Chapter 5. Cartilage

A. Introduction

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Image: Rutgers Biology

Image: Prologue Histology Resource, UCSF

Image: Yale School of Medicine

Cartilage is connective tissue specialized to offer very firm but resilient physical support to other tissues in the animal body. It is also one of the two principal tissues of the skeletal system.  Because fully functional cartilage can undergo internal growth, it serves as an effective model for bone development and an important component of most growing bones. 

Like other connective tissues, cartilage is composed of connective tissue cells embedded in an extracellular matrix. The extracellular matrix is composed of extracellular fibers embedded in an amorphous ground substance. Beyond this, however, cartilage has specialized features that sets it apart from other connective tissues. 

The cartilagenous matrix is composed of significant amounts of collagen embedded in a very firm ground substance. The ground substance is similar to that of connective tissue proper in many respects. The organic component of the ground substance consists of large proteoglycan aggregates composed of proteoglycans linked to the large nonsulfated glycosaminoglycan hyluronan (hyaluronic acid).  The proteoglycans are composed of large numbers of sulfated glycoasminoglycans linked to a core protein.  The concentration of these proteoglycans is significantly higher than that found in the ground substance of stromal connective tissues.  The extemely high negative charge associated with the sulfated glycosaminoglycans attracts cations, mostly Na+, which in turn attracts and holds high volumes of water (water is approximately 60 - 80% of the wet weight of cartilage), forming a compressible, weight-bearing gel capable of rapid fluid and gas exchange. Nutrient delivery and waste disposal depend on diffusion of substances through the hydrated extracellular matrix of this avascular tissue.

Three basic types of cartilage are found in the human: hyaline cartilage, elastic cartilage and fibrocartilage.  The type of collagen present in the matrix depends on the type of cartilage.  Most cartilage contains collagen type II and collagen type VI fibrils. The proteoglycan aggregates of the ground substance electrostatically binds to the collagen fibrils, imparting a degree of tensile strength to cartilage.

The cells embedded in the matrix are called chondrocytes. Chondrocytes are mature cartilage cells derived from precursor cells called chondroblasts. Chondrocytes exhibit two important functional characteristics:

  • They synthesize the organic components of the extracellular matrix (this activity is significantly reduced in older chondrocytes)
  • They are capable of undergoing proliferation - the cellular basis of the internal growth of cartilage.

The vascular supply of cartilage is supported by a dense connective tissue capsule called perichondrium. Nutrient delivery and waste disposal depend on diffusion of substances through the hydrated extracellular matrix, between the chondrocytes and the vasculature of the perichondrium.