DEVELOPMENT OF SKELETAL TISSUES 667 



formation of new cellular entities or by the fusion of osteoblasts, multinucle- 

 ated giant cells appear which aid in the dissolution of the previously formed 

 bone. These multinucleate cells are known as osteoclasts (fig. 314H). The 

 marrow-filled spaces between the trabeculae of spongy bone contain blood 

 spaces (sinusoids), developing red blood cells, looser connective tissues, and 

 fat cells (fig. 314H). When the trabeculae of spongy bone are resorbed, the 

 marrow-filled area increases in size. 



b. Endochondral and Perichondrial (Periosteal) Bone Formation 



While membranous bone development utilizes collagenous fibrils and ossein 

 as a foundation upon which the osteoblasts deposit bone salts, endochondral 

 that is, intracartilaginous bone development employs small spicules or larger 

 masses of cartilage as a basis for calcification. The small columns or spicules 

 of cartilage are produced as a result of erosion and removal of cartilage. This 

 erosion of cartilage is produced by perichondrial cells and vascular tissue 

 which invade the cartilaginous substance from the perichondrium. 



1) Endochondral Bone Formation. Endochondral bone formation occurs 

 as follows: 



(a) The initial step in erosion of cartilage is the migration within the 

 cartilage, in a manner not understood, of the scattered cartilage cells. 

 This migration brings about the arrangement of the cartilage cells and 

 their capsules into elongated rows (fig. 314F). Some deposition of 

 calcium within the cartilaginous matrix occurs at this time. 



(b) As this realignment of the cartilage cells is effected, vascular buds 

 from the inner layer of the perichondrium invade the cartilage, eroding 

 the cartilaginous substance and forming primary marrow cavities (figs. 

 31 3D; 314E, F). Large multinucleate cells or chondroclasts make 

 their appearance at this time and aid the process of dissolution of 

 cartilage. 



(c) Following this procedure, osteoblasts arise within the peripheral areas 

 of each vascular bud and begin to deposit bone matrix upon the 

 small spicules of calcified cartilage which remain. (See spicule "a," 

 fig. 314F.) The continual deposition of bone salts around these spicules 

 converts the greatly eroded cartilaginous mass into spongy or cancel- 

 lous bone (fig. 314F). 



2) Perichondrial (Periosteal) Bone Formation. As cancellous bone is 

 formed within the cartilaginous mass, the surrounding perichondrium of the 

 original cartilage now becomes the periosteum, and the cells of the inner layer 

 of the periosteum deposit circumferential layers of compact bone (perichon- 

 drial or periosteal bone formation) around the periphery of the cancellous 

 bone (fig. 314F). The latter action forms a cylinder of compact bone around 

 the spongy variety and around the cartilage which is being displaced (fig. 



