DIFFERENTIATION OF KIDNEY MESENCHYME
259
(Section II) and biochemical (Section III; Fig. 4) criteria. The control
tissue remains viable for a more limited period of up to 5 to 6 days of
cultivation (Vainio et al., 1965; Koskimies and Saxen, 1966).
II. Formation of Kidney Tubules
In the following, a short outline will be given of the morphogenesis of
kidney tubules; morphological parameters will serve as a reference for
the description of the changes detected at the subcellular level. The main
purpose of the experimental model system presented in the Section I
being to simulate normal development, some features of the morphogenesis seen in whole kidney rudiments cultivated in vitro may be briefly
mentioned.
In time-lapse films of the development of an isolated mouse embryonic
kidney in organ culture, the following events can be seen (Saxen et al.,
1965a,b; Saxen and Wartiovaara, 1966). In a 12-day-old kidney rudiment
the epithelial ureteric bud penetrates into the metanephrogenic mesenchyme (Fig. 5A). Subsequently, dichotomous branching of the growing
bud takes place. The onset of tubulogenesis is overtly expressed by the
appearance of cellular condensates around the inductively active tips of
the epithelial bud as randomly moving mesenchyme cells get trapped in
these areas and gradually lose their motility (Fig. 5B). The aggregates
grow by apposition of new cells and by cell proliferation. Judged by
radioautography, the incorporation of thymidine3
!! into the tubule cells
seems to be several times higher than into unaggregated mesenchyme
cells (Rapola et al., 1963; Sobel, 1966). During further cultivation the
condensates break up into smaller aggregates (Fig. 5C and D), which, in
turn, develop into S-shaped secretory tubules and join the branches of
the ureteric tree (Fig. 5E and F).
In the transfilter system shown in Fig. 6, a similar morphogenetic
response is seen in the metanephrogenic mesenchyme (Wartiovaara,
1966b). The loose mesenchyme cells (Fig. 7A) condense near the filter
and form aggregates of unorientated cells (Fig. 7B and C). The differentiation of the aggregate proceeds with radial orientation of the cells
(Fig. 7D) and the opening of a central lumen within the cell cluster (Fig.
7E). Finally, an S-shaped structure typical of the secretory tubule is
formed (Fig. 7F). The development of the secretory tubules in the
transfilter system differs morphologically from the whole kidney cultures
only in that the tubules remain as separate entities in the absence of the
ureteric tree.
259
(Section II) and biochemical (Section III; Fig. 4) criteria. The control
tissue remains viable for a more limited period of up to 5 to 6 days of
cultivation (Vainio et al., 1965; Koskimies and Saxen, 1966).
II. Formation of Kidney Tubules
In the following, a short outline will be given of the morphogenesis of
kidney tubules; morphological parameters will serve as a reference for
the description of the changes detected at the subcellular level. The main
purpose of the experimental model system presented in the Section I
being to simulate normal development, some features of the morphogenesis seen in whole kidney rudiments cultivated in vitro may be briefly
mentioned.
In time-lapse films of the development of an isolated mouse embryonic
kidney in organ culture, the following events can be seen (Saxen et al.,
1965a,b; Saxen and Wartiovaara, 1966). In a 12-day-old kidney rudiment
the epithelial ureteric bud penetrates into the metanephrogenic mesenchyme (Fig. 5A). Subsequently, dichotomous branching of the growing
bud takes place. The onset of tubulogenesis is overtly expressed by the
appearance of cellular condensates around the inductively active tips of
the epithelial bud as randomly moving mesenchyme cells get trapped in
these areas and gradually lose their motility (Fig. 5B). The aggregates
grow by apposition of new cells and by cell proliferation. Judged by
radioautography, the incorporation of thymidine3
!! into the tubule cells
seems to be several times higher than into unaggregated mesenchyme
cells (Rapola et al., 1963; Sobel, 1966). During further cultivation the
condensates break up into smaller aggregates (Fig. 5C and D), which, in
turn, develop into S-shaped secretory tubules and join the branches of
the ureteric tree (Fig. 5E and F).
In the transfilter system shown in Fig. 6, a similar morphogenetic
response is seen in the metanephrogenic mesenchyme (Wartiovaara,
1966b). The loose mesenchyme cells (Fig. 7A) condense near the filter
and form aggregates of unorientated cells (Fig. 7B and C). The differentiation of the aggregate proceeds with radial orientation of the cells
(Fig. 7D) and the opening of a central lumen within the cell cluster (Fig.
7E). Finally, an S-shaped structure typical of the secretory tubule is
formed (Fig. 7F). The development of the secretory tubules in the
transfilter system differs morphologically from the whole kidney cultures
only in that the tubules remain as separate entities in the absence of the
ureteric tree.