The transition of blood-formation by the foetal liver to the bone marrow is orchestrated by cells of the yolk sac
To encourage wildlife into your garden, you might make specialised nooks for different animals – a house for birds and a pond for frogs – to encourage them in. Developing bone marrow does something similar to invite immune cells, crafting tailored microenvironments for cells derived from the embryo's yolk sac to settle into after a brief stint in the early liver. But precisely how this recruitment happens was unclear, and understanding the process could explain childhood bone marrow syndromes and improve bone marrow engineering. A study found a type of blood vessel marked by a protein called VCAM1 guides stem and immune cells into place. When there, immune cells colonise particular areas of the bone marrow (pictured, mouse femur with different immune cell types coloured, unevenly distributed). Experiments showed that yolk-sac derived cells have different impacts on blood cell production in the liver and bone marrow, further showing the importance of location.
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