60
A . M O S C O N A , O. A . T R O W E L L A N D E . N . W I L L M E R
rapidly degraded by crystalline DNA-ase (Moscona, 1962). This
indicates that, in addition to other constituents, the viscous product
contains materials susceptible to DNA-ase. Such material might readily
come from destroyed cells, but actual evidence of its origin is lacking,
and other possibilities for the appearance of extra-cellular D N A have
been previously suggested (Medawar, 1957). Convincing evidence has
been obtained that the material susceptible to DNA-ase is not essential
for the orderly aggregation of dissociated embryonic cells (see Chapter
14) since its degradation with DNA-ase does not noticeably affect the
ability of cells to aggregate (Moscona, 1962); furthermore, recently
isolated cell products with aggregation-promoting activity do not
contain appreciable amounts of DNA, nor is their activity destroyed by
DNA-ase (Moscona, 1963b).
Rinaldini (1958) found birefringent fibres in the mucoid of trypsindissociated cells and suggested that they might be of collagenous nature.
Further support for this suggestion is not available. However it should
be recalled that, according to electron-microscopic evidence, formation
of fibres takes place at or near the cell surface (Fitton Jackson and Smith,
1957; Porter and Pappas, 1959) and that according to Merchant and
Kahn (1958) and Pumper (1958) there is a rapid accumulation of
fibrous proteins in the liquid medium of cell cultures.
Finally, it should be stressed that, owing to the selective effects of the
dispersive treatments or of subsequent cultivation, a cell suspension
may not represent accurately the original cell population of the tissue
from which it was prepared: it may lack cell types, or contain the
original range of types but in altered proportions. Such selective
changes can have profound effects on subsequent performance, since
specific cell functions usually depend on coexistence of different cell
types in certain quantitative relationships (Grobstein and Zwilling,
1953; Moscona, 1956; Zwilling, 1960).
C. CELL C U L T U R E S
When cells are prepared in suspension, either mechanically or by
enzymic disintegration of tissues, they tend to round up and to lose
many of their morphological characteristics. But, if such cells, suspended in a nutrient medium, are allowed to remain in stationary
flasks, they settle down to the bottom, spread out, and begin to assume
more characteristic forms. The may then grow actively as monolayers
and quickly form sheets of tissue which may become more than one
cell deep. The suspensions can be prepared to any required dilution
and, thus, inoculations of known numbers of cells can be made. After
the cells have settled on the glass and resumed their active growth, they
A . M O S C O N A , O. A . T R O W E L L A N D E . N . W I L L M E R
rapidly degraded by crystalline DNA-ase (Moscona, 1962). This
indicates that, in addition to other constituents, the viscous product
contains materials susceptible to DNA-ase. Such material might readily
come from destroyed cells, but actual evidence of its origin is lacking,
and other possibilities for the appearance of extra-cellular D N A have
been previously suggested (Medawar, 1957). Convincing evidence has
been obtained that the material susceptible to DNA-ase is not essential
for the orderly aggregation of dissociated embryonic cells (see Chapter
14) since its degradation with DNA-ase does not noticeably affect the
ability of cells to aggregate (Moscona, 1962); furthermore, recently
isolated cell products with aggregation-promoting activity do not
contain appreciable amounts of DNA, nor is their activity destroyed by
DNA-ase (Moscona, 1963b).
Rinaldini (1958) found birefringent fibres in the mucoid of trypsindissociated cells and suggested that they might be of collagenous nature.
Further support for this suggestion is not available. However it should
be recalled that, according to electron-microscopic evidence, formation
of fibres takes place at or near the cell surface (Fitton Jackson and Smith,
1957; Porter and Pappas, 1959) and that according to Merchant and
Kahn (1958) and Pumper (1958) there is a rapid accumulation of
fibrous proteins in the liquid medium of cell cultures.
Finally, it should be stressed that, owing to the selective effects of the
dispersive treatments or of subsequent cultivation, a cell suspension
may not represent accurately the original cell population of the tissue
from which it was prepared: it may lack cell types, or contain the
original range of types but in altered proportions. Such selective
changes can have profound effects on subsequent performance, since
specific cell functions usually depend on coexistence of different cell
types in certain quantitative relationships (Grobstein and Zwilling,
1953; Moscona, 1956; Zwilling, 1960).
C. CELL C U L T U R E S
When cells are prepared in suspension, either mechanically or by
enzymic disintegration of tissues, they tend to round up and to lose
many of their morphological characteristics. But, if such cells, suspended in a nutrient medium, are allowed to remain in stationary
flasks, they settle down to the bottom, spread out, and begin to assume
more characteristic forms. The may then grow actively as monolayers
and quickly form sheets of tissue which may become more than one
cell deep. The suspensions can be prepared to any required dilution
and, thus, inoculations of known numbers of cells can be made. After
the cells have settled on the glass and resumed their active growth, they