ABSTRACT
An important concept that has emerged from the study of
breast development is that the terminal ductal lobular unit
or TDLU, which had been identified as the site of origin of
the most common breast malignancy, the ductal carcinoma,
corresponds to a specific stage of development of the
mammary parenchyma, the lobules type 1 (Lob 1) (Russo
et al., 1991; Wellings, 1980; Wellings et al., 1975). This
observation is supported by comparative studies of normal
and cancer-bearing breasts obtained at autopsy. It was
found that the nontumoral parenchyma in cancer-associated
breasts contained a significantly higher number of hyper-
plastic terminal ducts, atypical Lob 1, and ductal carcino-
mas in situ originated in Lob 1 than those breasts of women
free of breast cancer. Lob 1 is affected by preneoplastic as
well as by neoplastic processes (Russo et al., 1991; Russo
and Russo, 1997). The finding that Lob 1, which are
undifferentiated structures, originate in the most undiffer-
entiated and aggressive neoplasm acquires relevance in the
light that these structures are more numerous in the breast
of nulliparous women, who are, in turn, at a higher risk of
developing breast cancer (Russo et al., 1992; Russo and
Russo, 1997). The Lob 1 found in the breast of nulliparous
women never went through the process of differentiation,
whereas the same structures, when found in the breast of
postmenopausal parous women did (Russo et al., 1992).
