170
E . N . W I L L M E R
have occurred and that the cells in these clones and strains (Fig. 6) have
some very different features from the cells in colonies of true fibroblasts,
freshly emerged from their original tissues. Another stage ofdedifferentiation or of adaptation to culture conditions has evidently occurred. What
is even more significant than this change in the character of fibroblasts
is that many of the clones and pure strains of cells which have been
derived from other sorts of tissue besides fibroblastic tissues (e.g. from
conjunctiva, liver, carcinoma of the cervix, epidermis, kidney etc.) all
have an extraordinary similarity to one another both in appearance
and properties (Fig. 7).
It is true that, in the last few years, some of these similarities have
turned out, under the critical test of recently developed immunity
tests, using the mixed agglutination reaction, (Franks, Gurner, Coombs
and Stevenson, 1962) to be cases of actual identity, presumably caused
by accidental contamination of the strains by cells of more actively
growing or fully adapted tissue-culture cells. Nevertheless, in spite of
this, there is no doubt, from observations on the "genuine" strains,
that when cells are completely isolated from the parent tissues into
these cell suspensions and pure-line cultures in vitro they undergo a
generalization of type which, in fact, is probably a specialization and
adaptation to the new conditions of life in vitro. Some specific properties
undoubtedly remain; for example, a long established strain of liver
cells still shows ability to store glycogen (Peppers, Westfall and Earle,
1959) and still possesses an arginase content characteristic of liver
(Westfall, Peppers, Evans, Sanford, Hawkins, Fioramonti, Kerr, Hobbs
and Earle, 1958), though its identification on morphological grounds
alone would be very ambiguous. Another liver strain showed only one
out of four enzymes characteristic of the normal liver (Perske, Parks
and Walker, 1957).
All these observations then lead to the view that a cell only shows its
full differentiation and function when in situ. Since, in vitro, cells
maintain something of their normal relationships with each other only
when they are explanted as organ cultures, it is only in such cultures
that the full gamut of cell types can even approximately be realized.
In tissue cultures in hanging drops, flasks, roller tubes etc., when outgrowth of the cells is being encouraged, the outgrowing cells have
already undergone some change and generalization. This change may
at first be reversible, if the proper conditions are re-established. If the
cells are further encouraged to grow or are maintained as cell suspenFIG. 7. T o p : A strain of human liver cells after 2 , 5 9 0 days in vitro. Middle: A clone of human
liver cells 1,239 days after it was cloned. (From Evans, Hawkins, Westfall and Earle, 1958.)
Below: A strain of human epidermal cells after 6 8 2 days in vitro. (From Perry, Evans, Earle,
Hyatt and Bedell, 1956.)
E . N . W I L L M E R
have occurred and that the cells in these clones and strains (Fig. 6) have
some very different features from the cells in colonies of true fibroblasts,
freshly emerged from their original tissues. Another stage ofdedifferentiation or of adaptation to culture conditions has evidently occurred. What
is even more significant than this change in the character of fibroblasts
is that many of the clones and pure strains of cells which have been
derived from other sorts of tissue besides fibroblastic tissues (e.g. from
conjunctiva, liver, carcinoma of the cervix, epidermis, kidney etc.) all
have an extraordinary similarity to one another both in appearance
and properties (Fig. 7).
It is true that, in the last few years, some of these similarities have
turned out, under the critical test of recently developed immunity
tests, using the mixed agglutination reaction, (Franks, Gurner, Coombs
and Stevenson, 1962) to be cases of actual identity, presumably caused
by accidental contamination of the strains by cells of more actively
growing or fully adapted tissue-culture cells. Nevertheless, in spite of
this, there is no doubt, from observations on the "genuine" strains,
that when cells are completely isolated from the parent tissues into
these cell suspensions and pure-line cultures in vitro they undergo a
generalization of type which, in fact, is probably a specialization and
adaptation to the new conditions of life in vitro. Some specific properties
undoubtedly remain; for example, a long established strain of liver
cells still shows ability to store glycogen (Peppers, Westfall and Earle,
1959) and still possesses an arginase content characteristic of liver
(Westfall, Peppers, Evans, Sanford, Hawkins, Fioramonti, Kerr, Hobbs
and Earle, 1958), though its identification on morphological grounds
alone would be very ambiguous. Another liver strain showed only one
out of four enzymes characteristic of the normal liver (Perske, Parks
and Walker, 1957).
All these observations then lead to the view that a cell only shows its
full differentiation and function when in situ. Since, in vitro, cells
maintain something of their normal relationships with each other only
when they are explanted as organ cultures, it is only in such cultures
that the full gamut of cell types can even approximately be realized.
In tissue cultures in hanging drops, flasks, roller tubes etc., when outgrowth of the cells is being encouraged, the outgrowing cells have
already undergone some change and generalization. This change may
at first be reversible, if the proper conditions are re-established. If the
cells are further encouraged to grow or are maintained as cell suspenFIG. 7. T o p : A strain of human liver cells after 2 , 5 9 0 days in vitro. Middle: A clone of human
liver cells 1,239 days after it was cloned. (From Evans, Hawkins, Westfall and Earle, 1958.)
Below: A strain of human epidermal cells after 6 8 2 days in vitro. (From Perry, Evans, Earle,
Hyatt and Bedell, 1956.)