Expression and role of laminin-1 in mouse pancreatic organogenesis

Previous studies have suggested that basement membrane alone may induce ductal differentiation and morphogenesis in the undifferentiated embryonic pancreas. The mechanism by which this induction occurs has not been investigated. Studies of other organ systems such as the lungs and mammary glands, where differentiation has been shown to be induced by basement membrane, have suggested a major role for laminin as a mediator of ductal or tubular morphogenesis and differentiation. We first defined the ontogeny of laminin-1 in the developing mouse pancreas. To determine the specific role of basement membrane laminin in pancreatic ductal morphogenesis and differentiation, we microdissected 11-day mouse embryonic pancreatic epithelium free from its surrounding mesenchyme and then suspended the explants in a 3-dimensional organ culture to allow us to assay cell differentiation and morphogenesis. When the pancreatic epithelium buds off the foregut endoderm, the pancreatic mesenchyme diffusely expresses laminin-1. This laminin subsequently organizes to the interface between the epithelium and the mesenchyme by E12.5. As gestation progresses, epithelial cells in direct contact with laminin-1 seem to differentiate into ducts and acini, whereas those spared intimate contact with laminin-1 appeared to organize into islets. Although basement membrane gel could induce pancreatic ductal morphogenesis of embryonic pancreatic epithelium, this induction was blocked when we added neutralizing antibodies against any of the following: 1) laminin (specifically laminin-1), 2) the "cross-region" of laminin-1, and 3) the alpha6 moiety of the integrin receptor, which is known to bind laminins. Immunohistochemistry, however, showed that pancreatic duct cell-specific differentiation (carbonic anhydrase II) without ductal morphogenesis was still present, despite the blockage of duct morphogenesis by the anti-laminin-1 neutralizing antibodies. Interestingly, there appeared to be a decrease in carbonic anhydrase II expression over time when the epithelia were grown in a collagen gel, rather than in a basement membrane gel. The pattern of laminin-1 expression in the embryonic pancreas supports the conclusion that laminin-1 is important in the induction of exocrine (ducts and acini) differentiation in the pancreas. Furthermore, our data demonstrate that 1) pancreatic ductal morphogenesis appears to require basement membrane laminin-1 and an alpha6-containing integrin receptor; 2) the cross-region of basement membrane laminin is a biologically active locus of the laminin molecule necessary for pancreatic ductal morphogenesis; 3) duct-specific cytodifferentiation, in the form of carbonic anhydrase II expression, is not necessarily coupled to duct morphogenesis; and 4) the basement membrane gel may contain components (e.g., growth factors) other than laminin-1 that can sustain both carbonic anhydrase II expression and, possibly, the capacity to form ducts, despite the absence of duct structures.