Research Article: Islet Formation during the Neonatal Development in Mice

Date Published: November 6, 2009

Publisher: Public Library of Science

Author(s): Kevin Miller, Abraham Kim, German Kilimnik, Junghyo Jo, Uchenna Moka, Vipul Periwal, Manami Hara, Kathrin Maedler.

Abstract: The islet of Langerhans is a unique micro-organ within the exocrine pancreas, which is composed of insulin-secreting beta-cells, glucagon-secreting alpha-cells, somatostatin-secreting delta-cells, pancreatic polypeptide-secreting PP cells and ghrelin-secreting epsilon-cells. Islets also contain non-endocrine cell types such as endothelial cells. However, the mechanism(s) of islet formation is poorly understood due to technical difficulties in capturing this dynamic event in situ. We have developed a method to monitor beta-cell proliferation and islet formation in the intact pancreas using transgenic mice in which the beta-cells are specifically tagged with a fluorescent protein. Endocrine cells proliferate contiguously, forming branched cord-like structures in both embryos and neonates. Our study has revealed long stretches of interconnected islets located along large blood vessels in the neonatal pancreas. Alpha-cells span the elongated islet-like structures, which we hypothesize represent sites of fission and facilitate the eventual formation of discrete islets. We propose that islet formation occurs by a process of fission following contiguous endocrine cell proliferation, rather than by local aggregation or fusion of isolated beta-cells and islets. Mathematical modeling of the fission process in the neonatal islet formation is also presented.

Partial Text: Insulin-secreting pancreatic beta-cells play a key role in the pathogenesis of diabetes mellitus. Autoimmune destruction of beta-cells results in type 1 diabetes. Functional loss of beta-cells leads to type 2 diabetes, which is one of the most prevalent chronic diseases worldwide. However, the beta-cell ultimately functions by forming an islet of Langerhans, which is a highly vascularized micro-organ [1], [2] consisting of various other endocrine cell types that function together to maintain normoglycemia, that include alpha-cells [3]–[6], delta-cells [3], [7], PP-cells [8], [9] and epsilon cells [10], [11]. It has been reported that cellular composition differs regionally (e.g. the dorsal versus ventral pancreas; 12–14) and the size differs among islets under normal conditions as well as disease states [15]. There is a striking plasticity of islet architecture among various species, also within the same species, and under different physiological conditions such as pregnancy, obesity and inflammation [15], [16]. Although the islet is the functional unit in the regulation of glucose homeostasis, little is known about how pancreatic islets are formed during development. The widely accepted model of formation is by the local aggregation of endocrine cells that migrate from the ductal epithelium in the late embryonic stage [17]–[20]. This aggregation model is based on observations using pancreatic tissue sections. We have previously shown that two-dimensional analysis (i.e. by thinly-cut sections) has certain limitations and could only capture part of larger structures [21], which potentially hampers deducing dynamic islet formation in the pancreas. In the fetal and newborn pancreas, endocrine progenitor cells proliferate by forming cord-like structures without distinct islet formation, and differentiation occurs within these cords. (Note that these are not the pancreatic duct; 22).

A large-scale optical analysis of the entire distribution of beta-cells in situ enabled us to carry out quantitative studies on islet development and further mathematically model this biological event. We propose a new model of islet formation based on the following observations: (1) Endocrine progenitor cells proliferate and differentiate contiguously forming branched cord-like structures in embryos, and beta-cell differentiation occurs within these branches [21]. (2) Beta-cells proliferate contiguously in late embryos and newborns forming an identical branching pattern without forming spherical-shaped islets. (3) Distinct spherical-shaped islets are observed at P3, accompanied with elongated interconnected islet-like structures. (4) The number of such interconnected islet-like structures decreases with further pancreatic development. (5) Alpha-cell lining appears to form the putative cleavage points where islet-like structures are connected to each other. (6) Subsequent beta-cell expansion may occur within islets resulting in increased islet volume and formation of spherical-shaped (not irregular-shaped) islets. (7) Islet fission appears to occur relatively randomly, which results in various sizes of islets in the adult. (8) Mathematical analysis of a large-scale data set (collected from >100 pancreata in situ at 9 time points in the developing pancreas: P1–P21) shows that islet development strikingly fits a lognormal probability density function up to P10. However, beginning at P12 and thereafter, a marked leftward deviation from the lognormal distribution was observed, indicating a regression in the number of elongated structures by fission events.



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