Bone marrow
“Bone Marrow”, B cell BCR production site
Not to be confused with spinal cord!
Bone marrow is the site of hematopoiesis. It ensures on the one hand the self-renewal of hematopoietic stem cells and on the other hand their engagement in the maturation processes into myeloid and lymphoid lineages. It is a particularly important structure in the maturation of B cells since they acquire their BCR there, and undergo negative selection also called central tolerance (see B cell ontogeny).
Structure
Bone marrow is a highly organised structure present in all bones, but especially in long bones (femur, humerus) and flat bones (sternum, iliac crest). There are several cell types, schematically:
- Osteoblasts that synthesise bone;
- endothelial cells , which delimit blood vessels;
- reticular cells which allow the different actors to be interconnected;
- sympathetic neurons that control the release of hematopoietic precursors.
These cellular actors build bone niches, which are specialised microenvironments for the self-renewal and differentiation of hematopoietic stem cells.
Primary functions
Hematopoiesis
The bone marrow is the site of self-renewal and differentiation of hematopoietic stem cells leading to the synthesis of all blood cells.
B cell ontogeny
B cells undergo a process of maturation in the bone marrow. They acquire their BCR after two stages of verifying its integrity . Firstly, verification of the heavy chain, then secondly, verification of the heavy chain – light chain assembly (see B cell ontogeny).
B cells that have an intact BCR then undergo, before leaving towards the periphery, a negative selection also called central tolerance, which reduces the number of self-reactive B cell clones, that is to say, those which have a BCR directed against self-antigens.
What should be remembered
Bone marrow is a highly organised structure present in all bones but especially in long and flat bones. It is the seat of hematopoiesis and plays a particularly important role in the ontogenesis of B cells since it is there that they acquire their BCR and undergo negative selection aimed at reducing the number of self-reactive B cell clones.
