Fig. 4. Lateral roof plate derivatives from rhombomere 1 show a temporal lag in molecular fate as compared with those from rhombomeres 2-8. (A-F) Dorsal view of whole embryos processed to detect Kcne2 mRNA (A-C) or Ttr mRNA (D-F). Broken lines demarcate the rhombic lip. Kcne2 or Ttr transcripts were undetectable in the medial hindbrain roof plate epithelium (hRPe) field at all time points, but were readily detectable in the lateral hRPe field, except in those lateral cells derived from rhombomere 1 (r1). (G) Dorsal view of E12.5 doubly transgenic En1::cre; Cre-responsive ßgal indicator embryos stained with X-gal to identify r1-derived cells. White broken lines indicate the levels of transverse sections shown in H-S (h,i,j,k); black broken lines demarcate the rhombic lip. (H-S) Serial transverse sections taken from doubly transgenic En1::cre; Cre-responsive ßgal indicator embryos; one set was processed for Ttr mRNA detection, the other for X-gal detection of ßgal. (H-K) Serial sections reveal that r1-derived ßgal+ hCPe (red arrowheads) lacks Ttr activity at E12.5, whereas caudal (r2-r8-derived) ßgal-negative (ßgal-) hCPe (white arrowheads) is Ttr+. (L-S) By E13.5, all hCPe was Ttr+, whether derived from r1 (red arrowheads, ßgal+) or from r2-r8 (white arrowheads, ßgal-negative). (T) Schematic of three hRPe fields at E11.5: field 1 (yellow) is medially located, and is Ttr- and Kcne2-; field 2 (light blue) is laterally located but caudally derived (r2-r8), and is Ttr+ and Kcne2+ from E9.5 onwards; field 3 (dark blue) is laterally located but rostrally (r1)-derived, and is Ttr+ and Kcne2+ after E12.5 only. CPe, choroid plexus epithelium; hb, hindbrain; ov, otic vesicle.