Primary motoneurons are reduced in smu and cyc;flh mutants. Lateral views of islet2 (A-D) expression at 17.5-20 h (17-22 somites) in wild-type (A); cyc;flh double mutant (B); smu mutant (C); and smu;syu double mutant (D) embryos. Primary motoneurons are ventral rows of islet-expressing cells (pmns); dorsal rows are Rohon Beard sensory neurons (RBs). Bracket in B demarcates the first 15 somites, considered as anterior in these studies. (E) Average number of islet1- or islet2-expressing spinal cord primary motoneurons plus total number of spinal cord primary motoneurons (islet1 plus islet2 expression) in wild-type embryos at 17-18 h (16-18 somites), and cyc;flh double mutants and smu mutants at 17-24 h. Some primary motoneurons still express both islet1 and islet2 at these stages. In wild-type embryos, primary motoneurons form in a rostrocaudal progression throughout somitogenesis, so counts were done at the same developmental time point. In smu and cyc;flh mutants, primary motoneuron numbers are roughly constant from 17-24 h. Error bars represent s.e.m.; n=number of embryos counted to calculate average. Scale bar: 250 μm.

The less Hh signal, the fewer primary motoneurons, but all embryos still form median floor plate. Lateral views of islet2 expression in blue (A,B,D,E) and α-collagen type II expression in red (A,B) at 18-21 h (18-22 somites) in wild-type uninjected embryo (A); MO-injected syu mutant (B); and embryos from a triple mutant cross (D,E). (B) entirely lacked primary motoneurons but still formed median floor plate. (D) has no islet2-expressing primary motoneurons and is presumably a cyc-/-;flh-/-;syu-/- triple mutant. (E) has only six islet2-expressing primary motoneurons but has a notochord (*) and is probably cyc-/-;flh+/-;syu-/- or cyc-/-;flh+/+;syu-/-. (C) shows total number of embryos from all the morpholino injection experiments that formed six or fewer islet2-expressing primary motoneurons. These are broken down into embryos from syu+/- parents (blue) or syu+/-;smu+/- parents (pink) injected with twhh-MO + ehh-MO and the one syu mutant injected with twhh-MO that formed six primary motoneurons (green). (F) The number of motoneurons in individual triple mutants. These were embryos from cyc+/-;flh+/-;syu+/- parents that had no notochord and five or fewer primary motoneurons. islet2 experiment, n=609: expect about 9.5 triple mutants; observed eight embryos with five or fewer motoneurons. islet1 experiment, n=549: expect about 8.5 triple mutants; observed seven embryos with five or fewer motoneurons. islet1 + islet2 experiment, n=367: expect about 5.7 triple mutants; observed four embryos with four or fewer motoneurons. Scale bar: 100 μm (A,B) and 250 μm (D,E).

Hh signals are required for primary motoneuron induction. Dorsal views of flat mounted preparations of islet1 expression in wild-type (A); smu mutant (B); cyc;flh double mutant (C) and cyc;flh;syu triple mutant (D) embryos at 11-12 h (five to six somites). At this stage, primary motoneurons (pmns) normally form two rows in the median neural plate and Rohon Beard (RB) sensory neurons are visible at the lateral edges of the neural plate. The arrow demarcates a single primary motoneuron in the triple mutant embryo in D. Scale bar: 250 μm.

Ehh is less effective than Shh and Twhh for primary motoneuron formation. Lateral views of islet1 + islet2 expression (both in blue) in wild-type embryo at 24 h (A); cyc;flh double mutant at 24 h (B); flh;syu double mutant at 18-19 h (C); and cyc;syu double mutant at 18-19 h (D). Scale bar: 50 μm.

ZFIN is incorporating published figure images and captions as part of an ongoing project. Figures from some publications have not yet been curated, or are not available for display because of copyright restrictions.