Fig. 2. Delta1 regulates neuroblast production by lateral inhibition. (A,B) Whole-mount view of dissected ears from stage HH12 embryos cultured for 24 hours in control (A) or DAPT-supplemented (B) medium then processed for Delta1 in situ hybridisation. In control embryos, Delta1 is expressed in scattered cells located in the anterior and medial region of the otic cup, corresponding to the delaminating neuroblasts. In DAPT-treated specimens, expression of Delta1 is more intense in the anterior region of the otic cup, and cells expressing Delta1 contact one another, but the size of the neurogenic patch is reduced (square brackets). (C-E) Transverse views of the otic cup of stage HH12 embryos cultured for 24 hours in control (C) or DAPT-supplemented (D,E) medium, and immunostained for three proteins: Delta1 (blue), Islet1 (green) and TuJ1 (red). In DAPT-treated embryos, an abnormally large number of Islet1-positive neuroblasts delaminate from the otic cup and accumulate in the underlying mesenchyme (arrows in D, compare with C). In control specimens (C), Delta1 protein is almost undetectable, whereas its expression is increased in the inner ear (D,E), as it is in the neural tube (asterisk in D), of DAPT-treated embryos. (E) In DAPT-treated embryos, Islet1- and TuJ1-positive neuroblasts are frequently found in the lumen of the otic cup (arrow). (F-I) Dorsal views (anterior is up) of stage HH11 embryos cultured for 4 hours in control (F,G) or DAPT-supplemented (H,I) medium then processed for whole-mount Delta1 in situ hybridisation. A short DAPT treatment induces a strong upregulation of Delta1 in the neural tube (asterisk) and in the otic cup (arrow in F,H). Closer examination of the anterior part of the otic cup shows that the delaminating neuroblasts are scattered in control embryos, but are more numerous and are frequently organised as cell clusters in DAPT-treated embryos (arrows in G,I). A, anterior; D, Dorsal.