ELECTRON-MICROSCOPIC STUDY OF THE TEXTURE OF THE BASEMENT MEMBRANE OF LARVAL AMPHIBIAN SKIN

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Abstract
Tail fins and head samples from embryos and larvae of urodeles (Amblystoma punctatum and opacum) and anurans (Rana pipiens, Xenopus laevis) were fixed in veronal-buffered osmic acid imbedded, in a mixutre of methyl-and isobutyl-methacrylate polymerized by UV irradiation, sectioned at about 0.1 [mu] with glass knives, and transferred to Formvar film on nickel screen of specimen holder (1) without removal of imbedding medium, (2) with imbedding medium dissolved by toluene with or without subsequent shadowing with chromium. A model EMU electron microscope was used, and magnifications were determined by calibration using (1) standard-size polystyrene granules and (2) diffraction grating. Detailed descriptions are given concerning the morphology of the larval skin and basement lamella. The latter is a 20-ply structure [plus or minus] 2), each ply consisting of approximately equal parts of ground substance and discrete parallel collagen fibers of about 500 A diameter and 500-55 A periodicity, about 5 fibers hight to each ply. Fiber orientation changes sharply by 90[degree] at the transition from one ply to the next, giving an orthogonal grid aspect to the surface view of the basement lamella. There was definite evidence that by Shumway stage 22 the basement lamella had been laid down, and the layer adhering to the underside of the epidermal cell contained fibers intersecting at near right-angles. Definite lamination was shown, in the head region after Shumway stage 25. During the formative stages most epidermal nuclei lay flat and closely applied to the basal cell surface, and the young basement lamella adhered closely to the basal surface. With the onset of metamorphosis mesenchyme cells appearing like fibrocytes moved in between the plies followingthe cleavage planes between layers. Further developmental studies on the formation of the basement lamella are in progress. The component fibers of the basement lamella are believed to consist of collagen, but the origin of the collagen or of the hyaline ground substance is not known. The epidermal cell apparently plays a fundamental role in the formation of the basement lamella. The possible factors involved in the organization of the fibers are discussed.

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