152
E . N . W I L L M E R
tissue culture, the cells may creep along them and assume elongate or
cylindrical forms, superficially quite unlike the flattened forms assumed
on glass or agar surfaces. Similarly, if an otherwise smooth glass surface
has grooves of the right size in it, cells meeting those grooves in the
course of their wanderings tend to spread out along them in an elongate
form. The surface of certain fish scales was shown by Weiss (1958) to
be excellent for aligning cells in this way (Fig. 3). Such aligned cells
may move in either direction along the grooves, but find it difficult
to escape from the grooves. In the body there are many examples of
cells, e.g. fibroblasts, being aligned and oriented by tensions and other
mechanical forces in such structures as developing bones, tendons etc.,
but an extreme form of this sort of thigmotactic behaviour is well
exemplified by the Schwann cells. Those in contact with "amyelinate
nerve fibres" have cytoplasm which creeps along and entirely encloses
numerous fibres. Those surrounding "myelinate nerve fibres" display
even stronger thigmotaxis in which the enormously extended lamelliform pseudopodium of each cell embraces a single axon again and again
in a tight spiral of concentric layers, thereby forming the myelin sheath.
Under culture and other conditions in which such mechanical
alignment of cells occurs, it may be extremely difficult to identify
particular cells. An amoebocyte lined up on a fish scale is morphologically very similar to a fibroblast.
If a solid plasma clot is provided in a tissue culture, the cells can
tunnel through it to a limited extent, one cell often following behind
another. Within the substance of the clot, the cells tend to assume a
cylindrical or spindle-shaped form. When they emerge on to the surface,
or reach the bottom of the clot and make contact with the glass, they
once more tend to spread out as flat as possible. As already mentioned,
lymphocytes in a clot move with a rapid squirming motion, as they do
in the tissues of the body; those on a flat surface, like that of agar gel,
show the "hand-mirror" form. The consistency of the clot naturally
makes a great difference to the speed with which cells can progress
through it, and also to the shapes which they assume in doing so. It is
also noticeable that mechanocytes growing in a solid or deep clot tend
to have more granules and droplets in their cytoplasm than do those
in less dense and shallower clots. Moreover, if the clot has been formed
under tension or is in some other way intrinsically orientated, the cells
not only follow the oriented molecules or fibrils (see p. 183) and grow out
preferentially in certain directions following the micro-structure, but
these oriented molecules also alter the shape of the cells. The mechanism
of the transfer of the orientation from the molecules of the substratum
to the direction of motion and the form of the cell poses some interesting
problems. Morphological form alone is therefore but a dangerous guide
E . N . W I L L M E R
tissue culture, the cells may creep along them and assume elongate or
cylindrical forms, superficially quite unlike the flattened forms assumed
on glass or agar surfaces. Similarly, if an otherwise smooth glass surface
has grooves of the right size in it, cells meeting those grooves in the
course of their wanderings tend to spread out along them in an elongate
form. The surface of certain fish scales was shown by Weiss (1958) to
be excellent for aligning cells in this way (Fig. 3). Such aligned cells
may move in either direction along the grooves, but find it difficult
to escape from the grooves. In the body there are many examples of
cells, e.g. fibroblasts, being aligned and oriented by tensions and other
mechanical forces in such structures as developing bones, tendons etc.,
but an extreme form of this sort of thigmotactic behaviour is well
exemplified by the Schwann cells. Those in contact with "amyelinate
nerve fibres" have cytoplasm which creeps along and entirely encloses
numerous fibres. Those surrounding "myelinate nerve fibres" display
even stronger thigmotaxis in which the enormously extended lamelliform pseudopodium of each cell embraces a single axon again and again
in a tight spiral of concentric layers, thereby forming the myelin sheath.
Under culture and other conditions in which such mechanical
alignment of cells occurs, it may be extremely difficult to identify
particular cells. An amoebocyte lined up on a fish scale is morphologically very similar to a fibroblast.
If a solid plasma clot is provided in a tissue culture, the cells can
tunnel through it to a limited extent, one cell often following behind
another. Within the substance of the clot, the cells tend to assume a
cylindrical or spindle-shaped form. When they emerge on to the surface,
or reach the bottom of the clot and make contact with the glass, they
once more tend to spread out as flat as possible. As already mentioned,
lymphocytes in a clot move with a rapid squirming motion, as they do
in the tissues of the body; those on a flat surface, like that of agar gel,
show the "hand-mirror" form. The consistency of the clot naturally
makes a great difference to the speed with which cells can progress
through it, and also to the shapes which they assume in doing so. It is
also noticeable that mechanocytes growing in a solid or deep clot tend
to have more granules and droplets in their cytoplasm than do those
in less dense and shallower clots. Moreover, if the clot has been formed
under tension or is in some other way intrinsically orientated, the cells
not only follow the oriented molecules or fibrils (see p. 183) and grow out
preferentially in certain directions following the micro-structure, but
these oriented molecules also alter the shape of the cells. The mechanism
of the transfer of the orientation from the molecules of the substratum
to the direction of motion and the form of the cell poses some interesting
problems. Morphological form alone is therefore but a dangerous guide