(a–d) In Sox172A-iCre/+;R26R/+ mice, all endoderm-derived lineages, as well as vascular endothelial cells and the hematopoietic system, constitutively express β-galactosidase (Engert et al., 2009). Serial sections of an E19.5 Sox172A-iCre/+;R26R/+ mouse lung show that X-gal labels PNECs (a,b; black arrowheads), whose identity is confirmed by serotonin expression (c,d; white arrowheads). The serotonin-positive cells are clearly all in the epithelium, which is entirely X-gal-positive, although there is some variation in staining level from cell to cell. (e–e’’’) A high-power view of a cluster of Ascl1-expressing PNECs in a section of an E16.5 Sox172A-iCre/+;R26RtdTomato mouse lung, in which endoderm-derived lineages express tdTomato. Only the occasional PNEC is serotonin-positive at this stage. The Ascl1-expressing PNECs are tdTomato-positive, i.e., endoderm-derived. (f–i’’) An endodermal contribution to putative NECs in Xenopus was investigated by performing focal DiI injections into the anterior endoderm at stage 14 (f), as described in Chalmers and Slack (2000). At stage 45, DiI labels the endoderm lining the orobranchial cavity (g,h), and serotonergic cells (putative NECs, arrowheads) in the orobranchial epithelium (i–i’’). (j–l) In Tg(sox17:creERT2;cmlc2:DsRed);Tg(-3.5ubi:loxP-GFP-loxP-mCherry) zebrafish, the endoderm is labeled with mCherry and (m–n’’) NECs in the gill filaments are mCherry-positive (arrowheads). 5-HT, serotonin; A, anterior; a, airway; Bac, branchial arch cartilage; Gf, gill filament; Obc, orobranchial cavity; P, posterior; tdTom, tdTomato. Scale-bars: 50 μm in a,c,g,k,m; 25 μm in b,d,h,i,l,n; 20 μm in e.