4. C E L L T Y P E S
165
than groups of cells whose general pattern of growth is often diagnostic.
There is also considerable difference of opinion both on the uniqueness
and the importance of the classes and on whether or not it is possible
for a cell of one type to change into another. There is certainly a great
deal of plasticity. For example, Lewis (1923a, b) described all transitions between typical fibroblasts and cells constituting mesothelial and
endothelial membranes. This is an interesting observation in that it
gives some indication as to how blood vessels and coelomic cavities
become lined by secondary epithelia in the body. It is as though the
epithelial cells which originally entered the mesohyl could no longer
return completely to the original type of epithelial structure but adopted the endothelial form as the next best thing. In vitro the occurrence
of such endothelial or mesothelial sheets is by no means uncommon
and the sheets can be distinguished from sheets of true epithelium by
the very large size of the constituent cells and often by the development
of large centrospheres. The fact that the cells of such a membrane in
the body form a boundary between two different media of course
determines a certain polarity within the cells, which may be superimposed upon the original polarity of the migrating cell.
As mentioned in the Introduction, a more or less continuous controversy has raged since about 1920 on whether or not fibroblasts and
macrophages are interconvertible cells, and another on whether
lymphocytes can become monocytes and then macrophages. These
controversies are discussed at length elsewhere (see Vol. 2, Chapters
1 and 2) but there are certain aspects of them which need to be kept
in the fore-front of any investigation of such transformations. First, it is
necessary to be absolutely certain of the nature of the cell, both before
and after the apparent transformation. As we have seen, the static,
morphological appearance of a single cell can be very deceptive. For
example, macrophages are so versatile in form in tissue cultures that at
any one moment they can well simulate the form of a fibroblast. Indeed,
it would be largely guess-work to attach names to most of the cells in
Fig. 1. Only when a single cell can be followed by the cine-camera
throughout the whole transformation does the evidence become at all
convincing and, even then, it must be supported by other evidence of
identification. For example, in the fibroblast-macrophage problem the
ability of the fibroblast to liberate tropocollagen and to produce
collagen fibres, or to liberate hexosamines and uronic acids and produce
extracellular mucopolysaccharides, to effect contact inhibition with
neighbouring cells, to respond favourably to treatment with embryo
juice, and unfavourably to doses of sodium arsenite which leave
macrophages unscathed, should all be established before the identity of
the cell can be assured. Similarly the development of intracellular acid
165
than groups of cells whose general pattern of growth is often diagnostic.
There is also considerable difference of opinion both on the uniqueness
and the importance of the classes and on whether or not it is possible
for a cell of one type to change into another. There is certainly a great
deal of plasticity. For example, Lewis (1923a, b) described all transitions between typical fibroblasts and cells constituting mesothelial and
endothelial membranes. This is an interesting observation in that it
gives some indication as to how blood vessels and coelomic cavities
become lined by secondary epithelia in the body. It is as though the
epithelial cells which originally entered the mesohyl could no longer
return completely to the original type of epithelial structure but adopted the endothelial form as the next best thing. In vitro the occurrence
of such endothelial or mesothelial sheets is by no means uncommon
and the sheets can be distinguished from sheets of true epithelium by
the very large size of the constituent cells and often by the development
of large centrospheres. The fact that the cells of such a membrane in
the body form a boundary between two different media of course
determines a certain polarity within the cells, which may be superimposed upon the original polarity of the migrating cell.
As mentioned in the Introduction, a more or less continuous controversy has raged since about 1920 on whether or not fibroblasts and
macrophages are interconvertible cells, and another on whether
lymphocytes can become monocytes and then macrophages. These
controversies are discussed at length elsewhere (see Vol. 2, Chapters
1 and 2) but there are certain aspects of them which need to be kept
in the fore-front of any investigation of such transformations. First, it is
necessary to be absolutely certain of the nature of the cell, both before
and after the apparent transformation. As we have seen, the static,
morphological appearance of a single cell can be very deceptive. For
example, macrophages are so versatile in form in tissue cultures that at
any one moment they can well simulate the form of a fibroblast. Indeed,
it would be largely guess-work to attach names to most of the cells in
Fig. 1. Only when a single cell can be followed by the cine-camera
throughout the whole transformation does the evidence become at all
convincing and, even then, it must be supported by other evidence of
identification. For example, in the fibroblast-macrophage problem the
ability of the fibroblast to liberate tropocollagen and to produce
collagen fibres, or to liberate hexosamines and uronic acids and produce
extracellular mucopolysaccharides, to effect contact inhibition with
neighbouring cells, to respond favourably to treatment with embryo
juice, and unfavourably to doses of sodium arsenite which leave
macrophages unscathed, should all be established before the identity of
the cell can be assured. Similarly the development of intracellular acid