15. E M B R Y O G E N E S I S in vitro
567
only nutrients, allow some differentiation, and the same thing is true
of amino acids; but the proteins of egg-white and of yolk give rise to
better differentiation and to better growth, in particular with regard
to the somites; (3) if the nutritive conditions are modified in the course
of an experiment, for example by beginning with the explant in a nonnutritive medium, differentiation can start off again after 12 h of
explantation. Differentiation and growth are better in a medium based
on albumin than in one with glucose only.
2. Enzymatic Activities Revealed by Vital Dyes in Tissue Cultures
By the use of colour tests for reducing enzymes, differences have been
observed in the enzymatic activities of different territories of the embryo.
Neotetrazolium chloride, potassium tellurite, and methylene blue,
applied to the embryo in in vitro culture, are examples of this technique
(Spratt, 1958).
Well-defined centres of reducing activity become manifest in the
course of the first phases of development. Hensen's node, from the
stage when axial organs begin to form, is one of the principal centres
of enzymatic activity. The somites, the notochord, and the neural tube
maintain these properties.
The two main centres of reducing activity, Hensen's node and the
cephalic nervous system, nevertheless manifest behaviour which varies
with the substrate provided them. No diastasic activity at all is apparent
in a non-nutritive medium (Ringer-agar). The reducing activity of
Hensen's node appears with eight different substrates of the sixteen
tested, in an aerobic environment. They are as follows: fructose, galactose, glucose, maltose, succinate, malate, pyruvate, and lactate. It
also appears with eleven substrates in an anaerobic environment—the
same ones as above plus maltose, glutamate, and a-keto-glutarate.
Dehydrogenases of the nervous system have more specific needs with
regard to substrates. Only five substrates were effective in an aerobic
environment—of these only fructose, maltose and galactose were particularly so; three sugars allow dehydrogenase activity in an anaerobic
environment, and more especially fructose.
Furthermore, Spratt has come to the conclusion that the reducing
activity of the blastoderm is stimulated by embryo extract, anaerobiosis,
and the addition of cyanide, and that it is inhibited or blocked by
heating to 60°C, or lowering the p H below 7.
The differences in reducing activity of regions of the blastoderm are
not due to differences in cellular density but to the activity proper of
individual cells. It is likely that they are due not to an increase in the
permeability of the cells, but rather to the number or properties of
their mitochondria.
567
only nutrients, allow some differentiation, and the same thing is true
of amino acids; but the proteins of egg-white and of yolk give rise to
better differentiation and to better growth, in particular with regard
to the somites; (3) if the nutritive conditions are modified in the course
of an experiment, for example by beginning with the explant in a nonnutritive medium, differentiation can start off again after 12 h of
explantation. Differentiation and growth are better in a medium based
on albumin than in one with glucose only.
2. Enzymatic Activities Revealed by Vital Dyes in Tissue Cultures
By the use of colour tests for reducing enzymes, differences have been
observed in the enzymatic activities of different territories of the embryo.
Neotetrazolium chloride, potassium tellurite, and methylene blue,
applied to the embryo in in vitro culture, are examples of this technique
(Spratt, 1958).
Well-defined centres of reducing activity become manifest in the
course of the first phases of development. Hensen's node, from the
stage when axial organs begin to form, is one of the principal centres
of enzymatic activity. The somites, the notochord, and the neural tube
maintain these properties.
The two main centres of reducing activity, Hensen's node and the
cephalic nervous system, nevertheless manifest behaviour which varies
with the substrate provided them. No diastasic activity at all is apparent
in a non-nutritive medium (Ringer-agar). The reducing activity of
Hensen's node appears with eight different substrates of the sixteen
tested, in an aerobic environment. They are as follows: fructose, galactose, glucose, maltose, succinate, malate, pyruvate, and lactate. It
also appears with eleven substrates in an anaerobic environment—the
same ones as above plus maltose, glutamate, and a-keto-glutarate.
Dehydrogenases of the nervous system have more specific needs with
regard to substrates. Only five substrates were effective in an aerobic
environment—of these only fructose, maltose and galactose were particularly so; three sugars allow dehydrogenase activity in an anaerobic
environment, and more especially fructose.
Furthermore, Spratt has come to the conclusion that the reducing
activity of the blastoderm is stimulated by embryo extract, anaerobiosis,
and the addition of cyanide, and that it is inhibited or blocked by
heating to 60°C, or lowering the p H below 7.
The differences in reducing activity of regions of the blastoderm are
not due to differences in cellular density but to the activity proper of
individual cells. It is likely that they are due not to an increase in the
permeability of the cells, but rather to the number or properties of
their mitochondria.