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A Note on the Effects of some Cyto toxic Substances
on Amphibian Embryos
by C. H. WADDINGTON 1
From the Institute of Animal Genetics, Edinburgh
WITH ONE PLATE
I N connexion with the experiments of Jurand (1958) on the action of TEM on
avian embryos, it seems worthwhile describing very shortly some observations
on the effects of this substance, and also of Myleran, on early amphibian embryos. Bodenstein (1954 and earlier) has described some of the cytological results
of the administration of various 'radio-mimetic' substances to amphibian embryos, including nitrogen mustards and TEM. The effects on the development
of the eye were particularly considered, and they have also been discussed and
illustrated by Tondury (1955). Two main types of damage were noted, the formation of giant cells, and of cellular degeneration accompanied by nuclear breakdown and the formation of deeply staining cytoplasmic lumps. The giant cells
presumably arise from the suppression of cell-division followed by the formation
of a restitution nucleus. These nuclei stain rather palely, and appear to be lacking in DNA. On the other hand, the cytoplasmic globules which are commonly
met with appear to consist of RNA, and to indicate an over-production of that
compound.
In some experiments with young amphibian embryos (Axolotl, Triturus
alpestris, and Xenopus) similar cytological pictures have been seen. The embryos
were exposed to solutions of TEM or of Myleran, at a concentration of 0-01 per
cent. The ages at exposure ranged from blastula to early tail-bud, and most of
the embryos were left in the solution until fixation some days later. The point of
interest, which does not emerge clearly from previous accounts, is that one often
finds a much greater degree of damage in the neural tube than in any of the other
tissues. Externally such embryos are short, and appear to have a 'hollow back'
In sections, one sometimes sees the neural tube very extensively abnormal, with
almost every cell affected, while the remaining tissues look reasonably healthy.
Figs. 1 and 2 of the Plate show two specimens from experiments with TEM, in
which nearly all the neural cells exhibit cytoplasmic aggregates of RNA and
some nuclear degeneration. In these experiments, the formation of giant cells
with pale nuclei was rare with TEM, but was well seen following exposure to
Myleran (Plate, fig. 3).
1
Author's address: Institute of Animal Genetics, West Mains Road, Edinburgh 9, U,K.
[J. Embryol. exp. Morph. Vol. 6, Part 2, pp. 363-364, June 1958]
364
C. H. WADDINGTON—CYTOTOXIC SUBSTANCES
Such tissue specificity is not always so well defined as in the specimens illustrated, but even when the damage is more widespread, it is, in these stages,
always the neural system and its derivatives such as the eye which are most
strongly affected. The ganglia of the head are as sensitive as the main body of
the central nervous system, but the sense placodes, such as the otic and nasal
invaginations, show the same resistance as the rest of the ectoderm.
The biological mode of action of these alkylating cytotoxic agents is as yet
too little understood for it to be possible to discuss profitably the chemical
reasons for the differences in sensitivity of different tissues. It is important to
note, however, that the tissue specificities may differ widely between one organism and another. The contrast between the great susceptibility of the mesoderm
in chick embryos, described by Jurand (1958), and the sensitivity of the neural
system in amphibian embryos is sufficient warning that the reaction to these
compounds cannot be interpreted in simple biological terms, but must be related
to specific, but as yet unknown, chemical properties of the cells exposed to them.
It may well be that, as Bodenstein (1954) has argued, these compounds inhibit
the synthesis of DNA, but if this is so. there must also be some other factors
involved in their action which determine the type of cell which will be most
severely affected.
REFERENCES
BODENSTEIN, D. (1954). Effects of radio-mimetic substances on embryonic development, with
special reference to nitrogen mustards. /. cell. comp. Physiol. 43, Suppl. 1, 179-205.
JURAND, A. (1958). The effects of TEM on chick embryos. / . Embryol. exp. Morph. 6,, 357-62.
TdNDURY, G. (1955). Einflufl chemischer Stoffe auf die embryonale. Zelle. Bull, schweiz. Akad.
med. Wiss. 11, 332-45.
E X P L A N A T I O N OF P L A T E
FIG. 1. Triturus alpestris larva, exposed to 001 per cent. TEM since late neurula stage. Neural
cells degenerating but other tissues healthy.
FIG. 2. Part of the brain (on left) together with head mesenchyme (on right) from a similar
embryo at higher magnification. Note darkly stained cytoplasmic accumulations of RNA in the
damaged neural cells.
FIG. 3. Part of the mid-brain of a Triturus, mid-tail-bud stage, exposed to 001 per cent.
Myleran since the early gastrula stage. Note the presence of some giant nuclei scattered amongst
those of normal size.
(Manuscript received 25: x: 57)
/. Emhryol. exp. Morph.
Vol. 6, Part 2
C. H. WADDINGTON