Phrynosoma intraepidermal receptor: A dorsal intraepidermal mechanoreceptor in horned lizards (Phrynosoma; Phrynosomatidae; Reptilia)

Horned lizards of seven species have intraepidermal sensory receptors on many dorsal body scales, some limb scales, and on head scales (supra- and infralabial, eyelid edges, frontal facing surfaces, and mental chin). These dome-shape scale receptors usually occur singly on keeled scales. Fre-quently, several receptor-bearing scales are grouped around an enlarged, receptor-bearing spine scale, thus forming multiple-scale complexes. Each dome-shape scale organ is delimited by a circular groove (figured in stereo-scopic SEM). Ultrastructural examination of these receptors in Phrynosoma modestum shows them to have characteristics of reptilian intraepidermal mechanoreceptors. Each dome-shape scale organ is covered by thinned a- and ß-layers of epidermal keratin. The ß-layer forms a thickened ring immediately outside the circular groove and is very thin within the groove. The epidermal portion of the domed receptor contains columnar cells with numerous desmosomal connections. Myelinated nerve fibers and melanophore processes traverse these columnar cells, extending upward from a dermal papilla to just below the a-layer of the dome-shape receptor. Distally, the nerve fibers enlarge to form discoidal nerve terminals, which are surrounded by desmosomes, forming a tonofibrillar basket. Dome-shape scale receptors may function to: (1) detect contact with objects during forward locomotion, (2) locate attacking ants on the lizard's skin and/or, during subterranean burrowing, (3) determine depth below the surface, or (4) sense vibrations through the soil from surface predators or surface rainfall.