NUCLEIC ACIDS AND SULPHYDRYL GROUPS
277
thiol-disulphide
ratio of the medium. Such a hypothesis can be eliminated
for a number of reasons. For instance, mercaptoethylgluconamide,
which produces the same drop in the redox potential of the medium as
mercaptoethanol, exerts very little effect on morphogenesis (occasionally,
the formation of the lens is suppressed). When eggs are treated for some
time with mercaptoethanol, they develop much better if they are
transferred to normal medium than to a dithiodiglycol-containing
medium (Brächet and Delange-Cornil, 1959). Finally, we shall see that
mercaptoethanol, a thiol, and lipoic acid, a disulphide, exert strikingly
similar effects on morphogenesis.
Another suggestion has been made recently by Tuft (1961): the
inhibition of gastrulation and neurulation by mercaptoethanol might be
the result of 'water imbalance'. In particular, the blastocoele cavity may
be so filled with fluid that it cannot collapse during gastrulation; its
continued presence would, of course, inhibit gastrulation movements in
a mechanical way. Such a simple explanation of our results is not very
likely, in our opinion, for the following reasons (Brächet, 1962; Malpoix
et al., 1963): an analysis of the gastrulation movements in mercaptoethanol-treated amphibian gastrulae, using both local staining of whole
gastrulae and careful study of expiants, has clearly shown that mercaptoethanol (M/100) has less effect on some morphogenetic movements than
on others. For instance, invagination movements are less affected than
epiboly, extension-convergence and ingression movements. Furthermore, mercaptoethanol, at the same concentration, inhibits the closure
of the explanted neural plate, which may form a ridge but never a tube.
This inhibition, although never so strong as in the whole embryo, clearly
shows that the swelling of the blastocoele and archenteron cavities
cannot be the sole explanation for the arrest of neural plate closure.
Another possibility is that mercaptoethanol, like many thiols, inhibits
the contractility of a fibrous protein, similar to the actomyosin of muscle.
Such a protein should undergo contraction in the presence of ATP and
Mg
2+
ions, and display adenosinetriphosphatase (ATPase) activity.
Since it is known that ATP accelerates the closure of the nervous
system in amphibian eggs (Ambellan, 1955, 1958) we decided to study
its effects on the mercaptoethanol-treated eggs.
We found (Brächet, 1962) that ATP (and, to a certain extent, ADP)
strongly counteract the inhibitory effects of mercaptoethanol on neural
tube closure in four different amphibian species. The best recovery was
obtained when late gastrulae were treated first with mercaptoethanol
(M/100), then with ATP (0-1 mg/ml). But it is interesting that no
beneficial effects of ATP were observed on the other abnormalities
induced by mercaptoethanol, such as lack of pigmentation, delay in
yolk utilization, absence of elongation, etc. The favourable effects of
277
thiol-disulphide
ratio of the medium. Such a hypothesis can be eliminated
for a number of reasons. For instance, mercaptoethylgluconamide,
which produces the same drop in the redox potential of the medium as
mercaptoethanol, exerts very little effect on morphogenesis (occasionally,
the formation of the lens is suppressed). When eggs are treated for some
time with mercaptoethanol, they develop much better if they are
transferred to normal medium than to a dithiodiglycol-containing
medium (Brächet and Delange-Cornil, 1959). Finally, we shall see that
mercaptoethanol, a thiol, and lipoic acid, a disulphide, exert strikingly
similar effects on morphogenesis.
Another suggestion has been made recently by Tuft (1961): the
inhibition of gastrulation and neurulation by mercaptoethanol might be
the result of 'water imbalance'. In particular, the blastocoele cavity may
be so filled with fluid that it cannot collapse during gastrulation; its
continued presence would, of course, inhibit gastrulation movements in
a mechanical way. Such a simple explanation of our results is not very
likely, in our opinion, for the following reasons (Brächet, 1962; Malpoix
et al., 1963): an analysis of the gastrulation movements in mercaptoethanol-treated amphibian gastrulae, using both local staining of whole
gastrulae and careful study of expiants, has clearly shown that mercaptoethanol (M/100) has less effect on some morphogenetic movements than
on others. For instance, invagination movements are less affected than
epiboly, extension-convergence and ingression movements. Furthermore, mercaptoethanol, at the same concentration, inhibits the closure
of the explanted neural plate, which may form a ridge but never a tube.
This inhibition, although never so strong as in the whole embryo, clearly
shows that the swelling of the blastocoele and archenteron cavities
cannot be the sole explanation for the arrest of neural plate closure.
Another possibility is that mercaptoethanol, like many thiols, inhibits
the contractility of a fibrous protein, similar to the actomyosin of muscle.
Such a protein should undergo contraction in the presence of ATP and
Mg
2+
ions, and display adenosinetriphosphatase (ATPase) activity.
Since it is known that ATP accelerates the closure of the nervous
system in amphibian eggs (Ambellan, 1955, 1958) we decided to study
its effects on the mercaptoethanol-treated eggs.
We found (Brächet, 1962) that ATP (and, to a certain extent, ADP)
strongly counteract the inhibitory effects of mercaptoethanol on neural
tube closure in four different amphibian species. The best recovery was
obtained when late gastrulae were treated first with mercaptoethanol
(M/100), then with ATP (0-1 mg/ml). But it is interesting that no
beneficial effects of ATP were observed on the other abnormalities
induced by mercaptoethanol, such as lack of pigmentation, delay in
yolk utilization, absence of elongation, etc. The favourable effects of