Extracellular Matrix

Original Editor - Lucinda hampton

Top Contributors - Lucinda hampton and Tolulope Adeniji  

Introduction

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 gives eg 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)

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The role of the extracellular matrix 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 tendon. Below various examples are given.

Tendon

Tendon are able to withstand tension. The ECM is made with this in mind. Collagen fibers of tendon are oriented parallel to the direction of the mechanical stress, which is only in one axis. The extracellular matrix is rich in collagen fibers and fibroblasts. Collagen is up to 65-80 % of the dry weight of the extracellular matrix, with elastin being only 1-2 %.

Cartilage

Chondrocytes synthesise the cartilage extracellular matrix, making 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

Osteoblasts synthesise bone extracellular matrix. 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

There is little amount of extracellular matrix in the nervous tissue. Hyaluronic acid and proteoglycans are more abundant than collagen, elastin and glycoproteins. During development, nervous extracellular matrix is synthesized and released by neurons and glial cells. The role of hyaluronic acid is structural and, together with other molecules, helps to form molecular scaffolds. A layer of extracellular matrix, known as perineuronal net, is found surrounding the membrane of neurons. This layer is composed of hyaluronic acid, tenascin and proteoglycans like chondroitin sulfate (up to 16 types). Perineural nets appear when the nervous tissue stops developing and it is thought that inhibit the cell motility and plasticity, and tissue remodeling. In fact, neuronal plasticity is activated when the perineural nets are damaged or removed. The composition of perineural nets changes during development. Type C chondroition sulfate is abundant in embryonary nervous tissue and gradually, decreases after birth, and disappears in adults, where it is replaced by type A chondroitin sulfate.

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