Permissive Interaction - Pancreas Development
Figure describes the development of pancreas in a mammal. In an organism like mouse the pancreas is first seen as a diverticulum of embryonic gut approximately 9th day of development. As the diverticulum grows it pushes into mesenchyme and branches making blind pockets called acini. The cells of these pancreatic acini distinguish into exocrine cells that produce digestive enzymes-the peptidases, proteases, amylase, lipase etc. Also, certain cells that separate out from pancreatic epithelium make clusters of cells surrounded by mesoderm.
Figure: Permissive Interaction
These cells are the endocrine cells, the islets of Langerhans, generating hormones such as insulin, glucagon and somatostatin. So the exocrine and endocrine cells arise from epithelium of embryonic gut. Growth of pancreas depends on the associated mesoderm. If in a 9-day old embryo, the pancreatic mesoderm and ectoderm are separated from one other, there will not be any additional development. But if they are recombined in a culture medium, both exocrine and endocrine cells differentiate normally. As well, instead of the pancreatic mesenchyme, if the mesenchyme from other regions like mesenchyme from salivary gland region is, substituted general pancreatic cells still differentiate. Somites which produce muscles and cartilage cells can as well induce pancreatic endoderm to develop normally. Even extracts of embryo can induce the pancreatic endoderm. All these results suggest that by 9th day of embryonic life of mouse, the pancreatic endoderm is already determined or committed and just only a non-specific signal from mesoderm is enough for it to complete the development. The mesoderm can be from any region but have to be in proximity to the endoderm.