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 (supr a- and infralabial, eyelid edges, frontal facing surfaces, and mental chin). These dome-shape scale receptors usually occur singly on keeled scales. Frequently, several receptor-bearing scales are grouped aroun d an enlarged, receptor-bearing spine scale, thus forming multiple-sca le complexes. Each dome-shape scale organ is delimited by a circular g roove (figured in stereoscopic SEM). Ultrastructural examination of th ese receptors in Phrynosoma modestum shows them to have characteristic s of reptilian intraepidermal mechanoreceptors. Each dome-shape scale organ is covered by thinned alpha- and beta-layers of epidermal kerati n. The beta-layer forms a thickened ring immediately outside the circu lar groove and is very thin within the groove. The epidermal portion o f the domed receptor contains columnar cells with numerous desmosomal connections. Myelinated nerve fibers and melanophore processes travers e these columnar cells, extending upward from a dermal papilla to just below the oc-layer of the dome-shape receptor. Distally, the nerve fi bers enlarge to form discoidal nerve terminals, which are surrounded b y desmosomes, forming a tonofibrillar basket. Dome-shape scale recepto rs may function to: (1) detect contact with objects during forward loc omotion, (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 r ainfall. (C) 1996 Wiley-Liss, Inc.