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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.
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.
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.