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Fetal Tissue Single-Cell RNA Analysis Reveals New Macrophage Subtypes

NEW YORK — Researchers have unraveled new subtypes and origins of macrophages in various tissues of human embryos.

For a study published in Cell on Tuesday, a team led by investigators from Fudan University in China used single-cell RNA sequencing to create a spatial and temporal molecular roadmap of macrophages using immune cells from medically aborted fetuses.

The authors noted that macrophages, crucial in human organ development, organogenesis, homeostasis, and disease progression, have not been well studied in human embryonic development. Moreover, they added, it is unclear that the findings of macrophage diversity and origin seen in murine studies apply to human tissues.

For this study, the researchers collected 367 prenatal samples from 19 different time points between four and 26 weeks post-conception, enabling them to examine nearly 300,000 cells. The samples came from tissues of 52 human embryos recovered from healthy pregnant women who underwent medical abortions at the Shenzhen University General Hospital and Shenzhen Baoan Maternal and Child Health Hospital. The authors wrote that crown-heel length and morphological features were used to determine each embryo's developmental stage.

By studying all the samples, the researchers found that immune cells in different organs diversified at various stages of embryonic development. They noted that macrophages showed the highest tissue-specific subtype diversity among all immune cells.

Meanwhile, using RNA sequencing and flow cytometry, the researchers identified microglia-like cells residing in tissues such as the fetal skin, testicle, and heart that were morphologically similar to the microglia seen in the central nervous system (CNS). This finding challenged the long-held view that microglia are confined and evolved to adapt only to the CNS.

Since the microglia-like cells were most abundant in the epidermis, the researchers conducted a detailed examination of the fetal skin samples and noted that these cells were prevalent in the skin of the back and head, compared with the skin of the limbs and abdomen. Depleting the microglia-like cells in the fetal skin affected the differentiation of neural crest cells along the melanocyte lineage.

The study also found a population called the proangiogenic macrophage (PraM) in the perivascular space. Unlike the other tissue-specific macrophages, PraMs didn't show tissue-specific signatures. However, the authors wrote that these also originated from yolk sac-derived macrophage progenitors like the microglia.

"Our study provides a comprehensive map of the heterogeneity and developmental dynamics of human macrophages and unravels their diverse functional roles during development," they concluded.

Highlighting the limitations of the analysis, they wrote that some of the macrophage subtypes may have been underrepresented due to low abundance and difficulty in isolating cells.

The researchers noted that follow-up research is necessary to find out if the microglia-like cells are also present in similar anatomical locations of mice and other species. Furthermore, they said more investigation was required to uncover the molecular mechanisms underlying the interactions between epidermis-resident microglia-like cells and neural crest cells.