The percentile volumes υ
A, υ
B and υ
D of A, B and D cells in the islets of Langerhans were histometrically estimated in the pancreases from 59 autopsy cases including maturity- and growth-onset diabetics and nondiabetics. Volumetry was performed microscopically by point counting method using a square-lattice eyepiece, then the total A, B and D cell volumes 
VA, 
VB and 
VD were calculated from the total islet volume 
Vi of the pancreas estimated separately. The measurement disclosed a prominent difference in 
VB between the three groups. Namely, it was only 0.156 and 0.388 cm
3 in the growth- and maturity-onset diabetics, much smaller values than 0.636 cm
3 in nondiabetics. The simultaneous investigation on clinical records revealed a negative correlation between 
VB and the maximum blood sugar level during GTT, showing that 
VB reflects grossly the degree of glucose tolerance of the individual. The smaller 
VB in diabetes was attributed to the diminution of 
Vi because υ
A, υ
B and υ
D were almost the same regardless of the presence or absence of diabetes. These results strongly suggested a failure of insulin production in diabetes due to quantitative deficiency of B cells.
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