Fig. 1. The nTS and AP do not form in Phox2b^lacZ/lacZ mutants. (A) Position of the mature nTS in the dorsal medulla and connections with the area postrema (AP) and cranial sensory ganglia. A fraction of sensory cells in the petrosal ganglion are postsynaptic to chemosensors of the carotid body. Most sensory cells in the nodose ganglion innervate sub-diaphragmatic organs, such as the gut. (B-I) Development of the nTS from Phox2b⁺ dorsal precursors in the medulla of wild-type embryos. (B-E) Immunohistochemistry for Phox2b, Islet1 and Lmx1b (B-D) and in situ hybridization for Rnx (E) show that Phox2b⁺ precursors at the level of r7 can be subdivided into motoneuron and nTS precursors according to their co-expression of Islet1 (C), or Lmx1b (D) and Rnx (E), respectively. Red cells in C correspond to Islet1⁺/Phox2b^-somatic motoneurons. (F-H) Immunohistochemistry for Phox2b and Lmx1b between E11.5 and E13.5 reveals the migration of both populations towards each other resulting in the formation of the dmnX (asterisk) and nA (arrowhead in F, not visible in G,H) (green), and of the nTS, whose cells co-express Phox2b and Lmx1b (yellow). Red cells correspond to several classes of Lmx1b⁺/Phox2b^- interneurons. (I) Schematic representation of the origin and migratory behavior of the dmnX, nA (red) and nTS (blue) precursors. The boxed areas are those photographed in B-H. (J-U) The nTS and AP do not form in Phox2b^lacZ/lacZ mutants. (J-M) A normal complement of dorsal cells detected by lacZ in situ hybridization are born and migrate ventrally in a homozygous (K,M) compared with a heterozygous (J,L) mutant between E10.5 and E11.5. Note that ventrally, lacZ expression is preserved in the neuroepithelium of Phox2b^lacZ/lacZ embryos (asterisk), where it persists until at least E13.5 (see Q), but not in the mantle layer. This reflects the fact that no bm/vm motor neurons are born (Pattyn et al., 2000b). (N,O) The dorsal emergence of lacZ⁺ cells continues normally in the homozygous mutants at E12.5 but no ventral accumulation occurs, and the dmnX (arrowhead, N) does not form. (P,Q) The entire dorsal vagal complex is absent at E13.5, as assessed by lacZ expression. (R,S) Dorsal view of a hindbrain at E18.5 showing the obex of the fourth ventricle (red arrow) caudally displaced and wider in a homozygous (S) compared with a heterozygous (R) mutant. (T,U) Transverse sections caudal to the obex stained by immunohistochemistry for Phox2b and in situ hybridization for peripherin, showing a loss of tissue in the homozygote (U) affecting the region where the dmnX, nTS and area postrema (i.e. the dorsal vagal complex) are found in the heterozygote (T). Note that the peripherin⁺ somatic motoneurons of the hypoglossal nucleus are preserved in the mutants. AP, area postrema; dmnX, dorsal motor nucleus of the vagus nerve; nA, nucleus ambiguus; nTS, nucleus of the solitary tract.