In tetrapods, it covers and protects the ends of long bones at the joints as articular cartilage,[1] and is a structural component of many body parts including the rib cage, the neck and the bronchial tubes, and the intervertebral discs.

Cartilage is composed of specialized cells called chondrocytes that produce a large amount of collagenous extracellular matrix, abundant ground substance that is rich in proteoglycan and elastin fibers.

In all vertebrates, cartilage is the main skeletal tissue in early ontogenetic stages;[3][4] in osteichthyans, many cartilaginous elements subsequently ossify through endochondral and perichondral ossification.

Due to the great stress on the patellofemoral joint during resisted knee extension, the articular cartilage of the patella is among the thickest in the human body.

The mechanical properties of articular cartilage in load-bearing joints such as the knee and hip have been studied extensively at macro, micro, and nano-scales.

For example, permeability varies throughout articular cartilage and tends to be highest near the joint surface and lowest near the bone (or “deep zone”).

Initially, there was a misconception that due to its predominantly water-based composition, cartilage had a Poisson's ratio of 0.5 and should be modeled as an incompressible material.

In addition to its role in load-bearing joints, cartilage serves a crucial function as a gradient material between softer tissues and bone.

Mechanical gradients are crucial for your body’s function, and for complex artificial structures including joint implants.

For example, the elastic modulus of human bone is roughly 20 GPa while the softer regions of cartilage can be about 0.5 to 0.9 MPa.

[16][17] When there is a smooth gradient of materials properties, however, stresses are distributed evenly across the interface, which puts less wear on each individual part.

The body solves this problem with stiffer, higher modulus layers near bone, with high concentrations of mineral deposits such as hydroxyapatite.

Collagen fibers (which provide mechanical stiffness in cartilage) in this region are anchored directly to bones, reducing the possible deformation.

Moving closer to soft tissue into the region known as the tidemark, the density of chondrocytes increases and collagen fibers are rearranged to optimize for stress dissipation and low friction.

Here cartilage is characterized by a dense extracellular matrix and is rich in proteoglycans (which dispel and reabsorb water to soften impacts) and thin collagen oriented parallel to the joint surface which have excellent shear resistant properties.

Aging in calcified regions also generally leads to a larger number of mineral deposits, which has a similarly undesired stiffening effect.

Over the last years, surgeons and scientists have elaborated a series of cartilage repair procedures that help to postpone the need for joint replacement.

Complete healing of cartilage after injury or repair procedures is hindered by cartilage-specific inflammation caused by the involvement of M1/M2 macrophages, mast cells, and their intercellular interactions.

[24] These gels have exhibited great promises in terms of biocompatibility, wear resistance, shock absorption, friction coefficient, flexibility, and lubrication, and thus are considered superior to polyethylene-based cartilages.

A two-year implantation of the PVA hydrogels as artificial meniscus in rabbits showed that the gels remain intact without degradation, fracture, or loss of properties.

Tumors arising from other tissues may also produce a cartilage-like matrix, the best-known being pleomorphic adenoma of the salivary glands.

Cartilage tissue can also be found among some arthropods such as horseshoe crabs, some mollusks such as marine snails and cephalopods, and some annelids like sabellid polychaetes.

It is a vesicular cell-rich cartilage due to the large, spherical and vacuolated chondrocytes with no homologies in other arthropods.

The endosternite cartilage forms close to Hh-expressing ventral nerve cords and expresses ColA and SoxE, a Sox9 analog.

[28] The sabellid polychaetes, or feather duster worms, have cartilage tissue with cellular and matrix specialization supporting their tentacles.

The model organisms used in the study of cartilage in sabellid polychaetes are Potamilla species and Myxicola infundibulum.