Europ. J. CancerVol. 12, pp. 747-748. Pergamon Press 1976. Printed in Great Britain
Cell Proliferation in the Mammary Glands of the Mouse During Prolonged Unilateral Lactation G. H. ZEILMAKER and C. M. P. 1VL VERHAMME Department of Endocrinology, Growth and Reproduction, Erasmus University, Rotterdam, The Netherlands
lactating glands was very low. This was quantitated by determining the percentage of labelled nuclei in 4-10 samples of 100 epithelial cells per mammary gland [average 1-3%, median 0.5%, range 0-8%, 18 animals (Fig. 2)]. A low degree of labelling in lactating glands during normal lactation in mice  has been described earlier. In the thelectomized glands of the same mice, intact small and distended alveoli as well as degenerated and disorganized alveoli were present. Labelling was intense in small intact alveoli and in disorganized alveoli, in which the nuclei were no longer situated in the periphery [average percentage of labelled nuclei in 4-10 randomly chosen samples of epithelial cells: 13%, median 9.5%, range 1-60%, 18 animals (Fig. 1)]. In 3 animals the histology of the lactating and thelectomized glands both showed the presence of many degenerated alveoli and labelled alveolar nuclei, probably as a result of inadequate milk production. These animals were not included in the data. The present observations are in line with the hypothesis  that in thelectomized glands, exposed to a strong hormonal stimulus, the distended alveoli degenerate as a result of the stagnation of secretion products. The adipose tissue, present only in the thelectomized glands as a result of this degeneration process, provides space for the occurrence of epithelial cell proliferation, leading to the formation of new distended alveoli. These alveoli may originate by reorganization of the remnants of degenerated alveoli into small tubular alveoli . Alveolar degeneration occurring after cessation of milk removal following thelectomy or suppression of young has been observed earlier [8, 9]. The results of this study indicate that in the mammary gland of susceptible mice an increase in cell proliferation correlates with the formation of preneoplastic and neoplastic lesions.
IT HAS been shown that lactation has a profound influence on mammary tumorigenesis in mice [1, 2]. In unilaterally lactating mice, in which both the lactating and the thelectomized glands are exposed to the same hormones, tumours invariably occur first in the thelectomized glands. This influence on tumorigenesis is attributed to a protectiv e effect of regular milk removal on preneoplasfic nodule formaficn. The transformation of nodules into tumours is not affected by lactation in intact glands [3, 4]. In order to investigate whether nodule genesis in the thelectomized glands is the result of an increase in cell proliferation induced by thelectomy, an autoradiographic study was initiated in pit aitary-graft-bearing mice subjected to prolonged unilateral lactation. Eight to 12 vceeks old nulliparous (C3Hx 020) F1 hybrid mice were allowed to become pregnant. On pregnancy days 15-19 two isologous pituitary glands were grafted under the left kidney capsule and the nipples were excised unilaterally. Following parturition prolonged lactation was induced [3, 5]. After 5 weeks of continuous milk production 21 lactating mice were injected twice daily during 3 consecutive days with 25#Ci (Methyl-H3) thymidine (Amersham, 15-30 C/mMol). One day later the animals were sacrificed and the 4th lactating and thelectomized mammary glands were fixed separately in Bouin solution. Deparaffinized sections (5 pro) were prepared for autoradiography by the dipping technique (Ilford K2 emulsion). The exposure time was 5 weeks. In the lactating glands very little of the interstitial fat-tissue was present and the structure of the alveoli was normal. Depending on the degree of distention of the alveoli the epithelial cells and their nuclei were more or less flattened. "]?he degree of labelling in the Accepted 14 April 1976. 747
G. H. Zeilmaker
and C. M. P. M. Vehmme
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