THE INHERITANCE OF BIOCHEMICAL CHARACTERISTICS 313 



seen. Elastin fibres, also a product of the fibrocytes, like collagen, contain 

 much glycine and proHne, traces of tyrosine and no tryptophan. But whilst 

 collagen is more or less "normal" in the amounts of arginine, lysine, 

 aspartic acid and glutamic acid which it contains, elastin contains only very 

 small amounts of these polar amino acids. The ground substance, also 

 synthesized by the fibrocytes, consists of mucoproteins the prosthetic 

 groups of which consist of chondroitin sulphate joined to galactosamine 

 by a glycoside linkage. The ground substance also contains free muco- 

 polysaccharides, the chief one being hyaluronic acid (acetylglucosamine 

 -f glucuronic acid). 



In the type of connective tissue known as cartilage, the ground substance 

 is more dense than that of ordinary connective tissue and besides collagen 

 and elastin fibres it contains a chondromucoid the prosthetic group of which 

 is chondroitin sulphate. In the case of bony tissue, its character is a result 

 of the biochemical differentiation of the large cells of mesench^Tnatous 

 origin, the osteoblasts. The beginning of this differentiation of a mesen- 

 chymatous cell into an osteoblast, depends on the position of the cell in 

 the bone-forming region and is marked by the appearance of alkaline 

 phosphatase in the nucleus and the accumulation of large amounts of 

 glycogen in the cytoplasm. The mitochondria increase in number and the 

 amount of RNA in the cytoplasm also increases. Between the cells, the 

 interstitial substance is made up mainly of reticular fibres. As differentiation 

 continues, phosphatase activity appears in the cytoplasm and finally reaches 

 a maximum and so does the accumulation of glycogen. The number 

 of mitochondria continues to increase. The basophihc character of the 

 cytoplasm also increases. Bundles of osteogenous collagen appear in the inter- 

 stitial substance, distinct from normal collagen in giving a positive reaction 

 for alkaline phosphatase and in being highly metachromatic due to the pre- 

 sence of polysaccharides. The osteoblast having reached its final form the 

 phosphatase activity in the nucleus and cytoplasm decreases sharply and the 

 cytoplasmic glycogen disappears whilst the basophilia attains a maximum. 

 The organic matrix of the bone, strongly metachromatic and later to be 

 filled with bone salts, appears in the region of the non-nucleated extremity 

 of the osteoblast. What we have just described is the most simple type of 

 ossification and is known as the intramembranous type. In most bones, the 

 production by the osteoblasts of the calcifiable matrix of the bone takes 

 place on a cartilaginous former. The bone is constantly eroded and new 

 osseous material laid down. The erosion is due to the activity of the osteo- 

 clasts, mesenchymatic cells differentiated to possess proteolytic activity. 

 In fact, in the case of osteoblasts and osteoclasts, it would be preferable to 

 speak of biochemical modification rather than of biochemical differentia- 

 tion. In reality osteocytes, osteoblasts and osteoclasts are examples of 

 reversible modifications of the same type of connective cell. 



