The protective action of vitamin A upon various tissues in the avitaminotic rat, and the sensitivity of these tissues to vitamin A deficiency.

The experiments reported here were undertaken to compare the sensitivity of some rat tissues to relative or absolute vitamin A deficiency and to prophylactic doses of the vitamin. That there must be considerable differences in the requirements of various tissues and processes in the rat for vitamin A is suggested by the computed requirements of the entire animal found in the literature, based on such phenomena as growth, tooth colour, vaginal smears, storage, and longevity, recently summarized by Rubin & De Ritter (1954). The figures given range from 10 to 1000 i.u./kg bodyweightlday . In the present experiments the changes compared were those that occur in the incisor teeth (Wolbach & Howe, 1933), the medulla oblongata (Irving & Richards, 1938-9), the maxillary fundic alveolar bone (Irving, 1949) and the nasolacrimal duct (Irving, Pindborg, Fitzhugh, Weinmann & Schour, 1952). These were chosen as examples of two hard tissues, epithelia and nerve tissue. A preliminary account of some of these findings has already been published (Irving & Richards, 1954).