Regulating tooth size and spacing. Thin sections of Metriaclima zebra (7 days post-fertilization; 3–4-teeth stage). (A)-(C) eda expression within the mesenchyme (black arrowheads) surrounding the tooth germs (red dashed circle or black arrows; numbers represent the order of appearance for the tooth shown). Note the expression of eda is restricted to the mesenchyme and is not present in the epithelium. (D)-(F) The expression of wnt7b is restricted to the epithelium (D), (E) (black arrowheads) either side of the thickened dental epithelium (black arrow) and in inter-row space (white arrow). (F) A dentary tooth germ (red dashed circle, tooth 1) devoid of wnt7b expression. Expression is restricted to the non-dental epithelium (black arrowhead); numbers 1–3 refers to the order of tooth appearance in the series; asterisk denotes the dental epithelium that will express shh (see Figure 3G–I) and lacks wnt7b. wnt7b expression is present lingually restricting the second odontogenic band (white arrow, (D) and (F); see Figure 5). Sections cut at a thickness of 25 μm. Scale bar in (A) is 20 μm; all panels are to the same scale. Mc, Meckel's cartilage; pmc, premaxillary cartilage.