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Pig skin vs human skin
Your points regarding the pig animal model with respect to blood
cells and physiology are well taken, but I feel some defense
should be made with respect to the pig skin model. Specifically,
use of pigs for the hot particle experiments carried out at
Brookhaven National Laboratory refereed to by Alan Roecklein
several days ago.
Pig skin is similar to human skin in terms of color, hair
follicles, sweat glands, and subcutaneous fat (Hall, 1988, p.55).
The skin color for white pigs is convenient for identification of
erythema. The presence of hair follicles is important, since
skin stem cells (basal cells) lining the hair follicle bulb is
thought to aid in the repopulation of the skin after irradiation
(Hopewell 1991). While I am not a biologist, I think that the
dog model you propose has fur, which is different than hair in
terms of the follicle bulb structure. I don't think dogs have
sweat glands in their skin, hence the pig is again a better model
for skin irradiation.
You suggest that the skin of the swine is much thicker than that
of humans, and that pig hide can be tanned. My parents have
repeatedly (and justifiably) tanned my hide as well, so it isn't
limited just to pigs. More seriously, the suggested average
value for the thickness of the epidermis is 70 um (ICRP 1974),
and the measured value for 3-4 month old swine used in the
Brookhaven study was 80 um. While we did not measure the
thickness of the dermis layer, the 3-4 month old pigs used by
Hopewell (1991) had a dermal thickness of ~1400 um, which is
again similar to humans. Hopewell also discusses that for the
pigs used in his group's studies, the density of the basal cells,
cell kinetics, collagen fibers, and vascular supply are all
similar to that of humans. Forbes (1969) also suggests that the
skin of the swine is more human like than any other readily
I think your comments demonstrate that there is no perfect animal
model for the human; but for the skin, pig skin is the best we
have at the present time.
Forbes, P.D., "Vascular Supply of the Skin and Hair in Swine,"
Chapter XXVII in _Advances in Biology of Skin_, Vol. IX, Hair
Growth, Montagna, W. and R.L. Dobson (editors), Pergamon Press,
Oxford, 419-432, 1969.
Hall, E.J., _Radiobiology for the Radiologist_, third edition.
J.B. Lippincott Company, Philadelphia, 1988.
Hopewell, J.W., "Biological Effects of Irradiation on Skin and
Recommended Dose Limits". Radiation Protection Dosimetry, Vol.39,
No.1-3, pp.11-24, 1991.
ICRP. Report of the Task Group on Reference Man. ICRP Publication
23, Annals of the ICRP, Pergamon Press, NY 1974.