Your bone marrow makes about 500 billion blood cells every day
Tucked inside spongy bone, marrow makes up roughly 5% of an adult's body mass, around 3.7 kg in a 73 kg person. It turns out some 500 billion blood cells a day. A newborn's bones hold only active red marrow, but with age much of it gradually turns to fat.
Marrow is a semi-solid mix of blood-forming cells, fat and supporting stromal tissue. In adults the productive red marrow sits mainly in the central skeleton, in the ribs, vertebrae, sternum and pelvis, while the long bones of the limbs fill with fatty yellow marrow. Stem cells here give rise to all three classes of circulating blood cell: red cells, white cells and platelets. Lymphocytes begin in the marrow too, though some, such as T cells, must travel to the thymus to finish maturing, which is why marrow and thymus together count as primary lymphoid organs.
The surrounding stroma is more than packing. Stromal cells release colony-stimulating factors that steer blood cell formation, macrophages hand over iron for haemoglobin, and endothelial cells line the sinusoids through which new cells enter the bloodstream. Those vessels act as a gate: only mature cells carry membrane proteins such as aquaporin and glycophorin that let them pass. Blood-forming stem cells can also slip through, which allows them to be harvested from blood. Cells even cluster by type, with red-cell precursors gathering around vessels and granulocytes at the edges.
Since 2003 marrow has also been known as a place where T cells meet blood-borne antigens and get activated, a finding extended in 2013 by imaging inside mouse skulls. It serves as a long-term refuge for memory B and T cells and plasma cells, housed in niches built by stromal cells, which matters for how vaccines keep working. Its mesenchymal stem cells can become bone, cartilage, muscle and fat cells.
Because marrow divides so rapidly, radiation and chemotherapy hit it hard, weakening immunity, and much of radiation sickness stems from marrow damage. Leukaemia, multiple myeloma and aplastic anaemia disrupt it, and transplants can treat some of these diseases. Doctors sample it with a hollow needle, usually from the crest of the hip bone. In a healthy sample, myeloid cells outnumber red-cell precursors by about three to one.
Source: Bone marrow
