Transfilter induction of kidney tubules: correlation with cytoplasmic penetration into Nucleopore filters

The presence of cell contacts in transfilter mouse kidney tubule induction by spinal cord was investigated. The interacting tissues were separated by Nucleopore® membrane filters of various pore sizes. Filters with a mean pore size of 0·2 μm or more allowed tubule formation, whereas 0·13 μm pore filters prevented it. There was an inverse correlation between pore size and minimum time of transfilter culture required for induction to occur. The differences in the diffusion rates through these filters do not support long range diffusion as a mechanism for induction. Electron microscopy of the cultures showed abundant cytoplasmic penetration deep into filters with 0·2 μm or larger pores. Processes from mesenchymal and spinal cord cells were closely apposed within the filter channels. No extracellular matrix or membrane vesicles were seen between the processes. In a few instances shallow penetration was seen in 0·1 μm type filters, but no contacts were observed. The presence of close cell appositions in those filters which allow kidney tubulogenesis suggests that close cellular interactions, rather than long range diffusion of signal substances, is the most likely communicative mechanism in this transfilter induction.

FINE STRUCTURE OF DIFFERENTIATING MOUSE PANCREATIC EXOCRINE CELLS IN TRANSFILTER CULTURE

Fine structural observations have been made in the 11-day embryonic mouse of exocrine cells in pancreatic epithelium developing in tissue culture transfilter from salivary gland mesenchyme of the 13-day embryonic mouse. After 2 days in culture, the exocrine cells show increased cytoplasmic density, abundant ribosomes in aggregate or "rosette" form, and expanded profiles of rough-surfaced endoplasmic reticulum. After 3 and 4 days in culture, the cells exhibit continued expansion of the profiles of endoplasmic reticulum, increased amounts of Golgi membranes, and large areas of light density (prozymogen granules). After 5 days in culture, dense zymogen granules are present in the most highly differentiated cells. In addition, at the filter-epithelial surface, at 2 days, small fibers can be discerned which, after 4 days in culture, show obvious periodicity and are thought to be collagen. The significance of these changes, in relation to the mesenchymal effect, to the onset of specific synthesis and to the stabilization of differentiation is discussed.