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First published online 18 March 2003
doi: 10.1242/jcs.00376
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Research Article |

1 Department of Anatomy & Cell Biology, University of Toronto, Toronto,
Ontario, M5S 1A8, Canada
2 Department of Molecular & Medical Genetics, University of Toronto,
Toronto, Ontario, M5S 1A8, Canada
* Present address: INSERM Unit 403, Hôpital E. Herriot, Lyon, 69437,
France
Author for correspondence (e-mail:
jane.aubin{at}utoronto.ca)
Accepted 21 January 2003
Mesenchymal stem cells give rise to osteoprogenitors that proliferate and
differentiate into identifiable preosteoblasts, osteoblasts, bone lining cells
and osteocytes. To identify and establish a molecular profile for the more
primitive and uncharacterized cells in the lineage, relatively rare (<1%)
osteoprogenitors present in primary cultures of fetal rat calvaria cell
populations were identified by a replica plating technique. Since the cell
number was limited in each colony sampled, we used global amplification PCR to
analyze the repertoire of genes expressed in osteoprogenitors. We established
a molecular fingerprint and a developmental sequence based on simultaneous
expression patterns for both known osteoblast-associated markers (collagen
type I, alkaline phosphatase, osteopontin, bone sialoprotein, PTH1R and
osteocalcin) and potential regulatory molecules (i.e. FGFR1, PDGF-R
and
PTHrP). By analysis of 99 osteoprogenitor and osteoblast colonies captured by
replica plating at different developmental stages, we found: (1) a
recognizable cohort of cells considered more primitive than committed
osteoprogenitors; (2) a cohort of early progenitors transiently expressing
bone sialoprotein; and (3) that mRNAs for FGF-R1, PDGF-R
and PTH1R were
expressed earlier than other markers and tended to increase and decrease in
relative concert with the osteoblast-specific markers. The observations
suggest that within the osteoblast differentiation sequence both discrete
stages and continua of changing marker expression levels occur with variation
in expression for any given marker. This combined approach of replica plating
and global amplification PCR allows molecular fingerprinting of definitive
primitive osteoprogenitors and will aid in identifying novel developmental
stages and novel differentiation stage-specific genes as these cells progress
through their differentiation sequence.
Key words: Osteoprogenitors, Osteoblasts, Differentiation, PCR, Replica plating
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