13. D I F F E R E N T I A T I O N
469
intracellular or in the governing circumstances among cells, i.e. supracellular? At what point in the time course of differentiation is the
capacity or probability of cell division affected? Does irreversibility of
differentiation set in and, if so, at what point in the time course and by
what mechanisms? H o w do reduced probability of division, and stabilization of differentiative direction, relate in time and mechanism to
the onset of specific synthesis? H o w does the time course of differentiation in one cell relate to that in others following the same path, and
what interactions, if any, occur between cells at different points in the
time course? D o such interactions play any role in governing the
populational synchrony or asynchrony of differentiation which underlies orderly growth and development? These are the kind of questions
to which answers are most needed at the present time, both to understand differentiative mechanisms and to control them in the interest of
more effective culture technology. It is important to note that answers
may not be the same for each instance of differentiation studied. Until,
however, answers are available for at least a representative series of cell
types, statements about the general nature of differentiation in vitro
remain ill-founded speculation.
I V . C U L T U R A L F A C T O R S A N D D I F F E R E N T I A T I O N
It has been known since the beginning of Tissue Culture that under
suitable conditions differentiation of cells and tissues is possible in vitro.
The degree, and in some measure the character, of the differentiation
is influenced by the culture environment supplied. Among the significant cultural factors are the type of culture (including physical factors,
substrate properties, gaseous exchange, etc.), the overall composition of
the nutrient medium, and certain specific nutritional components—
precursors, hormones, or vitamins—which favor particular differentiations.
Culture techniques which provide early attachment of a nondispersed embryonic explant to a solid substratum have generally
yielded differentiation more readily than techniques emphasizing dispersal of cells in fluid or suspension cultures. Roughly, these extremes
represent the difference between so-called organ and cell cultures,
though the extremes are ramifying into a growing number of intermediate procedures as media and techniques are refined. It now seems
likely that the loss of differentiated properties noted in early tissue
cultures, particularly striking in the outgrowth zone and less marked in
the relatively undisturbed center of such cultures, arose largely from the
alteration of intercellular and intertissue relations. The importance of
these homotypic and heterotypic interactions for differentiation will be
469
intracellular or in the governing circumstances among cells, i.e. supracellular? At what point in the time course of differentiation is the
capacity or probability of cell division affected? Does irreversibility of
differentiation set in and, if so, at what point in the time course and by
what mechanisms? H o w do reduced probability of division, and stabilization of differentiative direction, relate in time and mechanism to
the onset of specific synthesis? H o w does the time course of differentiation in one cell relate to that in others following the same path, and
what interactions, if any, occur between cells at different points in the
time course? D o such interactions play any role in governing the
populational synchrony or asynchrony of differentiation which underlies orderly growth and development? These are the kind of questions
to which answers are most needed at the present time, both to understand differentiative mechanisms and to control them in the interest of
more effective culture technology. It is important to note that answers
may not be the same for each instance of differentiation studied. Until,
however, answers are available for at least a representative series of cell
types, statements about the general nature of differentiation in vitro
remain ill-founded speculation.
I V . C U L T U R A L F A C T O R S A N D D I F F E R E N T I A T I O N
It has been known since the beginning of Tissue Culture that under
suitable conditions differentiation of cells and tissues is possible in vitro.
The degree, and in some measure the character, of the differentiation
is influenced by the culture environment supplied. Among the significant cultural factors are the type of culture (including physical factors,
substrate properties, gaseous exchange, etc.), the overall composition of
the nutrient medium, and certain specific nutritional components—
precursors, hormones, or vitamins—which favor particular differentiations.
Culture techniques which provide early attachment of a nondispersed embryonic explant to a solid substratum have generally
yielded differentiation more readily than techniques emphasizing dispersal of cells in fluid or suspension cultures. Roughly, these extremes
represent the difference between so-called organ and cell cultures,
though the extremes are ramifying into a growing number of intermediate procedures as media and techniques are refined. It now seems
likely that the loss of differentiated properties noted in early tissue
cultures, particularly striking in the outgrowth zone and less marked in
the relatively undisturbed center of such cultures, arose largely from the
alteration of intercellular and intertissue relations. The importance of
these homotypic and heterotypic interactions for differentiation will be