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

Pancreatic organogenesis has been a classic example of epitheliomesenchymal interactions. The nature of this interaction, and the way in which endocrine, acinar and ductal cell lineages are generated from the embryonic foregut has not been determined. It has generally been thought that mesenchyme is necessary for all aspects of pancreatic development. In addition islets have been thought to derive, at least in part, from ducts. We microdissected 11-day embryonic mouse pancreas and developed several culture systems for assays of differentiation: (i) on transparent filters; (ii) suspended in a collagen I gel; (iii) suspended in a basement membrane rich gel; (iv) under the renal capsule of an adult mouse. Epithelia were grown either with or without mesenchyme, and then assayed histologically and immunohistochemically. Epithelium with its mesenchyme (growth systems i-iv) always grew into fully differentiated pancreas (acinar, endocrine, adn ductal elements). In the basement membrane-rich gel, epithelium without mesenchyme formed ductal structures. Under the renal capsule of the adult mouse the epithelium without mesenchyme exclusively formed clusters of mature islets. These latter results represent the first demonstration of pure islets grown from early pancreatic precursor cells. In addition, these islets seemed not to have originated from ducts. We propose that the default path for growth of embryonic pancreatic epithelium is to form islets. In the presence of basement membrane constituents, however, the pancreatic analage epithelium appears to be programmed to form ducts. Mesenchyme seems not to be required for all aspects of pancreatic development, but rather only for the formation of acinar structures. In addition, the islets seem to form from early embryonic epithelium (which only express non-acinar genes). This formation occurs without any specific embryonic signals, and without any clear duct or acinus formation.

A transition in transcriptional activation by the glucocorticoid and retinoic acid receptors at the tumor stage of dermal fibrosarcoma development.

In transgenic mice harboring the bovine papillomavirus genome, fibrosarcomas arise along an experimentally accessible pathway in which normal dermal fibroblasts progress through two pre-neoplastic stages, mild and aggressive fibromatosis, followed by a final transition to the tumor stage. We found that the glucocorticoid receptor (GR) displays only modest transcriptional regulatory activity in cells derived from the three non-tumor stages, whereas it is highly active in fibrosarcoma cells. Upon inoculation into mice, the aggressive fibromatosis cells progress to tumor cells that have high GR activity; thus, the increased transcriptional regulatory activity of GR correlates with the cellular transition to the tumor stage. The intracellular levels of GR, as well as its hormone-dependent nuclear translocation and specific DNA binding activities, are unaltered throughout the progression. Strikingly, the low GR activity observed in the pre-neoplastic stages cannot be overcome by exogenous GR introduced by co-transfection. Moreover, comparisons of primary embryo fibroblasts and their transformed derivatives revealed a similar pattern--modest GR activity, unresponsive to overexpressed GR protein, in the normal cells was strongly increased in the transformed cells. Likewise, the retinoic acid receptor (RAR) displayed similar differential activity in the fibrosarcoma pathway. Thus, the oncogenic transformation of fibroblasts, and likely other cell types, is accompanied by a striking increase in the activities of transcriptional regulators such as GR and RAR. We suggest that normal primary cells have a heretofore unrecognized capability to limit the magnitude of induction of gene expression.