Cartilage: Definition, Cells & ECM

Cartilage Definition

• It's a tough, flexible form of connective tissue
• Characterized by an ECM with a high concentration of GAGs and proteoglycans
• GAGs and proteoglycans interact with collagen and elastic fibers
• The cartilage ECM can bear mechanical stress without permanent distortion
• This tissue is present in the trachea, ears, and nose
• Cartilage provides shock-absorbing and sliding regions with joints
• It facilitates movement
• Guides development and growth of long bones, before and after birth

Cartilage Cells & ECM

• Cells are called chondrocytes, from "condros" (cartilage) + "kytos" (cell)
• Chondrocytes synthesize and maintain ECM components
• They are located in matrix cavities called lacunae
• Principle components of all cartilage ECM include: collagen, hyaluronic acid, proteoglycans, and various glycoproteins
• Semi-rigid consistency of cartilage is attributable to water

3 Major Forms of Cartilage

• Hyaline cartilage is the most common, glassy type
• Elastic cartilage is more pliable and distensible
• Possesses abundant elastic fibers in addition to collagen II
• Fibrocartilage is present in body parts subjected to pulling forces
• Has a matrix highly rich in networks of type I collagen
• Cartilage is avascular, receiving nutrients from capillaries in adjacent connective tissue
• Chondrocytes have low metabolic activity
• Cartilage lacks nerves and lymphatic vessels

Perichondrium

• It is a sheath of dense connective tissue
• Surrounds cartilage in most places
• Forming an interface between the cartilage and the tissues supported by the cartilage

Description of Hyaline Cartilage

• Hyaline cartilage is homogeneous and semitransparent
• Localized in articular surfaces of movable joints in adults
• It is located in the walls of the respiratory tract (nose, larynx, trachea, bronchi)
• It's present in the ventral ends of ribs and in the epiphyseal plate of long bones (for growth)
• In the embryo, hyaline cartilage forms the temporary skeleton that is gradually replaced by bone
• Aggrecan is the most abundant proteoglycan
• Water makes up 60-80% of the weight of fresh hyaline cartilage
• Chondronectine is the multiadhesive glycoprotein

Chondrocytes in Hyaline Cartilage

• Chondrocytes occupy little of the hyaline cartilage mass
• Young chondrocytes (chondroblasts) at the periphery of the cartilage have an elliptic shape
• Deeper in the cartilage, chondrocytes are round and appear in groups of up to eight cells, called isogenous aggregates
• A major regulator of hyaline cartilage growth is pituitary-derived growth hormone or somatotropin

Elastic Cartilage

• Elastic cartilage contains an abundant network of elastic fibers, plus collagen type II
• It's found in the auricle of the ear, the walls of the external auditory canals
• Also in the auditory (Eustachian) tube, the epiglottis, and the cuneiform cartilage in the larynx
• Covered with perichondrium

Interesting Pathology

• Relapsing Polychondritis cases exhibit a thickened and deformed pinna
• Underlying cartilage destruction results in a floppy ear

Fibrocartilage Attributes

• Fibrocartilage is a combination of hyaline cartilage and dense connective tissue
• It's located in intervertebral discs
• Also in attachments of certain ligaments, and in the pubic symphysis
• Regions with chondrocytes and hyaline matrix are separated by other regions
• These regions containing bundles of type I collagen and scattered fibroblasts
• Lacks a surrounding perichondrium

Chondrogenesis Information

• Mesenchyme is the precursor for all types of cartilage
• Mitosis and early differentiation produces a tissue with condensations of rounded cells, chondroblasts
• Chondroblasts are then separated by their production of matrix components
• These components swell with water and form the extensive ECM
• Multiplication of chondroblasts within the matrix gives rise to isogenous cell aggregates
• These aggregates are surrounded by a condensation of territorial matrix
• In mature cartilage, interstitial mitotic activity ceases
• Chondrocytes become more widely separated by their production of matrix

Mechanisms of Cartilage Growth

• Interstitial growth involves the mitotic division of preexisting chondrocytes; increases length
• Appositional growth results from chondroblasts depositing collagen/matrix on the surface of pre-existing cartilage; increases girth or width

Cartilage Repair

-Damaged cartilage undergoes slow and incomplete repair, primarily by activity of cells in the perichondrium, except in young children

• In extensively damaged areas, the perichondrium produces a scar of dense connective tissue instead of forming new cartilage