Calcium Orthophosphates as Bioceramics - Calcium Orthophosphates: Applications in Nature, Biology and Medicine - page 386

to that of the natural extracellular matrix that assists proliferation,
differentiation and biosynthesis of cells. In addition, scaffolds placed
at the regeneration sites will prevent disturbing cells from invasion
into the sites of action [671, 672]. The role of scaffolds has been
perfectly described by Andrés Segovia (1893-1987), a Spanish
classical guitarist: “When one puts up a building one makes an
elaborate scaffold to get everything into its proper place. But when
one takes the scaffold down, the building must stand by itself with no
trace of the means by which it was erected. That is how a musician
should work.”
The idea behind tissue engineering is to create or engineer
autografts by either expanding autologous cells in vitro guided by

a scaffold or implanting an acellular template and allowing

the patient's cells to repair the tissue guided by the scaffold. The
first phase is the

in vivo

formation of a tissue construct by placing
the chosen cells and scaffolds in a metabolically and mechanically
supportive environment with growth media (in a bioreactor), in
which the cells proliferate and elaborate extracellular matrix. It is
expected that cells infiltrate into the porous matrix and consequently
proliferate and differentiate therein. In the second phase, the construct
is implanted in the appropriate anatomic location, where remodeling
in vivo

in vitro

is intended to recapitulate the normal functional architecture
of an organ or a tissue [673, 674]. The key processes occurring
during both phases of the tissue formation and

maturation are: (1) cell proliferation, sorting and differentiation, (2)
extracellular matrix production and organization, (3) biodegradation
of the scaffold, (4) remodeling and potentially growth of the tissue.
To achieve the goal of tissue reconstruction, the scaffolds must meet
several specific requirements [158, 159, 667]. A reasonable surface
roughness is necessary to facilitate cell seeding and fixation [675,
676]. A sufficient mechanical strength and stiffness are mandatory to
oppose contraction forces and later for the remodeling of damaged
tissues. A high porosity and an adequate pore dimensions (Tables 4.1
and 4.5) are very important to allow cell migration, vascularization,
as well as a diffusion of nutrients [384]. Namely, scaffolds should
have a network of interconnected pores where more than ~60 % of
the pores should have a size ranging from ~150 μm to ~400 μm and
at least ~20 % should be smaller than ~20 μm [12, 92, 384, 394, 395,
452-459, 504, 677-684]. Scaffolds must be manufactured from the
materials with controlled biodegradability and/or bioresorbability,
such as calcium orthophosphate bioceramics, so that a new
bone will eventually replace the scaffold [685]. Furthermore, the

in vitro and in vivo