14. Cartilage structure function

Cartilage: Structure and Function

Cartilage is a tough, durable connective tissue with an extracellular matrix (ECM) containing a high concentration of glycosaminoglycans (GAGs) and proteoglycans. It plays essential roles in supporting and cushioning various body structures.

1. Composition of Cartilage ECM:
   • Collagen: The properties of collagen in cartilage depend on electrostatic bonds between type II collagen fibrils, hyaluronan, and sulfated GAGs on proteoglycans.
   • Hyaluronan and GAGs: These negatively charged molecules contribute to the high water content within the dense meshwork of thin collagen type II fibers, making cartilage an effective shock absorber.

1. Cell Types in Cartilage:
   • Chondrocytes: Cartilage consists primarily of chondrocytes embedded within the ECM. These cells reside in matrix cavities called lacunae.
   • Vascular Supply: Cartilage lacks direct blood vessels, and chondrocytes receive nutrients from the surrounding connective tissue (perichondrium). Large blood vessels can traverse cartilage but provide limited nutrients due to hypoxia.
2. Perichondrium:
   • The perichondrium is a dense irregular connective tissue that surrounds cartilage.
   • It has two distinct layers:
     • Outer Fibrous Layer: Contains fibroblasts producing collagenous fibers.
     • Chondrogenic Layer: Remains undifferentiated and can form chondroblasts.

1. Types of Cartilage:
   • Hyaline Cartilage:
     • Most common type, homogenous, and semitransparent.
     • Found in articular surfaces of movable joints, respiratory passages (nose, larynx, trachea, bronchi), ventral ends of ribs, and epiphyseal plates of long bones.

1. • • Key features:
       • Type II collagen and aggrecan
       • Presence of chondrocytes and chondroblasts
       • Chondronectin (similar to fibronectin) mediates adherence of chondrocytes to the ECM.
       • Basophilic matrix stained blue due to GAGs.
       • Territorial matrix (around chondrocytes) differs from inter-territorial matrix (richer in collagen).
     • Isogenous aggregates: Groups of around 8 chondrocytes resulting from mitotic division.

1. • Elastic Cartilage:
     • Similar to hyaline cartilage but contains elastic fibers (collagen type II).
     • Yellowish color due to elastin.
     • Found in the auricle of the ear, walls of the external auditory canal, epiglottis, and cuneiform cartilage in the larynx.

1. • Fibrocartilage:
     • Intermediate between dense connective tissue and hyaline cartilage.
     • Found in intervertebral discs and pubic symphysis.
     • Contains collagen type I, fewer proteoglycans, and lacks a distinct perichondrium.

1. Chondrogenesis:
   • Formation of cartilage from embryonic mesenchyme:

1.               Mesenchymal cells round up, multiply, and become densely packed.

2.               During rapid proliferation, they are called chondroblasts.

3.               ECM production encloses cells in lacunae and separates chondroblasts.

4.               Cartilage differentiation occurs outward, with central cells resembling chondrocytes and outer cells resembling chondroblasts.

5.               Superficial mesenchyme develops as the perichondrium.

6.      Interstitial Growth:

1. • Interstitial growth involves the mitotic division of preexisting chondrocytes during the early phase of cartilage formation.
   • It increases tissue mass from within and occurs in the following locations:

0.                                                       Epiphyseal Plates of Long Bones: These growth plates allow for longitudinal bone growth during development.

1.                                                       Articular Cartilage: The cartilage covering the articulating surfaces of movable joints.

1. Appositional Growth:
   • Appositional growth occurs at the cartilage’s periphery.
   • The process involves chondroblasts differentiating in the inner layers of the perichondrium (the connective tissue surrounding cartilage).
   • These chondroblasts proliferate and eventually become chondrocytes as they surround themselves with cartilaginous matrix.