8 4 



HISTOLOGY 



concluded that there is but one method of bone formation, whether or 

 not it takes place in relation with cartilage, but he was unaware of the 

 existence of cells, and believed that bones were produced from an ossifying 

 juice derived from the blood. 



Development of bone. Bone formation begins with the production 

 of a layer or spicule of matrix which stains red with eosin. As to the 

 origin of this matrix there is the same difference of opinion which obtains 

 in regard to other intercellular products. It has been asserted that it 

 proceeds from osteogenic fibers, which are modified white fibers of the 

 connective tissue. Frequently a spicule of matrix is seen to fray out 

 into the connective tissue, as shown in the lower part of Fig. 71. Between 

 the osteogenic fibers, calcareous granules may then be deposited until 



Osteoblasts. Calcifying connective tissue bundles. Bone matrix. Bone cells. 



FIG. 71 FROM A SECTION OF THE MANDIBLE OF A HUMAN EMBRYO OF FOUR MONTHS. X 240. 



the fibers are lost in a homogeneous calcified matrix. According to 

 this opinion the matrix is essentially an intercellular formation. Others 

 consider that the matrix is produced by a transformation of the exoplasm 

 of bone-forming cells, or osteoblasts. 



Osteoblasts are derived from mesenchymal or young connective 

 tissue cells through an increase in their protoplasm and a shortening of 

 their processes. They are found in contact with the surface of spicules 

 of bone, arranged in an epithelioid layer (Fig. 72). There is great varia- 

 tion in their shape. Often they are pyramidal, but they may rest upon 

 the bone either by a broad base or a pointed extremity. Their round 

 nuclei may be in the part of the protoplasm next to the bone, or away 

 from it as far as possible. Active osteoblasts tend to be cuboidal or 

 columnar, but as bone production ceases they may become quite flat. 

 They form bone only along that surface which is applied to the matrix. 

 As the strand of bone grows broader through their activity, it encloses 

 here and there an osteoblast, which thus becomes a bone cell (Fig. 72). 

 Apparently bone cells do not divide, and if they produce matrix, thus 



