Fetal rat pancreases, Day 16 through Day 22 postcoitum, were analyzed by immunoassay for insulin to assess islet cell development in vivo. The rise in pancreatic insulin concentration observed during gestation was relatively constant, amounting to 50 to 100 per cent per day, and was accompanied by increases in the total islet mass of the pancreas.

A difference was found in the potential of the pancreas for islet differentiation in organ culture which was related to the developmental state of the pancreas at the time of explantation. Highly differentiated islets of twenty-two day fetal pancreas were maintained in organ culture, while large increases in the islet mass occurred following organ culture of relatively undifferentiated eighteen day fetal pancreas.

Using the present organ culture system, explants can be produced which are composed of 25 per cent islet cells and contain 17 mU. of insulin activity per milligram.

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