DEVELOPMENT OF THE BILE CANALICULAR SYSTEM IN RAT LIVER: WITH SPECIAL REFERENCE TO BILE CANALICULAR ACTIN FILAMENTS AND Mg²⁺, Ca²⁺-ATPase ACTIVITY

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Developmental changes of the bile canaliculi (BC) in rat (fetal 13th-21st day, postnatal and adult) livers were cytochemically examined by fluorescence, light and electron microscopy. Bile canalicular actin filaments (F-actin), consisting of the circumferential pericanalicular F-actin and microvilli (MV) core F-actin, were observed by 7-nitrobenz-2-oxa-1, 3-diazole phallacidin (NBD-ph) fluorescence microscopy and electron microscopy. Mg²⁺, Ca²⁺-ATPase activity, which is presumed to be involved in bile acid secretion across the bile canalicular membrane, was detected by a modified Wachstein and Meisel method at both light and electron microscopic levels. Bile canalicular F-actin sparsely appeared from the 13th day of gestation. Since then, the amount of bile canalicular F-actin gradually increased accompanying the bile canalicular formation by 2 weeks after the birth when the development was almost completed. Mg²⁺, Ca²⁺-ATPase activity appeared on the fetal 19th day in less than 10% of the BC. The number of Mg²⁺, Ca²⁺-ATPasepositive BC rapidly increased around the time of birth (fetal 21st day-postnatal 1st day). The rate of AT-Pase positive BC reached 40-70% during this period. On the 5th day after birth, most BC showed Mg²⁺, Ca²⁺-ATPase activity. These findings indicated that the expression of bile canalicular F-actin and ATPase can be regarded as a structural marker and a functional marker for bile secretion, respectively. Structuraldevelopment of BC began earlier but at a rather slower rate than functional maturation.