Fig. 1. Expression of Cyp1B1 in the chick embryo. (A,B) Expression in the ectoderm and mesoderm (black arrowheads) of the posterior primitive streak of a HH4 embryo. Black lines denote plane of sections. (C-F) 4s: transcripts are localised to all of the newly formed somites (D, black arrow), as well as to the mediolateral ectoderm of the sinus rhomboidalis (F, white arrow). Black lines denote planes of section shown in D-F. (G) 6s: at HH9-, expression persists in the somites but is lost from the posterior ectoderm. (H) HH10: transcripts are abundant in the neuroepithelium of the mid-hindbrain boundary (MHB) region (white arrowhead), in addition to weak expression in r2 (blue arrow) and the paraxial mesoderm adjacent to the developing hindbrain (black arrow). (I) HH10: flat-mount preparation showing that the expression extends across the DV axis of the MHB and r2. (J,K) HH10: transverse sections though the hindbrain showing strong expression in both ectoderm and mesoderm, possibly including neural crest-derived cells immediately adjacent to the MHB (blue arrow). (L) HH12+: MHB expression has narrowed to a thin band in the posterior midbrain, and an additional band of neuroepithelial expression is seen at the anterior midbrain (blue arrowhead). Expression is evident in the ectoderm immediately anterior to the otic vesicle (white arrowhead). Black lines denote planes of section shown in N-P. (M) Expression is enriched in the dermomyotome (white arrow). (N,O) Transverse sections showing strong expression of Cyp1B1 in the ectoderm abutting the entire mesencephalic vesicle (white arrows). (P,Q) Cyp1B1 message in the mesoderm adjacent to the hindbrain persists at HH12+ (blue arrow), as does expression in the somites. Cyp1B1 expression is also strong in the notochord (black arrows). The section in Q is more posterior than can be shown in L. (R) 30s: new sites of Cyp1B1 expression emerge, including a highly localised expression pattern in the developing eye (white arrowhead) and the epiphysis (blue arrow). (S) Longitudinal sections show expression in lens vesicle (black arrow) and neural retina (blue arrows). There is continued expression in all of the somites. (T) Transverse section at the level indicated in R shows expression throughout the notochord (black arrow) and somites. (U-W) 41s: other sites of expression include a pocket of mesodermal cells lateral to r2 (blue arrow), the sinus venosus of the heart and asymmetrically in the endoderm of the pharyngeal arches (V, blue arrows). Cyp1B1 is also expressed in the mesoderm of the forelimb bud (W, white arrow). (X) Dorsal view of Raldh2 expression in a 12s embryo. (Y) Cyp26C1 is expressed in r2 (black arrow) in a 12s embryo.