Zone model for intensity profiles. Zones with fixed length are calculated for the accepted Zo-1 profiles. Firstly, two peaks are identified, and the center of the profile is found. Zone 1 has a length of 5 pixels (0.5 μ m), and is centered between the peaks. Its length was chosen according to the empirical determination of canalicular width (0.8 - 2.5 μ m). The integral intensity of zone 1 reveals the amount of Bsep in the center of the canaliculus. Zones 2a and 2b (5 pixels each) are centered on the peaks and indicate protein concentration in the canalicular membrane. To analyze the internalization of transport proteins, intracellular zones were defined. Zones 3a and 3b (5 pixels each) describe intracellular fluorescence intensities close to the canalicular membrane. Zones 4a and 4b are situated closer to the centers of hepatocytes and measure 10 pixels each. Zone 5 covers the fluorescence intensities in the interior of the canaliculus between the peaks, while zone 6a and 6b combine the intracellular intensities of the profile from the maximum values of the peaks to the ends.