Development of adhesive toe-pads in the tree-frog (Phyllomedusa trinitatis)

Toe-pad development in Phyllomedusa trinitatis is described using light microscopy, SEM and TEM. Pad development in forelimbs was slightly in advance of hindlimbs. The first sign of a difference between pad (ventral) skin and dorsal skin occurred at Gosner stage 37 as a convolution of the basal side of the epidermis. By stage 39, pad basal cells were elongated in shape, with highly convoluted basal sides, whereas the dorsal epidermis had a basal layer of cuboidal cells with relatively straight bases. Mucous glands developed from pad epidermis earlier than at the dorsal side, and penetrated deeply into a matrix-rich dermis with a sparse cell population. Specialization of digit tip shape first became apparent at stage 38, with appearance of the distal circumferal groove. Widening of toe tips occurred at stage 39. Surface cells of the digital pads began to be shed by stage 40, in advance of epidermis elsewhere. Specialized pad surface cells were fully developed by stage 46, the end of metamorphic climax. During growth to adulthood, pads increased in overall area, but pad surface cells did not alter their dimensions. However, pad epidermis did become several cell layers thicker, much more so than in most other species described so far. Pad cells also had a surface area higher than in many other described species. The value of developmental studies in aiding our understanding of the multiple origins of adhesive pads in different anuran lineages is discussed.