Unlike most tissues, articular cartilage does not have blood vessels, nerves, or lymphatics. It consists of a highly organized extracellular matrix (ECM) with a sparse population of highly specialized cells (chondrocytes) distributed throughout the tissue.
The primary components of theECMarewater, proteoglycans, and collagens, with other proteins and glycoproteins present in smaller amounts. The structure and composition of the articular cartilage vary throughout its depth (Fig. 1),
from the articular surface to the subchondral bone. These differences include cell shape and volume, collagen fibril diameter and orientation, proteoglycan concentration and water content. The cartilage can be divided into four zones: the superficial tangential zone; the middle or transitional zone; the deep zone; and the calcified zone. The superficial tangential zone forms the smooth, nearly frictionless gliding surface. The thin collagen fibrils there are arranged parallel to the surface, and the chondrocytes are elongated with the long axis parallel to the surface. Here the proteoglycan content is at its lowest concentration and thewater content at its highest. The middle (transitional) zone contains collagen fibers with less apparent organization, and the chondrocytes have a more rounded appearance. The deep zone contains the highest concentration of proteoglycans and the lowest water content. The collagen fibers are organized vertical to the joint surface, and the chondrocytes are arranged in a columnar fashion. The deepest layer, the calcified zone, separates hyaline cartilage from the subchondral bone. Histologic staining with hematoxylin and eosin shows a wavy bluish line, called the tidemark, which separates the deep zone from the calcified zone. In addition to these articular surface-tobone zonal distinctions, the ECM is divided into pericellular, territorial, or interterritorial regions (Fig. 2).
The pericellular matrix is a thin layer that completely surrounds each chondrocyte. It contains proteoglycans and other noncollagenous matrix components. This pericellular matrix serves as an important biomechanical buffer between the territorial matrix and the cell. The territorial matrix surrounds the pericellular matrix, and it is characterized by thin collagen fibrils that form a fibrillar network that is distinct from the surrounding interterritorial matrix. The interterritorial matrix is the largest of the matrix regions and contributes most of the material properties of the articular cartilage. It encompasses all of the matrix between the territorial matrices of the individual cells or clusters of cells and contains large collagen fibers and most of the proteoglycans.