Ground substance, cartilage

Our own skin is made up of specialized cells which become filled with microfibrils of keratin as they move outward and become the relatively dry nonliving external surface (Box 8-F). Internal epithelial cells secrete protein and carbohydrate materials that form a thin basement membrane around the exposed parts of the cells. The connective tissue that lies between organs and which also includes tendons, cartilage, and bone consists of a relatively small number of cells surrounded by a "matrix" consisting of the protein fibers collagen and elastin in a "ground substance" rich in proteoglycans (Chapter 4).616 618 in bone, the calcium phosphate is deposited within this matrix. 

A. Proteoglycans are found in the extracellular matrix or ground substance of connective tissue, synovial fluid of joints, vitreous humor of the eye, secretions of mucus-producing cells, and in cartilage. 

The body s connective tissues can be classified as connective tissue proper (subdivided into loose and dense connective tissue proper), fluid connective tissues (subdivided into blood and lymph) and supporting connective tissues (subdivided into the cartilage and the bone). Fibroblasts are the most abundant permanent residents of the connective tissue proper and are the only cells always present in it. Fibroblasts secrete hyaluronans (a polysaccharide derivative) and proteins, both of which interact in the extracellular fluid to form proteoglycans that make ground substances viscous. They also secrete protein subunits that interact to form large extracellular fibres that could create loose/open framework or densely packed framework. 

Chondroitin sulfates are among the principal zoopolysaccharides of the ground substance in mammalian tissue and in cartilage. Cattle nasal cartilage has been the chief source for the preparation of chondroitin sulfate (f), generally extracted by calcium chloride 48) or potassium chloride solution 49). 

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