A transgenic marker mouse line labels Cajal-Retzius cells from the cortical hem and thalamocortical axons.

Cajal-Retzius (CR) cells are among the earliest born cortical neurons and are required for normal cortical development in rodents and humans; however, their embryonic origin has been controversial. Recent genetic lineage studies and direct visualization of migration of CR cells have demonstrated multiple germinative sources for CR cells on the edges of the developing pallium. We generated transgenic mice using 5' untranslated regions of the Frizzled10 gene in order to mark the cortical hem (the most caudomedial edge of the telencephalic neuroepithelium) and found that these mice faithfully reproduce the previously described expression pattern of Frizzled10 mRNA in the cortical hem, dorsal thalamus and dorsal neural tube. In the cortical hem, expression of LacZ mRNA was confined to the ventricular zone and perdurance of LacZ protein served as lineage marker for CR cells derived from the hem during embryonic life. When these marker mice were crossed with FoxG1 (BF1) mutants, they confirmed the previous finding that in these mice the cortical hem is expanded leading to increased production of CR cells from the medial wall of the cortex.