286
JEAN BRÄCHET
(mixtures of lipoic acid 20-5 μ-g/ml and oxaloacetate 1 mg/ml, in NiuTwitty medium). At first, no immediate effect and no favourable
influence on the closure of the neural plate are noticed; but, after one or
two days, a very favourable effect is usually observed on the elongation
of the embryo. On the other hand, we never observed any effect of
oxaloacetate in mercaptoethanol-treated embryos.
The present results thus suggest that the biochemical
mechanisms
underlying nervous system formation and tail differentiation are different.
Such a conclusion would be in agreement with the idea that cephalic
and caudal inductions are mediated by chemically different substances.
Comparable results have been obtained with explanted chick embryos
(Pohl and Brächet, 1962). It was found that both mercaptoethanol and
lipoic acid completely inhibit the closure of the neural tube, exactly as in
the amphibians. Again, this inhibition can be overcome by the addition
of ATP. ATP alone accelerates considerably the closure of the neural
tube in normal embryos. Sections show that this favourable effect on
morphogenesis is linked to a marked increase in mitotic activity,
especially in the nervous system. Another similarity between amphibian
and avian embryos is that oxaloacetate and succinate (0-5 mg/ml)
counteract the inhibitory effects of lipoic acid, but not those of
mercaptoethanol.
The effects of lipoic acid (5-20 jLtg/ml) have also been studied on the
alga Acetabularia. It was found that lipoic acid (20-10 /xg/ml) inhibits
morphogenesis (cap formation) in nucleate and anucleate halves.
Nucleate fragments, however, can occasionally form a small cap. A
striking effect of lipoic acid is that it produces a marked etiolation
(yellowing) of the algae; but there is no direct relationship between this
loss of chlorophyl synthesis and cap formation. Low concentrations of
lipoic acid (5 /xg/ml) have a slight inhibitory effect on cap formation in
large algae (whole or anucleate fragments); on the other hand, they
always stimulate cap formation in young algae and in regenerating
nucleate halves. As in the case of amphibian embryos, mercaptoethanol
and lipoic acid, if added together, have a marked additive effect.
Reversal is almost complete when the treated and completely blocked
algae are placed in normal sea-water, even after a 6-week treatment
(unpublished observations).
It can be concluded that lipoic acid exerts the same 'morphostatic'
effects as mercaptoethanol and that the two substances are very useful
reagents for arresting morphogenesis without injuring the organisms.
D. Biochemical Effects of ^-Mercaptoethanol on Developing Organisms
Our experiments have so far been limited to an analysis of the
biochemical effects of mercaptoethanol on amphibian eggs and
JEAN BRÄCHET
(mixtures of lipoic acid 20-5 μ-g/ml and oxaloacetate 1 mg/ml, in NiuTwitty medium). At first, no immediate effect and no favourable
influence on the closure of the neural plate are noticed; but, after one or
two days, a very favourable effect is usually observed on the elongation
of the embryo. On the other hand, we never observed any effect of
oxaloacetate in mercaptoethanol-treated embryos.
The present results thus suggest that the biochemical
mechanisms
underlying nervous system formation and tail differentiation are different.
Such a conclusion would be in agreement with the idea that cephalic
and caudal inductions are mediated by chemically different substances.
Comparable results have been obtained with explanted chick embryos
(Pohl and Brächet, 1962). It was found that both mercaptoethanol and
lipoic acid completely inhibit the closure of the neural tube, exactly as in
the amphibians. Again, this inhibition can be overcome by the addition
of ATP. ATP alone accelerates considerably the closure of the neural
tube in normal embryos. Sections show that this favourable effect on
morphogenesis is linked to a marked increase in mitotic activity,
especially in the nervous system. Another similarity between amphibian
and avian embryos is that oxaloacetate and succinate (0-5 mg/ml)
counteract the inhibitory effects of lipoic acid, but not those of
mercaptoethanol.
The effects of lipoic acid (5-20 jLtg/ml) have also been studied on the
alga Acetabularia. It was found that lipoic acid (20-10 /xg/ml) inhibits
morphogenesis (cap formation) in nucleate and anucleate halves.
Nucleate fragments, however, can occasionally form a small cap. A
striking effect of lipoic acid is that it produces a marked etiolation
(yellowing) of the algae; but there is no direct relationship between this
loss of chlorophyl synthesis and cap formation. Low concentrations of
lipoic acid (5 /xg/ml) have a slight inhibitory effect on cap formation in
large algae (whole or anucleate fragments); on the other hand, they
always stimulate cap formation in young algae and in regenerating
nucleate halves. As in the case of amphibian embryos, mercaptoethanol
and lipoic acid, if added together, have a marked additive effect.
Reversal is almost complete when the treated and completely blocked
algae are placed in normal sea-water, even after a 6-week treatment
(unpublished observations).
It can be concluded that lipoic acid exerts the same 'morphostatic'
effects as mercaptoethanol and that the two substances are very useful
reagents for arresting morphogenesis without injuring the organisms.
D. Biochemical Effects of ^-Mercaptoethanol on Developing Organisms
Our experiments have so far been limited to an analysis of the
biochemical effects of mercaptoethanol on amphibian eggs and