Beta-cell proliferation in normal and streptozotocin-treated newborn rats: site, dynamics and capacity

Distribution of endocrine cells displaying immunoreactivity for one or more peptides in the pancreas of the adult vervet monkey (Cercopithecus aethiops)

BACKGROUND

Our previous studies on the Chacma baboon revealed that the most striking difference between islets of the ventral and dorsal regions of the pancreas was their content of A and pancreatic polypeptide (PP) cells with A cells predominating in the tail and PP cells in the uncinate and head. Cells displaying dual immunoreactivity for both glucagon and PP were also observed. The objective of this study was to establish baseline parameters of the adult Vervet monkey (Cercopithecus aethiops) pancreas so that it could be used as a primate model to investigate possible therapies for diabetes.

METHODS

Vervet-monkey pancreas was divided into uncinate, head, and tail regions, and the tissue processed for immunolabelling for pancreatic peptides using avidin-biotin-peroxidase as marker. Dot-blotting and absorption controls for antibody specificity were included because of the shared amino acid sequences in pancreatic polypeptide (PP), neuropeptide Y (NPY), and peptide YY (PYY). Endocrine cell distributions and the percentages of each cell type per region were calculated for each monkey.

RESULTS

A significant difference in the percentages of PP cells (P = 0.02) was observed between uncinate and tail regions, the distribution of NPY cells was similar to that of the PP cells, and all other distributions were similar to those reported in the literature for most animals studied. Cells displaying dual immunoreactivity for glucagon and PP or NPY, PYY and NPY, or PP, PP and somatostatin and glucagon and insulin were identified and mapped throughout the pancreas. Most co-localizations occurred in the uncinate region. Co-localization of glucagon and insulin has not, to our knowledge, been reported previously in the adult pancreas.

CONCLUSIONS

Pancreatic endocrine cell distribution in the adult Vervet monkey was found to be very similar to most other animals studied. The occurrence of cells displaying dual immunoreactivity for a number of different combinations of pancreatic peptides suggests an interesting plasticity of endocrine cells even in the adult animal.

Beta-cell growth in adolescent and adult rats treated with streptozotocin during the neonatal period

Regeneration of neonatal beta cells after subtotal streptozotocin (STZ)-induced destruction is incomplete but nevertheless leads to rapid remission of hyperglycaemia. To study the proliferative and functional capacity of regenerated beta cells in adolescent and adult rats after early beta-cell damage and to determine the time point after birth which is decisive for regeneration, beta-cell growth and metabolic capacity were analysed in rats treated during the neonatal period with STZ (100 micrograms/g body weight) and studied over 3 to 20 weeks. Using immunocytochemistry combined with morphometry we found that the regenerated beta cells continue to increase in number till week 6 of life, when they reached values of more than 50% of those of controls. After week 6, the regenerated beta cells had enlarged in size but failed to further increase their number, although their proliferative activity, determined by bromodeoxyuridine (BrdU) pulse labelling, was still higher at 6 and 10 weeks than that of normal rats. The inability of regenerated beta cells to further increase their number coincided with a deterioration of their function (week 10, male rats; week 20, female rats). When beta cells were destroyed on day 2 or 5 instead of the day of birth, regeneration of beta cells markedly decreased and the rats were already on the threshold of development of glucose intolerance at 3 weeks of age. We concluded that the partially regenerated beta-cell population in rats treated with STZ on the day of birth ceases to grow 10 to 20 weeks later. This growth arrest may be due to the sustained functional demand to which these beta cells are exposed in order to compensate for their reduced number. Beta-cell capacity for regeneration declines rapidly during the first days of life.

Duct- to islet-cell differentiation and islet growth in the pancreas of duct-ligated adult rats

We investigated the growth of islet beta and alpha cells in adult rats which had undergone partial pancreatic duct ligation. Whereas the non-ligated head portion of the pancreas remained unaffected in terms of histology and cell population dynamics, the ligated tail part of the pancreas showed pronounced changes in histology and cell growth. These changes included replacement of exocrine acini by ductal complexes and significant growth of islet cells. Using immunocytochemistry and morphometry, we found that the beta-cell population had nearly doubled within 1 week and that a smaller, but also significant growth of the alpha-cell population had occurred. In addition, small islets and islet-cell clusters were more numerous in the pancreatic tail, indicating islet neogenesis. The bromodeoxyuridine (BrdU) pulse labelling index of beta and alpha cells increased five fold and threefold, respectively, in the tail. However, the observed beta-cell labelling index remained below 1% which was largely insufficient to explain the increased number of beta cells. This indicates that recruitment from a proliferating stem-cell compartment was the main source for the beta-cell hyperplasia. A tenfold-elevated BrdU labelling index (18%) was observed in the duct-cell compartment which was identified by specific immunostaining for cytokeratin 20. Transitional cytodifferentiation forms between duct cells expressing cytokeratin 20 and beta cells expressing insulin, or alpha cells expressing glucagon, were demonstrated by double immunostaining. Pancreatic duct ligation also induced the expression of the beta-cell-specific glucose transporter type 2 (GLUT-2) in duct cells, indicating their metaplastic state. We concluded that in this adult rat model, the proliferation and differentiation of exocrine duct cells represents the major mechanism of endocrine beta-cell neogenesis. Our study thus demonstrates that in normal adult rats islet-cell neogenesis can be reactivated by stimulation of pancreatic duct cells.