Lineage-specific morphogenesis in the developing pancreas: role of mesenchymal factors

Pancreatic development and maturation of the islet B cell. Studies of pluripotent islet cultures

Pancreas organogenesis is a highly regulated process, in which two anlage evaginate from the primitive gut. They later fuse, and, under the influence of the surrounding mesenchyme, the mature organ develops, being mainly composed of ductal, exocrine and endocrine compartments. Early buds are characterized by a branching morphogenesis of the ductal epithelium from which endocrine and exocrine precursor cells bud to eventually form the two other compartments. The three compartments are thought to be of common endodermal origin; in contrast to earlier hypotheses, which suggested that the endocrine compartment was of neuroectodermal origin. It is thus generally believed that the pancreatic endocrine-lineage possesses the ability to mature along a differentiation pathway that shares many characteristics with those of neuronal differentiation. During recent years, studies of insulin-gene regulation and, in particular, the tissue-specific transcriptional control of insulin-gene activity have provided information on pancreas development in general. The present review summarizes these findings, with a special focus on our own studies on pluripotent endocrine cultures of rat pancreas.

Transdifferentiation of human islets to pancreatic ductal cells in collagen matrix culture

Transdifferentiation is a change from one differentiated phenotype to another, involving morphological and functional phenotypic markers. Stability of the cellular phenotype is probably related to the extracellular milieu, as well as cytoplasmic and nuclear components that interact to control gene expression, and the conversion of cell phenotype is likely to be accomplished by selective enhancement of gene expression, which controls the terminal developmental commitment of cells. In this paper, we show the induction of cultured human islets cells to alter their usual phenotypic expression and attain morphological and functional characteristics of duct cells. Islets were isolated by collagenase digestion of pancreata that were removed from cadaveric organ donors. The islets were purified on a two-step density gradient of bovine serum albumin and were then placed into a three-dimensional rat-tail collagen gel matrix supplemented with NuSerum epithelial growth factor and cholera toxin. During the initial 96 h of culture, the islets underwent a cystic transformation that was associated with (1) the maintenance of immunoreactivity for neuron-specific enolase, an endocrine cell marker, but a progressive loss of insulin gene expression, (2) a loss of immunoreactivity for insulin protein, and (3) the appearance of CK-19, a marker for ductal cells. After the transformation was complete, the cells had the ultrastructural appearance of primitive duct-like cells. Cyst enlargement after the initial 96 h was associated, at least in part, with cell replication, as reflected in the 1500% increase in the incorporation of tritiated thymidine. These experiments are consistent with the transdifferentiation of an islet cell to a ductal cell. The exact mechanisms involved still need to be fully elucidated.