13. D I F F E R E N T I A T I O N
467
properties which characterize a given cell type form a closely coupled
complex in vivo, it is not inconceivable that the complex may become
uncoupled in vitro (Ebner, Hageman and Larson, 1961; Hilfer, 1962),
and that certain aspects of differentiation may be persistent whereas
others are lost.
Negative results with even the most sensitive procedures for detection
of specialized biosynthetic pathways do not exhaust theoretically the
possible existence of differentiated states. It has long been assumed that
there are covert states of differentiation which, by current methods, are
undetectable at the time of analysis but which are revealed in the
subsequent behavior of the cells. For example, a tissue removed to
culture from an embryo at an earlier time may fail to differentiate
nervous tissue, whereas the same tissue removed at a later time, though
overtly unaltered and showing no signs of differentiation, may produce
abundant nervous tissue in culture. Between the two times a differentiative change, unrecognizable by other methods, is assumed to have
occurred. Such "determination", "stabilization", or latent differentiation could be based, among other possibilities, on some alteration of
intracellular controls antecedent to specific cytoplasmic synthesis, and
hence undetectable without knowledge of the nature of the control. This
hypothesis has been invoked to account for the apparent persistence of
differentiative tendencies in overtly dedifferentiated cultured cells—a
question which will be returned to below. For present purposes it is
sufficient to note that stability of developmental tendency has been used
as a criterion—and presumed to be a particularly sensitive one—of
differentiative states.
I I I .
C U R R E N T P R O B L E M S OF D I F F E R E N T I A T I O N IN
VITRO
Current approaches to differentiation in vitro have two general foci.
The first is the practical matter of maintaining cells in vitro in a differentiated state, so that their specialized metabolism may be used as substrate in studies of other problems. Virus-cell interactions, comparative
nutritional requirements of different tissues and organs, specific syntheses of special interest such as antibody production, the mechanism of
hormonal action at the level of cells and tissues—these are examples of
problems whose solution will be aided as culture technology advances
in the direction of maintained and controlled differentiation in vitro. In
general, methods devised for the rapid proliferation of cells, qua cells,
have proved inappropriate for maintaining or promoting differentiation, while methods devised for the culture of tissues, and of tissue
complexes as organs, have been more successful. The problem for the
467
properties which characterize a given cell type form a closely coupled
complex in vivo, it is not inconceivable that the complex may become
uncoupled in vitro (Ebner, Hageman and Larson, 1961; Hilfer, 1962),
and that certain aspects of differentiation may be persistent whereas
others are lost.
Negative results with even the most sensitive procedures for detection
of specialized biosynthetic pathways do not exhaust theoretically the
possible existence of differentiated states. It has long been assumed that
there are covert states of differentiation which, by current methods, are
undetectable at the time of analysis but which are revealed in the
subsequent behavior of the cells. For example, a tissue removed to
culture from an embryo at an earlier time may fail to differentiate
nervous tissue, whereas the same tissue removed at a later time, though
overtly unaltered and showing no signs of differentiation, may produce
abundant nervous tissue in culture. Between the two times a differentiative change, unrecognizable by other methods, is assumed to have
occurred. Such "determination", "stabilization", or latent differentiation could be based, among other possibilities, on some alteration of
intracellular controls antecedent to specific cytoplasmic synthesis, and
hence undetectable without knowledge of the nature of the control. This
hypothesis has been invoked to account for the apparent persistence of
differentiative tendencies in overtly dedifferentiated cultured cells—a
question which will be returned to below. For present purposes it is
sufficient to note that stability of developmental tendency has been used
as a criterion—and presumed to be a particularly sensitive one—of
differentiative states.
I I I .
C U R R E N T P R O B L E M S OF D I F F E R E N T I A T I O N IN
VITRO
Current approaches to differentiation in vitro have two general foci.
The first is the practical matter of maintaining cells in vitro in a differentiated state, so that their specialized metabolism may be used as substrate in studies of other problems. Virus-cell interactions, comparative
nutritional requirements of different tissues and organs, specific syntheses of special interest such as antibody production, the mechanism of
hormonal action at the level of cells and tissues—these are examples of
problems whose solution will be aided as culture technology advances
in the direction of maintained and controlled differentiation in vitro. In
general, methods devised for the rapid proliferation of cells, qua cells,
have proved inappropriate for maintaining or promoting differentiation, while methods devised for the culture of tissues, and of tissue
complexes as organs, have been more successful. The problem for the