An ultrastructural study was carried out on the epidermis of Agalychnis callidryas tadpoles during limb development. Larval epidermis consisted of four cell layers: basal, lower intermediate, upper intermediate, and surface or apical layers. Basal cells represented the stem compartment of intermediate cells: both belong to the skein cell (SC) lineage, described in several anuran species, on account of the conspicuous intracytoplasmic tonofilament bundles. Apical cells were secretory in nature and released mucus on the body surface. Intermediate SCs exhibited a hydrated central cytoplasm and peripheral tonofilament bundles. They closely resembled the epidermal ball-like cells, Kugelzellen (KZn) of Xenopus laevis tadpoles, and possibly shared their turgor-stiffness properties. In A. callidryas, the stratification of intermediated SCs on their stem cell layer provided the chance to study their cytodifferentiation in a suitable sequence, until basal cell differentiation shifted toward the keratinocyte lineage in premetamorphic stages. Present data assign A. callidryas to the anuran species with a constitutive SC population in larval epidermis, and demonstrate that KZn express the ultimate specialization of such cell line. SCs were arranged in the fashion of a random-rubble stone groundwork, and possessed long processes. These cytoplasmic outgrowths contained a tonofilament axial rod and held together contiguous cells. Ultrastructural findings suggest that this complex structure may impart compressive as well as sliding strengths to the larval epidermis, representing a possible adaption to the fresh water environment.
Keywords: Kugelzellen; hylid frogs; larval epidermis; skein cells; ultrastructure.
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