DIFFERENTIATION OF KIDNEY MESENCHYME
277
the metanephrogenic mesenchyme (Saxen et al., 1965a,b), but the occurrence of differential adhesion between the aggregated and unaggregated cells has been a more difficult phenomenon to demonstrate.
It was first found that no changes occur in the electrophoretic mobility
of the mesenchyme cells in the first 24 hours of cultivation (Saxen et al.,
1965b), during which period the determination of the mesenchyme takes
place. These results suggest that the basic change leading to the formation of the aggregates is not an alteration in the surface charge of the
cells.
With time-lapse cinematography the gradual spread of a trapping
effect could be demonstrated as moving mesenchymal cells became
caught into aggregates and gradually lost their motility within the cell
cluster (Saxen et al., 1965a). The aggregated cells acquired greater
mutual contact surfaces as a result of the disappearance of intercellular
gaps from within the developing aggregate (Fig. 18), which was demonstrated in electron microscopy (Wartiovaara, 1966b). The aggregate
acquired a spherical form with radially orientated, wedge-shaped cells
(Fig. 19).
If cells are considered as spheres with flexible surfaces, and mixed
populations of cells with differential adhesion are assumed to maximize
their adhesion, a group of more adhesive cells will segregate from the rest
and acquire a spherical form, as Pethica (1961) has pointed out. The
cells will assume a radial orientation within the sphere to minimize their
contact surfaces with the surrounding, less adhesive cells and to maximize
their contact surfaces with similar neighboring cells. In three dimensions
this leads to cells of conical shape with apices pointing to the center of
the sphere.
Increased cellular adhesion has been suggested to occur in many
morphogenetic events (Holtfreter, 1939; Curtis, 1962; Trinkaus, 1963;
Jones and Elsdale, 1963; Gustafson and Wolpert, 1963). Although direct
evidence for it is lacking, the following observations give indirect support
to the view that it occurs in kidney tubulogenesis: (1) the decrease in
cellular motility during aggregation; (2) the increase in mutual cell
contacts in the aggregates; and (3) the radial orientation of the aggregated cells and their wedge-shaped appearance, suggesting a conical form
in three dimensions.
Although the idea of an increase in cellular adhesion could be taken
as a working hypothesis to account for cell aggregation during tubulogenesis, the nature of this change needs clarification.
On the ultrastructural level, no connecting membrane units could be
277
the metanephrogenic mesenchyme (Saxen et al., 1965a,b), but the occurrence of differential adhesion between the aggregated and unaggregated cells has been a more difficult phenomenon to demonstrate.
It was first found that no changes occur in the electrophoretic mobility
of the mesenchyme cells in the first 24 hours of cultivation (Saxen et al.,
1965b), during which period the determination of the mesenchyme takes
place. These results suggest that the basic change leading to the formation of the aggregates is not an alteration in the surface charge of the
cells.
With time-lapse cinematography the gradual spread of a trapping
effect could be demonstrated as moving mesenchymal cells became
caught into aggregates and gradually lost their motility within the cell
cluster (Saxen et al., 1965a). The aggregated cells acquired greater
mutual contact surfaces as a result of the disappearance of intercellular
gaps from within the developing aggregate (Fig. 18), which was demonstrated in electron microscopy (Wartiovaara, 1966b). The aggregate
acquired a spherical form with radially orientated, wedge-shaped cells
(Fig. 19).
If cells are considered as spheres with flexible surfaces, and mixed
populations of cells with differential adhesion are assumed to maximize
their adhesion, a group of more adhesive cells will segregate from the rest
and acquire a spherical form, as Pethica (1961) has pointed out. The
cells will assume a radial orientation within the sphere to minimize their
contact surfaces with the surrounding, less adhesive cells and to maximize
their contact surfaces with similar neighboring cells. In three dimensions
this leads to cells of conical shape with apices pointing to the center of
the sphere.
Increased cellular adhesion has been suggested to occur in many
morphogenetic events (Holtfreter, 1939; Curtis, 1962; Trinkaus, 1963;
Jones and Elsdale, 1963; Gustafson and Wolpert, 1963). Although direct
evidence for it is lacking, the following observations give indirect support
to the view that it occurs in kidney tubulogenesis: (1) the decrease in
cellular motility during aggregation; (2) the increase in mutual cell
contacts in the aggregates; and (3) the radial orientation of the aggregated cells and their wedge-shaped appearance, suggesting a conical form
in three dimensions.
Although the idea of an increase in cellular adhesion could be taken
as a working hypothesis to account for cell aggregation during tubulogenesis, the nature of this change needs clarification.
On the ultrastructural level, no connecting membrane units could be