Biomedical Engineering Reference
In-Depth Information
7.1 Part I: Bone and Cartilage Repair
7.2 Bone
7.2.1 Structure
Bone is a specialized form of connective tissue responsible for several important
tasks in the body: it protects vital organs, facilitates movement, stores minerals,
and plays a role in calcium and phosphate homeostasis. In addition, bone houses
precursor cells that include hematopoietic as well as mesenchymal stem cells
(MSCs). This variety of tasks enables the bone to have a complex dynamic
structure with continual destruction and formation throughout life [
1
,
2
].
Bone tissue is structurally composed of an extracellular matrix (ECM) and
cells. The ECM includes minerals, organic materials, and water. The mineral part
of the bone matrix is mainly composed of calcium and phosphate in the form of
hydroxyapatite (HA) crystals. Other minerals include nonorganic salts of mag-
nesium, potassium, fluoride, and citrate in trace amounts [
3
,
4
].
Organic materials constitute about 30 % of dried bone weight of which the
main component is collagen I. Other collagen isoforms including types III, V, and
XII are also present in trace amounts [
5
,
6
]. Bone matrix is slightly metachromatic,
indicating the presence of glycosaminoglycan (GAG) which includes chondroitin
sulfate and keratin sulfate. GAGs are responsible for the bone's compressive
resistance. These macromolecules also play a role in matrix mineralization.
Noncollagenous proteins constitute about 5 % of bone matrix and are involved
in organization, turnover, and matrix mineralization. The most abundant non-
collagenous ECM protein in the bone is osteonectin, which binds to collagen
through its calcium-binding domain. Osteonectin expression occurs in the early
stage of osteoblastic differentiation [
5
-
8
]. Osteopontin (OPN) is a phosphorylated
sialoprotein involved in cell movement and regulation of matrix mineralization
[
9
]. In the absence of calcium, this protein has a random coil formation, but in the
presence of calcium, its beta sheet structure is seen. It has been shown that OPN is
involved in bone remodeling induced by mechanical stress [
10
]. Osteocalcin
constitutes about 15 % of noncollagenous protein in the bones. It is vitamin K
dependent and has an affinity for the Ca
2+
/HA complex. This protein is a late
marker of the osteogenic phenotype and plays a role in bone remodeling [
11
].
Fibronectin and vitronectin are other noncollagenous proteins present in bone
ECM that have crucial roles in cell signaling.
There are three main cells in bone tissue: osteoblasts, osteocytes, and osteo-
clasts. Osteoblasts are morphologically cuboidal and reside on bone surfaces.
These cells originate from bone progenitors in the bone marrow and periosteum in
response to inducers. The main function of osteoblasts is to produce organic ECM,
also known as osteoids. Osteoblasts also regulate matrix mineralization as well as
osteoclast activity. Osteoblasts have a high alkaline phosphatase activity [
12
-
14
].
