14. C E L L A G G R E G A T E S
491
thus be expected to provide an entering wedge into the whole problem
of cellular dynamics of histogenetic processes.
Dissociated embryonic-tissue cells spread out thinly on a plasma clot
tend to become individually attached (Moscona, 1952). Nerve cells,
heart muscle cells (Cavanaugh, 1955a, b ) , skeletal muscle cells
(Moscona, 1955) isolated singly from the embryo after the onset of their
functional determination may continue under such conditions to
differentiate and can achieve some of their typical characteristics.
However, most cells dispersed by tissue dissociation or by outgrowth
from tissue fragments usually acquire a generalized character devoid of
overt signs of differentiation (Willmer, 1958). For a display of their
typical functional traits a collective framework, an aggregate, is required.
Ebeling and Fischer (1922) and Drew (1923) described restitution
in vitro of histologically organized structures from embryonic avian
cells obtained by outgrowth from tissue fragments. Long-term
plasma-clot cultures of fibroblasts and epithelial cells were placed
side by side so as to bring the different cells together; in the merger
zone, clustering and aggregation of epithelial cells took place, resulting
in the formation of tubular structures surrounded by mesenchymal
stroma (see also Puck, Cieciura and Fisher, 1957). Such "pulling
together" and three-dimensional ordering of spread-out epithelial cells
in response to the presence of connective tissue elements was described
in a variety of in vitro systems (Ebeling, 1925; Toro, 1933; Borghese,
1950, 1958; Grobstein, 1953; Holtzer and Detwiler, 1953) and reviewed
by Grobstein (1955a, 1959) in connexion with his analysis of inductive
mesenchymal-epithelial interactions. The stimulus is thought to provide
conditions which cause a transition in the responding cells from a loose,
or "open" association to an aggregated or "closed" system, hence to
histotypic organization. Relatively simple, nutritional "cross-feeding"
effects between the two cell populations may be involved (Eagle, 1959);
however, there is evidence that more specific "inductive" events
(Grobstein, 1955a), and interactions at the fine-structure level between
the confronted cells (Weiss, 1958) play a role in their association and
organization into developmentally progressive complexes.
The above studies were done with cells in outgrowth cultures, or
with explants of organ rudiments. Further progress with these and
related problems can be made by starting with a suspension of totally
dispersed cells and by examining their resynthesis into aggregates.
Aggregation of dissociated cells can be studied in vitro in two ways: (1)
self-aggregation, which takes place spontaneously in stationary cultures,
i.e. in culture dishes, sitting drops, or hanging drops; (2) mediated
aggregation, in which the cells in suspension are concentrated and
491
thus be expected to provide an entering wedge into the whole problem
of cellular dynamics of histogenetic processes.
Dissociated embryonic-tissue cells spread out thinly on a plasma clot
tend to become individually attached (Moscona, 1952). Nerve cells,
heart muscle cells (Cavanaugh, 1955a, b ) , skeletal muscle cells
(Moscona, 1955) isolated singly from the embryo after the onset of their
functional determination may continue under such conditions to
differentiate and can achieve some of their typical characteristics.
However, most cells dispersed by tissue dissociation or by outgrowth
from tissue fragments usually acquire a generalized character devoid of
overt signs of differentiation (Willmer, 1958). For a display of their
typical functional traits a collective framework, an aggregate, is required.
Ebeling and Fischer (1922) and Drew (1923) described restitution
in vitro of histologically organized structures from embryonic avian
cells obtained by outgrowth from tissue fragments. Long-term
plasma-clot cultures of fibroblasts and epithelial cells were placed
side by side so as to bring the different cells together; in the merger
zone, clustering and aggregation of epithelial cells took place, resulting
in the formation of tubular structures surrounded by mesenchymal
stroma (see also Puck, Cieciura and Fisher, 1957). Such "pulling
together" and three-dimensional ordering of spread-out epithelial cells
in response to the presence of connective tissue elements was described
in a variety of in vitro systems (Ebeling, 1925; Toro, 1933; Borghese,
1950, 1958; Grobstein, 1953; Holtzer and Detwiler, 1953) and reviewed
by Grobstein (1955a, 1959) in connexion with his analysis of inductive
mesenchymal-epithelial interactions. The stimulus is thought to provide
conditions which cause a transition in the responding cells from a loose,
or "open" association to an aggregated or "closed" system, hence to
histotypic organization. Relatively simple, nutritional "cross-feeding"
effects between the two cell populations may be involved (Eagle, 1959);
however, there is evidence that more specific "inductive" events
(Grobstein, 1955a), and interactions at the fine-structure level between
the confronted cells (Weiss, 1958) play a role in their association and
organization into developmentally progressive complexes.
The above studies were done with cells in outgrowth cultures, or
with explants of organ rudiments. Further progress with these and
related problems can be made by starting with a suspension of totally
dispersed cells and by examining their resynthesis into aggregates.
Aggregation of dissociated cells can be studied in vitro in two ways: (1)
self-aggregation, which takes place spontaneously in stationary cultures,
i.e. in culture dishes, sitting drops, or hanging drops; (2) mediated
aggregation, in which the cells in suspension are concentrated and