fetal exposure - threshold effects

["Karam, Andrew" <Andrew_Karam@URMC.Rochester.edu>]

I am reading NCRP 128 (Radionuclide Exposure of the Embryo/Fetus) and
noticed (page 41, section 6.1.2) a part that reads "Typically, experiments
in rodents have yielded sigmoidal dose-response curves for external
malformations and skeletal defects, and no-effect levels of practical
thresholds are sometimes detectable.  In general, the threshold for
teratogenesis during the early or main inductive phase appears to be lower
(about 0.05 to 0.1 Gy) than during major organogenesis (threshold range 0.15
to 0.25 Gy)...Although insufficient data exist to construct unequivocal
dose-response curves for teratogenesis specific to the human embryo, there
are no reasons to believe that the foregoing threshold ranges are not
generally applicable."  

A comment to similar effect is made by Brent in "The Biological Basis of
Radiation Protection Practice (page 26):  "Prior to summarizing the effects
of irradiation at different stages of gestation it is important to emphasize
that all the effects described are produced at or above specific doses.
Since the vast majority of embryopathological effects are threshold
phenomena, subthreshold doses will not produce these effects."

My questions are (and please bear in mind that I am not an adherent to the
LNT model):  

Why would there exist a threshold for fetal exposure but not for adult
exposure?  

Why would a fetal dose-response curve be sigmoidal but an adult curve
linear?

Does this mean that the mechanism for fetal teratogenic effects differs
somehow from the mechanism for adult stochastic effects?


Please note that I am NOT trying to re-open the perennial
LNT/threshold/hormesis debate.  I'm just curious about the basis for
statements that seem, on the surface, to be mutually contradictory.  

Thanks in advance for any light anyone can help to shed.

Andy

(PS - please remember, too, if you respond that my academic background is in
geology - you don't need to use words of one syllable or less, but there's a
limit to the amount of jargon I can follow.)

Andrew Karam, CHP              (716) 275-1473 (voice)
Radiation Safety Officer       (716) 275-3781 (office)
University of Rochester        (716) 256-0365 (fax)
601 Elmwood Ave. Box HPH       Rochester, NY  14642

Andrew_Karam@URMC.Rochester.edu
http://Intranet.urmc.rochester.edu/RadiationSafety

We cannot prove that those are in error who tell us that society has
reached a turning point, that we have seen our best days.  But so said
all before us, and with just as much apparent reason.  On what principle
is it that, when we see nothing but improvement behind us, we are to
expect nothing but deterioration before us?    Lord Thomas B. Macaulay,
1830
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