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djublonskopf t1_jb2a0kb wrote

It actually varies by species! Penguins, on one extreme, have zero hollow bones, keeping a heavy, marrow-filled skeleton to help with deep diving.

For most other birds, the skeleton is actually quite marrow-rich when they are just hatchlings. As they grow into adulthood, the skeleton gradually becomes pneumatized, with air sacs displacing red marrow in many bones. However, this displacement is not 100%...small spots of marrow persist into adulthood in many bones that researchers would otherwise call "pneumatized" or "hollow".

A few bones tend to escape pneumatization entirely, especially the femur and the tibiotarsus. In most birds these red-marrow-filled leg bones will make up the bulk of blood cell production. Other common red-marrow sites in birds are the radius/ulna in the wing, and certain sections of vertebrae (especially the caudal vertebrae in the tail). Many other bones will retain just a small amount of marrow in some birds, and none in others, so there is variation from species to species...but in general those three sites (fibia/tibiotarsus in legs, radius/ulna in wings, vertebrae in tail) will play a big role in making red blood cells for most birds.

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[deleted] t1_jb1x76s wrote

Birds have bone marrow, just not in all of their bones.

They've some pneumatic bones which are hollow and filled with air, and actually connected to their respiratory system, through air sacs in their abdomen, giving them extremely efficient respiration which is likely an adaptation for dealing with low oxygen in flight and fluctuating air pressure etc.

Obviously the hollow bones give birds a weight advantage and their bones are also made form a tougher material than ours, so despite being light they're also extremely strong and springier - all of which is about adaptions to flight.

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Mikedc1 OP t1_jb25ym6 wrote

That's cool it seems way more complex and diverse than I thought. I thought all bones were calcium like matrices with small differences but turns out birds have some other composite

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[deleted] t1_jb2k9kb wrote

More or less the same composition but the material is denser, yet full of hollow spaces and struts. They tend to break quite jaggedly compared to mammalian bones, but are usually harder to break.

They’re totally evolved for flight.

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Supraspinator t1_jb2xd4l wrote

This is tangentially related. Ever wonder how B-cells (antibody producing white blood cells) got their name?

They are named after the Bursa of Fabricius, a lymphatic organ in birds that serves as the site of B-cell maturation. (Their sister cells, T-cells, mature in the thymus). Stem cells migrate from the liver to the Bursa of Fabricius, where they differentiate and mature into B-cells.

B-cells were first discovered in the Bursa fabricii of birds, that’s why they are named B-cells. The bursa equivalent in humans is the bone marrow.

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screen317 t1_jb924xl wrote

The real bursa equivalent is the spleen, where transitional B cell development occurs after central tolerance.

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SillyStallion t1_jb2r1bs wrote

What is amazing is that they have less red cell generating capacity, their red cells have half the life span of mammalian cells, less oxygen carrying capacity (nucleated red cells) and no splenic storage - it’s a wonder they haven’t had a significant evolutionary disadvantage

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green_print_business t1_jb403a7 wrote

The bones of birds are not completely empty. They are lighter in weight and have air pockets. The bone marrow in birds, like that of other animals, produces red and white blood cells, but the location and distribution of the marrow is different due to the unique structure of bird bones. HSCs are primarily found in the bone marrow of birds, which is a spongy tissue found within the bones. However, some studies have suggested that HSCs may be present in other tissues in certain bird species, such as the liver and spleen.

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Mikedc1 OP t1_jb47yq0 wrote

That's what I was thinking if their bones have to be at least partially empty then some of those stem cells must be somewhere else. That makes a lot of sense.

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