To refer to this article use this url: http://dpc.uba.uva.nl/ctz/vol76/nr04/art01
Introduction
Large cells characterized by translucent cytoloplasm and involved in secretory or mechanical roles have been described in the epidermis of amphibian larvae (Fox, 1988). Gregarious bufoniid tadpoles possess giant cells (Riesenzellen, RZn) with basal-apically arranged organelles and secretory granules resembling the products of serous glands in adult toad skin (Delfino, 1991; Delfino et al., 1995a, b). Giant cells synthesize alarm substances that, once released into the environ (Fox, 1988), trigger fright reaction in tadpoles (Pfeiffer, 1974).
Larval urodeles (Kelly, 1966; Fährmann, 1971a, b; Greven, 1980; Fox, 1988) as well as caecilians (Fox, 1986a, b; Breckenridge et al., 1987; Fox,1988) possess spheroid-ellipsoidal cells (Leydig cells, LCs) that exhibit a complex, peripheral cytoskeleton (Langerhans net) of tonofilaments (Fährmann, 1971a, b; Greven, 1980; Rosenberg et al., 1982; Kantorek and Cle-men, 1990). LC cytoplasm exhibits a central-peripheral polarization: the inner, perinuclear region (Hofcytoplasma) contains biosynthesis organelles, the outer region granules of various density. These granules may be discharged following LC rupture (Jarial, 1989) or released through merocrine processes (Quagliata et al., 2006).
Similar epidermal cells have been described in tadpoles of the South African clawed frog Xenopus laevis and referred to as Kugelzellen (KZn or ball cells), on account of their spheroid shape (Fröhlich et al., 1977). KZn have also been described as clear cells (Fox, 1988) or “vacuoles” (Seki et al., 1989) due to their remarkably transparent cytoplasm. KZn do not exhibit any ultrastructural patterns of secretory activity, but possess thick bundles of tonofilaments and a dense layer of peripheral cytoplasm (Fröhlich et al., 1977, Delfino and Malentacchi, 2006; Quagliata et al., 2006). KZn perform a mechanical function (Fröhlich et al., 1977) before undergoing degeneration during metamorphosis.
On account of the remarkable subcellular features outlined above, the present light and transmission electron microscope (LM and TEM) study investigates the larval epidermis of the western spadefoot frog Pelobates cultripes (Cuvier, 1829) and dart-arrow frog Phyllobates bicolor Duméril and Bibron, 1841, representative of distant branches: families Pelobatidae [97] and Dendrobatidae [462] of the anuran phylogenetic tree (Frost et al., 2006). Although these frogs have been investigated in previous studies on epidermal specialized cell lines (Delfino and Malentacchi, 2006; Quagliata et al., 2006), this is the first comparative report which analyses KZn in their tadpoles.