**2. Bone tissue**

Bone tissue is composed of two portions: 1. Organic, consisting of intrinsic bone cells (osteoblasts, osteoclasts and osteocytes) and the organic matrix synthesized thereby; 2. Inorganic, consisting of hydroxyapatite, deposited amorphously in an initial phase and that in a short space of time is converted into another crystalline hydroxyapatite. Organic matrix corresponds to 35% of the bone volume and inorganic matrix to 65%.

In spite of the resistance and hardness, bone tissue is very plastic and has a high capacity to remodel through various situations to which it is submitted, such as fractures, lesions and bone loss. The bone tissue regeneration process starts from important biological reactions, triggered by the actual tissue lesion. Grafting triggers a mechanism of migration of the bone cells belonging to the receptor bed to the inside of the graft, with the purpose of resorbing it and replacing it with neoformed bone.

<sup>2</sup>European Association of Tissue Banks. Common Standards for Tissues and Cells Banking: Berlin: European Association of Tissue Banks; 2004.

American Association of Tissue Banks. Standards for Tissue Banking. 11th ed. McLean : American Association of Tissue Banks; 2007

<sup>3</sup>Phillips GO, Strong DM, Versen RV, Nather A. Advances in Tissue Banking. Vol. 4. World Scientific . New Jersey, 2000.

Bancroft JD, Stevens A. Theory and practice of histological techniques. Fourth Edition. Churchill Livingstone. United Kingdom, 1999.

<sup>4</sup>Law n.9434 of February 5, 1997; Decree n.2268 of June 30, 1997;Administrative Ruling n.1686 of September 20, 2002; Resolution n. 220 of December 27, 2006; Administrative Ruling n. 2600 of October 21, 2009.

The cells belonging to the bone tissue are the osteoblasts, osteocytes and osteoclasts.

The osteoblasts are cuboid, elongated cells of mesenchymal origin that are located in the bone margins; their function is to produce the organic matrix of the bone tissue. In reduced activity these cells assume a more slender shape. The osteocytes are encapsulated osteoblasts, which after maturation became imprisoned inside the mineralized matrix, but that still maintain contact with other cells through cytoplasmic ramifications, thus maintaining physiologic functionality of the tissue (Junqueira, Carneiro, 1999; Davies 2000). This contact with surface cells such as the osteoblasts and lining cells is related bone structure maintenance and to the physiological responses that lead to tissue formation or resorption (Aubin et al., 2006).

The osteoclasts are giant cells with multiple nuclei and their function is related to resorption. In synergy with the osteoblasts they promote bone remodeling.

The interaction and the synergism among bone cells is called creeping substitution, and this occurs through three essential cellular events*:* osteogenesis (cellular event that favors the synthesis of bone matrix by the osteoblasts), osteoinduction (ability to induce the migration of mesenchymal cells and their differentiation into osteoblasts) and osteoconduction (ability of the tissue to serve as a mold or guide for the cellular processes involved in bone tissue repair).

Moreover, as is the case with others, the bone cells pass through the stages of the cell cycle, which range from formation to cell division (mitosis). Mitosis is susceptible to external interferences, and the cell can either enter a state of rest or continue to split cyclically (Urist, 1965; Enneking et al., 1975; Junqueira, Carneiro, 1999; Perren, Claes, 2002).
