SIKOV
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1014
Studies of factors that influence the response of the perinatal
animal to radiation include studies to evaluate the role of genetic
factors and of immunologic competence in the response of the neonatal
animal. Other studies have indicated the relevance of usually ignored
factors, such as season of the year.
Relatively few investigators have been interested in the long-term
effects of radiation on the intrauterine or neonatal animal. Many of the
groups involved in this area of research are represented at this
symposium, We have seen that the differences between the late effects
of irradiation of the perinatal animal and of the adult provide clues to
the unique nature of the response of the immature animal to radiation.
Studies with radionuclides have also pointed out differences and have
raised several, yet unanswered, questions about the mechanisms of
effect.
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Renewed interest in the effects of continuous or fractionated
irradiation was evident. Although many of the studies reported were
descriptive, this interest may signal coming advances since comparable
studies in the adult led to some of our more useful working hypotheses
on the mechanism of radiation effects. As an indication of the under-
standing that may result, studies combining split-dose techniques for
irradiation of the cerebrum with quantitative end points have suggested
that partial recovery occurs in the ‘fetal brain. Many morphological
studies are still being conducted, but there is a distinct trend toward
elegance of approach. The increasing use of the electron microscope
to investigate the ‘finer details of many of these morphologic changes
has been evident. We also saw the improved understanding that has
resulted from the use of fine beams of X rays or protons delivered to
narrowly defined areas of the brain. Much of the current work is
involved: with elucidating the mechanisms responsible for morphologic
changes described in earlier studies, ”
The increasingly quantitative approach contrasts markedly with
earlier work, For example, growth is being measured as an increase
in DNA rather ‘than in total weight. The effects of radiation on cell
proliferation are being subjected to definitive scrutiny through the use
of tritiated thymidine labeling. At least one group has carefully
measured the size of important functional compartments, which has
allowed them to propose quantitative explanations for age-dependent
differences in radiosensitivity.
The emphasis on quantitation also encompasses the physiological
and biochemical changes that result from irradiation during the
perinatal period. Reports included measurements of enzyme activities
as well as protein, DNA, and RNA content. A few laboratories are
carrying this a step further and are attempting to use such data to
define the effects at the molecular level.