NUCLEIC ACIDS AND SULPHYDRYL GROUPS
289
ventral fragments. Recent experiments in our laboratory have further
shown that the most radioactive proteins are found in the fraction of
basic proteins considered by Horn (1962) to be cytoplasmic histones.
This finding is in agreement with the observation that the yolk platelet
proteins incorporate large amounts of
35
>S-mercaptoethanol. These
observations have special interest when considered in the light of what
has been said above about the probable role of basic proteins (Vahs
1962) and yolk platelet breakdown (Rounds and Flickinger, 1958) in
neural induction: mercaptoethanol, by combining with the basic
proteins, might inhibit their release from the yolk platelets, with the
resulting observed inhibition of neural tube formation.
However, autoradiographic experiments on the localization of
Zb
Smercaptoethanol in the proteins of treated eggs (Brächet et al., 1963) do
not entirely support this hypothesis. During early stages, a distinct
animal-vegetal gradient of distribution is observed which corresponds
to the pigment granules gradient. The greatest radioactivity is found in
these granules, the balance being present in the yolk platelets. The
nuclei, mitotic figures and the hyaloplasm (in centrifuged eggs) display
no visible radioactivity. At later stages (gastrulation), one observes in
addition to this animal-vegetal gradient, a distinct penetration gradient
(from the outside to the inside), but no particularly marked radioactivity
is found in the blocked dorsal lip. It is worth mentioning that, in
agreement with the idea that mercaptoethanol combines with the basic
proteins of the egg, the nuclei do not become labelled before the
gastrula and neurula stages (even then the label is not so strong as in
the cytoplasm). According to Bloch (1962a, b) and Horn, the basic
proteins apparently migrate from the yolk to the nuclei. But experiments
on isolated and explanted medullary plates show that the incorporation
of
36
£-mercaptoethanol is stronger in the neuroblast than in the
chordomesoblast, a fact which suggests that, as shown by our previous
biological experiments, mercaptoethanol might inhibit the competence
of the reacting system more than the inducing activity of the organizer.
It might be, however, that the observations on explanted neural plates
are subject to an artefact due to the abundance, in the neuroblast, of
pigment granules which very actively take up mercaptoethanol.
Incidentally, the strong activity of these granules in these experiments
suggests that they may play a role in the defence mechanisms of the cell :
they could perhaps be the equivalent, in amphibian eggs, of the lysosomes
of other cells. In any event, the autoradiographic observations should be
repeated with species which are free of pigment granules.
Further experimental work is obviously needed before we can accept
the idea that mercaptoethanol blocks development by combining with
basic proteins. Our present results are, on the whole, in favour of this
289
ventral fragments. Recent experiments in our laboratory have further
shown that the most radioactive proteins are found in the fraction of
basic proteins considered by Horn (1962) to be cytoplasmic histones.
This finding is in agreement with the observation that the yolk platelet
proteins incorporate large amounts of
35
>S-mercaptoethanol. These
observations have special interest when considered in the light of what
has been said above about the probable role of basic proteins (Vahs
1962) and yolk platelet breakdown (Rounds and Flickinger, 1958) in
neural induction: mercaptoethanol, by combining with the basic
proteins, might inhibit their release from the yolk platelets, with the
resulting observed inhibition of neural tube formation.
However, autoradiographic experiments on the localization of
Zb
Smercaptoethanol in the proteins of treated eggs (Brächet et al., 1963) do
not entirely support this hypothesis. During early stages, a distinct
animal-vegetal gradient of distribution is observed which corresponds
to the pigment granules gradient. The greatest radioactivity is found in
these granules, the balance being present in the yolk platelets. The
nuclei, mitotic figures and the hyaloplasm (in centrifuged eggs) display
no visible radioactivity. At later stages (gastrulation), one observes in
addition to this animal-vegetal gradient, a distinct penetration gradient
(from the outside to the inside), but no particularly marked radioactivity
is found in the blocked dorsal lip. It is worth mentioning that, in
agreement with the idea that mercaptoethanol combines with the basic
proteins of the egg, the nuclei do not become labelled before the
gastrula and neurula stages (even then the label is not so strong as in
the cytoplasm). According to Bloch (1962a, b) and Horn, the basic
proteins apparently migrate from the yolk to the nuclei. But experiments
on isolated and explanted medullary plates show that the incorporation
of
36
£-mercaptoethanol is stronger in the neuroblast than in the
chordomesoblast, a fact which suggests that, as shown by our previous
biological experiments, mercaptoethanol might inhibit the competence
of the reacting system more than the inducing activity of the organizer.
It might be, however, that the observations on explanted neural plates
are subject to an artefact due to the abundance, in the neuroblast, of
pigment granules which very actively take up mercaptoethanol.
Incidentally, the strong activity of these granules in these experiments
suggests that they may play a role in the defence mechanisms of the cell :
they could perhaps be the equivalent, in amphibian eggs, of the lysosomes
of other cells. In any event, the autoradiographic observations should be
repeated with species which are free of pigment granules.
Further experimental work is obviously needed before we can accept
the idea that mercaptoethanol blocks development by combining with
basic proteins. Our present results are, on the whole, in favour of this