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CHAPTER 3: CARTILAGE

OVERVIEW

The three types of cartilage—hyaline, elastic, and fibrocartilage—are connective tissues that provide specialized support to both soft and hard tissues, allowing them to resist external pressure.

  • Hyaline cartilage is the most common. In adults, it is found arranged in rings and plates that help keep the walls of the large airways open, and it also lines the ends of bones at movable joints, producing resilient gliding surfaces.

  • Elastic cartilage facilitates speech by providing flexible support to the larynx.

  • Fibrocartilage links the vertebral bones together with shock-absorbing spacers.

As is the case with all connective tissues, the composition of the extracellular matrix (ECM) is the major determinant of the properties of cartilage, but cartilage differs from connective tissue proper in two significant ways: it is avascular and it contains only one cell type, the chondrocyte.

GENERAL FEATURES OF CARTILAGE

To understand the three types of cartilage, it is useful to consider the following four features (Figure 3-1A):

Figure 3-1:

Features of cartilage; hyaline cartilage. A. General features of cartilage in an adult. All cartilage, except fibrocartilage and articular (hyaline) cartilage, is covered with a dense connective tissue perichondrium, which contains blood vessels, progenitor cells, and chondroblasts. Chondrocytes reside in lacunae in the matrix, and their cell division forms isogenous (clonal) cell groups. The matrix immediately surrounding cells (territorial matrix) differs in composition from the interterritorial matrix between isogenous groups. B. Hyaline cartilage from the larynx. The perichondrium is composed of thick bundles of collagen and a deeper, more cellular layer that contains chondroblasts. Below the cellular layer of the perichondrium are individual chondrocytes separated by matrix, which quickly adopts the dark staining characteristics of mature matrix. The interterritorial matrix (ITM) between the isogenous groups of chondrocytes stains less darkly than the territorial matrix. Chondrocytes in a group are separated by septae (arrow). C. An aggrecan–hyaluronic acid complex in the matrix of hyaline cartilage. Each aggrecan core protein can carry about 100 glycosaminoglycans (GAGs), typically chondroitin sulfate and keratan sulfate, yielding a molecular weight of two to three million. Dozens of aggrecans may be linked to hyaluronic acid, forming a single polymer several microns long. D. Hyaline cartilage at an articular surface. Chondrocytes are found as individual cells in lacunae that extend up to the surface of this tissue, which covers the ends of bones at synovial joints.

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  • Chondrocytes. The only cells found in normal cartilage are chondrocytes, which are produced from progenitor cells called chondroblasts. When chondrocytes divide, the resulting daughter cells remain closely associated in clonal groups, called isogenous groups or cell nests, and are surrounded by the ECM they produce.

  • ECM. This material contains both fibrous proteins and ground substance. Fibrils formed from type II collagen are ...

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