In the US, more money is spent towards the detection and treatment of breast cancer (~$16.5 billion in 2010 dollars) than any other type of cancer [18
]. One of the reasons is that breast cancer is not a single disease, but a heterogeneous and phenotypically diverse set of diseases [19
]. Several molecular subtypes exist [20
], and each respond to treatment differently. Numerous studies have been conducted in an effort to better understand the etiology of the several types of breast cancer. Not only is breast cancer complex, but so is normal breast morphology. Breast tissue consists of multiple cell types, which must remain in close communication. Normal mammary gland growth involves intricate crosstalk between the epithelium and the surrounding stroma, to balance proper proliferation/apoptosis and remodel the gland at the different stages of life.
Rapid mammary gland development occurs in three distinctive life stages: fetal, peri-pubertal, and pregnancy [23
]. In girls, the fetal mammary bud begins to form late in the first trimester of pregnancy. During the last few weeks of pregnancy, the nipple and the primary epithelial ducts form and ducts branch outward into the stroma. Relatively little epithelial growth is observed until around the time of puberty when the gland growth is influenced by the release of pituitary and ovarian hormones. During thelarche, one of the earliest signs of puberty, female breast tissue enlarges and grows outward, making it noticeable. At the same time the mammary fat pad enlarges in size, and the rapidly extending epithelium form bulbous or club-like structures at the duct ends, termed terminal ductal lobular units (TDLU). The TDLU are structurally similar to terminal end buds (TEB) in rats and mice, present during the same life stage. These structures are undifferentiated and highly proliferative, and as such they are sensitive to the effects of carcinogens and other chemicals. The breast reaches a static stage some time after first menstruation, changing slightly with each additional menstrual cycle. demonstrates the progressive growth of the mammary epithelium within the fat pad of the mouse until it reaches the adult resting state. Thereafter, the gland remains in a fairly static stage throughout life, until a pregnancy occurs. At this time, morphological maturation is achieved and the gland continues to branch and fill with lobulo-alveoli to support the production and release of milk. EDC-induced disruption of normal development at any and all of these stages can cause permanent developmental abnormalities, impaired lactation, and influence risk for the development of breast cancer. It is important to note that male breast development also occurs in utero
, and is halted in boys by the androgen surge that occurs just before birth. Anti-androgenic chemical exposures in rodent models have caused reversal of that gender-specific response [24
]. It is important to note that EDC-induced changes affect risk for hormonally-responsive mammary tumors, such as the common ER+/PR+ subtype.
Fig. 2 Typical trajectory of mammary gland development in the CD-1 mouse from neonate to young adult. PND = Postnatal Day. Carmine-stained whole mount mammary gland samples were prepared using the NTP Animal Studies Protocol; Section XIII, Appendix 6, Section (more ...)
The use of rodents in laboratory studies has advanced our knowledge about the mechanisms controlling normal mammary gland growth, as well as fill knowledge gaps concerning spontaneous and chemical-induced carcinogenesis. The 2 year bioassay, involving adult
exposure to test chemicals, provides evidence of spontaneous carcinogenesis (in the entire body), whereas the use of a chemical carcinogen such as dimethylbenz-a-anthracene (DMBA) following an early life
chemical exposure tests the specific susceptibility of the mammary gland to form tumors. It is important to emphasize that dose levels and timing of exposure to EDCs may affect the severity, or lack thereof, of an effect on mammary gland growth and consequently breast cancer risk [23
]. EDC exposure can accelerate rodent mammary gland growth leading to unbalanced or unchecked proliferation of the gland. For example, prenatal exposure to DES accelerates mammary gland growth with increased lobular proliferation resulting in increased hyperplastic nests of epithelium and tumor multiplicity, as well as decreased tumor latency [27
]. Compared to controls, a virgin rodent with accelerated development of the mammary gland will have more TEBs compared to terminal ducts at weaning (3 weeks of age), and rat weanlings may exhibit extensive alveolar budding. As development progresses, exposed rodent mammary glands will exhibit a greater number of lobules, possibly with greater complexity, than unexposed animals. Yet, EDCs that cause delays in mammary growth can also increase sensitivity to carcinogens and increase risk for breast cancer. Delayed development of rodent mammary glands often manifests as decreased longitudinal growth of the epithelium and fewer TEBs compared to controls at the time of weaning. These changes may also be present in adolescent animals; fewer duct ends, decreased branching density and smaller gland outgrowth. At puberty, delayed glands may have more TEB than controls due to the slower pace of development. Although somewhat counter-intuitive, mammary gland growth delays can increase cancer risk by prolonging the presence of carcinogen-sensitive structures, such as TDLU or TEBs. demonstrates examples of accelerated and delayed mammary gland development, compared to vehicle-treated controls, in the rat.
Fig. 3 Examples of accelerated development (right) and delayed development (left) compared to a representative vehicle-treated control mammary gland at the time of weaning in the Long Evans rat. Carmine stained whole mount mammary gland. Specific examples of (more ...)
Although exposure to EDCs can increase breast cancer risk, some EDCs have been reported to have beneficial effects and protect against the development of breast cancer (see phytoestrogens). Exposure to EDCs can stimulate growth so that the gland is mature with a high ratio of fully differentiated structures compared to immature or un-differentiated structures or can reduce the proliferation and apoptosis ratio within the epithelium. As stated before, the beneficial effects of EDCs are highly dependent upon exposure levels and timing of exposure. Inhibition of mammary gland development early in life can be so severe that the mammary epithelial tree does not form past a nascent structure, leaving little proliferative potential [29
]. In addition to reduced risk for mammary tumor formation, these delays result in glands that are less responsive to ovarian and pituitary hormones, consequently reducing the functionality of the gland. Severe inhibition of mammary development has only been observed in rodents at high EDC exposures [30
]. For humans, these elevated levels of exposure may not be reached except in rare cases of high occupational or non-occupational accidental exposures (i.e. pollution). Yet, in wildlife and domesticated mammals, excessive exposures that result in severely stunted glands diminish sustenance to developing offspring, resulting in reduced postnatal survival because of minimal/no milk production [32
Although the ductal or lobular epithelia are often the sites of tumor production, mammary stroma growth patterns can also influence breast cancer risk. The communication of the multiple cell types in the mammary gland, i.e. stromal-epithelial interaction, is critical for normal development throughout life [8
]. Studies have linked increased stromal density in the breast to later development of epithelial based mammary tumors [33
]. When mammary tumor tissues were separated into epithelial and stromal compartments, recombination of normal epithelium with tumor stroma resulted in the growth of epithelial tumors while recombination of tumor epithelium with normal stroma resulted in the growth of normal tissue, indicating that stromal tissues have strong influences on overall gland growth and tumor status [11
]. Deleterious modifications to stromal tissues can occur along with accelerated or delayed overall mammary gland growth and is another way EDC exposure can influence the development of mammary tumors [16
]. In fact, thickening of the stromal compartment surrounding the epithelium has been found in the case of prenatal Bisphenol A exposure in rats [34
] and following prenatal exposure to perfluorooctanoic acid in mice [36
]. As expected, exposure to EDCs that alter mammary development may also cause an array of detrimental health effects in other organs. demonstrates a multitude of reproductive/hormonal effects from a handful of EDCs that are known to affect the mammary gland.
Summary of other EDC-induced reproductive effects