Bone Tissue - Meshless Methods in Biomechanics - page 224

the mineralized bone matrix, once it destroys the cytoplasmic extensions, stopping
the cellular metabolic process and the cellular fluid exchange. In the particular
case of the bone micro damage, the destruction of the cytoplasmic extensions is the
trigger button to activate the bone remodelling process. This process is also
activated if the osteocytes are unused [ 8 - 10 ].
The osteoclasts are plurinuclear cells responsible for the reabsorption and/or
elimination of the bone structure. These cells cause the demineralization of the
mineralized bone matrix and destroy the bone organic matrix. The osteoclasts cells
are formed by the fusion of precursor cells from the bone marrow or from other
haematopoietic tissue. These cells are located in superficial depressions (Howship
lacunas) which identify the bone reabsorption zones.
6.1.2.3 Reticular and Lamellar Bone
The bone tissue can be classified in reticular or lamellar bone tissue according with
the collagen fibre organization inside the bone matrix. The reticular bone tissue is a
neo-formed or immature bone, commonly referred as woven bone. In this bone
tissue the collagen fibre arrangement shows an anisotropic layout, since all fibres
are oriented in different directions (randomly), with no preferential orientation. In
the fetal development or in a fracture repair, the reticular bone is the first to be
formed. In this bone tissue active osteoblasts, excited by growing factors or
stimulated by fracture damage, build more solid bone structures.
In a posterior phase, by the remodelling process, the reticular bone gives place
to the lamellar bone. The lamellar bone is a mature bone tissue organized in thin
layers glued by collagen fibres oriented in perfect alignment with each other. The
osteocites in the interior of the lamellar bone are displayed in sandwich layers
between the lamellas. The lamellar bone has the same chemical composition and
material properties throughout the skeleton, regardless its mechanism of forma-
tion—intramembranous or endochondral—or its structural organization—cortical
(compact) or trabecular (soft) bone [ 1 ], Fig. 6.4 .
6.1.3 Cortical and Trabecular Bone
In adulthood the skeleton contains only two types of bone tissues, cortical bone
(compact bone) and trabecular bone (soft or cancellous bone) [ 1 ].
6.1.3.1 Cortical Bone
The basic functional unit of cortical bone is the osteon (also known as the Havers
system), which consists of concentric layers of bony lamellae surrounding a central
Haversian canal. The Haversian canal contains the blood capillaries that supply the