*2.3.2.2 The extra cellular matrix*

The Extra Cellular Matrix of bone is calcified and has an organic part and a mineral phase.


### *2.3.3 Bone tissue*

Most bones are made up of an outer layer that appears smooth and dense to the naked eye, compact bone tissue, and an internal area of trabecular (or cancellous) bone tissue. Bone marrow, red or yellow, is contained in the cavities between the spans of this structure. In the case of long bones, **Figure 1**, the cancellous bone is located at the extremities (epiphyses), the central part of the bone contains the bone marrow.

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

**Figure 2.** *Compact, cortical bone element [3].*

#### *2.3.3.1 Compact bone*

The osteon, or Havers' system, is the structural unit of compact bone. Each osteon is of elongated cylindrical shape, parallel to the longitudinal axis of the bone, **Figure 2**. The osteon is made up of a set of hollow cylinders (6 to 15 per osteon) made up of bone matrix and arranged concentrically around its central canal, or Havers' canal, which contains blood capillaries and nerve fibers. These canals are interconnected with the medullary cavity and with the surface of the bone by transverse or oblique canals, the Volkmann canals. Each matrix cylinder is a lamella of the osteon: the compact bone is often called the lamellar bone. In a given lamella, the collagen fibers are all parallel but the fibers of two adjacent lamellae are always oriented in different directions. This alternation strengthens the adjacent lamellae, which provides remarkable resistance to the torsional forces to which the bones are subjected. The osteocytes are found in small empty spaces at the junction of the lamellae, called lacunae. These gaps are connected to each other and to the central osteon canal by the canaliculi, very fine canals, which allow nutrients and waste to easily pass from one osteocyte to another. Thus, the osteocytes are well nourished even though the bone matrix is hard and impermeable to nutrients. The arrangement of the osteons, with the axis oriented in the direction of the mechanical stresses, gives the compact bone maximum strength. If we consider the compressive strength in relation to the density, bone is a reference structure in the field of cellular materials [1, 3].

#### *2.3.3.2 Cancellous bone*

Spongy (trabecular) bone tissue is mainly present in short bones and flat bones (sternum, iliac wings) as well as in epiphyses of long bones. It is formed by a threedimensional network of trabeculae (walls) of bone tissue, branched, delimiting a labyrinth of interconnected spaces reserved by the bone marrow and vessels, **Figure 3**. The shape and density of the alveolar cells depend on the intensity and direction of the stress that the bone must withstand. Cells tend to line up in the direction of greatest stress, and their density increases with the intensity of loading. Cancellous bone has no osteons, but its spans form irregular lamellae and osteocytes connected by canaliculi.

**Figure 3.** *spongy bone tissue. Side of the cube: 5 mm. Photo UMR 791 INSERM [3].*

The nutrients leave the medullary spaces between the spans and arrive at the osteocytes of the cancellous bone by diffusion through the canaliculi.

There are three phases in the life of the bone.

