66
A . M O S C O N A , O . A . T R O W E L L A N D E . N . W l L L M E R
T o establish a clone of cells of a particular type, although apparently
simple and highly desirable for many reasons, is a problem which is
loaded with difficulties, and in the apparent solution of which there has
been some rather loose thinking. At first sight, it would appear to be a
simple matter to isolate a single cell and establish it as a clone, and for
some cells this may be so. It has, however, already been stressed almost
ad nauseam that certain types of cell at least, and the normal fibroblast
is included among them, only tolerate complete isolation with difficulty and usually die at once. If such a cell adapts itself to this isolation
and survives under somewhat adverse conditions, it may be a rather
abnormal event, and the quesdon is raised as to whether the conditions
which the cell meets are just within the limits of its normal tolerance or
whether the cell itself is abnormal. If it is abnormal, was it always
so, or has it become adapted? That cells do become adapted in vitro,
or at least change their character, is shown by the observation that
two daughter cells derived from a cell division in a successful clone of
mouse L-strain cells each established a clone of its own, but the two
clones had very different properties, among which the development of
malignancy by one of them far more extensively than by the other
is perhaps the most conspicuous (Sanford, Likely and Earle, 1954;
Earle, 1957) (Fig. 7). If the cell which survives in isolation does not
become abnormal, to enable it to do so, but is different or "abnormal"
originally, is this the result of genetic mutation or phenotypic variation?
Puck, Cieciura and Robinson (1958) have succeeded in establishing
clones of apparently normal euploid human fibroblasts, but this of
course does not really answer these questions, especially as the plating
efficiency is generally rather low.
This raises the whole problem of the identification of cells in vitro
which will be discussed more fully in Chapter 4, but very few tissues
are composed of only one type of cell, and when the shapes of these
cells are modified, as they inevitably are by the process of isolation,
it is extremely difficult to know what type of cell is being selected for
cloning. Moreover, it is possible that the treatment which must be
given to the cell in order to isolate it may damage it or, at least, modify
it in such a way as to allow it to survive. In observation chambers,
isolated cells may sometimes be roughly classified by their type of
movement, e.g. in thoracic-duct lymph the monocytes can be clearly
distinguished from the more numerous lymphocytes by their characteristic movements, or again in a preparation of cells from chick-heart the
muscle cells can at first be distinguished from all the others by their
periodic contractions, but most other methods for identification of
cells, as by histochemical, cytochemical or cytological techniques, kill
the bird before it can lay the golden eggs. Thus the exact identification
A . M O S C O N A , O . A . T R O W E L L A N D E . N . W l L L M E R
T o establish a clone of cells of a particular type, although apparently
simple and highly desirable for many reasons, is a problem which is
loaded with difficulties, and in the apparent solution of which there has
been some rather loose thinking. At first sight, it would appear to be a
simple matter to isolate a single cell and establish it as a clone, and for
some cells this may be so. It has, however, already been stressed almost
ad nauseam that certain types of cell at least, and the normal fibroblast
is included among them, only tolerate complete isolation with difficulty and usually die at once. If such a cell adapts itself to this isolation
and survives under somewhat adverse conditions, it may be a rather
abnormal event, and the quesdon is raised as to whether the conditions
which the cell meets are just within the limits of its normal tolerance or
whether the cell itself is abnormal. If it is abnormal, was it always
so, or has it become adapted? That cells do become adapted in vitro,
or at least change their character, is shown by the observation that
two daughter cells derived from a cell division in a successful clone of
mouse L-strain cells each established a clone of its own, but the two
clones had very different properties, among which the development of
malignancy by one of them far more extensively than by the other
is perhaps the most conspicuous (Sanford, Likely and Earle, 1954;
Earle, 1957) (Fig. 7). If the cell which survives in isolation does not
become abnormal, to enable it to do so, but is different or "abnormal"
originally, is this the result of genetic mutation or phenotypic variation?
Puck, Cieciura and Robinson (1958) have succeeded in establishing
clones of apparently normal euploid human fibroblasts, but this of
course does not really answer these questions, especially as the plating
efficiency is generally rather low.
This raises the whole problem of the identification of cells in vitro
which will be discussed more fully in Chapter 4, but very few tissues
are composed of only one type of cell, and when the shapes of these
cells are modified, as they inevitably are by the process of isolation,
it is extremely difficult to know what type of cell is being selected for
cloning. Moreover, it is possible that the treatment which must be
given to the cell in order to isolate it may damage it or, at least, modify
it in such a way as to allow it to survive. In observation chambers,
isolated cells may sometimes be roughly classified by their type of
movement, e.g. in thoracic-duct lymph the monocytes can be clearly
distinguished from the more numerous lymphocytes by their characteristic movements, or again in a preparation of cells from chick-heart the
muscle cells can at first be distinguished from all the others by their
periodic contractions, but most other methods for identification of
cells, as by histochemical, cytochemical or cytological techniques, kill
the bird before it can lay the golden eggs. Thus the exact identification