Figure 1

The columella condensation arises from the proximal region of the second pharyngeal arch. HH18-31 (E3-E7) embryos stained for peanut agglutinin lectin (PNA). Transverse paraffin sections at the level of the second arch, 10-12 μm. A-G, I, J darkfield; H, K-N brightfield. (A) PNA positive cells in second arch derived proximal and distal ectomesenchyme have a uniform appearance (arrowheads), ventral to blood vessels at HH18. Non-chondrogenic mesenchyme cells are PNA negative (asterisk). (B-E) HH19-22 sections show little change in the proximal region, with cochlear duct ventral extension. (F-H) Between HH23 and HH24, continued cochlear duct extension, with reshaping of the proximal condensation between the two blood vessels (arrows, G, H). (I-K) HH25-27, the columella condensation begins to separate from the more distal ectomesenchyme cells that gives rise to hyoid skeleton (opposing arrows, J). (L-M) The columella inserts into the otic capsule to form the oval window/footplate (arrowed). The pharyngeal endoderm (arrowed, N) will later form the tympanic membrane in apposition with surface ectoderm. The basal plate of the chondrocranium also exhibits PNA staining (G-N). Abbreviations: bp-basal plate of chondrocranium, v-blood vessels, cd-cochlear duct, c-columella, fp-footplate, nt-neural tube, oc-otic capsule, ot-otocyst, pe-pharyngeal endoderm. Scale bars: A-E 100 μm, F-L 150 μm, M, N 200 μm.