Basement membrane exposure defines a critical window of competence for pancreatic duct differentiation from undifferentiated pancreatic precursor cells.

We previously showed that the undifferentiated pancreatic epithelium can differentiate into islets, ducts, or acini depending on its milieu and that laminin is necessary for pancreatic duct formation. Therefore we wanted to study the plasticity of laminin-induced duct differentiation the better to understand mechanisms of pancreatic duct lineage selection induced by basement membrane. Mouse embryonic pancreases were dissected at gestational day 11 (E11.5), and epithelium was isolated from its surrounding mesenchyme. Some epithelia were cultured in a collagen gel devoid of laminin. These epithelia were "rescued" at days 1-7 of culture by transferring them to a laminin-rich matrix (Matrigel) for 7 additional days. Other epithelia were instead first cultured in Matrigel, and then placed into collagen. Immunohistochemistry was performed for insulin, amylase, and carbonic anhydrase II. Pancreatic epithelia rescued from collagen into laminin during days 1-4 after harvest were still able to form ducts, whereas epithelia deprived of laminin for longer than this 4-day window were not. Pancreatic epithelia exposed to laminin for as little as 1 day, and then placed into collagen, still retained the ability to make ducts. Thus there is a clear cut-off in the development of the pancreatic epithelium at E11.5, after which laminin appears necessary to induce duct formation. We believe that such "windows of competence" in embryonic development imply that developmental programs in the embryo allow some flexibility.