A Cartilage is a type of connective tissue that helps in providing support and maintaining structure or morphology of the body. The cartilage is able to perform its function because of its firm consistency and ability to withstand compression and shearing forces; the cartilage can perform these functions because it is made up of cells, fibers, and ground substance. The ground substance of a cartilage is made of Collagen and elastic fibers that are embedded in the ground substance and this helps to impart tensile strength and elasticity. The combination of the ground substance and fibers make up a structure known as the matrix of cartilage. A Cartilage resembles ordinary connective tissue in the sense that the cells in it are widely separated by a considerable amount of intercellular material or matrix.

Differences between Cartilage and other Types of Connective Tissues

The Cartilage differs from other types of connective tissues because it lacks nerves, blood and lymphatic vessels. Another difference from other connective tissues is because the cartilage is entirely nourished by diffusion of nutrients from blood vessels of nearby body tissues this means a cartilage is avascular (it has no blood supply of its own); this is made possible by the fact that the matrix of the cartilage has a high water content of about 75% and also freely permeable even to some fairly large particles. Because of these differences, a cartilage is referred to as a modified connective tissue.

Location of Cartilages in the Body

Cartilage can be found in different parts of the body. Some cartilages form major parts of organs such as the auricles of the ears and the lower part of the nose.

Sites of Cartilage in the body

1. External ear (auricles)
2. Auditory tube
3. Epiglottis
4. Cartilage is found in some of the smaller laryngeal cartilages
5. Intervertebral discs
6. Cartilages are found at some articular points such as that of the pubic symphysis and at sites of attachment of certain tendons to bone
7. Lower part of the Nose
8. Larynx
9. Trachea
10. Bronchi
11. The Epiphyseal plate (that connect each epiphysis to the diaphysis)
12. The glenoidal labrum of the shoulder joint and the acetabular labrum of the hip joint are made of fibrocartilage

Well labelled Diagram of the area of transition between the perichondrium and the hyaline cartilage. As perichondrial cells differentiate into chondrocytes, they become round, with an irregular surface. Cartilage (interterritorial) matrix contains numerous fine collagen fibrils except around the periphery of the chondrocytes, where the matrix consists primarily of glycosaminoglycans; this peripheral region is called the territorial, or capsular, matrix.

Structure of a Cartilage

The cells of cartilage are called Chondrocytes and reside in small spaces called lacunae scattered throughout the matrix. The chondrocytes generally conform to the shape of the lacunae in which they are found. Deep in the cartilage the cells and their lacunae usually appear rounded in profile, whereas at the surface, they are elliptical, with the long axis parallel to the surface.

Chondrocytes most times form small clusters called Isogenous cell groups or Cell-nests which are the offspring of a single cell. In usual preparations, the chondrocytes show an irregular outline and appear shrunken and pulled away from the walls of the lacunae but with an Electron microscope, each cell completely fills the lacunar space and sends short processes into the surrounding matrix, however, neighboring cells do not touch one another.

The nucleus of chondrocytes is round or oval and contains one or more nucleoli. The cytoplasm shows the usual organelles as well as lipid and glycogen inclusions. In growing chondrocytes the Golgi complex and endoplasmic reticulum are well developed.

The matrix of fresh cartilage appears homogeneous because the ground substance and the collagen fibers embedded within it have the same refractive index. The Type of Collagen in cartilage is the Type II collagen because it appears to be less polymerized than in other tissues.

The ground substance of cartilage consists mainly of proteoglycans (the specific glycos-aminoglycans of cartilage are chondroitin-4- and chondroitin-6-sulfate, keratan sulfate, and a small amount of hyaluronic acid). The core protein is Aggrecan. These proteoglycans give the basophilic properties of the ground substance. A glycoprotein known as Chondronectin located within the ground substance helps in adhesion of type II collagen to chondrocytes. There is a calcium-binding glycoprotein called Chondrocalcin that has been demonstrated in the ground substance of hyaline cartilage.

The matrix around each lacuna and isogenous group stains more deeply than elsewhere, forming the territorial matrix while the less densely stained intervening areas form the inter-territorial matrix. Except for the free surfaces of articular cartilages, hyaline cartilage is enclosed by a specialized sheath of connective tissue called the perichondrium. The outer layers of the perichondrium consist of a well vascularized, irregular and dense connective tissue that contains elastic and collagen fibers and fibroblasts. Where it lies against cartilage, the perichondrium is more cellular and passes imperceptibly into cartilage. The slender collagen unit fibrils of the cartilage matrix blend with the wider, type I collagen unit fibrils of the perichondrium. Perichondrial cells adjacent to the cartilage retain the capacity to form new cartilage.

