A model of early molecular regionalization in the chicken embryonic pretectum

The pretectal region of the brain is visualized as a dorsal region of prosomere 1 in the caudal diencephalon, including derivatives from both the roof and alar plates. Its neuronal derivatives in the adult brain are known as pretectal nuclei. The literature is inconsistent about the precise anteroposterior delimitation of this region and on the number of specific histogenetic domains and subdomains that it contains. We performed a cross-correlated gene-expression map of this brain area in chicken embryos, with the aim of identifying differently fated pretectal domains on the basis of combinatorial gene expression patterns. We examined in detail Pax3, Pax6, Pax7, Tcf4, Meis1, Meis2, Nkx2.2, Lim1, Dmbx1, Dbx1, Six3, FoxP2, Zic1, Ebf1, and Shh mRNA expression, as well as PAX3 and PAX7 immunoreaction, between stages HH11 and HH28. The patterns analyzed serve to fix the cephalic and caudal boundaries of the pretectum and to define three molecularly distinct anteroposterior pretectal domains (precommissural, juxtacommissural, and commissural) and several dorsoventral subdomains. These molecular specification patterns are established step by step between stages HH10 and HH18, largely before neurogenesis begins. This set of gene-architectonic data constitutes a useful scaffold for correlations with fate maps and other experimental embryologic results and may serve as well for inquiries on homologies in this part of the brain.