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produced by normal fibroblasts.113,114
113,114
The content of
decorin was reduced
115
and hyaluronan levels were ele-
vated.
116,117
The expression of the OI dermal fibroblast
phenotype was evident in studies of cell behavior in cul-
ture. OI fibroblasts grew slower and achieved a lower
final density compared to normal cells.
118,119
Altered cell
morphology and engorged cytoplasmic vesicles were
also common.
119
The levels of fibronectin produced by
the OI fibroblasts were consistently elevated relative to
normal fibroblast production.
119
osteoblast cell lines.
125
The proliferative capacity of all
OI osteoblasts was significantly reduced in all types of
OI patients. Normal osteoblasts exhibited a decrease in
proliferation rate with increasing donor age
131
while OI
osteoblast proliferation was reduced at all donor ages.
OI osteoblast proliferative capacity was not significantly
different from older normal cells (>60 years of age).
125
Detailed growth parameter characterization of trabecu-
lar osteoblasts isolated from patients with OI and age-
matched controls revealed proliferative differences more
severe than those observed in OI fibroblasts.132
132
These
studies involved monitoring and modeling growth
curves over extended periods and fitting the data to bio-
logically relevant growth parameters: the lag time before
cells begin logarithmic growth, the maximal growth
rate, and the plateau value which reflects the maximum
cell density upon confluence. Normal osteoblasts had a
decrease in maximal growth rate with increasing age of
the donor, while OI osteoblast maximal growth rate was
decreased relative to age-matched normal cells. OI osteo-
blasts exhibited decreased maximal growth rates that
were unrelated to the age or phenotype of the subject,
and had significantly longer lag times until logarithmic
proliferation began. This may reflect a lengthened time to
develop and elaborate an extracellular matrix supportive
of proliferation.
132
Alterations in cell proliferation values
in the presence of a type I collagen mutation suggested
a possible correlation between collagen synthesis and
cellular growth rate. Collagen production and cell pro-
liferation in normal fibroblasts132
132
and osteoblasts
131
were
correlated significantly, while in OI-derived fibroblasts
and osteoblasts, collagen and proliferation was not cor-
related. A relationship between collagen metabolism and
osteoblast proliferation was further demonstrated by
reducing collagen production by treating normal human
osteoblasts with the proline analog 3,4 dehydroproline
(which destabilized collagen triple helix formation and
altered collagen metabolism) and measuring a dose
response on maximal growth rates.
132
These observations
suggest an active feedback system between type I colla-
gen synthesis and cellular proliferation.
Bone and Cells in the Osteoblastic Lineage in OI
Bone from individuals with OI has been character-
ized as immature woven bone with a poor lamellar pat-
tern, with observations of low trabecular bone volume,
an increased bone turnover rate and an increased num-
ber of osteoblasts.
120
Abnormal collagen fibers and an
altered mineral composition were variable and uncorre-
lated with OI phenotype. Ultrastructural observations of
dilated endoplasmic reticulum, swollen mitochondria,
intermitochondrial inclusions containing calcium and
phosphorus, and stromal calcification in the bone were
also observed in OI.
121,122
The osteoid was reduced in
quantity and collagen alpha-chains showed an increased
hydroxylysine content.
121
The number of osteoblasts
was increased, osteoid thickness was decreased, and
osteoclast number increased in type I OI.
123
The hyper-
cellularity of OI bone
120,123
does not appear to be univer-
sal, as some reports indicate no obvious difference in the
cell number of osteoblasts or osteoclasts.
122
In Vitro
Effects on Osteoblasts
Because of the more invasive nature of obtaining bone
samples for explant culture, fewer studies have been per-
formed on osteoblasts derived from subjects with OI.
The synthesis, processing and matrix incorporation of
collagen was decreased in OI osteoblasts compared to
age-matched controls.
98,124-126
Similar to observations in
fibroblasts, collagen from OI osteoblasts was often over-
modified.127
127
The metabolism of type I collagen was dif-
ferent between cell types as OI osteoblast collagen was
more electrophoretically delayed than OI fibroblast col-
lagen.
127
Extracellular signal transduction in certain path-
ways was aberrant in OI osteoblasts. TGF-β signaling that
stimulates collagen synthesis and alkaline phosphatase
activity was operant in normal human bone cells, but
not in OI bone cells.
128,129
In contrast, OI-derived bone
cells exhibited normal levels of 1,25(OH)
2
vitamin D
3
stimulation of osteocalcin synthesis and parathyroid hor-
mone stimulation of cAMP.
130
Osteoblast Differentiation
Not only was collagen production reduced, but also
other matrix component levels were altered in 22 OI
patients compared with 22 age-matched controls.
133,134
The total levels and matrix association of collagen, osteo-
nectin and four proteoglycans were significantly reduced
in OI osteoblasts.
125
Total protein synthesis, however, was
within the normal range. The levels of thrombospondin
and fibronectin synthesized by the OI osteoblasts were
elevated.
134
Although hyaluronan levels were reduced
compared to age-matched controls, the percentage of
matrix-associated hyaluronan was significantly elevated.
Different matrix component stoichiometries were evident
between OI Sillence classification types.
134
In Vitro
Effects in Other Molecular and Cellular
Domains
Significant differences in cellular proliferation were
observed between 22 OI osteoblast and 22 normal
