Cartilage

The Skeletal System

The skeletal tissues • Skeletal tissues consist mainly of cartilage and bone • They are specialized forms of connective tissue • Like all connective tissue they develop in embryos from mesenchyme

Functions of cartilage • Skeletal support in embryo prior to development of bony skeleton • Long bone elongation (endochondral ossification) • Articulating joints • Flexible support (ears, trachea, bronchi)

Cartilage Cartilage is composed of: • Cells (chondrocytes) which constitute only 2-5% of tissue volume • Cartilage cells are located in lacunae in the matrix • Cartilage is avascular and receives nutrients from vessels of perichondrium • Metabolites and nutrients diffuse via the matrix (low metabolic activity)

Types of cartilage There are three morphological types of cartilage: • Hyaline cartilage • Elastic cartilage • Fibrocartilage The differences depend on the matrix especially the type and amount of the embedded fibers

Hyaline cartilage • This is the most common type of cartilage • The name is derived from the Greek “hyalos”=glass • This is mainly found in developing embryos where it forms the model (template) for developing bones • In adults it is found in the respiratory tract, ventral part of ribs, and articulating joints (articular cartilage)

Hyaline cartilage

Hyaline cartilage

Cartilage cells • Chondroprogenitor cells (found in the perichondrium). These develop into chondroblasts • Chondroblasts (actively secreting matrix) • Chondrocytes (mature cells derived from the chondroblasts).

Chondrogenesis There are two different types of cartilage growth • Appositional growth (adding new cells from the perichondrium) • Interstitial growth (division of older cells to form isogenous or nest cells located in the same lacuna deeper in the matrix)

Hyaline cartilage cells

Hyaline cartilage matrix The main components of the matrix are: • Water (72-75%) • Proteoglycans (10%) • Collagen (type II) (16%) • Glycoproteins (e.g.chondronectin) 1.6% • Minerals (0.5%)

Proteoglycans of matrix The proteoglycans are a complex of protein and sulfated glycosaminoglycans (GAGs) and in particular: • chondroitin-4-sulfate • chondroitin-6-sulfate • keratan sulfate • (non-sulfated) hyaluronic acid

Cartilage matrix

Matrix staining properties • Basophilic after H & E staining (mainly due to sulfated glycosaminoglycans) • Periodic acid-Schiff (PAS) positive • Metachromatic The matrix surrounding lacunae stains deeper (territorial matrix) owing to more GAGs than in the interterritorial matrix

Hyaline cartilage metachromasia

Elastic cartilage • Very flexible and elastic • Contains large quantities of elastic fibers in matrix • Has yellowish color in fresh state • Elastic fibers can be stained with orcein • Found in external ear and epiglottis

Elastic cartilage

Fibrocartilage Fibrocartilage is found in areas subject to high mechanical stress or weightbearing such as: • intervertebral disks • pubic symphysis • temporo-mandibular joints • ligament connections to bones (e.g. Ligamentum teres femoris) • tendon insertions

Fibrocartilage • possesses large amounts of collagen fibers (little amorphous matrix) • white appearance because of collagen • acidophilic in H&E staining • lacks perichondrium • high tensile inelastic properties

Intervertebral disks

Secondary cartilage • This refers to cartilage that develops in association with specific bones formed by intramembranous ossification after the bones are already present (unlike cartilage associated with endochondral ossification) • The temporo-mandibular joint (TMJ) is an example of secondary cartilage

Cartilage regeneration • Cartilage in adults has very limited regenerative ability if injured or after wearand-tear of aging • This is result of limited number cartilage cells and minimal cell turnover • Absence of integral blood supply • Low metabolic activity