Newly identified patterns of Pax2 expression in the developing mouse forebrain

We determined the expression pattern of Pax2 in the forebrain of wild type mouse embryos between E10.5 and postnatal day (P) 15. Pax2 expression was detected in the septum of the basal forebrain, hypothalamus, eminentia thalami and in the subfornical organ. To evaluate Pax2 as a marker for septal cells, we examined Pax2 expression in Pax6Sey/Sey mutants, which have an enlarged septum. We found that Pax2 clearly marks a population of septal cells equivalent to that seen in wild types, indicating its utility as a marker of septal identity. These cells did not express the GABAergic marker calbindin nor the cholinergic marker choline acetyltransferase and were not detectable after P15.Pax2 is expressed in populations of cells within the developing septum, hypothalamus, and eminentia thalami. It seems especially useful as a marker of the telencephalic septum, because of its early, strong and characteristic expression in this structure. Further, its expression is maintained in the enlarged septum of Pax6Sey/Sey mutants.Pax2 is a member of the Pax family of transcription factors [1,2], characterised by the presence of a paired-type homeodomain [3,4]. Pax2 is expressed in a number of different organs in the developing mouse embryo, including the ureteric bud, kidneys [1,2] and otic vesicle [5]. In the developing nervous system, Pax2 is first detected at embryonic day (E) 7.5 in the neural plate, in the area of the presumptive midbrain-hindbrain region [6]. At E8.0, Pax2 displays a broad expression domain in this region, which by E9.5 is restricted to the isthmus at the midbrain-hindbrain boundary [6]. Pax2 expression in the isthmus ceases after E11 [7]. In the cerebellum, Pax2 is specifically expressed by a subset of cerebellar GABAergic interneurons and their precursors, from E12 until the end of cerebellar development (postnatal day 15) [8]. In the spinal cord, Pax2 expression is found in the intermediate zone as early as E10.5 [5,7]. In the developing eye,Pax2 expression