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Fig. 7. Commitment towards olfactory fate occurs concomitant with acquisition of
placodal morphology. (A) Schematic of HH13 quail embryo. Broken
line indicates plane of section in A'. (A') Section through
prospective olfactory placode; white lines indicate grafted ectoderm overlying
lateral plate (see C). (B) Schematic of HH14-15 quail embryo; broken
line indicates plane of section in B'. (B') Ultra-thin
section through olfactory placode (courtesy of Dr John Sechrist, Caltech).
(C) Red oval indicates placodal ectoderm grafted over lateral plate
mesoderm. (C') In section, broken white lines outline host trunk
ectoderm and the red line indicates transplanted ectoderm. no, notochord; da,
dorsal aorta. (D) Grafted HH8 ectoderm expresses Pax6 but not Hu.
(E) Grafted HH10 ectoderm expresses Pax6 and forms few neurons
(arrowhead). (F,F') Grafts of HH13 ectoderm, thicken and
differentiate as Hu+ neurons (cytoplasmic green); some are Dlx3+. Neurons
delaminate and ingress, migrating in loose association (F,F',
arrowheads), some reaching the dorsal aorta (da). Pax7+ (nuclear green,
asterisk) marks quail mesenchymal cells grafted with the presumptive placode
(F) and host dermamyotome (F'). Pax7 and Hu show no expression overlap
in these sections. (G-G'') Grafts of thickened placodal ectoderm
(HH14-15) express Dlx3 (blue) and Pax6 (not shown), invaginate and
morphologically resemble an endogenous olfactory pit. Arrowheads indicate Hu+
neurons that ingress from the placode and migrate as a chain. No Pax7+ cells
were detected in this graft.
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