Fig. 4. Delta functions are required for hypochord development. (A-F) Side views, anterior towards the left, of the trunk region of 24-26 hpf embryos probed for col2a1 expression to mark floor-plate (fp) and hypochord (hc) cells. (A) Wild-type embryo showing the normal pattern of floor-plate and hypochord cells. (B) Homozygous mutant aei^AR33 embryo with a deficit of hypochord cells. Floor plate appeared normal. (C) Wild-type embryo injected with dlc antisense morpholino oligonucleotides (MO). Hypochord cell number was reduced, whereas floor plate appeared normal. (D) Homozygous mutant aei^AR33 embryo injected with dlc MO. Most hypochord cells were absent, whereas floor plate appeared normal. (E-G) Dorsal views, anterior towards the top, of 9.5 hpf embryos from aei^AR33/+ intercross injected with dlc MO. notch5 expression was normal (E), whereas her4 expression was reduced (F) or absent (G) (compare with Fig. 2D). Embryos in F,G were double-labeled to reveal pax2.1 expression in pronephros (asterisks), which was normal. (H-K) Dorsal views, anterior towards the top, of 10.5 hpf embryos probed for isl1 expression to reveal prospective Rohon-Beard (RB) neurons and primary motoneurons (pmn). (H) Wild-type, uninjected embryo. (I) aei^AR33 mutant embryo showing small increase in the number of prospective primary motoneurons and RBs. (J) Embryo from aei^AR33/+ intercross injected with dlc MO. The neural phenotype is similar to that of the uninjected aei^AR33 mutant embryo shown in I. (K) Embryo from aei^AR33/+ intercross injected with dla MO. The number of prospective primary motoneurons and RB cells was greater than the embryos shown in I,J. Scale bar: 20 µm in A-D; 80 µm in E-K.

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