Bone marrow contains hematopoietic stem cells and stromal cells (mostly adipocytes). Marrow cellularity is the volume ratio of hematopoiesis and fat. Cellularity is age dependent — in newborns, all marrow is hematopoietic (shows 100% cellularity), with age hematopoiesis diminishes, and the amount of fat increases. In adults hematopoiesis takes place in axial skeleton only. Long bones contain only white (fatty) marrow with no hematopoiesis taking place (except proximal parts of the humerus and femur).
Normal cellularity of an adult hematopoietic bone marrow ranges between 30 – 70% and changes under pathological conditions — we talk of hypercellular (over 70%), normocellular (30 – 70%) or hypocellular (under 30%) bone marrow.
Under pathological conditions hematopoiesis may also expand into long bones of the limbs and even out of the marrow (into the liver, spleen and other organs).
Pluripotent stem cell: undifferentiated cell which divides and differentiates into 3 main hematopoietic lineages, i.e. erythropoiesis, myelopoiesis and thrombopoiesis.
Erythropoiesis: is organised into clusters known as erythroblastic islands (erythrons) which are dispersed in the intertrabecular space. They do not normally occur peritrabecularly.
Myelopoiesis: immature myelopoietic cells (myeloblasts and promyelocytes) are localised in the peritrabecular space, more mature granulopoietic cells (myelocytes, metamyelocytes, segments and rods) are situated closer to the centre of the intratrabecular space. Normal M:E (myeoloid-to-erythroid ratio) is approx. 2 – 3:1.
Thrombopoiesis: 3 – 5 separate megakaryocytes (thromocyte precursors) are usually sparsly dispersed in one field of view with 400× magnification ratio.
Bone marrow also produces monocytes and lymphocytes.
Other marrow cells: Other marrow cells form the microenvironment that is suitable for hematopoiesis; they include stromal cells i.e. adipocytes, osteoblasts and osteoclasts, fibroblasts, macrophages (a Fe supply), endothelial cells (forms vascular sinuses), and also lymphocytes, mast cells and plasma cells.
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