4. CELL T Y P E S
157
B. I N T R I N S I C F A C T O R S
It has already been noted that when a hanging-drop culture of chick
or human skin is set up in a plasma medium, epithelial cells, fibroblasts
(mechanocytes) and macrophages (amoebocytes) can all wander out
in recognizable form in one and the same medium, namely plasma.
Moreover, each of these classes of cells may be grown for a time in pure
culture, and two of them even as clones (Puck, Cieciura and Fisher,
1957), and the cells still retain something of their own methods of
growth, movement and behaviour; and, what is more interesting in
relation to the identity of the cells, optimum growth of the various types
demands different conditions of culture. For example, chick fibroblasts
do not normally continue to multiply in plasma alone, but require
supplementation of their medium with embryo extract or some similar
activating agent. Chick macrophages, on the other hand, prosper in
plasma or serum without any further additions (Jacoby, 1938). Epitheliocytes really only prosper when growing on a surface and they are
relatively indifferent to embryo juice.
Within the group of mechanocytes, there are several different races
which require different concentrations of embryo juice to bring about
their growth (Parker, 1933). For example there are some (e.g. periosteal fibroblasts) which are activated by low concentrations of embryo
juice (e.g. 5°/o) and respond more and more vigorously in concentrations up to about 40°/o, while others (cardiac mechanocytes) are hardly
activated any more by 4 0 % juice than they are by 15% juice (Jacoby
Trowell and Willmer, 1937).
C. STAGES IN T H E A L T E R A T I O N OF CELLS
We are thus faced with the situation that the cells which emerge from
the various tissues in fresh cultures become altered as compared with
their counterparts in the original tissue. The alterations, however,
appear to occur in steps. They are generally in the direction of a loss
of special characteristics and towards the assumption of a more generalized form. Epithelial cells of various kinds at first become "simpler"
epitheliocytes in culture, but they remain epitheliocytes and are distinguishable, at least for a time, from mechanocytes. Thus, in cultures
of the skin showing unorganized growth, it is not yet possible to pick
out among the out-growing epithelial cells those which were connected
with hair follicles, sweat glands or nail beds; they are all just epithelial
cells like those from the general keratinizing or cornifying surface.
Similarly, mechanocytes in cultures from bone, cartilage, connective
tissues and from some forms of muscle, though showing minor differences amongst themselves, are much more like each other than they are
157
B. I N T R I N S I C F A C T O R S
It has already been noted that when a hanging-drop culture of chick
or human skin is set up in a plasma medium, epithelial cells, fibroblasts
(mechanocytes) and macrophages (amoebocytes) can all wander out
in recognizable form in one and the same medium, namely plasma.
Moreover, each of these classes of cells may be grown for a time in pure
culture, and two of them even as clones (Puck, Cieciura and Fisher,
1957), and the cells still retain something of their own methods of
growth, movement and behaviour; and, what is more interesting in
relation to the identity of the cells, optimum growth of the various types
demands different conditions of culture. For example, chick fibroblasts
do not normally continue to multiply in plasma alone, but require
supplementation of their medium with embryo extract or some similar
activating agent. Chick macrophages, on the other hand, prosper in
plasma or serum without any further additions (Jacoby, 1938). Epitheliocytes really only prosper when growing on a surface and they are
relatively indifferent to embryo juice.
Within the group of mechanocytes, there are several different races
which require different concentrations of embryo juice to bring about
their growth (Parker, 1933). For example there are some (e.g. periosteal fibroblasts) which are activated by low concentrations of embryo
juice (e.g. 5°/o) and respond more and more vigorously in concentrations up to about 40°/o, while others (cardiac mechanocytes) are hardly
activated any more by 4 0 % juice than they are by 15% juice (Jacoby
Trowell and Willmer, 1937).
C. STAGES IN T H E A L T E R A T I O N OF CELLS
We are thus faced with the situation that the cells which emerge from
the various tissues in fresh cultures become altered as compared with
their counterparts in the original tissue. The alterations, however,
appear to occur in steps. They are generally in the direction of a loss
of special characteristics and towards the assumption of a more generalized form. Epithelial cells of various kinds at first become "simpler"
epitheliocytes in culture, but they remain epitheliocytes and are distinguishable, at least for a time, from mechanocytes. Thus, in cultures
of the skin showing unorganized growth, it is not yet possible to pick
out among the out-growing epithelial cells those which were connected
with hair follicles, sweat glands or nail beds; they are all just epithelial
cells like those from the general keratinizing or cornifying surface.
Similarly, mechanocytes in cultures from bone, cartilage, connective
tissues and from some forms of muscle, though showing minor differences amongst themselves, are much more like each other than they are