Fig. 1

Transcription factors expressed in the apical organ. a–l Lv-egr is expressed in the anterior two-thirds of the embryo from 10 to 12 hpf (hours post fertilization) (brackets in a–b) and then comes on in cells in the apical organ starting at 24 hpf (h). m–x Lv-hey is expressed in scattered cells of the vegetal plate from 10 to 12 hpf (arrowheads in n, o). Inset image of o shows posterior view of vegetal plate. One to two cells express Lv-hey in the elongating archenteron at 16 hpf (arrowhead in p). Then, at 24 hpf, Lv-hey is expressed in 2–4 cells in the apical organ (t–x). Lv-hey is also expressed in scattered mesodermal cells at 30 hpf onward (arrowhead in W) and in 1–2 cells in the gut endoderm (arrowhead in x). a′–l′ Lv-elk is expressed lightly in skeletogenic mesodermal cells from 10 to 18 hpf (a′–e′) (arrows). From 20 to 22 hpf, Lv-elk is expressed in the non-skeletogenic mesoderm (brackets in f′, g′). At 24 hpf, Lv-elk turns off in the non-skeletogenic mesoderm and turns back on in a subset of skeletogenic mesoderm (arrow in H′). At 28 hpf, Lv-elk is expressed in 1–2 cells of the apical organ (arrowheads in j′–l′). Scale bars = 50 μm