174
E . N . W I L L M E R
of cell, it has been shown that organ specificity is stronger than species
specificity, since aggregation produces tissues composed of a mixture
of cells from the different species, e.g. nodules of cartilage composed of
chick and mouse cells (see pp. 511-12).
One final point which may perhaps be emphasized in closing this
discussion on cell types is that differentiation is essentially a process
which depends to some extent on its own complexity. A single cell is
at the mercy of its environment on all sides. When cells cluster, local
conditions for each cell and even for parts of each cell are established;
the cells then begin to be much more dependent upon each other and
less and less upon the general environment. In other words, clusters
of cells tend to be self-differentiating while isolated cells are inevitably
dependent on their surroundings. It is therefore largely the size and
quality of the unit which determines whether or not it is self-differentiating.
By Tissue-Culture methods it is now becoming possible to vary the
micro-environments of cells in a more and more controlled manner,
and thus it seems probable that many of the unsolved problems of
differentiation will eventually be solved by such methods.
R E F E R E N C E S
Abercrombie, M . and Ambrose, E . J . (1962). T h e surface properties o f cancer cells:
A review. Cancer Res. 22, 525.
Abercrombie, M . and Johnson, M . L. (1942). T h e outwandering o f cells in tissue
cultures o f nerves undergoing Wallerian degeneration. J. exp. Biol. 19, 266.
Chevremont, M . and Chevremont-Comhaire, S. (1945). Recherches sur le d£terminisme de la transformation histiocytaire. Acta anat. 1, 95.
Costero, I. and Pomerat, C. M . (1951). Cultivation o f neurones from adult human
cerebral and cerebellar cortex. Amer. J. Anat. 89, 405.
Curtis, A . S. G. (1960). Cortical grafting in Xenopus laevis. J. Embryol. exp. Morph. 8,
163.
Dornfeld, E. J. and Owczarzak, A . (1957). Surface responses in cultured fibroblasts
elicited b y ethylenediaminetetracetic acid. J. biophys. biochem. Cytol. 4, 243.
Earle, W . R . (1957). Discussion on tissue culture in the study o f animal and human
tumours. J. nat. Cancer Inst. 19, 781.
Earle, W . R . (1958). Long-term cultivation o f animal tissue cells in large cultures.
Fed. Proc. 17, 967.
Earle, W . R . (1962). Some morphologic variations o f certain cells under controlled
experimental conditions. Nat. Cancer Inst. Monograph 7, Symposium. Analytic Cell
Culture.
Evans, V . J., Hawkins, N . M . , Westfall, B. B. and Earle, W . R . (1958). Studies on
culture lines derived from mouse liver parenchymatous cells grown in long-term
tissue culture. Cancer Res. 18, 261.
Franks, D . , Gurner, B. S., Coombs, R . R . A . and Stevenson, R . (1962). Results o f
tests for the species o f origin o f cell lines by means o f the mixed agglutination
reaction. Exp. Cell Res. 28, 608.
E . N . W I L L M E R
of cell, it has been shown that organ specificity is stronger than species
specificity, since aggregation produces tissues composed of a mixture
of cells from the different species, e.g. nodules of cartilage composed of
chick and mouse cells (see pp. 511-12).
One final point which may perhaps be emphasized in closing this
discussion on cell types is that differentiation is essentially a process
which depends to some extent on its own complexity. A single cell is
at the mercy of its environment on all sides. When cells cluster, local
conditions for each cell and even for parts of each cell are established;
the cells then begin to be much more dependent upon each other and
less and less upon the general environment. In other words, clusters
of cells tend to be self-differentiating while isolated cells are inevitably
dependent on their surroundings. It is therefore largely the size and
quality of the unit which determines whether or not it is self-differentiating.
By Tissue-Culture methods it is now becoming possible to vary the
micro-environments of cells in a more and more controlled manner,
and thus it seems probable that many of the unsolved problems of
differentiation will eventually be solved by such methods.
R E F E R E N C E S
Abercrombie, M . and Ambrose, E . J . (1962). T h e surface properties o f cancer cells:
A review. Cancer Res. 22, 525.
Abercrombie, M . and Johnson, M . L. (1942). T h e outwandering o f cells in tissue
cultures o f nerves undergoing Wallerian degeneration. J. exp. Biol. 19, 266.
Chevremont, M . and Chevremont-Comhaire, S. (1945). Recherches sur le d£terminisme de la transformation histiocytaire. Acta anat. 1, 95.
Costero, I. and Pomerat, C. M . (1951). Cultivation o f neurones from adult human
cerebral and cerebellar cortex. Amer. J. Anat. 89, 405.
Curtis, A . S. G. (1960). Cortical grafting in Xenopus laevis. J. Embryol. exp. Morph. 8,
163.
Dornfeld, E. J. and Owczarzak, A . (1957). Surface responses in cultured fibroblasts
elicited b y ethylenediaminetetracetic acid. J. biophys. biochem. Cytol. 4, 243.
Earle, W . R . (1957). Discussion on tissue culture in the study o f animal and human
tumours. J. nat. Cancer Inst. 19, 781.
Earle, W . R . (1958). Long-term cultivation o f animal tissue cells in large cultures.
Fed. Proc. 17, 967.
Earle, W . R . (1962). Some morphologic variations o f certain cells under controlled
experimental conditions. Nat. Cancer Inst. Monograph 7, Symposium. Analytic Cell
Culture.
Evans, V . J., Hawkins, N . M . , Westfall, B. B. and Earle, W . R . (1958). Studies on
culture lines derived from mouse liver parenchymatous cells grown in long-term
tissue culture. Cancer Res. 18, 261.
Franks, D . , Gurner, B. S., Coombs, R . R . A . and Stevenson, R . (1962). Results o f
tests for the species o f origin o f cell lines by means o f the mixed agglutination
reaction. Exp. Cell Res. 28, 608.