Coelomic epithelium is necessary for containing organ-specific diffusible signals

Introduction: During embryogenesis, developing organs are in direct apposition to one another. The coelomic epithelium (peritoneal anlage) has barrier properties and its removal adversely affects development. We hypothesize that coelomic epithelium contains signals within a developing organ to prevent inappropriate attenuation of signals. Methods: Pancreas was harvested from day 11 mouse embryos. Several experimental systems were used: apposition of organs with and without enveloping coelomic epithelium; transfilter experiments; cultures to allow regrowth of coelomic epithelium in the absence of underlying mesenchyme. Results: Differentiation of embryonic pancreas in the presence of mesenchyme, but absence of coelomic epithelium, led to decreased acinar tissue and overabundance of endocrine differentiation. These results suggest a loss of acinar-specific mesenchymal factors, and support coelomic epithelium as a barrier that contains signals. Isolated pancreatic epithelium was unable to differentiate significantly if coelomic epithelium-enveloped mesenchyme on the opposite side of a filter. Importantly, however, endocrine differentiation occurred if the coelomic epithelium had been removed from the mesenchyme on the opposite side of the filter, confirming a role for coelomic epithelium in containing signals. An instructive role for coelomic epithelium was ruled out using a culture system in which coelomic epithelium regrows over epithelium in the absence of mesenchyme. Here, no differentiation of the underlying epithelium was seen. Conclusions: These results support a novel concept in developmental biology, wherein coelomic epithelium serves to compartmentalize organs during growth, preventing diffusion out of critical morphogenic signals.