Universlty of Manitoba, ln Partîal Fulfiìlment - MSpace at the ...

The result of any teratogen¡c insult depends on the site of action
by the agent, and the developmenta¡ stage of the embryo. Thís appl les
to both the expression of genes, and the action of environmentâl agents
(saxá, t976).
ln general, the first period of embryonic development (up to the
formation of germ layers) shows little tendenöy to malfcrmations, with
embryonic death at high dose levels. lJîth the onset of morphogenesîs,
the embryo become¡ very suscept¡ble to teratogenlc influences, resultíng
ln major malformatlons. 0n reaching the fetal stêge, only structures
still undergoing differentlatlon (such as the braln, palate, and major
vessels) are stlll susceptlble. to abnormal development (falter, l968).
ln mammals, there ls the additional problem of whether an êgent
acts directly on the embryo or indírectty through the placenta, as
exempl ifíed by the activity of trypan blue. For experimenta¡ teratology,
the use of physical agents provîdes accurate control of timing and dosage,
though some effects may be secondary to tissue damage or alteration of
fiorphogenet i c processes.
X-ray treatment has revealed the sequence of crÌtical periods ín
development of the rat(.:lob,et €jL, 19352 Kàven, l!J8; Hicks,.t954), the mouse
(Russel l, 1950; 1956; r,rilson, .|954; Hícks, 1954), and the chick (Reyss-Brion,
1956; Hadj I tsky,l!62;¡1¡ rrmann and t/olff,1964). Hicks (1954, 1954) found
that irradíation of pregnant rats: at 9 days produced an open brain; at
l0 days produced forebrain, hindbraín, cord, and facial defects; at ll
days produced hydrocephalus, with braínstem and cord dèfects; êt l2 days
caused reductÌons of the brain and eyes; and after thls reduced the size
of fiber tracts. Hicks found that embryos showed a high regulative ability
before the onset of d i ffe ren t i a t i on, so that extensive t¡ssuè damage might
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