Cartilage

Objectives

Students should aim to recognize and differentiate the 3 types of cartilage, understand the composition of the extracellular matrix and the role of chondrocytes in producing and maintaining it.

Cartilage is a specialized form of connective tissue containing chondrocytes which secrete, and are surrounded by, an extensive intercellular matrix. Chondrocytes occur singly or in isogenous groups, composed of 2-8 cells derived by mitosis from a single chondrocyte. The cells are in the lacunae (cavities) within the matrix. Matrix stains more intensely immediately adjacent to the lacunae and the dark staining zone is called the capsule. The strength and durability of cartilage are properties of the matrix, which is an interlaced network of collagenous and/or elastic fibers in a ground substance, a gel of complex proteoglycans. The collagen is mostly Type II. How does this differ from dense C.T.?

There are three types of cartilage characterized by the composition of the intercellular matrix.

(1) hyaline, (2) elastic and (3) fibrocartilage.

Hyaline cartilage is found lining articular surfaces, and in the nasal septum, tracheal rings, costal cartilages, and the epiphyseal cartilage of growing bone. Study the trachea on slide #72 (trachea). Even numbered boxes usually contain slides stained for elastic tissues, odd numbered boxes have slides stained with hematoxylin and eosin. The former will have a brown coloration, the latter, the pink and blue you are familiar with. In this slide considerable fading has occurred, and the normal basophilia of freshly stained cartilage is almost absent. The trachea is a large open tube, with cartilage constituting the principal component of its wall.

Using the scanning lens, locate the cartilage rings. At higher power, identify single chondrocytes and locate isogenous groups. There are clear areas between many of the chondrocytes and the walls of their lacunae because of shrinkage of the cells brought about by fixation, and because some chondrocytes had lipid droplets which dissolved during histological preparation.

Identify the perichondrium. Note the gradation of cell shape changes from fibroblast-like in the outer layer to the round cell shape within the cartilage. The perichondrium is acidophilic due to the preponderance of collagen fibers. At the interface between perichondrium and cartilage, transitional cells can be seen which are still elongated but are beginning to be surrounded by matrix.

The cartilage matrix contains collagen fibers, but these are very difficult or impossible to see. Some areas may show some dark-staining material which could be indicative of elastic fibers, but these are best seen in a later slide. Note that the cartilage matrix should be slightly basophilic, especially around the chondrocytes, due to anionic properties of proteoglycans.

Elastic cartilage is found in the ear and epiglottis, where it provides a rigid but elastic, framework. Its principal components are elastic fibers but type II collagen is also present. Some elastic fibers may be present in the tracheal cartilage. Examine a section through infant ear, slide #11 (even boxes only), which has been stained for elastin with resorcin fuchsin. The purple staining interlaced fibers run in all directions, and appear as dots when cut transversely. The nuclei and cytoplasm of the cells are not stained well. The cartilage in this section is immature and therefore richer in cells than adult cartilage. Compare this arrangement of cells and fibers of elastic cartilage with that of hyaline cartilage of the trachea.

Fibrocartilage is found in intervertebral discs, the pubic symphysis, in menisci of joints, and often occurs where tendon and ligament are joined to bones. It resembles a dense C.T. which contains islands of chondrocytes embedded in cartilage matrix. Its appearance varies with its location. The major and characteristic intercellular component of this tissue consists of thick, interlaced collagenous fibers. The ground substance is much less abundant than in hyaline cartilage due to the preponderance of fibers.

Examine a section of human intervertebral disc, slide #12. Most of the tissue on this slide is fibrocartilage, with some dense C.T., and possibly fat, on the periphery. Examine the general structure of this tissue. It is difficult to distinguish from dense C.T. Careful examination will reveal that some cells are within lacunae. You may not be able to discern the difference between chondrocyte and fibroblasts by nuclear shape, but if a lacunus surrounds the cell, it is a chondrocyte; if not, it is a fibroblast. Interspersed among the deeper staining fibrous areas are less acidophilic areas where there are chondrocytes, accumulations of ground substance, and fewer coarse fibers. There may be wrinkles in the sections which stain deeply (artifacts). Compare the appearance of the cells of the cartilage islands in the fibrocartilage with those of hyaline cartilage and of dense connective tissue.