Chinese researchers have found that the very first blood cells in the human body begin forming earlier than scientists previously believed.

The findings, published online in the journal Nature, were made by researchers from the Institute of Hematology & Blood Diseases Hospital at the Chinese Academy of Medical Sciences (IHCAMS), Beijing Institute of Technology, and the Chinese PLA General Hospital. 

They could improve scientists' understanding of early human development and provide new insights for growing blood cells in the laboratory and studying developmental disorders, said Lan Yu, a researcher at the IHCAMS.

About three weeks after conception, an embryo enters a crucial stage of development known as gastrulation, when it begins laying down the basic blueprint for the body's tissues and organs.

Scientists previously believed that the first blood cells formed only after gastrulation. By analyzing a rare early-stage human embryo, however, the researchers found that blood production begins even earlier. Before gastrulation starts, the embryonic yolk sac – a temporary structure that nourishes the developing embryo – is already producing the first blood cells.

The team also created the first high-resolution spatial map of the human embryo before gastrulation. The map showed that these earliest blood cells originate from tissue outside the embryo itself, rather than from the embryonic cells long thought to produce them, helping resolve a longstanding question about how human blood first develops.

The study also found that the embryonic yolk sac is not a single, uniform blood-producing site. Instead, it contains two distinct "workshops" with different roles: one mainly produces cells that later develop into part of the immune system, while the other generates early red blood cells and cells that produce platelets, which help blood clot.