P88. Secondary neurulation: The neural tube directly formed from mesenchymal cells in the posterior region of body

Abstract

In the posterior region of vertebrate body (posterior to hind limb), the neural tube forms by the process called “secondary neurulation (SN)”. The SN exhibits several characteristic features. First, the SN proceeds by mesenchymal-to-epithelial transition (MET), the process contrasting with that of the primary neurulation in which an epithelial sheet of cells folds to form a neural tube. Second, the neural tube formed by SN has the anterior limit demarcated at the 27th somite-level where it anteriorly abuts on the neural tube formed by the primary neurulation. However, the mechanisms of SN remain largely unknown. With electroporation techniques, we have recently succeeded in introducing exogenous genes specifically into the region known to undergo SN (“prospective SN region”). EGFP-introduced SN-cells participate to the formation of neural tube with the anterior limit at the 27th somite-level. The prospective SN region specifically expresses Pax2 but not Tbx6L. To understand the role of Psx2 in SN, we have blocked the function of Pax2, and found that it abrogates the SN process where the MET fails to occur. In contrast, when ectopically expressed in early presomitic mesoderm (PSM), where Tbx6L would normally be present, Pax2 causes a formation of SN-like neural tube by repressing Tbx6L. Such phenomena seen in chickens are consistent with those reported for Tbx6 knockout mice. These findings suggest that during the SN processes Pax2 represses mesodermal genes (i.e. Tbx6L), and also regulates a set of genes important for cell epithelialization and tubular formation. We will also discuss a possible link of SN to Hox genes in forming the posterior body structure.