DIFFERENTIATION OF KIDNEY MESENCHYME
279
increase in the NADH-tetrazolium reductase activity that can be ascribed to the endoplasmic reticulum. It should be mentioned that the
aggregating cells are characterized by increased alkaline phosphatase
activity (Rapola et al., 1963), the significance of which is not known,
although it might be taken as an indication of an increased requirement
of energy needed for increased protein synthesis. The aggregated cells
also contain accumulated material for extracellular purposes in the distended rough surfaces of their endoplasmic reticulum. But at the aggregation stage, electron microscopy did not demonstrate any increase in
extracellular material on the surfaces of the aggregating cells, such as
might affect the adhesive properties of the cell surface. The later accumulation of the extracellular material to form the basement membrane on
the surface and its absence from the surface of unaggregated cells suggest
that these two types of cells differ in surface characteristics (Wartiovaara, 1966a).
The acquisition of resistance to certain viral infections that takes
place in the mesenchyme cells during aggregation (Vainio et al., 1963)
might also be taken as indirect evidence of alterations in the properties
of the surface membrane in the aggregating cells. It could be speculated
that a loss of virus receptors occurs or changes in surface permeability,
although several other explanations can also be proposed (Saxen, 1965).
As stated above, the formation of aggregates might create local cell
populations with the "critical mass" or "critical density" needed for
differentiation (Grobstein and Zwilling, 1953; Grobstein, 1955b). A
homotypic interaction between the cells in an aggregate can then be
thought to comprise a second phase in kidney tubulogenesis after the
priming heterotypic interaction between the inducing tissue and the
responding metanephrogenic mesenchyme leading to the aggregation
phenomenon.
The further differentiation of the aggregated cells can be considered
to constitute a second stage in the establishment of cellular contacts during tubulogenesis. This stage is characterized by the formation of junctional complexes between the apical cell membranes of the radially
oriented cells (Fig. 19). Loewenstein (1966) has suggested that junctional complexes may serve not only as intercellular attachment devices,
but also as barriers between a cell group and its surroundings as well as
sites of electronic coupling between cells, which allow free ionic diffusion
between the connected cells. In kidney tubulogenesis, the early appearance of the junctional complexes in the developing cell aggregates could
279
increase in the NADH-tetrazolium reductase activity that can be ascribed to the endoplasmic reticulum. It should be mentioned that the
aggregating cells are characterized by increased alkaline phosphatase
activity (Rapola et al., 1963), the significance of which is not known,
although it might be taken as an indication of an increased requirement
of energy needed for increased protein synthesis. The aggregated cells
also contain accumulated material for extracellular purposes in the distended rough surfaces of their endoplasmic reticulum. But at the aggregation stage, electron microscopy did not demonstrate any increase in
extracellular material on the surfaces of the aggregating cells, such as
might affect the adhesive properties of the cell surface. The later accumulation of the extracellular material to form the basement membrane on
the surface and its absence from the surface of unaggregated cells suggest
that these two types of cells differ in surface characteristics (Wartiovaara, 1966a).
The acquisition of resistance to certain viral infections that takes
place in the mesenchyme cells during aggregation (Vainio et al., 1963)
might also be taken as indirect evidence of alterations in the properties
of the surface membrane in the aggregating cells. It could be speculated
that a loss of virus receptors occurs or changes in surface permeability,
although several other explanations can also be proposed (Saxen, 1965).
As stated above, the formation of aggregates might create local cell
populations with the "critical mass" or "critical density" needed for
differentiation (Grobstein and Zwilling, 1953; Grobstein, 1955b). A
homotypic interaction between the cells in an aggregate can then be
thought to comprise a second phase in kidney tubulogenesis after the
priming heterotypic interaction between the inducing tissue and the
responding metanephrogenic mesenchyme leading to the aggregation
phenomenon.
The further differentiation of the aggregated cells can be considered
to constitute a second stage in the establishment of cellular contacts during tubulogenesis. This stage is characterized by the formation of junctional complexes between the apical cell membranes of the radially
oriented cells (Fig. 19). Loewenstein (1966) has suggested that junctional complexes may serve not only as intercellular attachment devices,
but also as barriers between a cell group and its surroundings as well as
sites of electronic coupling between cells, which allow free ionic diffusion
between the connected cells. In kidney tubulogenesis, the early appearance of the junctional complexes in the developing cell aggregates could