DESIGNING NANOFIBROUS SCAFFOLDS FOR TISSUE ENGINEERING - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 459

The matrix synthesized by osteoblasts consists of 25% organic matter (that
includes cells), 5% water and 70% inorganic mineral (hydroxyapatite). A typical
bone ECM predominantly consists of fi brillar type I collagen (about 100 nm in
diameter) packed in bundles, thereby increasing the tensile strength of the tissue.
Further, the bone hardness is maintained by the inorganic hydroxyapatite
well dispersed within the collagen bundles. Bone regeneration and home-
ostasis is governed by the mechanisms of osteoinduction, osteoconduction, and
osteogenesis:
1 . Osteoinduction involves the recruitment of immature cells and their stim-
ulation into osteoprogenitors. In the context of tissue engineering, osteo-
induction is the major process of bone healing during fractures.
2 . Osteoconduction involves the growth of bone on the surface. In terms of
tissue engineering or other bone healing implants, it is the provision of
scaffold for cells to migrate, differentiate and induce the process of bone
formation.
3 . Osteogenesis is the process of the formation of new bone by the bone
cells, osteoblasts. It is the ability of the graft to produce bone by virtue of
the cellular components.
Bone lesions/defects that are less than the critical size defect heal by conven-
tional treatment methods. However, large defects (larger than the critical size
defect) do not heal and hence need an appropriate treatment such as a bone
graft or their substitute. Every year, 28.6 million Americans incur a musculoskel-
etal injury [213]. The current state-of-the-art for treatment of bone defects is
via autografts, allografts, or xenografts. However, these strategies have been
limited due to limited availability (autografts), immune rejection and potential
for disease transfer (in case of allografts and xenografts).
Vascularised bone grafts are an alternative to the aforementioned methods.
The presence of vasculature allows for improved resistance to infection and hence
a better healing capacity. However, the disadvantages of donor site morbidity and
multiple surgeries still stand [214].
Apart from these, metal and ceramic-based implants have been used as an
alternative to tissue grafts. Metals have the ability to provide immediate mechan-
ical strength at the site of injury but fail to totally integrate with the tissue at that
site [215]. The bioactive ceramics that have found application in orthopaedics are
hydroxyapatite, tricalcium phosphate and bioglass [216]. Although such grafts can
show good osteoconduction and integration, they are limited by their brittle
nature and, hence, may not be suitable for load-bearing applications. Therefore,
tissue engineering has emerged as an exciting alternative to the aforementioned
approaches.
13.5.2.1.1 POTENTIAL CELL SOURCES FOR BONE TISSUE ENGINEERING. Mesen-
chymal stem cells (MSCs), also known as bone marrow stromal cells, have been