4. CELL T Y P E S
173
of neural tissues under somewhat similar conditions (Costero and Pomerat, 1951) allowed nerve cells and their supporting neuroglia, astrocytes and oligodendroglia, to differentiate in such a way that all these
cell types were clearly recognizable and functional. There was also
considerable differentiation among the nerve cells.
A striking feature of cells in the body is that their nuclei differ
rather conspicuously, at least when they are fixed and stained. In fact,
cells can often be identified by their nuclei alone. However, when
cells are being grown in vitro, in an unorganized manner, the nuclear
pattern becomes much more simple and, on the whole, much more
uniform, so that this criterion for the identification of cells is virtually
eliminated. Since the nuclear pattern as seen in the tissues in situ
mostly depends on the distribution of eu- and hetero-chromatin the
disappearance of the heterochromatin from the nuclei of cells in active
growth in culture can perhaps be regarded as an indication that the
appearance of heterochromatin is related to the process of differentiation. The idea that heterochromatin indicates those parts of the
chromosomes which are temporarily inactive is put forward elsewhere
(see p. 441) and this antithesis between the simple interkinetic nucleus of
the growing tissue culture and the patterned nuclei of the cells of the
original tissue is certainly suggestive. If this idea is correct, it would
indicate that cells, after being grown in culture, may be more totipotent than they were in situ in the body. Comparatively little attention
has, however, been given to the changes in interkinetic nuclear patterns
among cells growing under different conditions in culture, or to the
manner in which nuclear patterns change as cells emerge from the
parent tissues. Changes in the chromosome patterns in cultured cells
are, of course, well known.
Much has recently been done in studying the manner in which the
cells of a cell suspension can re-aggregate and then redifferentiate
(Chapter 14). Such systems are very useful in the study of cellular interrelationships and of the manner in which cells of different types recognize each other and behave accordingly. They can be equally useful
in examining the changes which cells undergo as they differentiate and
the study of nuclear differentiation could be well tackled by this method.
These methods of re-aggregation clearly show that the surface properties of cells of different types are quite different. For example, when
liver epithelial cells and skin epithelial cells are mixed together in
suspension and encouraged to re-aggregate, they all clump together
and stick to each other, as all good epithelial cells should; but after
a time the skin cells are found to be in one place and the liver cells in
another in the aggregates (Weiss, 1958). Furthermore, in many examples of aggregates made from suspensions containing more than one type
173
of neural tissues under somewhat similar conditions (Costero and Pomerat, 1951) allowed nerve cells and their supporting neuroglia, astrocytes and oligodendroglia, to differentiate in such a way that all these
cell types were clearly recognizable and functional. There was also
considerable differentiation among the nerve cells.
A striking feature of cells in the body is that their nuclei differ
rather conspicuously, at least when they are fixed and stained. In fact,
cells can often be identified by their nuclei alone. However, when
cells are being grown in vitro, in an unorganized manner, the nuclear
pattern becomes much more simple and, on the whole, much more
uniform, so that this criterion for the identification of cells is virtually
eliminated. Since the nuclear pattern as seen in the tissues in situ
mostly depends on the distribution of eu- and hetero-chromatin the
disappearance of the heterochromatin from the nuclei of cells in active
growth in culture can perhaps be regarded as an indication that the
appearance of heterochromatin is related to the process of differentiation. The idea that heterochromatin indicates those parts of the
chromosomes which are temporarily inactive is put forward elsewhere
(see p. 441) and this antithesis between the simple interkinetic nucleus of
the growing tissue culture and the patterned nuclei of the cells of the
original tissue is certainly suggestive. If this idea is correct, it would
indicate that cells, after being grown in culture, may be more totipotent than they were in situ in the body. Comparatively little attention
has, however, been given to the changes in interkinetic nuclear patterns
among cells growing under different conditions in culture, or to the
manner in which nuclear patterns change as cells emerge from the
parent tissues. Changes in the chromosome patterns in cultured cells
are, of course, well known.
Much has recently been done in studying the manner in which the
cells of a cell suspension can re-aggregate and then redifferentiate
(Chapter 14). Such systems are very useful in the study of cellular interrelationships and of the manner in which cells of different types recognize each other and behave accordingly. They can be equally useful
in examining the changes which cells undergo as they differentiate and
the study of nuclear differentiation could be well tackled by this method.
These methods of re-aggregation clearly show that the surface properties of cells of different types are quite different. For example, when
liver epithelial cells and skin epithelial cells are mixed together in
suspension and encouraged to re-aggregate, they all clump together
and stick to each other, as all good epithelial cells should; but after
a time the skin cells are found to be in one place and the liver cells in
another in the aggregates (Weiss, 1958). Furthermore, in many examples of aggregates made from suspensions containing more than one type