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Discussion
Limbless tadpoles of Pelobates cultripes and Phyllobates bicolor possess KZn in the basal epidermal layer, namely ephemeral cells (Fröhlich et al. 1977), destined to be substituted by typical keratinocytes of the anuran epidermis. In the ontogenetic range observed, KZn show no degeneration features at all, and retain their proliferative potential, at least in intermediate stage of maturation. KZn can regulate the cytoplasm turgor by active transmembranary transport of ions (Fröhlich et al., 1978) and act as pressure-elasticity modules, recalling the notochordal cells in their mechanical properties (Fröhlich et al., 1977). However, the hydrostatic pressure requires a stiffness component to perform a supporting role: this is accomplished by the tonofilament bundles in the peripheral cytoskeleton. Tonofilaments form compact bands in P. bicolor KZn, but in P. cultripes they are arranged in peculiar micro-trabeculatures, that foreshadow the Langerhans net of Leydig cells (Fährmann, 1971a, b; Greven, 1980; Rosenberg et al., 1982; Kantorek and Clemen, 1990). However, the LC peripheral filaments are assembled in a more solid arrangement, and in Salamandridae, the bundles form a regular network with polygonal meshes (Delfino and Malentacchi, 2006). The patterns of vesicle production described in KZn of P. cultripes represent further morpho-functional traits shared with Leydig cells, as suggested by similar features in the organelles involved (Rosenberg et al., 1982; Kantorek and Clemen, 1990). Homology between KZn and LCs is, nevertheless, a matter of debate, involving the three orders of extant amphibians with two possible contradictory meanings: Pflugfelder and Schubert (1965), and Nieuwkoop and Faber (1967) assign these cells to the same line. Fox (1988), on the contrary, maintains that Leydig cells have no homologous counterparts in Anura, while they are shared by urodele and Ichtyophis larvae. This suggests a relationship between Caudata and Gymnophiona which may be of phylogenetic significance (Fox, 1986b). In the light of the possible evolution by convergence of KZn and LCs, it is of interest that their typical ultrastructural traits: lucent cytoplasm and a peripheral felt-work of tonofilaments, have also been described in large epidermal cells of bony fishes (Whitear, 1986; Fox, 1989; Yokoya and Tamura, 1992). Possibly, the wide distribution of large clear cells among water-inhabiting vertebrates comes from their adaptive flexibility. Along with the mechanical function, these cells may regulate intraepidermal water flow (Fröhlich et al., 1977) and/or contrast dehydration (Kelly, 1966; Yokoya and Tamura, 1992).
Received: 17 May 2007
Accepted: 25 September 2007