4. CELL T Y P E S
145
merit in which the cell is finally located. Furthermore, the whole organization of the body is arranged to maintain these local micro-environments as constant as possible. As soon as an organ like the thymus is
removed from the body, this delicate dynamic equilibrium is inevitably
disturbed, and even more so when the organ is cut up into small pieces for
planting in hanging-drop cultures. As soon as the normal equilibrium
is upset, the cells no longer maintain their original relationships
either with each other or with the surroundings in which they have
been living. They, therefore, immediately begin to lose their detailed
specific qualities, since many of these qualities were dependent on the
immediate surroundings; the cells begin to adapt themselves to their
new surroundings, and in so doing, appear to behave as more
c
'generalized" cells since the surroundings in a culture tend to be more uniform
than those in the body.
This process of the generalization of cells in cultures showing "unorganized" growth may be a continuous process, but, as will be shown
later, it seems to involve several definable stages. However, before it is
possible to establish these stages and evaluate their significance, it is
necessary to examine some of the criteria which can be used for distinguishing between cells of various sorts as they occur in tissue or cell
cultures. In organ cultures, the same problem does not arise to anything like the same extent since the essence of a successful organ culture
is that the structure, and as far as possible the functions, of the organ
are maintained as nearly as possible as they were in the body, and there
is the minimum interference with the micro-environments of the
individual cells.
I I . T H E I M P O R T A N C E OF C E L L M O V E M E N T
In most of the organized tissues (as opposed to the fluids) in the adult
body, the constituent cells are relatively static; though a limited amount
of shuffling and migration of cells probably occurs. For example,
macrophages and leucocytes move about fairly extensively, but these
are normally "birds of passage" and not relevant to the present
discussion. Most tissues are habitually penetrated by wandering cells
of one sort or another. In the embryo, of course, the movement of the
actual tissue cells may be very extensive and active, but the massive
organ-forming movements of the early stages decrease as development
proceeds and may eventually stop. In some tissues, like the epithelium
of the villi of the intestine, cell movement may remain active and continuous throughout life, but, in general, cell movements in the adult
are on a very modest scale. In tissue cultures from any tissue, on the
145
merit in which the cell is finally located. Furthermore, the whole organization of the body is arranged to maintain these local micro-environments as constant as possible. As soon as an organ like the thymus is
removed from the body, this delicate dynamic equilibrium is inevitably
disturbed, and even more so when the organ is cut up into small pieces for
planting in hanging-drop cultures. As soon as the normal equilibrium
is upset, the cells no longer maintain their original relationships
either with each other or with the surroundings in which they have
been living. They, therefore, immediately begin to lose their detailed
specific qualities, since many of these qualities were dependent on the
immediate surroundings; the cells begin to adapt themselves to their
new surroundings, and in so doing, appear to behave as more
c
'generalized" cells since the surroundings in a culture tend to be more uniform
than those in the body.
This process of the generalization of cells in cultures showing "unorganized" growth may be a continuous process, but, as will be shown
later, it seems to involve several definable stages. However, before it is
possible to establish these stages and evaluate their significance, it is
necessary to examine some of the criteria which can be used for distinguishing between cells of various sorts as they occur in tissue or cell
cultures. In organ cultures, the same problem does not arise to anything like the same extent since the essence of a successful organ culture
is that the structure, and as far as possible the functions, of the organ
are maintained as nearly as possible as they were in the body, and there
is the minimum interference with the micro-environments of the
individual cells.
I I . T H E I M P O R T A N C E OF C E L L M O V E M E N T
In most of the organized tissues (as opposed to the fluids) in the adult
body, the constituent cells are relatively static; though a limited amount
of shuffling and migration of cells probably occurs. For example,
macrophages and leucocytes move about fairly extensively, but these
are normally "birds of passage" and not relevant to the present
discussion. Most tissues are habitually penetrated by wandering cells
of one sort or another. In the embryo, of course, the movement of the
actual tissue cells may be very extensive and active, but the massive
organ-forming movements of the early stages decrease as development
proceeds and may eventually stop. In some tissues, like the epithelium
of the villi of the intestine, cell movement may remain active and continuous throughout life, but, in general, cell movements in the adult
are on a very modest scale. In tissue cultures from any tissue, on the