FIGURE 8.16 • Section of mandible developing by the process of intramembranous ossification. This photomicrograph shows a section from a developing mandible stained with H&E. In this relatively early stage of development, the mandible consists of bone spicules of various sizes and shapes. The bone spicules interconnect with each other and form trabeculae, providing the general shape of the developing bone (no cartilage model is present). The numerous osteoblasts responsible for this growing region of spicules are seen at the surface of the newly deposited bone. The older, calcified portion of spicules contains osteocytes surrounded by bone matrix. In the right portion of the figure, adjacent to the bone spicules, the connective tissue is very cellular and is developing into the early periosteum. 250.
Newly formed bone matrix appears in histologic sections as small, irregularly shaped spicules and trabeculae.
With time, the matrix becomes calcified, and the interconnecting cytoplasmic processes of the bone-forming cells, now termed osteocytes, are contained within canaliculi. Concomitantly, more of the surrounding mesenchymal cells in the membrane proliferate, giving rise to a population of osteoprogenitor cells. Some of the osteoprogenitor cells come into apposition with the initially formed spicules, become osteoblasts, and add more matrix. By this process, called appositional growth, the spicules enlarge and become joined in a trabecular network with the general shape of the developing bone.
Through continued mitotic activity, the osteoprogenitor cells maintain their numbers and thus provide a constant source of osteoblasts for growth of the bone spicules. The new osteoblasts, in turn, lay down bone matrix in successive layers, giving rise to woven bone. This immature bone, discussed on page 223, is characterized internally by interconnecting spaces occupied by connective tissue and blood vessels. Bone tissue formed by the process just described is referred to as membrane bone or intramembranous bone.
Endochondral ossification also begins with the proliferation and aggregation of mesenchymal cells at the site of the future bone. Under the influence of different fibroblastic growth factors (FGFs) and bone morphogenic proteins (BMPs) (see page 219), the mesenchymal cells initially express type II collagen and differentiate into chondroblasts that, in turn, produce cartilage matrix.
Initially, a hyaline cartilage model with the general shape of the bone is formed.
Once established, the cartilage model (a miniature version of the future definitive bone) grows by interstitial and appositional growth (Plate 13, page 248). The increase in the length of the cartilage model is attributed to interstitial growth. The increase in its width is largely the result of the addition of cartilage matrix produced by new chondrocytes that differentiate from the chondrogenic layer of the perichondrium surrounding the cartilage mass. Illustrations 1 of Figure 8.17 show an early cartilage model.
The first sign of ossification is the appearance of a cuff of bone around the cartilage model.