hematopoiesis
Meaning of hematopoiesis: Every red cell, white cell and platelet traces back to one kind of stem cell, replaced constantly because none of them last.
Definition of hematopoiesis
Hematopoiesis is the making of blood cells: red cells, white cells and platelets. All of them descend from a small pool of blood forming stem cells that divide and mature through a series of steps before the finished cells enter the circulation. In adults the process is confined to the bone marrow, mainly in the spine, ribs, breastbone, pelvis and the upper ends of the long bones.
Where does it happen at different ages?
| Stage of life | Main site |
|---|---|
| Early in fetal life | The yolk sac, then the liver and spleen |
| Late fetal life and infancy | Bone marrow throughout the skeleton |
| Adulthood | Marrow of the spine, ribs, breastbone, pelvis and the upper ends of the long bones |
| Adulthood under heavy demand | Active marrow expands into the long bones, and the liver and spleen can restart production |
The retreat with age is a matter of red marrow being replaced by fat, not of marrow disappearing. Production returning to the liver and spleen in an adult is called extramedullary hematopoiesis, and it is a sign that the marrow is either failing or being pushed hard. That is why an enlarged spleen turns up in several long standing blood disorders.
What controls how many cells are made?
Each line has its own signal, and each signal is released in response to a shortage. The kidney senses low oxygen in the blood reaching it and releases erythropoietin, which drives red cell production; the liver supplies the hormone that governs platelet numbers; and a family of growth factors steps up white cell output during infection. This feedback is precise enough that the marrow can raise the output of one line many times over while leaving the others alone, which is how a bacterial infection produces a high neutrophil count without disturbing anything else.
What disrupts it?
Anything that damages the stem cells, crowds them out or starves them of raw material. Chemotherapy and radiation hit dividing cells hardest, so counts fall to a low point some days after treatment and recover as the surviving stem cells repopulate. Leukemia, myeloma and cancers spreading from elsewhere replace marrow with tumor. Shortages of iron, vitamin B12 or folate stall production for want of ingredients. Where the marrow cannot recover, bone marrow transplantation replaces the stem cell pool itself.
Also known as
Used in a sentence
Bone marrow biopsy was performed to evaluate whether hematopoiesis had recovered following treatment.
Good to know
Hematopoiesis can be temporarily suppressed by treatments such as chemotherapy, producing low blood counts that generally recover once the marrow resumes normal production.
Word origin
Where “hematopoiesis” comes from
Formed from Greek haima, meaning blood, and poiesis, meaning a making or production.