I.
PROBLEM
OF
THE
ORGANIZER
45
In the recent paper of Sirlin and Brahma (1959) the transfer of the
tracer in lens induction in the Xenopus embryo was studied with
quantitative high resolution autoradiography. When the optic cup
marked with dZ-3-phenylalanine-214
C was combined with the competent
ectoderm in the explant, the tracer was transferred preferentially and
intensively into the induced lens. The ratio of cytoplasmic counts to
nuclear counts in the lens was found to be higher than those in the optic
cup, while within the nucleus the identical trend of labelling of nuclear
components was observed both in the lens and optic cup. The authors
interpret the results as suggesting the transfer of both small molecules
and more complex molecules or cytoplasmic particles.
Thus, in spite of the difficulty inherent in the technique, isotope
experiments have also provided some positive suggestions as to the
passage of macromolecules from the inductor cells to the ectoderm cells.
VIII. Significance of Some Biochemical Events
Under the influence of Child's theory of physiological gradients it has
often been asserted that the organizer area may be the apex of the
physiological gradient of the early amphibian gastrula. The idea
appeared to be supported by Bellamy's finding (1919) that in the frog
gastrula developing in KCN solution, the dorsal lip region is most
susceptible. Later the point was called in question, when Holtfreter
(1943) showed that in this case KCN was probably not acting as a
metabolic inhibitor but was affecting cell affinity. Many workers were
interested in measuring the oxygen consumption of the organizer area
and in comparing it with that of other areas. In spite of some earlier
affirmative results (Brächet, 1935; Fischer and Hartwig, 1938; Barth,
1939), recent analysis has shown that if the oxygen consumption is
calculated on some basis which seems to represent the amount of
metabolically active cell material, there is virtually no significant
difference among various areas of the early gastrula (Sze, 1953). If the
data are calculated on a basis such as total nitrogen in which yolk is also
included, there are significant differences among areas. But in this case
the apex is present in the presumptive neural area and not in the
organizer area. Values decreasing in the following order were obtained:
presumptive neural ectoderm, presumptive epidermal ectoderm, dorsal
marginal zone (organizer area), ventral marginal zone and endoderm
(Sze, 1953). Within the mesoderm the organizer area represents the apex
of respiration. The overall pattern can be described as the superposition
of a steep gradient along the animal-vegetal axis, and a gentle gradient
along the dorso-ventral axis. A similar pattern is revealed in the
differential distribution of various substances or enzymatic activities
PROBLEM
OF
THE
ORGANIZER
45
In the recent paper of Sirlin and Brahma (1959) the transfer of the
tracer in lens induction in the Xenopus embryo was studied with
quantitative high resolution autoradiography. When the optic cup
marked with dZ-3-phenylalanine-214
C was combined with the competent
ectoderm in the explant, the tracer was transferred preferentially and
intensively into the induced lens. The ratio of cytoplasmic counts to
nuclear counts in the lens was found to be higher than those in the optic
cup, while within the nucleus the identical trend of labelling of nuclear
components was observed both in the lens and optic cup. The authors
interpret the results as suggesting the transfer of both small molecules
and more complex molecules or cytoplasmic particles.
Thus, in spite of the difficulty inherent in the technique, isotope
experiments have also provided some positive suggestions as to the
passage of macromolecules from the inductor cells to the ectoderm cells.
VIII. Significance of Some Biochemical Events
Under the influence of Child's theory of physiological gradients it has
often been asserted that the organizer area may be the apex of the
physiological gradient of the early amphibian gastrula. The idea
appeared to be supported by Bellamy's finding (1919) that in the frog
gastrula developing in KCN solution, the dorsal lip region is most
susceptible. Later the point was called in question, when Holtfreter
(1943) showed that in this case KCN was probably not acting as a
metabolic inhibitor but was affecting cell affinity. Many workers were
interested in measuring the oxygen consumption of the organizer area
and in comparing it with that of other areas. In spite of some earlier
affirmative results (Brächet, 1935; Fischer and Hartwig, 1938; Barth,
1939), recent analysis has shown that if the oxygen consumption is
calculated on some basis which seems to represent the amount of
metabolically active cell material, there is virtually no significant
difference among various areas of the early gastrula (Sze, 1953). If the
data are calculated on a basis such as total nitrogen in which yolk is also
included, there are significant differences among areas. But in this case
the apex is present in the presumptive neural area and not in the
organizer area. Values decreasing in the following order were obtained:
presumptive neural ectoderm, presumptive epidermal ectoderm, dorsal
marginal zone (organizer area), ventral marginal zone and endoderm
(Sze, 1953). Within the mesoderm the organizer area represents the apex
of respiration. The overall pattern can be described as the superposition
of a steep gradient along the animal-vegetal axis, and a gentle gradient
along the dorso-ventral axis. A similar pattern is revealed in the
differential distribution of various substances or enzymatic activities