CARTILAGE

Cartilage is a connective tissue composed of cells and fibres embedded in a firm, gel-like matrix which is rich in a mucopolysaccharide. it is more elastic than bone.

• Cartilage doesn't have blood supply nor it has lymphatics.it's nutrition diffuses through the matrix.
• It doesn't have nerves hence it is insensitive.
• Cartilage is surrounded by a fibrous membrane known as perichondrium which is similar to the periosteum in both structure and function. The articular cartilage doesn't have perichondrium so it's regeneration after injury is inadequate. 
• When cartilage dies, it forms into a bone like tissue.

TYPES OF CARTILAGE

1. HYALINE CARTILAGE: It is the most common form of cartilage. Hyalos is the Greek word for glass which describes the appearance of the tissue which is translucent, blueish-white and shiny. The cartilage is usually only 2-4mm thick. It is the embryonic form of cartilage. It is found in ribs, joints, nose. layrnx and trachea. Hyaline cartilage collagen fibres are primarily type II, extremely thin, invisible to microscope due to similar refractory properties to the matrix itself.

2. FIBROCARTILAGE: It is found where tendons and ligaments meet the bone, at the Pubic Symphysis, the Sternoclavicular joint and Annulus Fibrosus. The fibrocartilage is a very strong and pliable connective tissue. It is reinforced with collagen fibre bundles that run parallel to each other, allowing a low level stretch. Because of the abundance of collagen fibres, fibrocartilage is white in colour. It lacks a perichondrium and is composed of type II and type I collagen fibres. 

3. ELASTIC CARTILAGE: It is found in the external ear (auricle or pinna), the Eustachian tube and Epiglottis. Elastic's cartilage main role is purely structural, offering flexibility and resilience due to mixture of elastic fibres and type II collagen fibres. It is yellow in appearance without the organized structure of fibrocartilage when viewed under microscope.

COMPOSITION OF CARTILAGE

Cartilage is made up of highly specialized cells called chondrocytes and chondroblasts (chondro refers to cartilage), and other extracellular material which forms the cartilage matrix.

All connective tissue types within the human body are derived from the embryonal mesoderm. Bone, the strongest of the connective tissues, is the last to form and can remain in cartilage form well after birth. Increased cartilage to bone ratio enables a flexible and pliable new-born to exit the birth canal. A new-born has 300 bones, as opposed to the 206 of the normal adult, and all of these originate from cartilage.

From the 7th week of embryonic life, the process of ossification or osteogenesis slowly replaces cartilage with bone. This process continues into early childhood. Cartilage grows in two ways. In interstitial growth, chondrocytes proliferate and divide, producing more matrix inside existing cartilage throughout childhood and adolescence. In appositional growth, fresh layers of matrix are added to existing matrix surface by chondroblasts in the perichondrium. The perichondrium is a dense layer of connective tissue which surrounds most cartilage sites. Its outer layer contains collagen-producing fibroblasts, while the inner layer houses large numbers of differentiated fibroblasts called chondroblasts.

Chondroblasts: As long as they are free to move, chondroblasts produce the elements of the extracellular matrix (ECM). This cell type first forms a matrix of hyaluronic acid, chondroitin sulphate, collagen fibers, and water during embryonal development. Chondroblasts eventually become immobile after becoming surrounded by the matrix, and are then referred to as chondrocytes.

Chondrocytes: They are the immobile form of chondroblasts. They are surrounded by the matrix and contained within allotted spaces called lacunae. A single lacuna can contain one or more chondrocytes. Chondrocytes have varying roles according to the type of cartilage they are found in. In articular cartilage, found in the joints, chondrocytes increase joint articulation. At growth plates, chondrocytes regulate epiphyseal plate growth. While chondroblasts are ECM manufacturers, chondrocytes maintain the existing ECM and are a less active form of the same cell.

Fibroblasts: It is found in all types of connective tissue. In cartilage, these cells produce type I collagen. In certain situations, fibroblasts transform into chondrocytes.

Extracellular Matrix: There is significantly more matrix than cells in cartilage structure, as the low oxygen environment and lack of vasculature do not allow for larger numbers. Because of this, there is little metabolic activity, and little to no new growth in cartilage tissue – one of the reasons the elderly commonly suffer from degenerative joint pain. Cartilage does continue to grow slowly, however. This can be seen in the larger ears and noses of older individuals.

The ECM of cartilage contains three characteristic elements:

• Collagen

A protein-based collagen matrix gives form and strength to cartilage tissue through a mesh-like structure of fibrils. Although there are many different forms of collagen in the human body, the collagen found in cartilage is primarily type II, with an attached FACIT (short for fibril-associated collagen with interrupted triple helix) XIV collagen which determines the diameter of these fibers.

• Proteoglycans

Proteoglycans are large molecules that bind with water, providing flexibility and cushioning qualities. Proteoglycan monomers bond to hyaluronic acid by way of link proteins, as is the case with the large proteoglycan Aggrecan (chondroitin sulphate proteoglycan 

• Collagen and aggrecan in hyaline cartilage

The high numbers of negative charges such constructions provide, together with a large surface area, make it possible for proteoglycans to bind to large amounts of water. This creates high osmotic pressure, increases load-bearing, and constitutes the gel-like consistency of the ECM.

• Noncollagenous Proteins

Noncollagenous elements of the ECM are small in number and supposed to play a role in maintenance and organization of the cartilage structure on a macromolecular level.

Complied and written by Dr. Palak Shah