418
T . C
HSU
would be more suitable for microscopic observations. They prepared
tritiated 11-keto-pregnanolone and tritiated sodium acetate to demonstrate that labeling of yeast cells and paramecia was confined to cells
with a resolution as small as 0-5/x. The range of tritium ^-particles
averages about 1 -5/x, which is the main factor for its excellent resolution.
Tritium has a half-life of 12*26 years, a disintegration rate of 0-016°/o/
day as opposed to 0-000034 °/o/day for C
1 4
. Thus, in autoradiography,
one can expect to obtain better exposure in a relatively short time with
tritium than with C
1 4
.
Incorporating tritium into thymidine was accomplished by Hughes
(Hughes, Bond,Brecher, Cronkite, Painter, Quastler and Sherman, 1958;
Taylor, Woods and Hughes, 1957) and by Verly and Hunnebelle
(1957). Initially, Hughes attempted to use the tritiated thymidine
( H
3 - T D R ) for internal radiation of cancer cells. As the energy of
^-emission proved too small to produce severe cellular damage, his
group applied H
3 - T D R to autoradiographic studies, and opened a
new road to biochemical inquiries at the cellular level. Within minutes
after intravenous injection of H
3 - T D R into mice, Hughes et al. (1958)
found thymidine had left the blood stream and was present in tissues.
Within an hour the radioactive substance disappeared from the blood
and was no longer available for D N A synthesis. Thus the time period
during which injected H
3 - T D R is available for D N A synthesis is brief.
After a desired period of time, the organism can be sacrificed and the
tissues used for autoradiographic study. Cells bearing label would
indicate that they were actively synthesizing D N A while H
3 - T D R was
available.
One must remember that tritium is not completely free from damaging effects. Cautious investigators have repeatedly warned about the
harmful effects of the internal radiation of this isotope on tissues and
cells (Wimber, 1959; Plaut, 1959; Painter, Drew and Hughes, 1958;
Drew and Painter, 1959; Natarajan, 1961). Their studies include
chromosome breakages in plant root-tips and growth inhibition of
tissue cultured cells.
There is no question that prolonged treatment and/or high specific
activity of H
3 - T D R would produce harmful effects on living systems.
Results from such experiments should be cautiously interpreted. However, the degree of damage, as with any detrimental agent, depends
upon the duration of the treatment as well as the dose (specific activity)
used. Wimber (1959), using root-tips of Tradescantia, found that
chromosome fragmentations detected during anaphase began to rise
significantly 20 h after treatment with 1 /xc/ml H
3 - T D R . Similarly, in
root-tip cells of the onion, McQuade, Friedkin and Atchison (1956)
found that with C
1 4
- T D R treatment for extended periods of time,
T . C
HSU
would be more suitable for microscopic observations. They prepared
tritiated 11-keto-pregnanolone and tritiated sodium acetate to demonstrate that labeling of yeast cells and paramecia was confined to cells
with a resolution as small as 0-5/x. The range of tritium ^-particles
averages about 1 -5/x, which is the main factor for its excellent resolution.
Tritium has a half-life of 12*26 years, a disintegration rate of 0-016°/o/
day as opposed to 0-000034 °/o/day for C
1 4
. Thus, in autoradiography,
one can expect to obtain better exposure in a relatively short time with
tritium than with C
1 4
.
Incorporating tritium into thymidine was accomplished by Hughes
(Hughes, Bond,Brecher, Cronkite, Painter, Quastler and Sherman, 1958;
Taylor, Woods and Hughes, 1957) and by Verly and Hunnebelle
(1957). Initially, Hughes attempted to use the tritiated thymidine
( H
3 - T D R ) for internal radiation of cancer cells. As the energy of
^-emission proved too small to produce severe cellular damage, his
group applied H
3 - T D R to autoradiographic studies, and opened a
new road to biochemical inquiries at the cellular level. Within minutes
after intravenous injection of H
3 - T D R into mice, Hughes et al. (1958)
found thymidine had left the blood stream and was present in tissues.
Within an hour the radioactive substance disappeared from the blood
and was no longer available for D N A synthesis. Thus the time period
during which injected H
3 - T D R is available for D N A synthesis is brief.
After a desired period of time, the organism can be sacrificed and the
tissues used for autoradiographic study. Cells bearing label would
indicate that they were actively synthesizing D N A while H
3 - T D R was
available.
One must remember that tritium is not completely free from damaging effects. Cautious investigators have repeatedly warned about the
harmful effects of the internal radiation of this isotope on tissues and
cells (Wimber, 1959; Plaut, 1959; Painter, Drew and Hughes, 1958;
Drew and Painter, 1959; Natarajan, 1961). Their studies include
chromosome breakages in plant root-tips and growth inhibition of
tissue cultured cells.
There is no question that prolonged treatment and/or high specific
activity of H
3 - T D R would produce harmful effects on living systems.
Results from such experiments should be cautiously interpreted. However, the degree of damage, as with any detrimental agent, depends
upon the duration of the treatment as well as the dose (specific activity)
used. Wimber (1959), using root-tips of Tradescantia, found that
chromosome fragmentations detected during anaphase began to rise
significantly 20 h after treatment with 1 /xc/ml H
3 - T D R . Similarly, in
root-tip cells of the onion, McQuade, Friedkin and Atchison (1956)
found that with C
1 4
- T D R treatment for extended periods of time,