Vandenberg, LN, MV Maffini, PR Wadia, C Sonnenschein, BS Rubin and AM Soto. 2007. Exposure to environmentally relevant doses of the xenoestrogen bisphenol-A alters development of the fetal mouse mammary gland. Endocrinology 148(1):116-27.

Synopsis by Dr. Susan Nagel and Wendy Hessler  

 

This study of developmental exposure to bisphenol A, a molecule used to make a very common plastic, demonstrates that it causes harmful effects in mice at very low levels, exposures within the range that people commonly experience. The contaminant accelerates mammary gland development and alters how the gland’s tissues are formed and organized when the exposure takes place before birth.

Mammary glands in developing mice exposed to the widely used synthetic were more mature at birth than glands in the untreated mice. They had more ducts and duct extensions, more developed fat areas and additional cell changes associated with a more mature gland.

The consequences of this very early alteration in breast tissue development are likely to increase vulnerability to breast cancers later in life. The finding is consistent with earlier studies.

 

   Common sources of exposure to bisphenol ACommon sources of exposure to bisphenol A include polycarbonate bottles, like those pictured above, and canned food in cans that are lined with a resin made out of the contaminant. More...

 

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Previous studies have found that developmental exposure to BPA results in altered mammary gland growth in mice before puberty and as adults similar to those seen in by Vandenberg et al. This study clearly demonstrates that ten days of exposure to BPA at a dose within the range of current human exposure increases mammary gland growth and changes in mammary gland architecture before mice are even born. Therefore, these life-long changes occur long before they would possibly interfere with mammary gland function.

Context. There is a growing concern that pollutants in our fetal environment may affect our health once we become adults. This idea, called "fetal origins of disease," is gaining ground as more studies show how early exposure can alter developmental patterns and lead to disease, sometimes decades later in life.

For instance, one area of interest is that exposure to environmental estrogens from everyday products and chemicals may increase a woman’s risk of breast cancer later in life.

Another well-studied area is showing that laboratory animals exposed to BPA during development (during fetal life or around birth) have long-term health consequences. Early exposure decreases the age at puberty, increases adult prostate size, alters estrus cyclicity and alters mammary gland size.

BPA is an environmental estrogen found in 95% of Americans tested, including pregnant women and their newborn babies. Over 6 billion pounds of the synthetic chemical are produced each year globally. It is used in dozens of products, including the hard clear polycarbonate plastics made into baby bottles and water bottles, some dental sealants and in the plastic lining of metal food cans.

BPA acts like an estrogen in the body. While BPA was originally described as a very weak estrogen, recent work has shown that for some end points it is similar in activity to the body’s own estrogen, estradiol.

The mammary gland consists of a network of ducts, tube-like structures, that, when mature, will store milk and carry it to the nipple during nursing. Think of a reservoir where many streams and rivers feed into the reservoir that supplies one exit point below a dam.

The mammary gland begins developing before birth while a female fetus is in utero. In mice, this development begins around mid-gestation and continues until after birth.

What did they do? Mice were exposed to 0 or 250 ng/kg BPA from fetal day 8 to 18 via dosing to their pregnant mothers. That level of exposure is extremely low-- approximately 0.25 parts per billion--well within the range to which people are commonly exposed. Fetuses were delivered by Caesarian section on gestation day 18, a day before natural birth. Mammary glands were removed from the females and stained. The tissues were examined under a microscope for abnormal architecture and to distinguish cellular and extracellular fractions. To quantify structural differences, the researchers compared female mammary tissue from the fetal mice 1) in the untreated controls with those in the treated group and 2) growing between two males or two females in the untreated controls with those in the treated group.

 

 

What did they find? BPA changed the organization of the fetal mammary gland. Mammary glands from BPA exposed mice were more developed than mammary glands from control mice. The mammary gland consists of a network of ducts. In this study there was a significant increase in the mammary gland ductal branches and extension in BPA exposed fetal mice.

