Mechanisms of foregut development and malformations
Oesophageal atresia (OA) and tracheo-oesophageal fistula (TOF) are important human malformations of the foregut, the development of which is poorly understood. In this thesis, the mechanisms that underlie the development of OA/TOF, as well as normal tracheo-oesophageal development, were investigated. The rat model of OA/TOF, based on exposure of embryos to the anticancer agent Adriamycin, was adapted to the mouse in order to allow more in depth study of the cellular and molecular events that underlie the malformations. The Adriamycin-treated mouse was shown to have persistence of an undivided foregut at El 1.5, in contrast to saline controls in which the foregut was in the process of separating into a ventral structure (trachea) and a dorsal structure (oesophagus). This failure of tracheo-oesophageal separation was also a feature of the Sonic hedgehog (Shh) null mutant mouse. The study of the respiratory marker Nkx2.1 confirmed that the foregut normally divides along the dorsoventral boundary of respiratory/ gastrointestinal specification and that the fundamental defect in both the Adriamycin-treated and Shh null mutant mice is persistence of an undivided foregut in which both the respiratory and gastrointestinal lineages are represented, with a preservation of the dorsoventral pattern of expression of that marker. The study of expression of Shh showed this to have a dorsoventral pattern that was closely related to the separation boundary and which changed as separation progressed. Moreover, this dorsoventral expression pattern was disturbed in those Adriamycin-treated embryos that had failed to separate the trachea and oesophagus. The process of tracheo-oesophageal separation was also found to be associated with a distinct pattern of programmed cell death (PCD) in the dorsal foregut and at the dorsoventral boundary. PCD cells were present before any morphological evidence of separation, suggesting a possible role for PCD in controlling the separation process. When a PCD inhibitor was applied to an in vitro, whole embryo, culture system, the process of tracheo-oesophageal separation was arrested, suggesting that PCD is a requirement and not just a consequence of separation.