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Lobuloalveolar Differentiation and Tumorigenesis: Two Separate Activities of Mouse Mammary Tumor Virus¹

Institute of Pathological Anatomy and Histology, Laboratory of Experimental Oncology, University of Pisa, Via Roma 57, 56100 Pisa, Italy

The morphological growth of the mammary glandular tree from 3 to 18 months of age has been quantified in three groups of genetically identical virgin female mice: BALB/c mice free of milk-transmitted mouse mammary tumor virus (MMTV) infection and BALB/cfC3H and BALB/cfRIII mice carrying milk-transmitted MMTV infection of C3H or RIII origin, respectively. Mice were killed at 3-month intervals, and their mammary glands were prepared for stereoscopic examination as whole mounts. Any cluster of alveoli from 0.1 mm in diameter was counted and measured at 0.1-mm intervals. The small clusters of alveoli (0.3 mm) were used to measure the amount of lobuloalveolar differentiation. The larger clusters of alveoli, the hyperplastic alveolar nodules, and the mammary tumors were used to measure the frequency of tumor cell transformations. Lobuloalveolar differentiation in BALB/c mice was increased significantly by both C3H (p < 0.001) and RIII (p < 0.001) MMTV infections. RIII MMTV induced a significantly higher lobuloalveolar differentiation as compared to that induced by C3H MMTV (p < 0.001). C3H MMTV showed a significantly higher capability in inducing hyperplastic alveolar nodules and mammary tumors as compared to that of RIII MMTV (p < 0.001). The conclusions were that two different activities are carried out by MMTV, namely, lobuloalveolar differentiation and noduligenesis, and that these two MMTV activities are distinct from each other.

¹ Supported by Contract 82.00414.96 of the Italian National Research Council, Roma, Progetto Finalizzato CNR, "Control of Neoplastic Growth."

² To whom requests for reprints should be addressed.

Received 1/11/83. Accepted 8/17/83.