MIT Study Finds How Breast Milk Cells Change Over Time In Nursing Mothers

The researchers analysed human breast milk produced between three days and nearly two years after childbirth, and identified a variety of changes in gene expression in mammary gland cells.

New Delhi: In a first, researchers at the Massachusetts Institute of Technology (MIT) have performed a large-scale, high-resolution study of breast milk cells. This has allowed the researchers to track how breast milk cells change over time in nursing mothers. 

The study was recently published in the journal Proceedings of the National Academy of Sciences.

Analysis Of Breast Milk Produced Between 3 Days And Nearly 2 Years After Childbirth

The researchers analysed human breast milk produced between three days and nearly two years after childbirth, and identified a variety of changes in gene expression in mammary gland cells, the study said. 

What Causes Changes In Gene Expression In Mammary Gland Cells?

Some of the changes in gene expression in mammary gland cells were associated with factors such as hormone levels, illness of the mother or baby, and the mother starting birth control.

In a statement issued by MIT, Brittany Goods, one of the senior authors of the study, said the researchers were able to take this really long view of lactation that other studies have not really done, and shows that milk does change over the entire course of lactation, even after years of milk production. 

Dynamics Of Milk Production

The study is important because it could yield new ways to boost mothers' milk production or to improve the composition of infant formula, the statement said.

For months or years after childbirth, human mammary glands can produce more than a litre of milk in a day. Since the tissue itself cannot be biopsied or accessed during lactation, it has been difficult to study how mammary glands accomplish this feat. 

New studies have found that breast milk contains many cells from the mammary gland, which offers a non-invasive way to study the mammary gland cells.

The researchers collected breast milk samples from 15 nursing mothers, as part of the study. According to the study, each donor provided samples at multiple points of time, ranging from three to 632 days after giving birth. 

The scientists gathered information about the changes in health and lifestyle of the mothers occurring throughout the lactation period.

More than 48,000 cells were isolated from 50 samples, and were analysed using single-cell RNA-sequencing. This is a technology that can determine which genes are being expressed in a cell at a given moment in time. 

What Are The 10 Types Of Cells Found In Breast Milk?

The scientists found 10 types of cells in breast milk. There was a single population of fibroblast cells, two types of epithelial cells, and seven types of immune cells. Fibroblasts are the most common type of cells found in connective tissue, secrete collagen proteins that are used to maintain a structural framework for many tissues, and also play an important role in healing wounds. Epithelial cell is a type of cell that lines the surface of the human body. 

According to the study, macrophages constituted more than half of the immune cells found. Macrophage is a type of white blood cell which helps eliminate foreign substances by engulfing foreign materials and initiating an immune response. 

The immune cells appear to express genes that help make the mammary gland more tolerant of the milk proteins that they are producing, so they do not trigger an immune response. The scientists also found populations of B cells, T cells and other immune cells. However, their numbers were too small to do any in-depth studies of their functions. B cell is a type of lymphocyte that makes antibodies. T cells are also a type of lymphocyte, and can play different roles, ranging from acting as "killer cells" by attacking cells infected with a pathogen to acting as "helper cells" by supporting B cells to produce antibodies.

The most abundant cells the researchers found were lactocytes, which are a type of epithelial cell. A lactocyte is a milk-producing cell in the mammary epithelium. It secretes milk into alveoli. 

The lactocytes expressed many genes for proteins that are found in breast milk, such as lactalbumin, the study said. The cells also expressed genes for transporters needed to secrete milk proteins, micronutrients, fat, and other breast milk components.

Two Types Of Lactocytes

The scientists identified a particular cluster of cells among the lactocytes, which appears to be the primary producer of milk, and another cluster which plays more of a structural role in the mammary gland. 

The scientists observed that the proportion of lactocytes involved in milk production went down as time went on. However, the proportion involved in structural support went up. 

How Do Genes Responding To Prolactin Behave In Different Types Of Lactocytes?

The study found that the genes involved in responding to the hormone prolactin became more active in the milk-producing lactocytes. Prolactin is a hormone made by the pituitary gland, which causes the breasts to grow and make milk during pregnancy and after birth. 

The genes involved in responding to prolactin decreased in the structural lactocytes, the study found. 

These changes may be related to the changing nutritional needs of infants as they grow, the researchers theorised.

Goods said that this study paves the way for mapping out and better understanding some of the pathways that breast milk cells use to accomplish the tremendous amount of work they do.

What Is The Composition Of Breast Milk? 

The composition of cells in breast milk and events such as babies starting to go to daycare, starting formula, or the mother starting to use hormonal birth control were found to be linked.

Sarah Nyquist, the lead author of the study, said there are clearly changes in the composition of breast milk that are related to these lifestyle and health changes, such as infant illness or maternal hormonal birth control. 

She explained that these changes in lactation do not necessarily have a positive or negative impact on anyone's health, but they do occur and may lead researchers to insights into how mammary epithelial cells are producing milk, and the types of components they may be producing. 

In the future, the scientists aim to conduct studies that could help them find stronger links between environmental factors and milk composition, and also discover more about how milk naturally changes over time, all of which could eventually help scientists devise better infant formulas.