The regulation of haemopoietic stem cell and progenitor cell proliferation by humoral factors
Abstract
The mechanisms which regulate the growth fraction of the
haemopoietic stem cell (CFU-S) and granulocyte macrophage
progenitor cell (GM-CFC) have been investigated. In normal murine
bone marrow (NMBM) a small proportion of the CFU-S are synthesising
DNA (-10%). In contrast, in the bone marrow from mice regenerating
after treatment with cytotoxic drugs and in developing haemopoietic
tissues such as murine fetal liver a large proportion of the CFU-S
(-40%) are synthesising DNA. Medium conditioned by normal murine
and human bone marrow cells inhibited the proliferation of rapidly
cycling CFU-S from regenerating bone marrow. This inhibitor was
contained in a 50-100K daltons ultrafiltration fraction. In
contra-distinction medium conditioned by human fetal liver cells
stimulated the proliferation of CFU-S from NMBM. The stimulator
was produced by adherent cells and was contained in a 30-50K
daltons ultrafiltration fraction.
An alternative assay for the humoral regulators of CFU-S
proliferation was developed. Different numbers of haemopoietic
cells were injected into lethally irradiated mice. Five days later
they were injected with 2iCi of
125IUdR
and sacrificed 2 hours
later. There was a linear relationship between the log 125IUdR
uptake into the spleen and femur and the log cell dose injected.
Pre-treatment of haemopoietic cells with an S-phase specific
cytotoxic drug resulted in a reduction in the
125IUdR
incorporation
into the spleen. This enabled the kinetic properties of a
haemopoietic stem cell population to be assessed and the humoral
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factors which modulate the growth fraction of these cells to be
investigated.
At early stages of gestation (11-14 weeks) in human fetal
liver few GM-CFC are synthesising DNA, whereas later in gestation
(>14 weeks) a large proportion of GM-CFC are in S-phase, Moore and
Williams (1973b). Incubation of NMBM GM-CFC (approx 40% in DNA
synthesis) with a supernatant from an early human fetal liver
(11-14 weeks) reduced the proportion synthesising DNA to <5%. In
contrast, the proportion of murine GM-CFC synthesising DNA was not
affected by incubation with a supernatant from a late human fetal
liver (>14 weeks). GM-CFC that had been switched out of cycle by
incubation with a supernatant from an early gestation human fetal
liver were switched back into cycle following incubation with a
late human fetal liver supernatant. The inhibitor and stimulator
of GM-CFC proliferation were both produced by non-adherent cells
and were contained in >100K and 30-50K daltons ultrafiltration
fractions repectively. It is likely that changes in the relative
levels of a proliferation inhibitor and stimulator throughout
gestation might control the proportion of GM-CFC in cycle.
Type
Thesis, PhD Doctor of Philosophy
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