666 THE SKELETAL SYSTEM 



the cells. The increase in the intercellular substance separates the chondro- 

 blasts from each other, and the mass of cartilage expands as a whole. These 

 two types of growth are important processes involved in the increase in size 

 of many body structures. Cartilage formation in the human embryo begins 

 during the fifth and sixth weeks. 



3. Development of Bone 



Bone develops as the result of the calcification of previously established 

 fibrous or cartilaginous connective tissues. The transformation of fibrous con- 

 nective tissue into bone is called membranous or intramembranous bone for- 

 mation, and the process which transforms cartilage into bone constitutes 

 endochondral or intracartilaginous bone development. Membranous bone for- 

 mation occurs in the superficial areas of the body, particularly in or near the 

 dermal area of the skin whereas cartilaginous bone formation is found more 

 deeply within the substance of the body and its appendages. 



a. Membranous Bone Formation 



Membranous bone formation occurs as follows (fig. 314G): Thin spicules 

 or bars of a compact intercellular substance, known as ossein, gradually come 

 to surround collagenous (osteogenic) fibers which lie between fibroblast cells. 

 Later, these spicules of ossein become calcified by the action of specialized 

 cells, called osteoblasts, which surround the osseinated fibrils. Osteoblasts may 

 represent transformed fibroblasts or, more directly, transformed mesenchymal 

 cells. With the deposition of the bone salts, the tissue is converted from ossein 

 into bone. Thus, spicules of ossein and connective tissue fibers serve as the 

 basis for bone deposition and become converted into bony spicules. These 

 spicules are converted next into bony columns (trabeculae) by the formation 

 of layers (lamellae) of compact bone around the original bony spicule. Such 

 bony columns or trabeculae are characteristic of spongy bone (fig. 314A). 

 Some of the bone-forming cells become enclosed within the lacunar spaces 

 in the bone during the above process and are left behind as bone cells or 

 osteocytes (fig. 314A). The osteocytes within their respective lacunae tend 

 to be located between the layers of bony material (fig. 314A-D). 



After the primary trabeculae of spongy bone are formed, the surrounding 

 mesenchyme, which encloses the site of bone formation, becomes converted 

 into a membranous structure, known as the periosteum. The cells of the 

 inner layer of periosteum are transformed into osteoblasts and begin to de- 

 posit successive layers of compact bone around the initial framework of 

 spongy bone (peripheral growth). The latter activity results in an increase 

 in diameter of the bony area. 



The first bone thus formed occurs in a restricted area. As the bone grows, 

 the previously formed bone is torn down and resorbed, while new compact 

 bone is built up around the area occupied by the spongy bone. Either by the 



