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ABSTRACT 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.
Wartiovaara et al. (Sat,) studied this question.