Mouse Fgf8‐Cre‐LacZ lineage analysis defines the territory of the postnatal mammalian isthmus

The isthmus is recognized as the most rostral segment of the hindbrain in non‐mammalian vertebrates. In mammalian embryos, transient Fgf8 expression defines the developing isthmic region, lying between the midbrain and the first rhombomere, but there has been uncertainty about the existence of a distinct isthmic segment in postnatal mammals. We attempted to find if the region of early embryonic Fgf8 expression (which is considered to involve the entire extent of the prospective isthmus initially) might help to identify the boundaries of the isthmus in postnatal animals. By creating an Fgf8‐Cre‐LacZ lineage in mice, we were able to show that Fgf8‐Cre reporter expression in postnatal mice is present in the same nuclei that characterize the isthmic region in birds. The ‘signature’ isthmic structures in birds include the trochlear nucleus, the dorsal raphe nucleus, the microcellular tegmental nuclei, the pedunculotegmental nucleus, the vermis of the cerebellum, rostral parts of the parabrachial complex and locus coeruleus, and the caudal parts of the substantia nigra and VTA. We found that all of these structures were labeled with the Fgf8‐Cre reporter in the mouse brain, and we conclude that the isthmus is a distinct segment of the mammalian brain lying caudal to the midbrain and rostral to rhombomere 1 of the hindbrain. Fgf8‐Cre reporter expression in postnatal mice is present in the same structures that characterize the isthmic region in birds (e.g. trochlear nucleus, dorsal raphe nucleus, cerebellar vermis). We assert that the Fgf8‐Cre reporter defines a mammalian isthmic territory lying between midbrain and first rhombomere, and extending into the cerebellar vermis.