Cartilage

Department Of General Histology

Introduction Cartilage is characterized by an extracellular matrix (ECM) enriched with

glycosaminoglycans and proteoglycans, macromolecules that interact with collagen and elastic fibers. Variations in the composition of these matrix components produce three types of cartilage adapted to local biomechanical needs.

Cartilage is a specialized form of connective tissue in which the firm consistency of the ECM allows the tissue to bear mechanical stresses without permanent distortion. In the respiratory system cartilage forms a framework supporting soft tissues. Because it is smooth-surfaced and resilient, cartilage provides a shock-absorbing and sliding area for joints and facilitates bone movements. Cartilage is also essential for the development and growth of long bones, both before and after birth (see Chapter 8).

Cartilage consists of cells called chondrocytes (Gr. chondros, cartilage + kytos, cell) and an extensive extracellular matrix composed of fibers and ground substance. Chondrocytes synthesize and secrete the ECM and the cells themselves are located in matrix cavities called lacunae. Collagen, hyaluronic acid, proteoglycans, and small amounts of several glycoproteins are the principal macromolecules present in all types of cartilage matrix.

Because collagen and elastin are flexible, the firm gel-like consistency of cartilage depends on electrostatic bonds between collagen fibers and the glycosaminoglycan side chains of matrix proteoglycans. It also depends on the binding of water (solvation water) to the negatively charged glycosaminoglycan chains that extend from the proteoglycan core proteins.

As a consequence of different functional requirements, three forms of cartilage have evolved, each exhibiting variation in matrix composition. In the matrix of hyaline cartilage, the most common form, type II collagen is the principal collagen type (Figure 7–1). The more pliable and distensible elastic cartilage possesses, in addition to collagen type II, an abundance of elastic fibers within its matrix. Fibrocartilage, present in regions of the body subjected to pulling forces, is characterized by a matrix containing a dense network of coarse type I collagen fibers.

Distribution of cartilage in adults

(a): There are three types of adult cartilage distributed in many areas of the skeleton, particularly in joints and where pliable support is useful, as in the ribs, ears, and nose. Cartilage support of other tissues throughout the respiratory system is also prominent. The photomicrographs show the main features of

(b) hyaline cartilage, (c) fibrocartilage, and (d) elastic cartilage.

Hyaline cartilage A section of hyaline cartilage shows chondrocytes located in matrix lacunae. Preparation for sectioning usually causes shrinkage of the matrix which may cause the chondrocytes to pull away from the matrix and become distorted. The upper part of the figure shows the more eosinophilic perichondrium, an example of dense connective tissue consisting largely of type I collagen. There is a gradual transition and differentiation of cells from the perichondrium to the cartilage, with elongated fibroblastic cells becoming larger and more rounded chondrocytes with irregular surfaces contacting the matrix secreted by the cells.

Perichondrium Diagram of the area of transition between the perichondrium and the hyaline cartilage. In living cartilage chondrocytes essentially fill their lacunae. Closely associated groups of two or four lacunae indicate isogenous groups, or clones of chondrocytes derived from the same cell. Staining differences are apparent between the matrix immediately around each lacuna, called the territorial matrix, and that more distant from lacunae, the interterritorial matrix. Collagen is more abundant in the interterritorial parts of the matrix.

Molecular organization of hyaline cartilage ECM

Chondrocytes

Elastic cartilage

Photomicrograph of elastic cartilage from the epiglottis shows perichondrium (P) on both surfaces. Cell size and distribution in elastic cartilage is very similar to that of hyaline cartilage. With special staining for elastic fibers however, the matrix is seen to be filled with this material (arrows), providing greater flexibility to this form of cartilage.

Fibrocartilage

Micrograph of pubic symphysis shows staining variations in the matrix caused by varying concentrations of collagen (C). Lacunae (arrows) of chondrocytes are also seen. A section of intervertebral disk. X100. Masson trichrome.

axial aggregates of chondrocytes are separated by collagen. Fibrocartilage is also frequently found in the insertion of tendons on the epiphyseal hyaline cartilage.

Chondrogenesis

Embryonic mesenchyme is the precursor tissue of all types of cartilage.

Mitotic proliferation of mesenchymal cells and early differentiation gives rise to a tissue with condensations of rounded cells called chondroblasts.

Chondroblasts are separated from one another by their own production of various matrix components which collectively swell with water and form a great amount of ECM.

Multiplication of cartilage cells gives rise to isogenous aggregates, each surrounded by a condensation of territorial matrix. In mature cartilage this interstitial mitotic activity ceases and all chondrocytes typically become more widely separated by their production of matrix.

Chondrocytes in growing fibrocartilage

This TEM of fibrocartilage from a young animal shows three chondrocytes in their lacunae. RER is abundant in the cells, which are actively secreting their collagen-rich matrix. Fine collagen fibers, sectioned in several orientations, are prominent around the chondrocytes of fibrocartilage. Growing chondrocytes in hyaline and elastic cartilage have more prominent Golgi complexes and synthesize abundant proteoglycans in addition to collagens.

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