522
A . A . M O S C O N A
assorted and grouped selectively in distinct regions, retinal pigmentcells appear to provide, in some cases, an exception. Trinkaus (1961)
found that retinal pigment cells from the chick embryo, co-aggregated
by centrifugation with mesonephric cells, formed pigmented vesicles
within the nephric tissue; they were also found to remain associated
with perichondrial cells under similar conditions. Trinkaus does not
consider this to be due to passive entrapment; he proposes that any
given tissue cell has strong iso-affinities for like cells and hetero-affinities for
certain other cells, and that these hierarchies of affinity represent a
means of holding cells and tissues together. Accordingly, general connective tissue cells would be endowed with multiple hetero-affinities.
The relative "strength" of iso- and hetero-affinities between cells may
depend on their developmental and functional states; cells from older
embryos are less efficient in sorting out their regional grouping than
homologous cells from younger stages (Holtfreter, 1939; Lehman,
1951). This may be part of the syndrome of changes in contact properties
of cells with age which is reflected also in the age-dependent decline in
the ability of cells to aggregate (Moscona, 1961c).
Regionalization (Trinkaus and Groves, 1955; Trinkaus, 1961) is seen
in aggregates compounded of cells from two or more different tissues
(say, liver cells and limb-bud chondroblasts); in such combinations,
cells of one type usually tend to converge centrally (in this case, the
chondroblasts) while the others form an outer zone, with connective
tissue (or perichondral) cells in an intermediate position (Moscona,
1957a, 1960). Regionalization occurs both in rotation-compounded
aggregates, in self-aggregates and in pellets made by centrifugation. The
topography of the patterns depends on the cellular composition of the
aggregates (Trinkaus, 1961). It may simulate the relationships of the
homologous tissues in the organism or show little or no such similarity;
i.e. muscle cells tend to group around cartilage clusters; epidermal
cells tend to congregate at the surface of aggregates, but so do liver cells,
and kidney cells; however liver cells, co-aggregated with intestinal cells,
become surrounded by the latter. The completeness and precision of
the region grouping of cells depends in a large measure on the size of
the aggregates, in that larger aggregates may show a mosaic architecture devoid of the conspicuous layering which is typical of smaller
aggregates of similar composition. Fusion of aggregates results in
secondary regionalization patterns.
It is necessary to distinguish between regionalization in secondary
aggregates and the repatterning that may take place subsequently, due
to overgrowth, infiltration, differentiation, histolysis, or conditions
imposed by the culture. While the evidence is convincing that
regionalization of aggregates is achieved through movements of cells
A . A . M O S C O N A
assorted and grouped selectively in distinct regions, retinal pigmentcells appear to provide, in some cases, an exception. Trinkaus (1961)
found that retinal pigment cells from the chick embryo, co-aggregated
by centrifugation with mesonephric cells, formed pigmented vesicles
within the nephric tissue; they were also found to remain associated
with perichondrial cells under similar conditions. Trinkaus does not
consider this to be due to passive entrapment; he proposes that any
given tissue cell has strong iso-affinities for like cells and hetero-affinities for
certain other cells, and that these hierarchies of affinity represent a
means of holding cells and tissues together. Accordingly, general connective tissue cells would be endowed with multiple hetero-affinities.
The relative "strength" of iso- and hetero-affinities between cells may
depend on their developmental and functional states; cells from older
embryos are less efficient in sorting out their regional grouping than
homologous cells from younger stages (Holtfreter, 1939; Lehman,
1951). This may be part of the syndrome of changes in contact properties
of cells with age which is reflected also in the age-dependent decline in
the ability of cells to aggregate (Moscona, 1961c).
Regionalization (Trinkaus and Groves, 1955; Trinkaus, 1961) is seen
in aggregates compounded of cells from two or more different tissues
(say, liver cells and limb-bud chondroblasts); in such combinations,
cells of one type usually tend to converge centrally (in this case, the
chondroblasts) while the others form an outer zone, with connective
tissue (or perichondral) cells in an intermediate position (Moscona,
1957a, 1960). Regionalization occurs both in rotation-compounded
aggregates, in self-aggregates and in pellets made by centrifugation. The
topography of the patterns depends on the cellular composition of the
aggregates (Trinkaus, 1961). It may simulate the relationships of the
homologous tissues in the organism or show little or no such similarity;
i.e. muscle cells tend to group around cartilage clusters; epidermal
cells tend to congregate at the surface of aggregates, but so do liver cells,
and kidney cells; however liver cells, co-aggregated with intestinal cells,
become surrounded by the latter. The completeness and precision of
the region grouping of cells depends in a large measure on the size of
the aggregates, in that larger aggregates may show a mosaic architecture devoid of the conspicuous layering which is typical of smaller
aggregates of similar composition. Fusion of aggregates results in
secondary regionalization patterns.
It is necessary to distinguish between regionalization in secondary
aggregates and the repatterning that may take place subsequently, due
to overgrowth, infiltration, differentiation, histolysis, or conditions
imposed by the culture. While the evidence is convincing that
regionalization of aggregates is achieved through movements of cells