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Influence of Dietary Fat Levels on Development and Hormone Responsiveness of the Mouse Mammary Gland¹

Department of Anatomy, Michigan State University, East Lansing, Michigan 48824

Twenty-one-day-old female BALB/c mice were divided into three groups and fed a diet containing 0, 5, and 20% fat (corn oil). Ten days prior to sacrifice, one-half of the mice were given injections daily with saline (0.9% NaCl solution), and the remaining half, with 17ß-estradiol (1 µg) and progesterone (1 mg). After 3 mo on diet and 10 days of saline or estradiol:progesterone treatments, all mice were sacrificed, and mammary glands were excised and prepared for whole-mount evaluation (No. 4 glands), [³H]thymidine-autoradiographic analysis (No. 2 glands), and organ culture analysis (No. 2 glands). Whole-mount evaluation involved a rating for ductal and alveolar development on a scale of 1 to 6. [³H]Thymidine-autoradiographic analysis consisted of determining the total number of labeled epithelial cells per anterior 3 mm of gland. Organ culture analysis consisted of placing one gland of each gland pair in basal tissue culture medium, and the contralateral gland was placed in basal medium plus mammogenic hormones. These glands were cultured for 6 days and then analyzed for development by whole-mount evaluation (scale, 1 to 6) and for epithelial area (mm²) (via computer image analysis). In saline- and estradiol:progesterone-treated mice, there was a significant linear increase in the number of [³]-thymidine-labeled mammary epithelial cells as the fat content of the diet increased from 0 to 5 to 20% (P < 0.05). In saline- and estradiol:progesterone-treated mice, mammary gland development (assessed by whole-mount evaluation) was increased as the fat content of the diet increased from 0 to 5% (P < 0.05). In saline-treated mice, no significant difference in mammae development was observed between mice fed 5 or 20% fat diets; in estradiol:progesterone-treated mice, mammae development was marginally increased in mice fed the 20% fat diet compared to mice fed the 5% fat diet (P 0.07). When the mammary glands from saline- or estradiol:progesterone-treated mice were analyzed for hormone responsiveness in organ culture, the response of these glands to mammogenic hormones was significantly increased (assessed by mammae development and mammae epithelial area), as the fat content of the diet was increased from 0 to 5% (P < 0.05); as the fat content of the diet was increased from 5 to 20%, there was an increase in responsiveness to mammogenic hormones, but this increase was not as pronounced and often did not reach the 5% level of statistical probability. Thus, the results of this study provide evidence that the level of dietary fat can affect in situ mammary gland development and growth responsiveness to mammogenic hormones. The effect of dietary fat is quite striking when comparing mice fed 0 and 5% fat diets and less pronounced when comparing mice fed 5 and 20% fat diets.

¹ This research was supported by USPHS Research Grant CA-37613.

² To whom requests for reprints sould be addressed, at Department of Anatomy, Michigan State University, East Lansing, MI 48824.

Received 3/14/85. Revised 8/26/85. Accepted 8/28/85.