Yellow bone marrow is fatty marrow that gradually replaces red marrow in the shafts of long bones from childhood onward. By adulthood it fills the medullary cavities of the limb bones. It contains mainly fat cells and few blood-forming cells, but it keeps the ability to convert back to red marrow if the body needs more blood cells.
| Location | Medullary cavities of the shafts of long bones in adults, with increasing proportion in all bones with age |
|---|---|
| Secretes |
|
| Blood Supply | Nutrient arteries of bones |
Stores fat as an energy reserve and acts as a backup site for blood cell production when demand increases.
On MRI, fatty yellow marrow is bright on T1 images, so loss of that signal can indicate marrow infiltration by tumour, infection or reconversion. After major blood loss or in chronic anaemia, yellow marrow can reconvert to red marrow. Fat from yellow marrow can enter the circulation after long bone fractures and contribute to fat embolism syndrome.
Fat globules released after long bone fracture lodging in the lungs, brain and skin, causing breathlessness, confusion and a petechial rash.
Return of yellow marrow to red, blood-forming marrow in response to anaemia, high altitude or endurance training, visible on MRI.
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