Fig. 1. The pharyngeal pouch endoderm supports a two-dimensional web of supra-cellular actin cables. Side views of embryos (A-F) where Bmp7 expression (A-C) within the pharyngeal pouch endoderm has been used to highlight the elaboration of pouches along the proximodistal axis and phalloidin staining (D-F) has been used to visualise f-actin within the pouch endoderm. (A) At stage 14, two pharyngeal pouches (1pp and 2pp) have formed and begun to elongate along the proximodistal axis. (B) At stage 15, the third pharyngeal pouch (3pp) is evident. (C) At stage 18, all three pouches (1pp, 2pp and 3pp) have further elongated and display typical narrow slit-like pouch morphology. (D-F) Localised accumulation of actin is seen within the endoderm of each pouch as they undergo proximodistal elongation. (G) High magnification view of the third pharyngeal pouch (3pp) shows actin organised into a supracellular actin cable (red arrowhead), assembled along the apical margin of the endodermal cells (basal margin indicated by an asterisk). (H) Longitudinal section through the pharyngeal region, (at level indicated in F), showing pouches as distinct outpocketings of endoderm. The pharyngeal endoderm is described as pouch endoderm (PE) or interpouch endoderm (IPE). A two-dimensional web of supracellular cables, which follows the lumenal contours of the pouch endoderm, appears to be localised to regions where pouches are forming (red arrowhead) or have just formed, but at lower abundance in the interpouch endoderm (IPE). OV, otic vesicle; aa, arch artery. Anterior is towards the left.