Bone Healing and Failure - Engineered Bone: Tissue Engineering Intelligence Unit - page 49

macrophages and inflammatory cells in the early phases however both were also expressed by
mesenchymal and osteoblastic cells later during the healing process. TNF- α expression was
also detected at very high levels in hypertrophic chondrocytes. TNF- α and IL-1RII receptors
showed identical expression as their ligands. IL-1-RI receptors were expressed only during
initial inflammatory phase between days 1 and 3.
This important work has gone a long way to clarify the temporal roles of the various cytokines
and their receptors in fracture healing. It would appear therefore that strong induction of
TNF- α and IL-1 occurs within 24 hours following injury and is accompanied by a large influx
of immune cells to the fracture site. RANKL level peaks, and is maintained, slightly later than
the immediate inflammatory burst. This may mean that both TNF- α and IL-1 are involved in
its induction. Its production may also be stimulated by local osteoblasts and stromal cells at the
fracture site. The finding that TNF- α is not only synthesized by inflammatory cells but also by
mesenchymal cells, osteoblasts and hypertrophic chondrocytes would suggest that it has differ-
ent roles at different times during fracture healing. TNF- α may potentially regulate the initia-
tion of fracture healing including mesenchymal cell proliferation and differentiation in the
periosteum. The increasing expression of the ligands and receptors for both TNF- α and IL-1
on day 21 and day 28 following fracture suggests that these cytokines are most likely involved
in promoting bone remodeling at later times during bone healing. TNF- α may have a poten-
tial autocrine regulator of chondrocyte maturation or apoptosis. 11
This study 11 has also shown for the first time that OPG, RANKL and M-CSF function
during endochondral resorption in fracture healing in a similar manner to that which is ob-
served during bone remodeling. 35,36 Given the expression of OPG at moderately high levels in
unfractured bones and the sharp peak in expression during the phase of maximal cartilage
formation and then decline as endochondral resorption progresses it is reasonable to assume
that it has a role as a negative regulator of cells involved in calcified tissue resorption. RANKL
is expressed at very low levels in intact bones but shows a dramatic up-regulation immediately
after fracture in conjunction with M-CSF. Both remain elevated throughout the entire fracture
healing process. This would suggest that the expression of these cytokines is tightly coordinated
in their regulation of endochondral resorption. It is also apparent that both TNF- α and IL-l
play important roles in bone remodeling. However, given that low levels are expressed during
the period of endochondral resorption these factors may function differently here as compared
to their vastly increased expression from day 21 to 28 after fracture. This would suggest an
important role in the remodeling of woven bone.
Growth Factor Regulation of Fracture Repair
The majority of work carried out to date on the role of growth factors in bone repair has
concentrated on the following: acidic and basic fibroblast growth factor (FGF-1 and FGF-2),
PDGF and the TGF- β superfamily that includes; multiple isoforms of TGF- β s, BMPs, specifi-
cally BMP-2, -3, -4, and -7, and growth and differentiation factors (GDFs). 3,6,7,10,36-38 Re-
search has also centered on the expression of receptors to these growth factors during fracture
healing, growth factor signal effectors (particularly related to the TGF- β superfamily) and growth
factor inhibitors. 11, 40
Fibroblast Growth Factors
The FGFs are a family of structurally related polypeptides characterized by a high affinity to
heparin. The most common in normal adult tissue are acidic FGF and basic FGF (FGF-1 and
FGF-2). FGFs are abundant in bone extracellular matrix and transduce signals to the cyto-
plasm of cells via a family of transmembrane receptors with tyrosine kinase activity. Both FGF-1
and -2 can be detected in the early stages of fracture repair in the granulation tissue at the
fracture site, macrophages and other inflammatory cells being the most likely source. 7,41 Later
FGFs are expressed by mesenchymal cells, maturing chondrocytes and osteoblasts and have
been shown to enhance TGF- β expression in osteoblastic cells. 7