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To study the cell cycle and regulation by glucose of Bcell proliferation, B-cell-rich pancreatic islets of rat fetuses (22 days gestational age) were maintained in tissue culture at various glucose concentrations. The proliferating islet cells were synchronized with hydroxyurea and their rate of progress through the cell cycle studied by pulse labeling with 3H-thymidine and after exposure to colchicine. The time for a full cell cycle was estimated to 14.9 h and could be subdivided into a G1 phase of a duration of 2.5 h, an S phase of 6.4 h, a G2 phase of 5.5 h, and an M phase of 0.5 h. Although glucose significantly stimulated B-cell proliferation, the progression of cells through the cell cycle was similar at different glucose concentrations. The experiments furthermore suggested that only a limited pool of islet cells was able to proliferate. Glucose seemed to stimulate B-cell proliferation by increasing the number of cells entering the cell cycle. From the cell cycle data it was possible to calculate the rate of formation of new B-cells, which ranged from 4.2%/24 h in 2.7 mM glucose to 10.4%/24 h in 16.7 mM. When the accumulation of mitotic figures during colchicine treatment was used as an alternative method for estimation of newly formed B-cells the cell birth rates were found to be 3.1 and 6.0%/24 h at 2.7 and 16.7 mM glucose, respectively. The notion that only a small fraction of B-cells takes part in proliferation would explain the limited regenerative capacity of this cell type. It remains to be established whether such a limitation is of significance in the development of maturity-onset diabetes.
Ingemar Swenne (Wed,) studied this question.