Figure 1.

Scshh expression related to early pattern development of the lower jaw dentition. (a) Histology with Mallory's trichrome stain of a transverse section through the lower jaw before tooth formation, with a thickened basal layer of epithelium forming the primary OB (arrowheads) associated with a condensation of ectomesenchyme cells. All are located close to the depression in the jaw cartilage (ca) where the tooth families will be situated. (b) H&E section through the dental lamina (dl) with forming tooth germs deep into the oral epithelial surface (oe), alongside the jaw cartilage, same orientation as in (a). This shows one tooth germ with cusp tissue (arrowheads) below the differentiated cells of the inner dental epithelium around a dental papilla (dp). This is the first tooth of the family; deeper on the dental lamina is the early tooth germ of a sequential, alternate series tooth (arrow). (c) H&E of coronal section, right side with extensive dental lamina and five tooth germs (t₁–t₅) of both even and odd jaw positions (see the electronic supplementary material, fig. a). Tooth germ stages show the position of forming cusp tissue (t₁) in the first tooth germ, to the stage of incipient tooth germ (t₅). All form on the dental lamina in contrast to the later-developing, superficial dental papillae of skin denticles (arrowheads). (d–h) Oral surface views of whole-mount in situ-hybridized ssh expression in a stage 30–31 series. All show intense expression localization at the oral–aboral boundary to form the primary odontogenic field and later OB. The characteristic cupid's bow shape is at the earliest time with labiolingual intensity gradient of ssh expression in the epithelium representing the primary dental field (d). (e,f) A later stage of odontogenic field expression, equivalent to the OB in (a). There is a posterior extension of the OB (right arrow) and reduction to a thinner intense zone at the outer margin; the difference between the left and right sides may be due to an asymmetric timing for the downregulation of shh. Weaker expression of an inner parallel but defined band may account for the position of the later development of the lingual side of the dental lamina (figure 2l,m). At the stage in (d), there is a gap in the high-intensity dental field expression at the symphysis (sy). The next stage (e–h) shown in close up (g,h), where a midline joining of the shh expression bands forms into groups of cells making the symphyseal subfield (white arrows) in the jaw midline. The line X–Y is shown in section (i) with a distinct absence of expression in the epithelial cells in the midline between the left and right OBs (le.ob and rt.ob). (g,h) Increased magnifications of (e,f) stage OB, where expression is localized to clusters of epithelial cells in the anterior region to form the putative tooth family clones (white arrow); line X–Y is the field of (h,i). (i–k) Sections cut at 7 μm through the stage in (e,f) confirm the basal epithelial location of expression (arrowhead). (l,m) Sections cut at 50 μm through the posterior region of the stage in (e,f) show restriction of intense expression to part of epithelial OB in (a), with sharp junction at the aboral–oral boundary (arrow) located immediately above the jaw cartilage (ca), where a dental lamina will develop before any tooth germs form (figure 3a).