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First published online December 26, 2008
Journal of Experimental Biology 212, 155-162 (2009)
Published by The Company of Biologists 2009
doi: 10.1242/jeb.019232
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Ultrastructure and physical properties of an adhesive surface, the toe pad epithelium of the tree frog, Litoria caerulea White

Ingo Scholz, W. Jon P. Barnes*, Joanna M. Smith{dagger} and Werner Baumgartner

Department of Cellular Neurobionics, Institute of Biology 2, RWTH-Aachen, Kopernikusstrasse 16, 52056 Aachen, Germany


Figure 1
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  		Fig. 1. Vertical section of a 4-sided pyramidal indenter of an atomic force microscope. Calculation of the cross-sectional area of the indenter tip at the indentation depth zi was as follows:

Formula

Formula

Formula
where w is the width of the AFM indenter tip at the indentation depth and {theta} equals the tip angle of the indenter (centreline-to-face).

 

Figure 2
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  		Fig. 2. (a) Immature White's tree frog, Litoria caerulea (snout–vent length, approximately 40 mm). (b–d) Scanning electron microscopy (SEM) of toe pad epithelium. (b) Low-power micrograph of whole pad of a juvenile frog. (c) Medium-power micrograph showing a mucous pore and (largely) hexagonal epithelial cells separated from each other at their distal ends by channels. (d) Higher power micrograph indicating the presence of nanostructuring on the `flat' surface of the epithelial cells.

 

Figure 3
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  		Fig. 3. (a,b) Nanostructural features of the adhesive surface of toe pad epithelial cells. (a) High-power scanning electron micrograph showing a surface view of the (largely) hexagonal nanostructures that form a dense array on the external surface of a toe pad epithelial cell. (b) High-power transmission electron micrograph showing one of the channels that separate adjacent epithelial cells and a side view of the nanostructures, which are themselves separated from each other by narrow channels. The inset shows similar nanostructures on a toe pad of the hylid tree frog, Scinax ruber. Here the nanostructures are associated with filaments running at right angles to the cell surface.

 

Figure 4
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  		Fig. 4. Freeze-fracture image of a toe pad showing a side view of parts of two epithelial cells. Note that cytoskeletal elements are concentrated in the outer `nanopillar' layer (top of picture), with only a loose lattice of cytoskeletal material beneath.

 

Figure 5
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  		Fig. 5. (a) Three-dimensional reconstruction of the surfaces of parts of three toe pad epithelial cells, showing the rough surface of each cell and the deep channels that separate them. (b,c) AFM images (b, height and c, deflection) of part of one of these cells, indicated by the outline in a. The height image (of which b is a part) was used for the three-dimensional reconstruction (a), while the deflection image (c) clearly shows the dense array of peg-like nanopillars that constitutes the adhesive surface of the epithelial cells. (d) Enlargement of part of the deflection image showing more detail of the appearance of the nanopillars. The arrows indicate gaps connecting the dimples with the surrounding channels. For the height image (b), the colour gradient covers the range 0–500 nm, while for the deflection images (c and d) the range is 0–6.5 nm.

 

Figure 6
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  		Fig. 6. (a–c) Height profiling of toe pad epithelium. (a) Deflection image showing the line from which the height profile (b) was taken. The two crosses delineate a columnar nanopillar that lay precisely on the profile line, showing that these peg-like structures have a small dimple at their centres. (c) Average values (means ± s.d.) of the width of the nanopillars and the depth of the dimples based on 199 measurements from height profiles such as that shown in b.

 

Figure 7
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  		Fig. 7. Material stiffness as measured by the AFM. (a,b) Blue lines show typical experimental curves for cantilever deflection plotted against distance in the z-axis for toe pad epithelium (a) and glass (b). Green lines are fits to the best theoretical curves using Eqn 3. The y-axes can be converted to force by multiplying by the spring constant of the cantilever (0.03 N m–1 in these experiments). The curve in a represents an effective elastic modulus Eeff of 250 kPa.

 

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© The Company of Biologists Ltd 2009