For comparison, picture a one-year-old tree with three branches each about 20 inches long. By the time this tree is two years old, it will likely have five or six branches, and the branches will be 30 inches long. In this study, the mammary gland ductal network in BPA exposed fetal mice looked more like “two-year-old” sapplings while the ductal network in control fetal mice was similar to “one-year-old” sapplings.

A well-characterized phenomenon in litter-bearing animals, known as the 'womb mate effect,' shows that mice who develop between two males (2M) have different behavior and physiology in adulthood relative to mice who develop between two females (0M). This difference is thought to be due to hormone levels that pass between neighboring fetuses.

Mammary gland ducts from 2M females showed increased ductal area and extension relative to 0M females. It is likely that the increased estrogen levels in 2M females resulted in the increased growth. Importantly, the increase in growth in BPA exposed mice was only detected between 0M control and 0M exposed females. BPA exposure served to eliminate the normal variation in ductal area due to fetal position.

In order to form the interior open space of the ducts or tubes, some cells must die by a normal and essential process called programmed cell death. While BPA increased the size and number of the ducts, it decreased by 60% the normal programmed cell death necessary for formation of the hollow portion of the duct, called the lumen. Consequently, while none of the BPA exposed mammary gland ducts showed lumen formation, 38% of control ducts did.

A large portion of the mammary gland is made up of fat cells or adipocytes. An important function of the fat cells during mammary gland development is to send signals to other cells that make up the tubes or ducts, which are called epithelial cells. If these cells do not receive the proper signals from the fat cells then they do not form correctly.

BPA exposure increased the maturation of fat cells in the fetal mammary gland. The number cells with fat droplets inside of them was increased by 70% in mammary gland fat pads from bisphenol A exposed fetal mice, although this increase did not reach statistical significance. However, the average fat cell had 18% more fat droplets in exposed mice. These data suggest that the fat cells present were more mature in exposed mice.

The distribution of fat cells within the mammary gland was also altered by BPA exposure. While the overall number of fat cells was reduced by 15%, the number of fat cells next to the tubes or ductal cells was doubled in exposed fetal mice. The increased density of fat cells next to the ducts may have resulted in increased growth due to local signaling.

This same pattern of altered organization was seen in the distribution of collagen, a protein outside of the cell that is essential in maintaining structure. Fetal BPA exposure changed the distribution of collagen in the mammary gland. While the overall amount of collagen in the mammary gland was reduced by 40%, the amount next to the tubes or ductal cells was increased by 20%.

What does it mean? According to Vandenberg et al., this study suggests “that alterations in mammary gland phenotypes observed at puberty and adulthood in perinatally exposed mice have their origins in fetal development.”

Exposure to BPA in utero at an environmentally relevant dose altered the development of the mammary gland. This alteration was evident at the time of exposure before birth.

Previous studies have shown that these alterations result in permanently different mammary glands in adulthood. This study follows another from this group demonstrating that developmental exposure to 2.5 µg/kg BPA increases changes in the mammary gland that are precursors to cancer (ductal hyperplasia) in adult rats (Murray 2006).

In this study, fetal mice that developed between two males (and have a slightly higher level of estradiol) had enhanced growth of the mammary gland relative to females who had developed between two females. Interestingly, BPA exposure obliterated this difference, presumably due to the small, but physiological, increase in total estrogenic activity due to BPA exposure

Human twins also circulate higher levels of endogenous estrogens than singletons. Twins have an increased risk of developing breast cancer and endometriosis in adulthood. While it cannot be assessed directly in humans if the increased exposure to estrogens during development is the cause, it is consistent with other findings in laboratory animals.

Taken together, these findings are very significant. In addition to endogenous estrogens, humans and wildlife are exposed to an increased ambient level of xenoestrogens from our environment during development and throughout life. This paper supports the growing body of literature that extremely small changes in the level of estrogens, like bisphenol A, during fetal life have permanent and often detrimental health effects.

References

Murray, TJ, MV Maffini, AA Ucci, C Sonnenschein and AM Soto. 2006. Induction of mammary gland ductal hyperplasias and carcinoma in situ following fetal bisphenol A exposure. Reproductive Toxicology. Published online Oct. 24, 2006