A comparative study of the mechanisms by which X-irradiation and genetic mutation cause loss of vibrissae in embryo mice.

INTRODUCTION It has long been known (e.g. Warkany & Schraffenberger, 1947; Wilson, 1949; Russell & Russell, 1954; Russell, 1957) that X-irradiation can phenocopy mutation and that in mice (Russell, 1950) there are sensitive periods during embryonic development for the production of these effects. It has also been known (Van Scott & Reinertson, 1957) that X-irradiation of adult mouse hair follicles during the active growth phase can cause degeneration. Another group of workers (Fraser & Hall, 1958; Dun, 1958; Kindred, 1964) have produced reductions in the number of facial vibrissae by irradiating random-bred foetal mice with acute doses of 200 r. In these last experiments vibrissa development was only disturbed if the dosage was given between 11 and 13 days of foetal life. Fraser & Hall (1958) have also altered vibrissa number in an apparently similar manner by the introduction of the Tabby gene into a previously random mouse stock. The initial effect of the gene was to cause reduction of vibrissa number and increase of variance, but by further selective breeding of the variant animals both increase and decrease of vibrissa number were produced in Tabby individuals and in their non-mutant sibs (Dun & Fraser, 1959). The development of facial vibrissae in mice has been investigated by Griineberg (1943), Rawles (1947), Falconer, Fraser & King (1951), Davidson & Hardy (1952), Dun (1959) and others. These workers have made observations on the timing of initiation and on the process of development in normal mice and in several mutants with abnormal vibrissae. However, the only detailed study is that of Davidson & Hardy (1952), who have divided normal vibrissa development into eight stages, calling the first histological differentiation of the epidermal follicle rudiment stage 1 (each rudiment is composed of an epidermal follicle and a dermal papilla) and the emergence of the hair at the surface approximately 5 days later stage 8 (Text-fig. 1). The facial vibrissae of mice are divided