INTERCELLULAR SUBSTANCES AND WALLS 87 



each chitobiose repeating unit, 10.3 A in length, would coincide 

 with each three amino acid residues in the protein. 



The final process which we will consider is, like others mention- 

 ed, distinct from the traditional botanical concept of cell wall 

 formation. Nevertheless, osteogenesis contains all the essential 

 elements of a wall-forming process. Bone formation begins with 

 the differentiation of mesenchymal cells into osteoblasts, proceeds 

 through their active deposition of the intercellular substance which 

 makes up the organic matrix of bone, and is completed with the 

 deposition of bone mineral on the matrix. Functionally, the osteo- 

 blast situated in the calcified interstitial substance becomes the 

 bone-maintaining osteocyte. We will not concern ourselves with 

 the many interactions and interconversions among connective 

 tissue cells, but only with the intercelluar substances elaborated 

 by the bone-forming cell. Most (90-96 per cent) of the dry, lipid- 

 free organic matter of bone consists of the protein collagen, which 

 is deposited in the form of fibrils (0.3— 0.5[x in diameter) or fibrillar 

 aggregates. Collagens are noted for their X-ray diffraction patterns, 

 and form a characteristic double cross-banding with a period 

 of 640 A. The interfibrillar ground substance consists of amor- 

 phous mucopolysaccharides such as hyaluronic acid and chondro- 

 itin sulfate, which on hydrolysis yield glucosamine or galactosamine, 

 sulfuric acid and glucuronic acid. The amount of mucopolysacch- 

 aride in bone is small, 0.1-0.4 per cent, but it is nevertheless an 

 important component of the organic matrix. 



The third polymeric matrix component is reticulin, a fibrous 

 glycoprotein whose polysaccharide moiety contains mannose, 

 galactose, fucose and, perhaps, hexosamine. It is of interest in its 

 intermediate position relative to the protein and polysaccharide 

 constituents. The mineral composition of bone salt is approximated 

 by hydroxyapatite : 



3 Ca 3 (P0 4 ) 2 • Ca(OH) 2 



Crystallographically, hydroxyapatite is hexagonal with a unit 

 cell 9.4 Ax 9.4 Ax 6.9 A. 



Bone crystals are submicroscopic (representative dimensions: 

 500 A x 250 A x 85 A). Thus, they are only a few unit cells in thick- 



