Expression and role of laminin-1 in mouse pancreatic organogenesis

Previous studies have suggested that basement membrane alone may induce duc tal differentiation and morphogenesis in the undifferentiated embryonic pan creas. The mechanism by which this induction occurs has not been investigat ed. Studies of other organ systems such as the lungs and mammary glands, wh ere differentiation has been shown to be induced by basement membrane, have suggested a major role for laminin as a mediator of ductal or tubular morp hogenesis and differentiation. We first defined the ontogeny of laminin-1 i n the dec-eloping mouse pancreas. To determine the specific role of basemen t membrane laminin in pancreatic ductal morphogenesis and differentiation, we microdissected 11-day mouse embryonic pancreatic epithelium free from it s 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 pancreat ic mesenchyme diffusely expresses laminin-1. This laminin subsequently orga nizes 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 c ontact with laminin-1. appeared to organize into islets. Although basement membrane gel could induce pancreatic ductal morphogenesis of embryonic panc reatic epithelium, this induction peas blocked when we added neutralizing a ntibodies against, any of the following: 1) laminin (specifically laminin-1 ),2) the "cross-region" of laminin-1, and 3) the alpha 6 moiety of the inte grin receptor, which is known to bind laminins. Immnnohistochemistry, howev er showed that pancreatic duct cell-specific differentiation (carbonic anhy drase II) without ductal morphogenesis was still present, despite the block age of duct morphogenesis by the anti-laminin-1 neutralizing antibodies. In terestingly there appeared to be a decrease in carbonic anhydrase II expres sion 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 emb ryonic pancreas supports the conclusion that laminin-1 is important iu the induction of exocrine (ducts and acini) differentiation in the pancreas. Fu rthermore, our data demonstrate that 1) pancreatic ductal morphogenesis app ears to require basement membrane laminin-1 and an alpha 6-containing integ rin receptor; 2) the cross-region of basement membrane laminin is a biologi cally active locus of the laminin molecule necessary for pancreatic ductal morphogenesis; 3) duct-specific cytodifferentiation, in the form of carboni c anhydrase II expression, is not: necessarily coupled to duct morphogenesi s; and 4) the basement membrane gel may contain components (e.g., growth fa ctors) other than laminin-1 that can sustain both carbonic anhydrase II exp ression and, possibly the capacity to form ducts, despite the absence of du ct structures.