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CHARLES Ε.
WILDE
not necessarily probable) a means whereby the directive cues for a
specific differentiation could act directly on the cytoplasmic synthetic
machinery. The genetic material would, under these circumstances
reside more or less ineffectually in cytoplasm effectively directed into a
unique synthetic pathway.
The diffusion kinetics of low molecular weight compounds are such
that, in response to externally applied substrate cues, the synthetic
pathways within any one cell must act to trap differentially selected
molecules while the fixation of the preferred pathway must occur within
a relatively short period of time. This has been confirmed for Vitamin A
induced metaplasia by Weiss and James (1954).
Pigment cell induction by phenylalanine, however, poses greater
problems. If the diffusing substrate concept is to hold, it must additionally be postulated that selected cells are more receptive to external cues
than are their neighbours, since all of the explanted cells are not
converted. While variants in receptivity are probable, such a concept
requires further study, for the statement itself only adds an indeterminate
parameter to the problem of differentiation.
External, environmentally supplied macromolecular cues may range
from the transfer of pieces of organized cytoplasm from cell to cell,
through ingestion of organized but specific particulate protoplasmic
structures, to macromolecules such as nucleic acids and proteins delivered to the cell as solutions or suspensions. In the first group evidence
for the uptake of organized cytoplasm is available but still scant.
Unpublished data from the author's laboratory from time lapse cinematographic studies indicate such transfers. The processes of phagocytosis
and pinocytosis are other means of transfer with an implied intracellular
digestion of the ingested material.
Macromolecules in solution or suspension have been invoked by
several investigators as means of transfer of differentiative cues. The
literature in embryonic induction is replete with such data. The reader
is referred to the recent report of Yamada (1958) for a review. The type
conversions of bacteria (Avery et al., 1944) give firm evidence of the
effect of macromolecular components on the differentiative behaviour
of this group of organisms.
Great interest for processes of cellular differentiation lies in recent
reports on the effects on cellular differentiation of organized particulate
fractions of specific cell types presented to the environment of receptive
cells. Benitez, Murray and Chargaff (1957) reported that liver microsomes elicited a morphological change in tissue culture fibroblasts, such
that the cells assumed a morphological expression similar to cultured
neurones. However, if type specific differentiative behaviour is to be
expected, a liver cell habitus should have appeared in the converted
CHARLES Ε.
WILDE
not necessarily probable) a means whereby the directive cues for a
specific differentiation could act directly on the cytoplasmic synthetic
machinery. The genetic material would, under these circumstances
reside more or less ineffectually in cytoplasm effectively directed into a
unique synthetic pathway.
The diffusion kinetics of low molecular weight compounds are such
that, in response to externally applied substrate cues, the synthetic
pathways within any one cell must act to trap differentially selected
molecules while the fixation of the preferred pathway must occur within
a relatively short period of time. This has been confirmed for Vitamin A
induced metaplasia by Weiss and James (1954).
Pigment cell induction by phenylalanine, however, poses greater
problems. If the diffusing substrate concept is to hold, it must additionally be postulated that selected cells are more receptive to external cues
than are their neighbours, since all of the explanted cells are not
converted. While variants in receptivity are probable, such a concept
requires further study, for the statement itself only adds an indeterminate
parameter to the problem of differentiation.
External, environmentally supplied macromolecular cues may range
from the transfer of pieces of organized cytoplasm from cell to cell,
through ingestion of organized but specific particulate protoplasmic
structures, to macromolecules such as nucleic acids and proteins delivered to the cell as solutions or suspensions. In the first group evidence
for the uptake of organized cytoplasm is available but still scant.
Unpublished data from the author's laboratory from time lapse cinematographic studies indicate such transfers. The processes of phagocytosis
and pinocytosis are other means of transfer with an implied intracellular
digestion of the ingested material.
Macromolecules in solution or suspension have been invoked by
several investigators as means of transfer of differentiative cues. The
literature in embryonic induction is replete with such data. The reader
is referred to the recent report of Yamada (1958) for a review. The type
conversions of bacteria (Avery et al., 1944) give firm evidence of the
effect of macromolecular components on the differentiative behaviour
of this group of organisms.
Great interest for processes of cellular differentiation lies in recent
reports on the effects on cellular differentiation of organized particulate
fractions of specific cell types presented to the environment of receptive
cells. Benitez, Murray and Chargaff (1957) reported that liver microsomes elicited a morphological change in tissue culture fibroblasts, such
that the cells assumed a morphological expression similar to cultured
neurones. However, if type specific differentiative behaviour is to be
expected, a liver cell habitus should have appeared in the converted