linda R. Adkison PhD, in Elsevier's Integrated Review Genetics (Second Edition), 2012
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