1. The effect of hydrocortisone on the development of fetal rat skin in organ culture, and on its repair after exposure to a mixed beam from a mercury lamp, are described.
2. The addition of hydrocortisone (7.5 µg/ml) to the culture medium (HC medium) caused accelerated differentiation and keratinisation of the epidermis followed by atrophic changes as in vivo.
3. Explants were grown for 2 days in either normal or HC medium and then irradiated with light from an Hanovia lamp.
4. Irradiation of the control explants produced severe necrosis in both epidermis and dermis and much disorganisation of the dermal intercellular material; 2 days after exposure the s. corneum with adherent cellular debris had become either completely detached from the denuded dermis, or raised to form a fluid-filled blister. Epidermal regeneration had begun by the 4th day after irradiation and was usually complete by the 6th day.
5. Hydrocortisone modified the response to irradiation as follows: (1) reduced and retarded cellular breakdown, (2) prevented vesication, (3) preserved the organisation of the dermal intercellular material, (4) hastened epithelialisation, (5) accelerated the differentiation of the new epidermis. Effects (2), (3), and (4) were probably secondary to (1).
6. Experiments with various light filters showed that the effective wavelengths for producing lesions in the skin explants were those below 3000 A.
7. It is suggested that the beneficial effect of hydrocortisone on the repair of irradiated skin explants might be due, at least in part, to a reduced proteolytic activity in the damaged tissue through a stabilising action of the hormone on the lysosomes.
8. The relationship of these findings to clinical observations is discussed.