4. C E L L T Y P E S
153
to identification; it is, however, often a factor of the utmost importance
in relation to physiological function, and therefore cannot be neglected
as irrelevant.
As already mentioned, some classes of cells, e.g. mechanocytes,
freshly emerged from connective tissues etc., must be provided with a
surface upon which to extend themselves and to creep before they
display any growth activity and, when they are thus creeping, the rate
of growth (as measured by the mitotic index) often runs closely parallel
with the velocity of cell migration (Jacoby, Trowell and Willmer, 1937).
When suspended in a fluid medium, without contact with surfaces,
such cells become round and inert. This, however, does not apply to
some other classes of cells. For example, lymphosarcoma cells can be
grown in suspension cultures. (Owens, Gey and Gey, 1954). Moreover it
has recently been found possible to maintain other strains of cells, some
of which (e.g. strain-L) were originally derived from mechanocytes
(fibroblasts), in a rapidly growing condition while they are kept in actual
suspension in a chemically defined medium (Waymouth, 1956; Healy,
Fisher and Parker, 1954; Earle, 1962) (seep. 69). Clearly these cells
have changed their character or growth properties in the process of
culture.
So far, the extrinsic factors discussed have been mostly mechanical
or physico-chemical, but recently it has been shown that chemical
factors in the environment can also alter the shape and form of cells
in rather specific ways which are yet consistent with the viability of the
cells. Both cholesterol (3 mg/100 ml of the medium) and linolenic acid
(up to 1 mg/100 ml of the medium) added to a plasma medium allow
fibroblast-like cells from the aorta to flatten readily into broad, sheetlike cells on the surface of the clot or of the glass on which they are
growing. But, in certain mixtures of the two, the cells assume a much
more spindly and fibre-like form (Rutstein, Ingenito, Craig and Martinelli, 1958). This may well be an example of new constituents entering
the cell surface and thus altering its mechanical properties and probably
its permeability (Willmer, 1961). Certainly the presence of cholesterol
alone assists the accumulation of fat in the cytoplasm of the cell and
linolenic acid counteracts this effect (Rutstein et al., 1958) thus perhaps
indicating an alteration of the chemical or structural properties of
the surface. ^-Lipoproteins can take the place of linolenic acid and are
much more potent in their anti-cholesterol action. The suggestion has
been made that the proportions of phospholipins to steroids and other
fats in the surface layers of cells alter the packing of the molecules in the
surface, and hence alter not only the chemical properties but the mechanical properties also (Willmer, 1961).
The form of lymphoblasts (from a lymphosarcoma strain) in cultures
153
to identification; it is, however, often a factor of the utmost importance
in relation to physiological function, and therefore cannot be neglected
as irrelevant.
As already mentioned, some classes of cells, e.g. mechanocytes,
freshly emerged from connective tissues etc., must be provided with a
surface upon which to extend themselves and to creep before they
display any growth activity and, when they are thus creeping, the rate
of growth (as measured by the mitotic index) often runs closely parallel
with the velocity of cell migration (Jacoby, Trowell and Willmer, 1937).
When suspended in a fluid medium, without contact with surfaces,
such cells become round and inert. This, however, does not apply to
some other classes of cells. For example, lymphosarcoma cells can be
grown in suspension cultures. (Owens, Gey and Gey, 1954). Moreover it
has recently been found possible to maintain other strains of cells, some
of which (e.g. strain-L) were originally derived from mechanocytes
(fibroblasts), in a rapidly growing condition while they are kept in actual
suspension in a chemically defined medium (Waymouth, 1956; Healy,
Fisher and Parker, 1954; Earle, 1962) (seep. 69). Clearly these cells
have changed their character or growth properties in the process of
culture.
So far, the extrinsic factors discussed have been mostly mechanical
or physico-chemical, but recently it has been shown that chemical
factors in the environment can also alter the shape and form of cells
in rather specific ways which are yet consistent with the viability of the
cells. Both cholesterol (3 mg/100 ml of the medium) and linolenic acid
(up to 1 mg/100 ml of the medium) added to a plasma medium allow
fibroblast-like cells from the aorta to flatten readily into broad, sheetlike cells on the surface of the clot or of the glass on which they are
growing. But, in certain mixtures of the two, the cells assume a much
more spindly and fibre-like form (Rutstein, Ingenito, Craig and Martinelli, 1958). This may well be an example of new constituents entering
the cell surface and thus altering its mechanical properties and probably
its permeability (Willmer, 1961). Certainly the presence of cholesterol
alone assists the accumulation of fat in the cytoplasm of the cell and
linolenic acid counteracts this effect (Rutstein et al., 1958) thus perhaps
indicating an alteration of the chemical or structural properties of
the surface. ^-Lipoproteins can take the place of linolenic acid and are
much more potent in their anti-cholesterol action. The suggestion has
been made that the proportions of phospholipins to steroids and other
fats in the surface layers of cells alter the packing of the molecules in the
surface, and hence alter not only the chemical properties but the mechanical properties also (Willmer, 1961).
The form of lymphoblasts (from a lymphosarcoma strain) in cultures