The pancreases of eight normoglycemic and of nine alloxan-diabetic rabbits, and those of their respective litters, sacrificed within one week after birth, were studied by light and electron microscopy and were assayed for insulin by the double-antibody method. The insulin concentrations of the pancreases from the neonatal rabbits averaged 0.56 U. per gram of wet tissue in rabbits born to normoglycemic mothers and 0.73 U. in those born to alloxan-diabetic animals. Since ultrastructurally in the neonatal rabbits of both groups mostly paid B cell granules were encountered, it is concluded that pale granules contain assayable insulin. This agrees with previous immunofluorescent studies, in which it was also shown that the pale granules contain insulin. The relatively lower number of B cell granules as well as the more prominent display of vesicular units of the endoplasmic reticulum in rabbits born to alloxan-diabetic animals suggest that a state of hyperfunction, induced by hyperglyceinia transmitted through the placenta by the mother rabbit, exists in the B cells of this group of neoaates. On the other hand, the absence of blood sugar changes in the diabetic mother animals during pregnancy is in keeping with former observations that insulin fails to cross the placental barrier in rabbits (and other mammals).

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