Human embryos have a mysterious “yolk sac” and it plays a key role in human development

In the early stages of embryonic development, the chicken embryo is nourished solely by the golden-colored yolk. While mammals like us don’t need this personal snack, our developing cells spend resources building a yolk sac. Previously thought to be a useless relic, this evolutionary remnant has been discovered by researchers at the Wellcome Sanger Institute.

Mapping the evolution of the yolk sac

A team led by dermatologist Muzlifa Haniffa conducted a study as part of the international Human Cell Atlas initiative to examine human yolk sac tissue. The study aims to understand how this organ develops during the early stages of pregnancy and its role in the development of the immune system. Haniffa explains, “This is the first time we are showing the multiple organ functions of the yolk sac – we have seen the baton from the yolk sac to the liver, to the bone marrow.”

The source of our first blood cells

Studies in various animal models have suggested that the yolk sac is responsible for producing the first blood cells, including both red blood cells for oxygen transport and white blood cells for immune response. These cells travel from the yolk sac to the liver and eventually settle in the bones to form bone marrow. However, research on the human yolk sac is limited and many questions remain unanswered.

Detailed atlas of human yolk sac cells

To understand the peculiarities of the functioning of human yolk sac tissues, researchers sequenced the RNA strands actively produced by its cells. These data allowed them to create an extensive library of materials generated by nearly 170,000 human yolk sac cells. The resulting atlas included 15 categories of tissue types involved in blood cell production and a variety of critical early functions.

Critical role in early development

Analyzing samples obtained 4-8 weeks after conception, the researchers constructed a timeline of activity that revealed the yolk sac’s critical role in the development of many systems. In addition to producing blood cells, clotting agents, and metabolic enzymes, the yolk sac also plays an important role in breaking down sugars and lipids, fighting toxins, and developing and dispersing immune cells.

Differences in human yolk sac tissues

Although the yolk sac functions similarly in most animals, researchers have identified significant differences in the cellular and biochemical pathways of human yolk sac tissues compared to typical laboratory models such as mice. Understanding these processes at the cellular level may contribute to advances in disease research and artificial tissue and organ culture.

Dr. Haniffa emphasizes the importance of the findings, stating, “Studying how the yolk sac develops during the first weeks of pregnancy is fundamental to understanding the development of the immune system.” This groundbreaking study sheds light on the previously underappreciated role of the yolk sac and opens new avenues for research.

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