A transplantable rat Leydig cell tumour.

Abstract. Growth rate and morphology were studied in a transplantable rat Leydig cell tumour (H-540) grown in intact, castrated and hypophysectomized rats. The plasma levels of pituitary hormones and testosterone were measured in the same rats. The results can be summarized as follows: 1. The growth curves of tumours in intact and castrated rats were S-shaped and very similar during the observation period of three weeks. 2. In hypophysectomized rats, the onset and progression of tumour growth were delayed, compared with intact and castrated rats. 3. The thymidine labelling index as well as the size of the S and G2 phase compartments were decreased in tumours greater than 10 g compared with smaller tumours and tumours grown in hypophysectomized rats. 4. Testosterone concentrations in plasma correlate well with increasing tumour weight up to approximately 10–15 g in intact and castrated rats. 5. Plasma testosterone levels in tumour-bearing hypophysectomized rats were 7-fold higher than those of corresponding intact and castrated rats. 6. In castrated rats, suppression of LH production occurs by very small tumours (< 2 g), whereas FSH levels show a gradual decrease with increasing tumour size. PRL production was independent of castration, tumour weight, and testosterone levels. 7. In spite of no major differences in cell morphology, the morphometric analysis revealed a reduction in tumour cell size and nuclear size in hypophysectomized rats compared with intact and castrated rats. It is concluded that pituitary hormones stimulate tumour growth, but surprisingly appear to reduce the secretion of testosterone from these tumours.