We recently described a tissue culture system in which 21.5-day-old fetal rat islets underwent an in vitro maturation of insulin stimulus-secretion coupling over a period of 7 days. During the same period, the acinar part of the explanted fragments degenerated and the islets became isolated, seeming to increase in mass. In the present study, we have utilized these characteristic morphologic changes in an attempt to evaluate the extent that apparent islet growth reflects multiplication of preformed beta cells or the neogenesis of these cells from ductular or acinar cells. In the first days of culture, continuity between islets and ducts could be demonstrated, and the islets appeared to “bud” from the ducts. During this time, only insulin-and glucagon-positive cells could be demonstrated immunocytochemically, and the 3H-thymidine incorporation index of the beta cells (expressed as the percentage of beta cells labeled during 24 h of exposure to 3H-thymidine and 3 h of “chase”) in the “budding” islets was 28.7 ± 2.6. After 7 days in culture, i.e., after maturation of the insulin stimulus-secretion mechanism, the islets were no longer associated with ductular epithelium. At this stage, insulin-, glucagon-, and occasional somatostatin-positive islet cells could be demonstrated, and the 3H-thymidine incorporation index of the beta cells was significantly decreased to 16.7 ± 2.8. These observations are taken to support previous suggestions of a possible neogenesis of beta cells from duct epithelium in the rat. This tissue culture technique appears well suited for further detailed studies of this neogenesis.

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