DIFFERENTIATION OF KIDNEY MESENCHYME
281
branes and the appearance of electron-dense material in the lumen (Figs.
19 and 20). This stage coincides with the appearance of thiamine pyrophosphatase (TPPase) activity in the apical regions of the cells, which
marks a change in the Golgi apparatus as the TPPase is restricted to the
Golgi lamellae (Novikoff and Goldfisher, 1961). The developing pretubule is also characterized by an increase in the number of cytosomes
and related particles in the apical cytoplasm and by the appearance there
of acid phosphatase activity, which may have a role to play in the
formation of the lumen, since it is possible that the adhesiveness and
physical properties of cell surfaces can be modified by the release of
endogenous enzymes (Weiss, 1965).
IV. Discussion
The main events in kidney tubule differentiation and some experimental results on its inhibition are summarized in Fig. 21, and will be
discussed below. In doing so, it must be stressed that, although the results
have been gained from a great variety of experiments performed by
many different workers, they were all carried out in the same laboratory
and with the same standardized basic techniques. Consequently, they can
be temporally correlated as in the figure, and it remains to be critically
discussed whether any causal relationships can be demonstrated between
these different events contributing to morphogenesis and chemodifferentiation.
A. Induction
As pointed out in Section I, differentiation of the kidney tubule is one
example of the common control mechanisms whereby contact between
two intimately related tissues controls their subsequent differentiation.
Two alternative mechanisms for such short-term tissue interaction have
to be considered; either the whole pathway to the complex end result is
triggered and determined during a short-term initial pulse, or only the
first step of a long-chain reaction has been activated and this is followed
by secondary interactions between the tissue components.
The fundamental event triggered by heterotypic induction is not
known, but the first morphologically detectable indication of the onset of
differentiation is a clustering of mesenchymal cells to form cell aggregates. Accordingly, we may consider that this condensation is the immediate result of induction. Is it the principal effect of heterotypic
induction in this system? It is known that even if the inductor is removed shortly before or at the time of the appearance of the condensates,
281
branes and the appearance of electron-dense material in the lumen (Figs.
19 and 20). This stage coincides with the appearance of thiamine pyrophosphatase (TPPase) activity in the apical regions of the cells, which
marks a change in the Golgi apparatus as the TPPase is restricted to the
Golgi lamellae (Novikoff and Goldfisher, 1961). The developing pretubule is also characterized by an increase in the number of cytosomes
and related particles in the apical cytoplasm and by the appearance there
of acid phosphatase activity, which may have a role to play in the
formation of the lumen, since it is possible that the adhesiveness and
physical properties of cell surfaces can be modified by the release of
endogenous enzymes (Weiss, 1965).
IV. Discussion
The main events in kidney tubule differentiation and some experimental results on its inhibition are summarized in Fig. 21, and will be
discussed below. In doing so, it must be stressed that, although the results
have been gained from a great variety of experiments performed by
many different workers, they were all carried out in the same laboratory
and with the same standardized basic techniques. Consequently, they can
be temporally correlated as in the figure, and it remains to be critically
discussed whether any causal relationships can be demonstrated between
these different events contributing to morphogenesis and chemodifferentiation.
A. Induction
As pointed out in Section I, differentiation of the kidney tubule is one
example of the common control mechanisms whereby contact between
two intimately related tissues controls their subsequent differentiation.
Two alternative mechanisms for such short-term tissue interaction have
to be considered; either the whole pathway to the complex end result is
triggered and determined during a short-term initial pulse, or only the
first step of a long-chain reaction has been activated and this is followed
by secondary interactions between the tissue components.
The fundamental event triggered by heterotypic induction is not
known, but the first morphologically detectable indication of the onset of
differentiation is a clustering of mesenchymal cells to form cell aggregates. Accordingly, we may consider that this condensation is the immediate result of induction. Is it the principal effect of heterotypic
induction in this system? It is known that even if the inductor is removed shortly before or at the time of the appearance of the condensates,