Fig. 5. Expression of selected genes in canal-forming primmorphs in response to retinoic acid. Primmorphs were first incubated for 5 days on non-coated dishes and then, in order to induce canals, on galectin-coated dishes for 10 days. Subsequently, these primmorphs remained either untreated for 5 days (C; lane a) or were treated with 1 µmol l^–1 (lane b), 3 µmol l^–1 (lane c), or 50 µmol l^–1 of retinoic acid (lane d) as indicated. RNA was then extracted and 5 µg of total RNA per lane were size-separated. After blot transfer, hybridization was performed using the following probes: SDRXR (the putative sponge retinoid X receptor), SDCASPR (S. domuncula caspase), SDLIM4 (sponge LIM/homeodomain protein) or the sponge SDCYP4 (CYP4A related cytochrome P-450 protein). The levels of expression of the respective genes were estimated by northern blot and the intensities of the different transcript bands were determined relative to the intensity of the controls (cultivated for a total of 15 days in solution).

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