Some observations on the biological assay of vitamin A and its precursors by the vaginal-smear method

Abstract

The dekeratinizing effect of vitamin A on the vaginal epithelium of the rat was reported by Evans & Bishop (1922), and various attempts have been made since 1930 to use it as a basis for a biological assay of the vitamin and its precursors. I n particular, Pugsley, Wills & Crandall(1944) have described a method suitable for routine purposes, wherein a quantitative response is given by the number of days needed by depleted rats to become re-depleted after dosing with the vitamin. The method has several attractive features: notably that the response is more specific than in other methods of vitamin A assay, and that the method is economical in that the same animals may be used repeatedly. A further advantage is that the criterion of vitamin A depletion, the characteristic cornified-cell smear, is easily recognized, whereas the corresponding criterion in the assay based on body-weight is poorly defined (see, for example, Bliss & Gyorgy, In spite of these advantages, however, the vaginal-smear method does not appear to be widely used. Recently we employed it in a series of assays of the biological activity of materials containing isomers of vitamin A: our experience is reported here, to draw attention to the method and to the fact that we found it possible to assume a simpler relationship between dose and response than that used by Pugsley et al. (1944), so that the analysis of the results was also simpler. Some points affecting the design of the assay will also be discussed.