Relationship of Lrrn1 expression to neuromere boundaries. (a-e, g-h) Flatmounted or (f, i-k) vibratome sections showing Lrrn1 expression in the CNS of chick (a-d, f-k) and mouse (e) embryos. (a) Hindbrain preparation showing strong Lrrn1 expression restricted to a ventral domain with a clear dorsoventral boundary (white arrowhead). Note strong expression from rhombomere 2 (r2) caudally, but downregulation in rhombomere 1 (r1) and at interrhombomeric boundaries. (b) At HH18 downregulation has occurred in the core of the rhombomeres but strong expression is seen at boundaries, for example, at the r2/3 and r3/4 boundaries (black arrowheads) and in bilateral longitudinal stripes (white arrowheads). (c) At HH22 two further longitudinal stripes are evident medially (white arrowheads) and downregulation can be seen in r3 (black arrow). (d) A similar pattern persists at HH25. (e) Expression of mouse Lrrn1 in the hindbrain at e9.5 with a strong ventral distribution, bearing little relationship to that in the chick. (f) Frontal section of a HH18 chick embryo shows an additional site of expression in the ventral retina, but not in the lens primordium. (g) Hemisected, flatmounted preparation of the CNS with the eye removed and viewed from the ventricular surface. Strong Lrrn1 expression is evident in the midbrain (MB) and telencephalon (Tel). Downregulation can be seen at the mid-hindbrain boundary (MHB), in the thalamus at the presumptive ZLI (pZLI) and in the optic stalk (OS). (h) At HH19 downregulation has occurred throughout the basal plate of the forebrain and midbrain and is clearly distinguishable at principal neuromeric boundaries: telencephalic-diencepahlic boundary (TDB), mid-diencephalic boundary (ZLI), diencephalic-midbrain boundary (DMB), and the mid-hindbrain boundary (MHB). (i) Longitudinal section taken through the hindbrain at HH17 (white line in (a) marks plane of section). Positions of rhombomeres (r2–r5) are marked, confirming the existence of downregulation at interrhombomeric boundaries (black arrowheads). (j) Subsequent expression at the interface between rhombomeres can be seen at HH19 (black arrowheads). (k) Longitudinal section through the forebrain at the same stage (white line in (h) marks plane of section) showing downregulation at the TDB and ZLI. Scale bars = 200 μm.