Breastmilk

Breastmilk is Dynamic

Cellular and transcriptional diversity over the course of human lactation

This recent 2022 paper in the Proceedings of the National Academy of Sciences by Dr. Nyqiust and colleagues is a site for sore eyes.

It offers a remarkable, high-resolution portrait of how the cellular landscape of human breast milk (hBM) shifts over time. The authors capture something both scientifically rich and uniquely human: the dynamic, living composition of milk as it adapts to the changing needs of mother and child.

The abstract: "Human breast milk is a dynamic fluid that contains millions of cells, but their identities and phenotypic properties are poorly understood. We generated and analyzed single-cell RNA-sequencing (scRNA-seq) data to characterize the transcriptomes of cells from hBM across lactational time from 3 to 632 d postpartum in 15 donors. We found that the majority of cells in hBM are lactocytes, a specialized epithelial subset, and that cell-type frequencies shift over the course of lactation, yielding greater epithelial diversity at later points. Analysis of lactocytes reveals a continuum of cell states characterized by transcriptional changes in hormone-, growth factor-, and milk production-related pathways. Generalized additive models suggest that one subcluster, LC1 epithelial cells, increases as a function of time postpartum, daycare attendance, and the use of hormonal birth control. We identify several subclusters of macrophages in hBM that are enriched for tolerogenic functions, possibly playing a role in protecting the mammary gland during lactation. Our description of the cellular components of breast milk, their association with maternal–infant dyad metadata, and our quantification of alterations at the gene and pathway levels provide a detailed longitudinal picture of hBM cells across lactational time. This work paves the way for future investigations of how a potential division of cellular labor and differential hormone regulation might be leveraged therapeutically to support healthy lactation and potentially aid in milk production." (Nyquist et. al. 2022)



For decades, breast milk has been recognized as more than nutrition. It is a living fluid, replete with cells that serve immunologic, structural, and metabolic roles. Yet the precise identities of these cells, how they change over time, and how they respond to maternal and infant circumstances have remained largely mysterious. This work fills some of that gap by generating a longitudinal, cell-level atlas of milk, one that connects gene expression to real-world maternal and infant factors.

The key finding is the central role of lactocytes, the milk-producing epithelial cells that dominate breast milk’s cellular population. While they are abundant throughout lactation, their nature changes in subtle but meaningful ways. Early in lactation, lactocytes are heavily focused on production; over time, structural variants of these cells become more prevalent, and overall epithelial cell diversity increases. Rather than existing as static cell types, lactocytes occupy a continuum of gene transcriptional states, gradually shifting their expression of hormone and growth factor related genes, as if tuning their activity to meet evolving demands. Such is the beauty of human evolutionary dynamics!

Let me paraphrase some of their work: lactation progresses through three main stages: 1) colostrum (0–3 days postpartum) 2) transitional milk (6–14 days) 3) mature milk (>15 days). From the earliest days of pregnancy, to lactation, and finally involution, the mammary gland undergoes major remodeling in structure and cell composition, guided by hormonal signals. During active lactation, mammary cells synthesize and transport the diverse components of human breast milk while responding to maternal and infant cues to sustain milk production. Human milk contains 7 immune cell clusters, including B cells, dendritic cells, T cells, two macrophage clusters, neutrophils, and eosinophils as well as three nonimmune top-level clusters, including luminal cluster 2 (LC2) , luminal cluster 1 (LC1), and fibroblasts, likely shed from the mammary gland during feeding. These cells can be cultured, and in animal models, milk-derived immune cells enter the offspring’s bloodstream and tissues. Immune cells, such as macrophages, which were the predominant immune cell, may protect the breast from infection and provide antibodies and cytokines that shape infant immunity. Somatic cells include epithelial cells and a small number of stem cells; the dominant epithelial cells, lactocytes, produce and secrete key milk components including human milk oligosaccharides, lactose, fats, micronutrients, hormones, and cytokines.

Intriguingly, the authors also uncover correlations between certain cell subclusters and maternal lifestyle or hormonal factors. One epithelial group, labeled “LC1”, was more common as lactation progressed and appeared in higher abundance among mothers whose infants attended daycare and those using hormonal birth control. These associations hint at a remarkable responsiveness of milk cells to both the internal hormonal milieu and external environmental exposures, underscoring lactation’s adaptive nature.

Immune cells within milk also tell a compelling story. Macrophages, present in several distinct subclusters, appear enriched for tolerogenic functions, suggesting a role in preventing excessive immune activation in the mammary gland. This aligns with the idea that successful lactation requires a careful balance: protecting the gland and infant from pathogens while avoiding inflammation that could disrupt milk production.

By pairing high-resolution sequencing with maternal infant metadata, the study bridges molecular biology and lived experience. The result is not just a catalog of cell types, but a dynamic narrative of milk as it responds to postpartum time, hormonal status, and environmental exposures. It reframes lactation as a biologically fluid process that is sensitive to an array of physiological and lifestyle cues.

The implications are far-reaching. This foundational map of milk’s cellular composition could guide precision interventions to support mothers struggling with lactation, inform more biologically faithful infant formula design, and help optimize milk banking practices. For researchers, it opens a pathway to explore how diet, stress, or illness might further shape milk’s cellular landscape, and how these changes influence infant development and immune programming.

The gene-expression data, while richly informative, is part one where part two is the key functional validation to confirm the protein roles suggested by transcriptional profiles.

It reminds us that breast milk is not a static product but a living, responsive medium. Its cellular diversity and function shifting in concert with the evolving needs of mother and child.

Something that we will NEVER find in cow milk based formula.

By mapping this dynamic biology with such precision, the study deepens our appreciation for lactation’s sophistication and adaptability. It also plants the seeds for advances in maternal and infant care, from targeted nutritional support to improved formula composition, iterative in nature. In doing so, it brings us a step closer to understanding not just what milk contains, but how it changes and why that matters. In truth, a reminder that in biology, as in life, change is the essence of resilience.

Change and understanding remain the key to a prosperous society and medical field.

Dr. M

Nyquist PNAS