entiation of epithelial cells. Deficiency of vitamin A causes squamous metaplasia of columnar epithelium (11, 13). Excess vitamin A causes conversion of some nor mally keratinizing epithelia to a columnar, secretory type (1, 9). In addition, vitamin A has been shown to influ ence the morphology of myxoviruses (4). In view of these effects, it was of interest to determine whether high doses of v...
