Extracellular Matrix

Original Editor - Lucinda hampton

Top Contributors - Lucinda hampton and Tolulope Adeniji  

Introduction

ECM components of cartilage

The extracellular matrix (ECM) is the non-cellular component present within all tissues and organs, providing the essential physical scaffolding for the cellular constituents. The ECM holds water, ensures suitable hydration of the tissue, participates in a selective barrier to the external environment and also eg gives tensile strength, limits overstretch of the organs. 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)

Varies 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.

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.

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 behaviors and characteristics of cells such as proliferation, adhesion, migration, polarity, differentiation, and apoptosis.