Fig. 3. Expression of bts1 during gastrulation and early somitogenesis, as revealed by whole-mount in situ hybridization at the stages indicated (% of epiboly). (A-F) Dorsal views, anterior towards the top; (G-J) sagittal views, anterior towards the left. Open arrowheads indicate the blastoderm margin, black arrowheads the mid-hindbrain domain, and small arrows point at hypoblastic expression. (A',C',D',D'') Sagittal sections at the levels indicated, dorsal towards the right, anterior towards the top. bts1 expression is first detected at 30% epiboly (A,A') along the ventral and lateral margins of the blastoderm (arrows) and in the yolk syncitial layer (small arrow). During gastrulation (B-F), expression is maintained in the posterior epiblast up to a sharp limit at the mid-hindbrain level, and in hypoblast cells bordering the prechordal plate (small arrow). From the end of gastrulation (F-H), bts1 expression is confined to the mid-hindbrain level and tail bud and extinguishes from the rest of the epiblast. Additional expression sites during later somitogenesis (I,J) include the otic vesicle, somites and diencephalon.
