Human Breast Milk Is Full of Living Cells. Scientists Just Discovered What Some of Them Do

Breast milk contains far more than nutrients and antibodies.

Human milk is stranger than you think. It’s not just a mixture of fats, proteins, sugars and antibodies. It’s also home to thousands of living cells, including immune cells and cells involved in producing milk. And these cells are doing far more than simply floating around.

Now, scientists have found a way to look deep inside them, revealing how different cells process nutrients and carry out surprisingly different biological activities.

Studying maternal milk isn’t easy because of its diversity. Human milk is rich in fat and contains a mixture of different cell types. Conventional analysis can show its overall chemical composition but it’s much harder to establish which cells use particular nutrients or what they do with them. 

Now, researchers at the University of Western Australia have now combined several powerful techniques to investigate these individual cells in extraordinary detail. They found that immune cells and milk-producing cells handle nutrients very differently, revealing a level of metabolic specialization that conventional methods largely miss.

“By bringing together imaging, metabolic tracing and single-cell analysis, we can connect cell identity with structure and function in ways that were previously difficult to achieve,” Kai Chen, one of the study authors and a senior research fellow at the University of Western Australia (UWA), said in a statement.

Looking inside milk cells, one layer at a time

To see what was happening inside human milk, the researchers adapted an approach called correlative light, electron and ion microscopy. Essentially, they combined different ways of looking at the same biological material, with each technique revealing something the others couldn’t.

First, confocal microscopy helped identify different cell populations, including milk-producing cells called lactocytes and immune cells such as neutrophils, macrophages and dendritic cells.

Electron microscopy then revealed the cells’ intricate internal architecture, including tiny granules and compartments where fats accumulate.

Next came a technique called NanoSIMS, which allowed researchers to map elements and isotopes at extraordinarily small scales. By tracing specially labelled nutrients, they could investigate where those nutrients ended up inside the cells.

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Finally, the team integrated these observations with single-cell RNA sequencing, which measures gene expression in individual cells and provides clues about the biological processes taking place within them.

Together, these techniques produced a detailed atlas of human milk cells; the list includes cells such as lactocytes, neutrophils, macrophages and dendritic cells. The researchers also observed bacteria-like structures and evidence of a defensive process called NETosis, in which neutrophils can trap microbes.

But the most revealing findings came when the scientists investigated what these cells were doing with nutrients.

Sugar, fat, and protein reveal different cellular jobs

Macrophages are among the immune system’s cleanup crews. They engulf microbes, damaged cells and other unwanted material. But the researchers discovered something interesting about how macrophages in human milk handle sugar.

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To investigate, they tracked glucose containing carbon-13, a stable, non-radioactive isotope of carbon.

The results showed that macrophages stored much of the glucose-derived carbon in tiny fat droplets. These cells also switched on genes involved in processing fats.

The team then tracked amino acids, the building blocks of proteins. They found that some lactocytes (milk-secreting cells) showed particularly strong signs of protein production and secretion-related activity. Other lactocytes had different metabolic profiles, suggesting that even cells of the same general type aren’t necessarily doing identical jobs.

“The study also revealed high levels of sulfur and iron within immune cells called neutrophils that contribute to the killing of bacteria and help protect babies from infection,” Donna Geddes, one of the researchers and a human lactation scientist at UWA, said.

Taken together, the findings suggest that the living cells in human milk are a remarkably varied community. Some are involved in milk production, while others have metabolic features associated with immune defense and nutrient handling.

“With our new approach, we can look inside individual cells at the nanoscale and begin to understand how their structure, chemical composition and metabolism are connected,” Qiongxiang Lin, first author of the study and a PhD candidate at UWA, said.

A new wat to study maternal and infant health

Why does all this matter? Because scientists are still working to understand exactly what the living cells in human milk contribute to breastfeeding and infant development.

Decades of research have shown that breast milk supports infant growth, helps protect against infections, and contributes to the development of the immune system. But despite these well-established benefits, scientists still don’t fully understand how all its components work together.

The researchers believe this approach could help explain how living cells contribute to human milk composition and volume, and how their activities change between mothers, throughout lactation and under different health conditions.

One possibility is that a mother’s health could influence the kinds of cells present in her milk or how those cells process nutrients.

The next goal is to investigate whether maternal health conditions influence the cellular and metabolic characteristics of milk. One particular interest is gestational diabetes, which develops during pregnancy.

“We are particularly interested in understanding whether conditions such as gestational diabetes alter the cellular and metabolic characteristics of human milk,” K. Swaminathan Iyer, one of the study authors and deputy head of UWA’s School of Molecular Science, concludes.

The study is published in the journal ACS Nano.