The morphogenetic role of midline mesendoderm and ectoderm in the development of the forebrain and the midbrain of the mouse embryo

The anterior midline tissue (AML) of the late gastrula mouse embryo compris es the axial mesendoderm and the ventral neuroectoderm of the prospective f orebrain, midbrain and rostral hindbrain, In this study, we have investigat ed the morphogenetic role of defined segments of the AML by testing their i nductive and patterning activity and by assessing the impact of their ablat ion on the patterning of the neural tube at the early-somite-stage, Both ro stral and caudal segments of the AML were found to induce neural gene activ ity in the host tissue; however, the de novo gene activity did not show any regional characteristic that might be correlated with the segmental origin of the AML. Removal of the rostral AML that contains the prechordal plate resulted in a truncation of the head accompanied by the loss of several for ebrain markers, However, the remaining tissues reconstituted Gsc and Shh ac tivity and expressed the ventral forebrain marker Nkx2.1. Furthermore, anal ysis of Gsc-deficient embryos reveals that the morphogenetic function of th e rostral AML requires Gsc activity. Removal of the caudal AML led to a com plete loss of midline molecular markers anterior to the 4th somite, In addi tion, Nkx2.1 expression was not detected in the ventral neural tube. The ma intenance and function of the rostral AML therefore require inductive signa ls emanating from the caudal AML, Our results point to a role for AML in th e refinement of the anteroposterior patterning and morphogenesis of the bra in.