Fig. 5. QCPN staining indicates that FNP outgrowths occur at dorsoventral compartment boundaries. (A) Schematic view of a stage 25 host FNP illustrating two types of boundaries. One type of boundary is created between quail and chick tissues (i.e. a heterospecific junction); these are indicated with blue arrows. Another type of boundary is created by the juxtaposition of dorsal and ventral ectoderm; we refer to these as dorsoventral compartment boundaries and they are indicated with red arrowheads. (B) QCPN immunostaining identified ectodermal domains comprised of quail FEZ and chick FNP ectoderm. The colored lines indicate both the polarity and source of the ectoderm. Dorsal ectoderm is green and ventral is yellow, and quail tissues are indicated with darker lines. C, the transplanted FEZ; D, a new FEZ created by the juxtaposition of chick dorsal and quail ventral ectoderm (shown at higher magnification in C,D. Note that the junction of chick dorsal and quail dorsal ectoderm (blue arrowhead) did not result in an outgrowth. (C) Quail-positive ectoderm spanned the dorsoventral boundary of the ectopic outgrowth (quail nuclei are black; the broken red line demarcates the epithelial-mesenchymal interface). (D) The juxtaposition of quail ventral ectoderm and chick dorsal domains resulted in the creation of a chimeric FEZ. mes, mesenchyme; et, egg tooth.