13. D I F F E R E N T I A T I O N
483
systemic influence has been removed, but the intermediate levels of
homotypic and heterotypic influence persist. The information which
has been reviewed seems ample to justify the conclusion that these
intermediate shells and their mechanisms—at the moment at the
threshold of understanding—are important to both theoretical and
practical grasp of differentiation.
Technically, then, the objective seems clear—the behavior of cells
and tissues in the intermediate ground between cell and organ culture
must be explored so as to identify the essential factors which account
for the differences of behavior observed by the two techniques.
Beginnings have been made, in the attempt to adapt defined media to
organ culture, in the demonstration of continued differentiation of dispersed cells under special conditions, in the development of methods
for separation of tissue components and cells, and in the interaction of
components of rudiments across membranes. These methods make
possible the gradual dissection of organ culture procedures, so as to
identify the products of the interactive processes within organs. These
products, molecules probably in complex and specific spatio-temporal
patterns, must be assumed to provide the essential microenvironment
or matrix for cytodifferentiation.
T o the degree that this microenvironment can be duplicated in
systems simpler than organ cultures, cytodifferentiation will be able to
be controlled, and thus rendered more accessible to the tools of intracellular analysis. In the process, hopefully, information will be gained on
the ways in which microenvironment impinges on intracellular controls.
At the moment, the molecular biologist has the choice of following
events in individual cells, with the attendant advantages and limitations
of cytochemical, ultrastructural, or autoradiographic analysis, or of
applying sometimes more discerning chemical methods to populations
which may be heterogeneous with respect to differentiation—often both
as to kind and stage. Neither alternative is entirely satisfactory. Much
needed are culture methods which provide relatively large populations
of cells undergoing synchronized differentiation of one kind—making
far easier the task of working out the details and interactions of synthetic
and other metabolic pathways. Populations of this sort, too, should
provide definitive answers to the fundamental questions raised earlier
with respect to the time course of differentiation, its reversibility, the
dissociability of its several properties, its relation to mitosis, and its
propagability. Without these answers, as has been said, there is no way
to choose among the many postulated mechanisms of differentiation.
Continued study of differentiation in culture, therefore, will simultaneously contribute to understanding of differentiation, and to the
strengthening of culture technology. For some time to come, as from its
483
systemic influence has been removed, but the intermediate levels of
homotypic and heterotypic influence persist. The information which
has been reviewed seems ample to justify the conclusion that these
intermediate shells and their mechanisms—at the moment at the
threshold of understanding—are important to both theoretical and
practical grasp of differentiation.
Technically, then, the objective seems clear—the behavior of cells
and tissues in the intermediate ground between cell and organ culture
must be explored so as to identify the essential factors which account
for the differences of behavior observed by the two techniques.
Beginnings have been made, in the attempt to adapt defined media to
organ culture, in the demonstration of continued differentiation of dispersed cells under special conditions, in the development of methods
for separation of tissue components and cells, and in the interaction of
components of rudiments across membranes. These methods make
possible the gradual dissection of organ culture procedures, so as to
identify the products of the interactive processes within organs. These
products, molecules probably in complex and specific spatio-temporal
patterns, must be assumed to provide the essential microenvironment
or matrix for cytodifferentiation.
T o the degree that this microenvironment can be duplicated in
systems simpler than organ cultures, cytodifferentiation will be able to
be controlled, and thus rendered more accessible to the tools of intracellular analysis. In the process, hopefully, information will be gained on
the ways in which microenvironment impinges on intracellular controls.
At the moment, the molecular biologist has the choice of following
events in individual cells, with the attendant advantages and limitations
of cytochemical, ultrastructural, or autoradiographic analysis, or of
applying sometimes more discerning chemical methods to populations
which may be heterogeneous with respect to differentiation—often both
as to kind and stage. Neither alternative is entirely satisfactory. Much
needed are culture methods which provide relatively large populations
of cells undergoing synchronized differentiation of one kind—making
far easier the task of working out the details and interactions of synthetic
and other metabolic pathways. Populations of this sort, too, should
provide definitive answers to the fundamental questions raised earlier
with respect to the time course of differentiation, its reversibility, the
dissociability of its several properties, its relation to mitosis, and its
propagability. Without these answers, as has been said, there is no way
to choose among the many postulated mechanisms of differentiation.
Continued study of differentiation in culture, therefore, will simultaneously contribute to understanding of differentiation, and to the
strengthening of culture technology. For some time to come, as from its