Cartilage

Cartilage consists of chrondrocytes set in matrix which is rich in proteoglycans (this accounts for the solid, but flexible consistency of cartilage). There are three types of cartilage, depending on the amount and nature of fibres in the ground substance:

• Hyaline cartilage contains few fibres. It is the most common type and is found in the nasal septum, larynx, tracheal rings and most articular surfaces of joints. It has a matrix with a translucent appearance because the refractive index of its collagen is similar to that of the ground substance within which it is buried. It also provides support to walls of some respiratory airways.
• Fibro-cartilage contains abundant collagen fibres, giving it great tensile strength. It is found in the intervertebral discs and some joints such as the pubic symphysis.
• Elastic cartilage contains elastic fibres. They occur in the external ear, external auditory meatus, epiglottis and parts of the laryngeal cartilage rings. It contains chondrocytes embedded in a matrix dominated by elastic fibres making it firm but flexible. It attaches bone to tendon and it is found in the menisci of synovial joints and in intervertebral discs.

Except where hyaline cartilage serves as articular cartilage and is exposed to synovial fluid, cartilage is enclosed by a layer of dense connective tissue – the perichondrium. This is essential for cartilage growth. Chondrocytes are flattened near the perichondrium and more round in deeper regions. Chondrocytes in hyaline cartilage are often arranged in pairs or groups of four to six.

The matrix is a firm yet resilient gel, with physical attributes of plastic. It is not as rigid as matrix of bone and gives cartilage a sold, firm consistency. It is 70-75% water, with the rest being collagen (15-20% for tensile strength) and teoglycans (2-10%) for resilience.