14. C E L L A G G R E G A T E S
515
that similar lentoids tend to arise also in the small aggregates by rotation
of long term monolayer-cultured retina-cells (Moscona, 1960).
There is considerable evidence that various metabolic features of
cells become modified by growth in monolayers. Most of the detailed
information on this refers primarily to cells of adult origin and
represents usually a decline in enzyme activity or in the synthesis of
characteristic products. With regards to embryonic cells Peck (1960)
reported that chick neural-retina cells, dissociated with elastase and
grown dispersed in a plasma clot, tended to form pigment. There is a
decline in the synthesis of keratin by skin cells grown in monolayer.
Cell cultures prepared from skin (epidermis and dermis) of 15-day
mouse embryos showed, after 2 weeks, no identifiable keratin;
harvested and aggregated by rotation such cells produced only minute
clusters which failed to form recognizable epidermal differentiations
(Moscona, 1960). The question might be asked whether this loss of
histoformative traits reflected changes in the epithelial or the dermal
cells, or both. From other evidence, mentioned later, it might be concluded that the epidermal cells are not irreversibly altered; since it
appears that connective tissue cells are required for epidermal development (McLoughlin, 1961; Wessels, 1962), the possibility exists that in
monolayers these cells become modified first and are then unable to
support epidermal differentiation.
Changes in biosynthetic activities of cells grown in monolayer were
convincingly demonstrated in studies on cultures of chondrocytes.
Dissociated vertebral cartilage cells from 10-day chick embryos grown
in monolayer rapidly lose their ability to synthesize chondroitin sulphate
and after about 9 days show no evidence of its formation; even when
spun into a pellet and grown in organ culture, such lumps of modified
cells form no cartilage (Holtzer et al., 1960). The same is true for 13-day
long-bone cartilage cells in monolayer cultures (Kuroda, 1963), in
which the capacity to synthesize a cartilaginous matrix in aggregates
formed by rotation is lost in about 9 days, concurrently with a decline
in their ability to form such aggregates (see p. 504). Whether such
"dedifferentiation" (Holtzer et al., 1960) in skin or in cartilage-cell
cultures affects all cells in the population simultaneously or shows a
progressive selection, is not yet clear; the situation in this respect may
not be identical for all cellular systems or experimental conditions.
Neither is it clear whether the monolayer-modified cells regress to a
"unspecialized" state or rather undergo a new type of specialization, a
metaplasia, in response to life in culture. The precise causal or modifying
factors, extrinsic and intrinsic, involved in these cellular changes and
their correlation with mitotic activity and cell growth have yet to be
defined. It remains to be seen whether they reflect "an irreversible loss
515
that similar lentoids tend to arise also in the small aggregates by rotation
of long term monolayer-cultured retina-cells (Moscona, 1960).
There is considerable evidence that various metabolic features of
cells become modified by growth in monolayers. Most of the detailed
information on this refers primarily to cells of adult origin and
represents usually a decline in enzyme activity or in the synthesis of
characteristic products. With regards to embryonic cells Peck (1960)
reported that chick neural-retina cells, dissociated with elastase and
grown dispersed in a plasma clot, tended to form pigment. There is a
decline in the synthesis of keratin by skin cells grown in monolayer.
Cell cultures prepared from skin (epidermis and dermis) of 15-day
mouse embryos showed, after 2 weeks, no identifiable keratin;
harvested and aggregated by rotation such cells produced only minute
clusters which failed to form recognizable epidermal differentiations
(Moscona, 1960). The question might be asked whether this loss of
histoformative traits reflected changes in the epithelial or the dermal
cells, or both. From other evidence, mentioned later, it might be concluded that the epidermal cells are not irreversibly altered; since it
appears that connective tissue cells are required for epidermal development (McLoughlin, 1961; Wessels, 1962), the possibility exists that in
monolayers these cells become modified first and are then unable to
support epidermal differentiation.
Changes in biosynthetic activities of cells grown in monolayer were
convincingly demonstrated in studies on cultures of chondrocytes.
Dissociated vertebral cartilage cells from 10-day chick embryos grown
in monolayer rapidly lose their ability to synthesize chondroitin sulphate
and after about 9 days show no evidence of its formation; even when
spun into a pellet and grown in organ culture, such lumps of modified
cells form no cartilage (Holtzer et al., 1960). The same is true for 13-day
long-bone cartilage cells in monolayer cultures (Kuroda, 1963), in
which the capacity to synthesize a cartilaginous matrix in aggregates
formed by rotation is lost in about 9 days, concurrently with a decline
in their ability to form such aggregates (see p. 504). Whether such
"dedifferentiation" (Holtzer et al., 1960) in skin or in cartilage-cell
cultures affects all cells in the population simultaneously or shows a
progressive selection, is not yet clear; the situation in this respect may
not be identical for all cellular systems or experimental conditions.
Neither is it clear whether the monolayer-modified cells regress to a
"unspecialized" state or rather undergo a new type of specialization, a
metaplasia, in response to life in culture. The precise causal or modifying
factors, extrinsic and intrinsic, involved in these cellular changes and
their correlation with mitotic activity and cell growth have yet to be
defined. It remains to be seen whether they reflect "an irreversible loss