ABSTRACT
In 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 Töndury (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).
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
EXPLANATION OF PLATE
Fig. 1. Triturus alpestris larva, exposed to 0 01 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 0 01 per cent. Myleran since the early gastrula stage. Note the presence of some giant nuclei scattered amongst those of normal size.
Fig. 1. Triturus alpestris larva, exposed to 0 01 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 0 01 per cent. Myleran since the early gastrula stage. Note the presence of some giant nuclei scattered amongst those of normal size.
