Development of most of the bones of the body is preceded by the formation of a temporary model of hya-line cartilage. The chief peculiarity of this method of ossification is that the cartilage is provisional and is replaced by bony tissue, which is formed in the my already described,(successive formation of osteoblasts, osteoid and bone matrix). The intermediate stage of cartilage is advantageous because the cartilage can grow rapidly enough to match the very fast growth of the fetus when the skeleton is being formed. All long bones of the body are formed by the intracartilaginous method of ossification. In addition, all other components of skeleton, which do not develop by the intramembranous method of ossification, are formed by intracartilaginous ossification (e.g.the short
bones, ribs and vertebrae).
The endochondral method of osteogenesis can best be understood by considering the development of a typical long bone.Mesenchyme condenses in the 5th week of development to form the primordium of the long bone.The mesenchymal model is replaced by a model of hyaline cartilage during the 6th week.The cartilaginous model resembles very closely to the shape of the bone that will finally be formed.During the 8th week,the process of ossification begins by the appearance of a primary (or diaphysial) center of ossification in the middle of the shaft. In this center of ossification the cartilage cells multiply and increase in size with corresponding enlargement of their lacunae. The cartilage matrix (forming the wall of the enlarged lacunae) becomes calcified by the deposition of calcium salts. As a result, the cartilage cells in the lacunae become deprived of their nutrition. Consequently, these cells die, disintegrate and disappear, leaving their enlarged lacunae empty. When these events are taking place in the center of the shaft of cartilaginous bone model, the peri-chondrial covering of the shaft changes into periosteum and the cells in its inner layer transform into osteoblasts. These periostea! osteoblasts lay down a thin layer of compact bone around the shaft, which is called periosteal bony collar.
Accompanying these changes, vascular buds derived from the inner layer of periosteum penetrate into the calcified cartilage matrix.These vascular buds consist of a network of actively growing capillaries associated with mesenchymal cells. Some of these mesenchymal cells become chondro- clasts (cartilage destroying cells), while others differentiate into osteoblast. The chondroclasts break down and remove the calcified cartilage matrix present between cartilage lacunae, Consequently,adjacent lacunae merge with each other and, thus, larger
spaces are formed within the center of ossification.These spaces become
occupied by the osteoblasts,which arrange themselves along the remain-
ing spicules and strands of the calcified cartilage. Through the activity of the. osteoblasts, layers of bone tissue are deposited on the cartilaginous strands For this reason the newly formed endo- chondral bon is characteristically spongy.However,the bony layers gradually fuse with one another and very soon also unite with the compact bone of the periostea! collar. In addition to chondroclasts and osteoblasts. multi-nucleated giant cells called osteo- clasts aIso appear in the vascular buds, As their name indicates, the osteo-clasts possess the capability of resorp- the osteoblasts produce an irregular system of intercommunicating spaces called primary marrow spaces, which become occupied by primary bone marrow.The bone shaft, which ossifies by the activity of the primary center of ossification,is called diaphysis.
During the fetal life, the cartilage at each end of the bone continues to grow rapidly and ossification process extends from the primary center of
ossification toward the bone ends,
After birth, secondary (or epiphysial) centers of ossification appear in the
.bone ends.These centers are estab-lished in a manner similar to that of the
primary center of ossification. After the appearance of secondary centers of
ossification, a plate of cartilage remains near each end of the bone, intervening
between the bony shaft and the bony ends.These cartilaginous plates are
called epiphysial cartilages or growth plates. The bone ends, which ossify
from the secondary centers of ossi- fication, are called epiphyses.
Growth Of Intramembmous
Bones
These bones grow in diameter by continued marginal ossification from
Osteoblast -rich connective tissue at themargins (the future site of sutures). The intramembranous bone increase in thickness by further deposition of peri-osteal compact bone at the inner and outer bone surfaces,
The osteoblasts which remain on the outer surface of an intra-membranous bone assume a fibro-blast-like appearance and persist as the deep cells of the periosteum. Nor-mally these cells remain dormant dur-ing rest of the life. However, under ap- propriate stimulus (e.g. bone injury), these cells can resume morphological characteristics and functional cap-abilities of osteoblasts.
characteristics and functional capabilities of osteoblasts.