EVIDENCE FOR INDIRECT EFFECTS OF RADIATIONS OF HEAVY IONS AND ELECTRONS ON HAIR DEPIGMENTATION*

Hair cortical cell counts (HCCC), a new sensitive in vivo assay with possible applications for biological dosimetry

In the region of a growing hair follicle where terminal keratinization takes place, the nuclei of the medulla and cortex are gradually degraded. In the zone between rapid cell proliferation in the matrix and the complete loss of the nuclei in the formed hair, the number of medullary cells in the hair proved to be a sensitive-radiation-dose-dependent endpoint (Potten et al. 1990). Nuclei in the cortex of the hair are similarly degraded but are more difficult to quantitate since they form a circumferential ring of cells round the medulla. However, using the confocal microscope to obtain discrete optical sections just above and below the medulla, the number of cortical cells was reproducibly counted. The maximum reduction in cortical cell count occurred on the 3rd day after irradiation. Here, the dose-response curve (0.5-4.0 Gy) is presented, for the population of cortical cells in awl guard hairs of the mouse for 137Cs gamma-rays at 4 Gy/min, which was exponential with no significant shoulder, (the D0 was 2.4 +/- 0.2 Gy and the extrapolation number 1.2 +/- 0.1). The cortical cells exhibited a slightly higher D0 value than the medullary cells but differed in that there was no evidence of a shoulder. The effects of 0.5 Gy could be detected. The degree of sensitivity, and the fact that cortical cells are readily detected in human hair follicles, unlike the medullary cells, make this a potentially valuable human biological dosimeter.

Hair medullary cell counts following low-dose-rate gamma- and high-energy neutron irradiation

Young adult Balb/c mice with hair follicles synchronously in the middle of the hair growth cycle received whole-body or partial-body doses of gamma-radiation or neutron radiation. The hair follicles were analysed either 3 days after irradiation in the dose-response experiments, or at various times after a constant dose in the time-course experiments, for changes in the number of cells in the forming medulla of the hair in the region just above the germinal matrix of the growing (anagen) hair follicle. Time-course experiments showed that 3 days after irradiating growing follicles (2 or 4 Gy of gamma-rays or 1 or 2 Gy of neutrons), the maximum reduction in the hair medullary cell count (HMCC) was observed. Survival curves were obtained for gamma-rays over a range of dose-rates (4.0-0.0023 Gy/min) using total doses between 0.5 and 5.0 Gy. A survival curve was also obtained for 62 MeV neutrons at a dose-rate of 0.31 Gy/min and doses of 0.1-2.0 Gy. The D0 for the HMCC dose-response curve following caesium gamma-irradiation at 4.0 Gy/min was 2.1 +/- 0.2 Gy with n = 1.7 +/- 0.2. The dose-response curve for low-dose-rate gamma-rays was best fitted by a simple exponential function with no evidence of a shoulder. The D0 was 3.0 +/- 0.1 Gy at a dose-rate of 0.04 Gy/min. The data for 62 MeV neutrons (at 0.35 Gy/min) had a small but significant shoulder n = 1.5 +/- 0.1 and a D0 of 1.0 +/- 0.1 Gy. These data further illustrate the sensitivity of this assay and its potential application as a biological dosimeter.

Unique biological aspects of radiation hazards--an overview

Low orbit, geostationary, and deep-space flights differ from one another with respect to particle radiation environment, participating population size, mission duration, and biological risks other than radiation. It is proposed that all of these factors be considered in the setting of safety standards and, in particular, that the rem-dose concept is applicable only to radiations having low and intermediate linear energy transfer (electrons, protons, and helium ions), whereas the incidence of microlesions is a more meaningful indicator of the hazard due to higher-Z, high energy (HZE) particles. A microlesion is the biological injury inflicted in a specific tissue by a single HZE particle, and it is still in need of quantitative biological definition for specific mammalian tissues. If for example, a microlesion is taken as due to a HZE particle track 10 cell diameters long with LET > 200 KeV/micrometer in its core and > 25 rad dose in its penumbra at a distance of 10 micrometers, then the microlesion dose rate in geostationary orbit, for example, is about 9,000 microlesions per cm3 of tissue per month.