Why Is the Inside of a Bone Alive? A Factory Producing Blood

The tissue contains stem cells capable of becoming any kind of blood cell, and they divide continuously to replace what the body loses. Red cells last around four months and are replaced at roughly two million per second. Platelets last about a week. White cells of several kinds are produced in response to demand, with production ramping up during infection. That output is enormous in total, amounting to hundreds of billions of cells daily throughout life, and it is regulated by hormones released elsewhere in response to conditions, including one from the kidneys that responds to low oxygen and drives red cell production upwards.

The tissue comes in two forms whose balance changes with age:

• •Red marrow, which actively produces blood cells
• •Yellow marrow, which is largely fat and produces nothing under normal conditions
• •A newborn has red marrow throughout the skeleton
• •It is progressively replaced by yellow marrow from the limbs inwards
• •An adult retains red marrow mainly in the pelvis, spine, ribs, sternum and skull
• •Yellow marrow can revert to red under severe demand, such as major blood loss

Tissue dividing rapidly is exactly what is damaged by several kinds of insult, which is why marrow features so prominently in medicine. Radiation damages dividing cells preferentially, so marrow failure is a leading cause of death in radiation exposure and a limiting factor in radiotherapy. Chemotherapy works by attacking rapidly dividing cells and therefore attacks marrow as a side effect, which is why treatment causes anaemia, infection risk and bleeding, and why doses are limited by marrow recovery rather than by the tumour. Several drugs and toxins suppress it. Autoimmune conditions can attack it. And cancers arising within it, including leukaemia and myeloma, crowd out normal production directly.

Marrow has been food for as long as people have broken bones open, and the archaeological evidence for that is substantial. Cut and percussion marks on animal bones at sites hundreds of thousands of years old show carcasses being broken specifically to reach it, and it is calorie-dense, fatty and available from bones after the meat has been stripped, which makes it a rational target for a scavenging or hunting population. Several researchers have argued that access to it was an important early source of the fat that a growing brain requires. It remains a delicacy across many cuisines, roasted in the bone and eaten with a spoon, and it is the basis of several traditional preparations including the dumplings and broths of central Europe.

Replacing the tissue is possible and is among the more remarkable procedures in medicine. Stem cells are collected either from a donor's pelvis under anaesthetic or, far more commonly now, from the bloodstream after a drug pushes them out of the marrow, which avoids surgery entirely. The recipient's own marrow is destroyed by chemotherapy or radiation, and the donated cells are given by infusion into a vein, from where they find their way into the bone cavities and begin producing. Matching is done on immune markers rather than blood group, which is why finding a donor can be difficult and why registries of volunteers matter. The main danger afterwards is the donated immune system attacking the recipient's tissues.

Stem cells in the cavities of bones produce hundreds of billions of blood cells daily, with red cells replaced at around two million per second. Red marrow is progressively replaced by fatty yellow marrow from the limbs inwards, leaving an adult with production concentrated in the pelvis, spine and ribs. Rapidly dividing tissue is preferentially damaged by radiation and chemotherapy, which is why marrow recovery limits cancer treatment.