Extracellular Matrix

Original Editor - Lucinda hampton

Top Contributors - Lucinda hampton and Tolulope Adeniji  

Introduction

ECM components of cartilage

The connective tissues non cellular portion is known as the extracellular matrix (ECM), giving the physical scaffolding for the cells. The ECM is able to hold water, provide appropriate hydration of the tissue and form part of a selective barrier to the external environment. The ECM also initiates crucial biochemical and biomechanical cues for tissue morphogenesis, differentiation and homeostasis.

In human, the main components of the extracellular matrix are:

1. Fibrous elements (e.g. collagen, elastin, reticulin),
2. Link proteins (e.g. fibronectin, laminin), and
3. Space filling molecules (e.g. proteoglycans, glycosaminoglycans)

Various Roles of ECM

The role of the ECM depends on its nature and composition. eg, the matrix may be mineralised in bone to resist compression or dominated by tension resisting collagen fibres in tendons. Below various examples are given.

Tendon

The ECM of tendon is composed predominantly of collagen, which accounts for ~ 60–85% of the dry weight of the tissue. Approximately 95% of the collagen is type I. The collagen provides excellent mechanical strength to the tendon along one axis! allowing them to be able to withstand tension.

Cartilage

Chondrocytes synthesise the cartilage ECM and makes it both stiff and elastic.

1. Mainly composed of type II collagen, up to 25 % of dry weight, with types IX and XI collagens present in lower proportions.
2. The second most abundant molecules are glycosaminoglycans, such as hyaluronan and aggrecan, which join together to form macromolecular complexes.

Collagen counteracts tensile loads and glycosaminoglycans decrease the mechanical pressures.

Bone

Glycosaminoglycans

Osteoblasts synthesise bone ECM. Consists of:

1. Type I collagen mixed with a matrix of calcium phosphate crystal (which is up to 70 % of the dry weight). Collagen allows bone to be elastic enough to avoid bone fragility and not to be easily broken whilst calcium phosphate crystal provides stiffness and hardness.
2. Proteoglcans and glycoproteins, which are less abundant, but vital for the organization of collagen fibers, mineralization and resorption of bone. Chondroitin sulfate accounts for 67-97 % of the bone glycosaminoglycans.

Skin

Skin layers

The ECM of skin is composed of various polysaccharides, water and collagen proteins, giving the skin its unique properties, ie great tensile strength of plus substantial elasticity and compressibility. The ECM molecules, which are secreted by fibroblasts and epidermal cells, are importantly.

• Fibrous structural proteins, eg collagens, elastin and laminin, which give the ECM strength and resilience

• Proteoglycans, eg dermatan sulfate and hyaluronan, which are large, highly hydrated molecules that help cushion cells in the ECM.

Blood

The extracellular matrix of blood is called plasma, making blood unique among connective tissues because it is fluid. This fluid, which is mostly water, suspends the formed elements and enables them to circulate throughout the body within the cardiovascular system.

Nervous Tissue

Glial cells, neuron blue

Macroglia provide most of the the ECM in the nervous system, with the ECM playing an important role in supporting and signaling. ECM components are key mediators of glial activation and have the capacity to evoke both regenerative and degenerative effects on glia and neurons. The production of ECM components changes drastically during reactive gliosis, a reaction that happens in response to nervous tissue damage. There is only a small amount of extracellular matrix in the nervous tissue.

Clinical Implication of ECM and Physiotherapy Practice

While Knowledge of ECM is important to physiotherapy, its clinical application in manual therapy and wound healing cannot be overemphasized. The ECM components of clinical significance to physiotherapists include

1. Glycosaminoglycans, GAGs, (named proteoglycans when attached to a protein core)
2. Glycoprotein
3. Fibrous proteins

Some important examples of the ECMs relevance include:

1. ECM is motion-dependent (ECM requires a motion for its production)
2. Half-life of GAGs and Proteoglycans is between 1,7 to 7 days.
3. Isometric contraction at an early stage of rehabilitation of patients with pain-induced splinting is essential to keep dense connective tissue lubricated.
4. In a period greater than a month, loss of weight-bearing and movement on joint may reduce proteoglycans by 40%
5. Hyaluronan or hyaluronic acid is a form of GAGs that is responsible for resisting mechanical stress within cartilage and a good presence of it also aids in wound healing.
6. In the final stage of tendon healing (the remodeling phase), the fibrous protein, collagen I, synthesis dominates, and the ECM becomes more aligned. This is an important time to apply appropriate loads to the new tissue. See more on Tendon Physiology. This phase begins 1–2 months after injury, continuing potentially for more than a year. This repaired tissue appears scar-like, never completely regaining the biomechanical properties it had prior to injury. .

Conclusion

As shown above, the ECM is also a reservoir of bioactive molecules, being a highly dynamic entity that is of great importance, determining and controlling the most fundamental behaviours and characteristics of cells such as proliferation, adhesion, migration, polarity, differentiation, and apoptosis.