Overview – Cartilage
Plate 5-1. Hyaline Cartilage
Plate 5-2. Elastic Cartilage and Fibrocartilage
Hyaline cartilage has unique and diverse characteristics. Like all cartilage, it is a specialized form of connective tissue that consists of cells and extracellular material. Cartilage cells, called chondrocytes (C), are embedded in an Extracellular Matrix (M), and it is the Matrix that endows cartilage with its unique characteristics. As depicted at right, Hyaline Cartilage is covered by a tough, fibrous sheath of Dense Irregular Connective Tissue called the Perichondrium (P). The Perichondrium shown here contains a graded series of cells caught at different stages of development. The cells at the periphery of the Perichondrium are flat fibroblasts (F) that, in time, differentiate to form the egg-shaped chondroblasts (CB), which in turn develop into round, mature chondrocytes. The chondrocytes, which often contain large lipid droplets (L), are responsible for the synthesis, secretion, and maintenance of the Extracellular Matrix. Should the chondrocytes die or become damaged, the cartilage Matrix degenerates and can undergo calcification. Although the Perichondrium can act as a source of new chondrocytes to a limited extent, cartilage does not regenerate well unless the animal is quite young.
The Extracellular Matrix of Hyaline Cartilage, located around and between the chondrocytes, is all-important and its structure, complex. The Matrix contains a feltwork of randomly oriented Collagen fibers that provide structural strength. These Collagen fibers are embedded in an amorphous, gellike Ground Substanceendowed with two key properties. First, the ground substance binds to Collagen; second, it has a tremendous affinity for water.
The Ground Substance is composed largely of Chondromucoprotein, a copolymer of proteins and mucopolysaccharides secreted by chondrocytes. Like all cartilage, Hyaline Cartilage is avascular, and hence the diffusion of nutrients from the outer limits of the perichondriurn into the core of the cartilage depends on the water-holding properties of its Chondromucoprotein Ground Substance, since water facilitates diffusion. With aging, as the concentration of hydrophilic Chondromucoprotein in the Matrix surrounding a Chondrocyte falls, so does its water content. Consequently, the rate of diffusion of nutrients to and metabolic wastes from the Chondrocyte decreases rapidly, and the aging Chondrocyte starves and dies. This process forms a vicious circle of degeneration, because the dead Chondrocyte can no longer maintain its surrounding Matrix, which may then undergo calcification and hinder diffusion of nutrients to nearby chondrocytes.
Because cartilage is avascular, many small arterioles (A) and capillaries (Ca) are closely associated with the Perichondrium. In the micrographs at right, the myelinated axons of a small nerve (N) are present, as are several obliquely sectioned Skeletal Muscle fibers (MF). Several fat cells (FC), or adipocytes, laden with lipid, are scattered throughout the tissues beneath the cartilage.
Plate 5-1, Figures A and B. Matched set of light and electron micrographs of serial sections taken through the mouse trachea. A, Arteriole; C, Chondrocyte; Ca, capillary; CB, Chondroblast; F, Fibroblast; FC, fat cell; L, lipid droplet; M, Matrix; MF, muscle Fiber; N, nerve; P, Perichondrium. Figure A, 1,250 X;
Elastic Cartilage And Fibrocartilage
In addition to Hyaline Cartilage, described in the previous plate, there are two other classes of cartilage: elastic cartilage (Figure A) and Fibrocartilage (Figures B and C). Elastic Cartilage is found in such places as the ear, the external auditory tube, and the eustachian tube. The Matrix of Elastic Cartilage is criss-crossed by numerous branched elastic fibers rich in the protein Elastin. These elastic fibers lend a high degree of flexibility and resilience to the Matrix, endowing it with what polymer chemists call “memory.” When bent, Elastic Cartilage will rapidly bounce back to its original form. Figure A is a low-magnification electron micrograph of a cross section cut through the external ear of the monkey. The outer margins of the cartilage, like those of hyaline cartilage, are defined by a fibrous Perichondrium (P) made of dense connective tissue. The Perichondriumcontains Collagen fibrils (CF), fibroblasts (F), and chondroblasts (CB). The mature chondrocytes (C) occupy spaces in the Matrix (M) called lacunae. When viewed with the light microscope, Elastic Cartilage, if not specially stained to reveal the elastic fibers, looks very much like Hyaline Cartilage. Under the electron microscope, however, the elastic fibers are evident as clusters of electron-dense fibrils (arrows).
Although Hyaline Cartilage and Elastic Cartilage share many morphologic features, the third kind of cartilage, Fibrocartilage, is unique. Its Extracellular Matrix has far more Collagen and less Chondromucoprotein than do the matrices of hyaline and Elastic Cartilage. In many ways, Fibrocartilage resembles dense connective tissue. It lacks a true Perichondrium, and often, as in the intervertebral disk, forms a structural and functional junction where cartilage and Ligamentmeet. Figure B is a relatively high magnification light-microscopic image of a field of Fibrocartilage in the intervertebral disk. Photographed here with the Nomarski interference microscope, the large Collagen fibers (arrows) stand out in bold relief. The chondrocytes (C) look more like fibroblasts than cartilage cells, which is to be expected because their primary job is the biosynthesis of Collagen. The chondrocytes are often fusiform, have dense Cytoplasm, and frequently appear dark and retracted, indicating that they are probably inactive. The territorial Matrix(arrowhead) immediately surrounding each Chondrocyte contains tightly packed Collagen fibers arranged in concentric circles around their cell of origin. Farther out in the interterritorial Matrix, the Collagen fibers (arrow) exhibit a more random orientation and are woven into a dense feltwork of great strength. Consequently, Fibrocartilage is resistant to compression and is used to good advantage in such weight-bearing structures as the intervertebral disk.
An active Fibrocartilage Chondrocyte and its fibrous Matrix are illustrated by electron microscopy in Figure C. Here, the Cytoplasm is packed with Ribosome-studded Cisternae of the rough Endoplasmic Reticulum (RER), reflecting the cell’s intense biosynthesis of protein for export. Many Mitochondria (MC) are present to supply ATP for the cell’s biosynthetic activities. The lobed Nucleus (N) is outlined by a thick margin of Heterochromatin (H), indicating that only a small portion of the cell’s genome is transcriptionally active. The cell surface sends many fine filopodial processes (FP) into the Matrix. The fibrous nature of the Matrix is apparent in this electron micrograph.
Plate 5-2 Cartilage
Plate 5-2, Figure A. Electron micrograph of Elastic Cartilage from the ear. C, Chondrocyte; CB, Chondroblast; CF, Collagen fibrils; F, Fibroblast; M, Matrix; P, Perichondrium; arrows, elastic fibers. 1,100 X Figure B. High-magnification light micrograph (Nomarski interference) of Fibrocartilagefrom the monkey intervertebral disk. C, Chondrocyte; arrow, Collagen fibers; arrowhead, territorial Matrix. 2,000 X Figure C. Electron micrograph of Chondrocyte within Fibrocartilage of the monkey intervertebral disk. FP, filopodial process; H, Heterochromatin; MC, mitochondrion; N, Nucleus; RER, rough Endoplasmic Reticulum; arrow, Collagen Fibril. 7,300 X