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MAURICIO A. LANDE, JOSÉ A. ZADUNAISKY; The Structure and Membrane Properties of the Frog Nictitans. Invest. Ophthalmol. Vis. Sci. 1970;9(7):477-491.
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The nictitating membrane of the frog is a semitransparent tissue which appears as an adaptation to terrestrial life and serves a protective function. The optical and electron microscopic studies indicate that it does not contain smooth muscle and that it is formed by an external stratified epithelium similar to that of frog skin, a thick stromal region containing vessels, nerves, and collagen organized in lamellae with fibers of uniform size with an arrangement resembling the corneal stroma, and an inner mucoid epithelium similar to the one of the conjunctiva. This membrane maintains a potential difference, positive inside (40 ± S.E. 5.6 mV. in vivo and in vitro. By means of 22Na fluxes a transport of sodium from outside to inside (0.568 µEq h-1. cm-2) was found which is inhibited by ouabain and stimulated by vasopressin. This transport is located in the outer epithelium. The membrane does not swell remarkably in vitro, nor does it show a temperature reversal phenomenon. An extremely well-organized stroma and loss of pigmentation are the causes for its semitransparency. The amount of incidental light scattered by single fibers predicts complete opacity for the frog nictitans as much as in the case of the transparent cornea. The difference in fiber size and interfiber distance between the stromas of the nictitating membrane and that of the cornea could be related to their different degrees of transparency.
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