276
JEAN BRÄCHET
mercaptoethanol-treated organizers were put together with normal
ectoderm fragments and vice versa demonstrated that mercaptoethanol
acts more effectively on the competence of the ectoderm than on the
inducing power of the organizer. These observations are in good agreement with those made on whole embryos, in which the neural plate fails
to form or close, while chorda and somites differentiate relatively well
(Brächet, 1958a, 1960; Brächet and Delange-Cornil, 1959).
At lower concentrations of mercaptoethanol (M/300 or M/1,000, for
instance) development proceeds further, but after 3 or 4 days of
continuous treatment, one obtains strongly microcephalic embryos
which are more or less delayed in their development. A striking feature
of these embryos is an almost complete absence of melanophores and a
complete lack of pigmentation of the retina. Mercaptoethanol, probably
by virtue of its reducing properties, thus exerts a profound inhibition of
pigment formation, in the eye as well as in the skin. The results in the case
of Xenopus are especially striking. The tadpoles that form in the presence
of mercaptoethanol have blue eyes: in this case, mercaptoethanol
inhibits the formation of melanin in a fairly specific way without inhibiting the formation of pigment cells. This effect is a reversible one, and
the eyes and melanophores become black after a few days in normal
medium.
The great toxicity of dithiodiglycol, as compared to mercaptoethanol,
is especially remarkable in the case of Pleurodeles neurula stages. In a
M/10,000 solution, cytolysis may occur within 24 hours. But, in contrast
to mercaptoethanol, dithiodiglycol does not markedly delay or modify
development before it exerts its lytic effects. At lower concentrations
(M/30,000 to M/100,000), dithiodiglycol has, if anything, a slightly
stimulating effect: in particular, the head sometimes shows overdevelopment, and the tail may be longer than in the controls. The
tadpoles always show a higher degree of motility than the controls.
There is thus no doubt that mercaptoethanol
and dithiodiglycol
exert
opposite effects in all respects. The absence of mitoses, the microcephaly,
the elongation of the body, which are characteristic of the embryos
treated with mercaptoethanol, are replaced after treatment with
dithiodiglycol by intense mitotic activity, over-development of the
head, reduction of the body and lordosis. Dithiodiglycol has no particular
effect, in contrast to mercaptoethanol, on pigment formation.
How does mercaptoethanol exert its inhibitory effects on morphogenesis? A number of hypotheses, which can be ruled out on the basis of
simple biological experiments, will now be discussed. Other explanations
will be proposed at the end of this section, when the material relative to
the biochemical effects of mercaptoethanol has been presented.
The simplest hypothesis is that morphogenesis is controlled by the
JEAN BRÄCHET
mercaptoethanol-treated organizers were put together with normal
ectoderm fragments and vice versa demonstrated that mercaptoethanol
acts more effectively on the competence of the ectoderm than on the
inducing power of the organizer. These observations are in good agreement with those made on whole embryos, in which the neural plate fails
to form or close, while chorda and somites differentiate relatively well
(Brächet, 1958a, 1960; Brächet and Delange-Cornil, 1959).
At lower concentrations of mercaptoethanol (M/300 or M/1,000, for
instance) development proceeds further, but after 3 or 4 days of
continuous treatment, one obtains strongly microcephalic embryos
which are more or less delayed in their development. A striking feature
of these embryos is an almost complete absence of melanophores and a
complete lack of pigmentation of the retina. Mercaptoethanol, probably
by virtue of its reducing properties, thus exerts a profound inhibition of
pigment formation, in the eye as well as in the skin. The results in the case
of Xenopus are especially striking. The tadpoles that form in the presence
of mercaptoethanol have blue eyes: in this case, mercaptoethanol
inhibits the formation of melanin in a fairly specific way without inhibiting the formation of pigment cells. This effect is a reversible one, and
the eyes and melanophores become black after a few days in normal
medium.
The great toxicity of dithiodiglycol, as compared to mercaptoethanol,
is especially remarkable in the case of Pleurodeles neurula stages. In a
M/10,000 solution, cytolysis may occur within 24 hours. But, in contrast
to mercaptoethanol, dithiodiglycol does not markedly delay or modify
development before it exerts its lytic effects. At lower concentrations
(M/30,000 to M/100,000), dithiodiglycol has, if anything, a slightly
stimulating effect: in particular, the head sometimes shows overdevelopment, and the tail may be longer than in the controls. The
tadpoles always show a higher degree of motility than the controls.
There is thus no doubt that mercaptoethanol
and dithiodiglycol
exert
opposite effects in all respects. The absence of mitoses, the microcephaly,
the elongation of the body, which are characteristic of the embryos
treated with mercaptoethanol, are replaced after treatment with
dithiodiglycol by intense mitotic activity, over-development of the
head, reduction of the body and lordosis. Dithiodiglycol has no particular
effect, in contrast to mercaptoethanol, on pigment formation.
How does mercaptoethanol exert its inhibitory effects on morphogenesis? A number of hypotheses, which can be ruled out on the basis of
simple biological experiments, will now be discussed. Other explanations
will be proposed at the end of this section, when the material relative to
the biochemical effects of mercaptoethanol has been presented.
The simplest hypothesis is that morphogenesis is controlled by the