Functions of Cartilage

1. Articular Cartilages provide the bone ends with smooth surfaces between which there is very little friction
2. Articular cartilages also act as shock absorbers
3. Costal cartilages that connect the ventral ends of the ribs to the sternum increases elasticity of the chest wall
4. Fibrocartilage often helps in insertion of tendons into bones

Facts and Characteristics of Cartilages

1. Cartilage is sufficiently firm to maintain its form but it is not rigid like bone. It can be bent but always returns to its original form when the bending force is removed.
2. Cartilage develops embryologically from mesenchyme. Some mesenchymal cells differentiate into cartilage-forming cells known as Chondroblasts which are responsible for the production of the intercellular matrix as well as the collagen fibers that form the intercellular substance of cartilage. Chondroblasts that become trapped within the matrix develop into Chondrocytes while some mesenchymal cells that surround the developing cartilage form the perichondrium this perichondrium contains cells that are capable of transforming themselves into Chondrocytes when required.
3. Newly formed cartilage grows by multiplication of cells throughout its substance a kind of growth that is referred to as Interstitial growth. This type of growth is possible only when the matrix is sufficiently pliable to allow movement of cells through it, because as cartilage matures the matrix hardens and the cartilage cells can no longer move widely apart and interstitial growth is impeded or stopped. When the growth is stopped, anytime a cartilage cell divides the daughter cells remain close together and forms Cell nests and further growth of cartilage only takes place by addition of new cartilage over the surface of existing cartilage a form of growth called Appositional growth (a common type of growth in bones).
4. Cartilage has very limited ability for regeneration following injury or disease. Any defect in a cartilage is usually filled in by fibrous tissue.
5. During fetal life cartilage is much more widely distributed than in the adult. The ends of most long bones are cartilaginous at the time of birth, and are gradually replaced by bone and this replacement is completed only after full growth of the individual at about 18 years of age. This process of replacement of cartilage by bone is called Ossification ( this is different from Calcification which involves the matrix becoming hardened due to the deposition of calcium salts in it).
6. Cartilage is usually described as an avascular tissue but there is the presence of cartilage canals, through which blood vessels may enter cartilage this is a well-documented fact. Each cartilage canal contains a small artery surrounded by numerous venules and capillaries. In addition to diffusion, cartilage cells also receive their nutrition in cartilage canals. Cartilage canals may also play a role in the ossification of cartilage by carrying bone forming cells into it.

Types of Cartilage based on types and abundance of Fibers in the Matrix

The Classification of cartilage into hyaline, elastic, and fibrous types is based on differences in the abundance of fibers as well as the type of fibers in the matrix.

1. Hyaline Cartilage
2. Elastic Cartilage
3. Fibrous Cartilage

Diagram of a Hyaline cartilage showing Chondrocytes surrounded by homogeneous matrix. Perichondrium is seen at the left end of the picture

Hyaline Cartilage Structure and Function

The Hyaline cartilage is the most common type of cartilage and forms the cartilages of the ribs that join at the sternum (costal cartilages). It got its name from its appearance ( because it is transparent – hyalos = glass). The hyaline cartilage is also found at the articular points of joints, and it is the cartilage that forms the nose, larynx, trachea, and bronchi. It is also present in the growing ends of long bones. The hyaline cartilage makes up most of the skeleton in the Fetus because the skeleton is first laid down as hyaline cartilage.

Diagram of Elastic cartilage showing the Chondrocytes surrounded by bundles of elastic fibres. The section has been stained by Verhoeffs method in which elastic fibres are stained bluish black

Elastic Cartilage Structure and Function

Elastic cartilage is a variant of hyaline cartilage but the main difference is the fact that elastic cartilage contains branched elastic fibers in its matrix. Collagen fibers of the type found in hyaline cartilage are also present in elastic cartilage but are masked. Deep in the cartilage, elastic fibers form a dense, closely packed mesh that blinds the ground substance, forming a loose network beneath the perichondrium and continuous with it. The chondrocytes are similar to those of hyaline cartilage and elaborate the elastic and collagen fibers. The Elastic cartilage is more flexible than hyaline cartilage and is found in the external ear, auditory tube, epiglottis, and in some of the smaller laryngeal cartilages.

Fibrous Cartilage showing Cartilage cells being embedded in thick bundles of collagen fibers

Fibrous Cartilage Structure and Function

Fibrous (or fibrocartilage) cartilage is a type of cartilage that represents a transition between dense connective tissue and cartilage. It consists of typical chondrocytes enclosed in lacunae that are suspended in only a small amount of ground substance in the immediate vicinity of the cells. The chondrocytes lie singly, in pairs, or in short rows between bundles of dense collagen fibers of typical type I collagen because the unit fibrils of show the 64-nm banding pattern. Fibrous cartilage always lacks a perichondrium and blends into hyaline cartilage, bone, or dense fibrous connective tissue. It is found in the intervertebral discs, in some articular cartilages, in the pubic symphysis and at sites of attachment of certain tendons to bone.

Medical Application of Cartilage growth

Cells of cartilage can give rise to either benign tumors called Chondroma or slow-growing malignant tumors called Chondrosarcoma. These tumor cells produce normal matrix components. Chondrosarcomas rarely metastasize and are generally removed surgically.