Alma Howard

Different modes of cell sterilization: cell killing and early differentiation

Investigations in plants and parallel observations in animals suggest that specific interference with endocellular control of differentiation due to cytoplasmic growth during mitotic delay is invol...

Induced Resistance in a Desmid Closterium moniliferum

When exposed to fast electrons at high dose rate, all cells of Closterium moniliferum survive (form colonies) up to a threshold dose of about 10 krad. If previously exposed to a dose between 0.14 and 10 krad they can survive higher threshold doses. We call this induced resistance and measure its extent by the ratio of doses to give the same killing effect with and without a previous exposure. We propose that induced resistance is due to the induction, by the first dose, of a repair mechanism which may be an extension of that which repairs sublethal damage. Resistance was optimally induced by doses between 1 and 5 krad, and developed rapidly with time after the first dose, being clearly apparent 1 hr after 2 krad and 3 to 4 hr after 19 krad. It developed slowly or not at all in the cold, suggesting that it is a metabolic process. (auth)

Radiation depigmentation of mouse hair, the influence of local tissue oxygen tension on radiosensitivity.

Experiments were conducted on Strong F or DBA-1 mouse skin in telogen (resting stage) to determine the effect of local tissue oxygen tension on sensitivity to 300-kVp x-rays. Radiation response was...

EFFECT OF NITROGEN AND HYDROGEN GAS UNDER PRESSURE ON THE RADIOSENSITIVITY OF THE BROAD BEAN ROOT

In the course of experiments on the modification of the sensitivity of bean roots to X-rays by hydrogen gas, control groups of roots were irradiated in nitrogen at a pressure of 50 atm. We found that pressure alone, either of hydrogen or nitrogen, had only a slight effect, if any, on the growth of the roots. As already reported (1, 2), we observed that the presence of 50 atm of H2 during irradiation markedly reduced radiosensitivity. An additional and unexpected finding, which appeared in two separate experiments, was a reduction in radiosensitivity due to 50 atm of nitrogen, which, in the presence of 1 atm of air, appeared to be an even better protector than hydrogen under pressure. Personal communications from L. H. Gray and John Read reveal that each of these investigators has observed a reduced radiosensitivity under nitrogen pressure (at 50 atm and 14 atm, respectively), although in neither case was it wholly consistent. We have now carried out experiments which confirm beyond doubt that, in the presence of 1 atm of air, nitrogen at pressures of 21 atm or more has a protective action which is equivalent to the complete removal of oxygen.

Induced Resistance in Closterium: Indirect Evidence for the Induction of Repair Enzyme

Cells of the desmid Closterium moniliferum are more resistant to killing by radiation if they are previously exposed to relatively small doses of ionizing radiations (14-MeV neutrons, 290-kVp X ray...

Cell kinetics in the sebaceous glands of the mouse. II. The glands during hair growth.

The sebaceous glands of the mouse have been studied during hair growth initiated either spontaneously or artificially.

K values and gain factors of fast neutrons and x-rays for a mammalian cell system (cfu) in vivo.

The relationship between radiosensitivity and oxygen tension is defined by two parameters; m, the maximum ratio of radiosensitivity obtained by variation of the oxygen tension, and K, the oxygen tension at which the radiosensitivity equals (m + 1)/2. It is shown here that the ratio of K values for two radiations, e.g., X-rays and neutrons, can be determined from the sensitivities of cells in anoxic, well-oxygenated, and one intermediate condition, irrespective of the absolute oxygen tension of the latter. For mouse bone marrow stem cells irradiated in dead mice and in mice breathing air or breathing 5.8% O2 in N2, the ratio

Neutrons and the Oxygen Effect: Corroboration of the Greater Effectiveness at High LET for Sensitization by Oxygen at Low Concentrations

K_{x}/K_{n}

Mutation and Genetics

was 1.46 ± 0.45. This ratio implies a Gain Factor for neutrons, with respect to cells at low (but not zero) oxygen tension, which is slightly greater than that expected if Kx is less than, or equals, Kn. The findings are discussed with reference to fast-neutron radiotherapy.

Synthesis of nucleoprotein in bean root cells.

HENDRY, J. H., AND HOWARD, A. Neutrons and the Oxygen Effect: Corroboration of the Greater Effectiveness at High LET for Sensitization by Oxygen at Low Concentrations. Radiat. Res. 75, 529-540 (1978). Another comparison has been made between the sensitivity of cells at low oxygen concentration to 290-kV X rays or 14-MeV neutrons. The K values for oxygen with young sporelings of the fern Osmunda regalis were 35 4- 5 juM for X rays, and 23 4- 7 ,M for neutrons. The former value is higher than those reported for other cells. The trend toward a lower value for neutrons is consistent with two of three previous comparative measurements. This consensus of experimental evidence indicates that the component of the dose from neutrons, which demonstrates an oxygen effect, tends to show more affinity for sensitization by oxygen in the competitive-type reaction which leads to fixation of damage than does the dose from X rays. This qualitative feature should be considered in proposed mechanisms for the reduction in OER with an increase in ionization density, and these are discussed.