Thymus metabolises progesterone-possible enzymatic marker for T lymphocytes

FEMALES of the athymic mutant strain of nude mice show severe deficiencies in reproductive function. The ovaries and the uterus are abnormally small¹, puberty is delayed², oestrous cycles are abnormal and mature females are usually sterile^1,3. These abnormalities can be prevented by grafting thymic tissue at birth^2,3. Similar abnormalities result from neonatal thymectomy of normal female mice, and can be corrected by thymus re-grafting up to 7 d age^2,4. These observations suggest an involvement of the thymus in gonadal development and function. Conversely, certain sex hormones are known to cause atrophy of the thymus⁵. Hitherto, however, there has been no evidence implicating the thymus of the infantile mouse in sex hormone metabolism. We report here the presence of 20α-hydroxysteroid dehydrogenase (20α-SDH) activity in the mouse thymus, an enzyme engaged in progesterone metabolism.