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LAURI SAXEN ET AL.
Wilt, 1965), amphibian ectoblast (Brachet and Denis, 1963), lens
(Yamada and Roesel, 1964), and muscle (Yaffe and Feldman, 1964). The
suppressing effect of actinomycin on RNA synthesis in kidney explants
reaches a maximum at the time of the earliest appearance of the cell
condensates (Vainio et al., 1965). It thus seems likely that the first
morphologically detectable differentiation, condensation, is causally
linked with genomic activation, but that once the genome has been
activated subsequent tubular morphogenesis cannot be prevented by
intereference of actinomycin with RNA synthesis.
As the initiation of tubule formation seems to be a result of genomic
activation, we may ask whether this initial activity programmes and
stabilizes the subsequent events of tubule differentiation or whether the
new landmarks of differentiation at later stages are expressions of new
activation of the genetic material. It may be noted that when actinomycin is added to the incubation mixture after the critical time, after which
it does not prevent the formation of condensations, the tubules develop
to a certain morphological maturity as well (Jainchill et al., 1964). The
results with the LDH isozymes, however, suggested that genomic activation is needed for the synthesis of B subunits (Koskimies, 1967a). As
pointed out in Section III,B, actinomycin inhibited the synthesis of B
subunits if given some 18 hours before a shift in the LDH isozyme pattern, but the drug no longer inhibited tubule formation at this stage of
development. This suggests that the genome activation initiating tubule
formation is followed by subsequent activations which are actinomycinsensitive, but eventually reach a stage of stability which cannot be
reversed with actinomycin, i.e., by inhibiting rRNA and/or mRNA synthesis.
The subsequent maturation and growth of the tubule is accompanied
by morphological and biochemical changes. The opening of the tubule
lumen is combined with reorganization of the shape and internal structure of the cells, and some new classes of proteins appear at the same
time (see Sections III,B and C). If the production of a special protein
is taken as a marker of differentiation, we have evidence that differentiation continues after the tubules have reached their maturation in shape
and size. This maturation was attained around the sixth day of cultivation. Yet histochemically demonstrable ATPase only appeared around
the seventh day and kidney-specific antigen at about the twelfth day of
cultivation.
When the appearance of a new protein is recorded during tubule
maturation, we must keep in mind that we are not necessarily dealing
LAURI SAXEN ET AL.
Wilt, 1965), amphibian ectoblast (Brachet and Denis, 1963), lens
(Yamada and Roesel, 1964), and muscle (Yaffe and Feldman, 1964). The
suppressing effect of actinomycin on RNA synthesis in kidney explants
reaches a maximum at the time of the earliest appearance of the cell
condensates (Vainio et al., 1965). It thus seems likely that the first
morphologically detectable differentiation, condensation, is causally
linked with genomic activation, but that once the genome has been
activated subsequent tubular morphogenesis cannot be prevented by
intereference of actinomycin with RNA synthesis.
As the initiation of tubule formation seems to be a result of genomic
activation, we may ask whether this initial activity programmes and
stabilizes the subsequent events of tubule differentiation or whether the
new landmarks of differentiation at later stages are expressions of new
activation of the genetic material. It may be noted that when actinomycin is added to the incubation mixture after the critical time, after which
it does not prevent the formation of condensations, the tubules develop
to a certain morphological maturity as well (Jainchill et al., 1964). The
results with the LDH isozymes, however, suggested that genomic activation is needed for the synthesis of B subunits (Koskimies, 1967a). As
pointed out in Section III,B, actinomycin inhibited the synthesis of B
subunits if given some 18 hours before a shift in the LDH isozyme pattern, but the drug no longer inhibited tubule formation at this stage of
development. This suggests that the genome activation initiating tubule
formation is followed by subsequent activations which are actinomycinsensitive, but eventually reach a stage of stability which cannot be
reversed with actinomycin, i.e., by inhibiting rRNA and/or mRNA synthesis.
The subsequent maturation and growth of the tubule is accompanied
by morphological and biochemical changes. The opening of the tubule
lumen is combined with reorganization of the shape and internal structure of the cells, and some new classes of proteins appear at the same
time (see Sections III,B and C). If the production of a special protein
is taken as a marker of differentiation, we have evidence that differentiation continues after the tubules have reached their maturation in shape
and size. This maturation was attained around the sixth day of cultivation. Yet histochemically demonstrable ATPase only appeared around
the seventh day and kidney-specific antigen at about the twelfth day of
cultivation.
When the appearance of a new protein is recorded during tubule
maturation, we must keep in mind that we are not necessarily dealing