Abstract
The progression of mammary gland development in Sprague‐Dawley rats between 5 and 440 days of age was studied. Tumorigenesis of the mammary gland after administration of 7,12‐dimethylbenz(a)anthracene (DMBA) was also investigated during different prepubertal and postpubertal stages. The mammary gland starts as a single primary duct on day 5 and with age branches and grows into a complete gland. The growth of the gland was observed to start from the nipple area and progress into the empty fat pad. This growth was in the form of club‐shaped end‐buds which, after the onset of puberty, became gradually transformed into alveolar buds. [^3^H]‐thy‐midine labelling index (LI.) was high (19%) in prepubertal glands, but it eventually declined to 8% at 60‐65 days. The postpubertal gland at the age of 150 days was determined to be practically mitotically static, but at 440 days, there was another increase in LI. There was a direct relationship between mammary tumorigenesis by DMBA and LI. in the mammary gland at the time of treatment with the carcinogen. Up to 55 days of age, tumorigenesis was 90‐100%, and LI. was also high during this period. The carcinogen was practically ineffective at 150 days, when the mammary gland was non‐proliferative. Though the total tumor induction was high in the prepubertal rats, the majority of the tumors (up to 84%) were benign fibro‐adenomas, while almost all the tumors in postpubertal rats were adenocarcinomas. Thus, a relationship was observed between the presence of ovarian hormones and the induction of malignant tumors. It was concluded that cell proliferation index at the time of carcinogen treatment is important for tumor induction, but whether or not the tumor is malignant is determined by the presence or absence of ovarian hormones.