234 VITAMIN D GROUP 



known as osteoblasts are fibroblast derivatives which possess the double 

 function of bone formation and bone destruction. Although it is in both 

 instances the same cell, the term osteoblast is applied to those cells which 

 are producing bone, and the term osteoclast is applied to those in the process 

 of bone resorption. As death of cartilaginous cells proceeds, there is an 

 invasion of the matrix by capillaries proceeding upward from the diaphysis. 

 These invade the empty lacunae. Some of the matrix trabeculae are resorbed 

 by activity of the osteoblasts, giving rise to an increasing size of the marrow^ 

 space and a reduction in the number but an increase in size of the trabecu- 

 lae. Into the enlarged lacunae the blood vessels grow, continuously carrying 

 with them connective tissue, osteoblasts, and osteoclasts. Upon the larger 

 trabeculae the osteoblasts lay down true bone. Between the osteoblasts and 

 the cartilage matrix, a new layer of tissue appeal's, gradually thickens, and 

 surrounds the contours of the cartilage projections. It is this tissue which 

 under favorable conditions begins to calcify as it is deposited and thus 

 becomes bone. Physiologically, there is a lag in calcification, resulting in 

 formation of an uncalcified osseous material known as osteoid. This appears 

 in a limited degree under physiological conditions but becomes enlarged 

 when there is local failure to supply calcium and phosphate. When this 

 failure is marked and generalized, there is an increase in the width of the 

 osteoid border. This is a picture typical of diaphyseal rickets and of oste- 

 omalacia. By this complex process (bone formation and bone resorption) 

 the bones become more hollow because of the increase in size of the marrow 

 spaces, and stronger because of the widening and ossification of the trabecu- 

 lae and the increased strengthening of the cortex. 



Although the morphological changes associated with endochondral bone 

 formation have been carefully studied and adequately described, the chem- 

 ical changes related to this process are not so well understood. Mineraliza- 

 tion of bone matrix and osteoid consists of the deposition of calcium and 

 phosphate, plus, to a lesser degree, carbonates, fluorides, and perhaps other 

 anions combined wdth small amounts of calcium, sodium, magnesium, and 

 potassium. The nature of the mineral deposit has been the subject of much 

 investigation and controversy. The most acceptable concept has been that 

 the mineral matter of bone has a crystalline structure resembling that of 

 the apatite minerals, and that dissolved in this, perhaps as an adsorbate 

 or as a solid solution, are the calcium salts hydroxide, fluoride, carbonate, 

 etc. It seems clear that an adequate concentration of calcium and inorganic 

 phosphorus both in the bone matrix and the tissue fluid is essential for this 

 process, but the exact level at which calcification occurs is as yet poorly 

 defined, and the mechanism itself is not clear. 



Studies on calcification of endochondral cartilage in vitro have added 

 much to our knowledge of this subject. Inhibition of this process may occur 



