Streaming pancreas: islet cell kinetics.

Thirty male young adult rats aged 7 weeks, weighing 200 g, were injected with 0.5 microCi/g body weight tritiated thymidine (specific activity 5.0 Ci/mM). The rats were then killed in groups of five, at the following times: 1 hour, and 14, 28, 42, 56, 90, and 120 days. The pancreata were dissected, embedded in paraffin, and cut into 5 microns thick sections which were then dipped into liquid emulsion. The labelling index of acini and islet cells was estimated in each autoradiogram, and their distance from intercalated duct was measured. Acini and islet cells streamed away from the intercalated duct at a daily velocity of 0.52 microns. Acinus labelling index which was initially 0.78% declined with time. Islet cell labelling index was initially 0.38%. In the following days it climbed upward reaching on the 42nd day its peak value of 0.78%. From then onward it declined to a low of 0.38%. Since cells were labelled only once, and labelling was instantaneous, it is concluded that labelled acinus cells entered the islet, and that acinus cells are actually islet cell precursors. The pancreas is structured kinetically like other exocrine glands, being a two compartment cell renewal system which continuously renews its cells. The pancreocyte is a descendant of an intercalated duct progenitor. After leaving the duct it turns into an acinus cell, gradually approaching the Langerhans islet. After crossing its border it becomes an islet cell, and proceeds toward the islet center. Its population size dwindles exponentially until at islet center all cells are eliminated.