**3. Bone development**

Osteogenesis and ossification refers to the process of bone formation that leads to the formation of the bone skeleton in the embryo (**Figure 4**). Bone growth is another form of ossification that continues into adulthood as long as the subject continues to grow. In adults, ossification is mainly used for rearrangement and to bone consolidation. The embryo's skeleton is made up entirely of fibrous membranes and hyaline (joint) cartilage until the 6th week of gestation. Then most of these structures are gradually replaced by bone tissue. Two bone formation processes then exist: intramembranous ossification and endochondral ossification.

#### **3.1 Inside the membrane ossification**

Intramembranous bone is formed from a fibrous membrane. The bones produced are flat bones. The ground substance of the bone matrix is deposited between the collagen fibers, inside the fibrous membrane, to form cancellous

*Multi-Scale Modeling of Mechanobiological Behavior of Bone DOI: http://dx.doi.org/10.5772/intechopen.95035*

#### **Figure 4.**

*Hierarchical structure of cortical bone constituents and associated porosity. HA = hydroxyapatite, NCP = non-collagenous proteins [4].*

bone. Plates of compact bone eventually enclose the diploe. The successive stages of ossification are:


Ossification starts from hyaline cartilage and leads to endochondral bone or cartilaginous bone. This process is more complex than the previous one because the cartilage must be disintegrated as the ossification progresses. The majority of the bones of the skeleton are formed by this ossification. The osteoblasts that lie below the periosteum secrete a bone matrix modeled on hyaline cartilage, thus forming a bone sheath. The deterioration of the cartilaginous matrix forms cavities, which allows the entry of a bud which is at the origin of the point of primary ossification: it contains an artery and a nourishing vein, lymphatic vessels, neurofibers, red marrow elements, osteoblasts and osteoclasts. Bone matrix is deposited around the remains of cartilage.

### **3.2 Bone growth**

Bone growth takes place mainly during childhood and adolescence. Most bones stop growing in early adulthood.

## *3.2.1 Growth in length of bones*

Long bones elongate as a result of the interstitial growth of epiphyseal cartilages and the replacement of cartilage with bone material. Growth in length is accompanied by an almost continuous rearrangement of the extremities (epiphyses), in order to maintain the correct proportions between the diaphysis and the epiphyses.

### *3.2.2 Growth in thickness or diameter of bones*

The bones thicken with the efficient activity of the periosteum. Osteoblasts, located below the periosteum, secrete a bone matrix on the outer surface of the bone; osteoclasts, located on the endostate of the diaphysis, destroy the bone surrounding the medullary cavity. The resorption is generally less important than the formation of bone material, the bone thickens and its diameter increases.
