Role of Bone Marrow in Haematopoiesis

In adults, haematopoiesis primarily occurs in the bone marrow, producing 5-10 x 10^11 blood cells daily. Bone marrow is classified as active (red) or inactive (yellow) based on the presence of haemoglobin or adipocytes, respectively.

Bone Marrow Structure

Bone marrow is a highly vascular tissue comprising blood sinuses and stroma. It weighs nearly as much as the liver and includes various cell types at different developmental stages.

Hematopoietic Stem Cells (HSCs) and Cell Lineages

HSCs are the common progenitors for all blood cells, characterized as colony-forming units (CFUs). Key lineages include myeloid (CFU-GEMM) and lymphoid (CFU-L) progenitors.

Cytokines in Haematopoiesis

Cytokines, such as colony-stimulating factors and erythropoietin, regulate the proliferation and differentiation of stem/progenitor cells. Thrombopoietin specifically stimulates megakaryocyte development into platelets.

Thrombopoiesis and Platelet Production

Thrombopoiesis involves the CFU-MK progenitor developing into megakaryocytes, which produce 10^11 platelets daily through cytoplasmic fragmentation. Thrombopoietin enhances megakaryocyte proliferation and platelet function.

Monocyte and Lymphocyte Production

Immature monocytes (monoblasts, promonocytes) develop into mature monocytes that migrate into tissues as macrophages. T cells mature in the thymus, while B cells receive differentiation signals from various sites, including the bone marrow.

Granulopoiesis

Granulocytes (neutrophils, eosinophils, basophils) arise from the CFU-GEMM lineage. Their granules contain enzymes and molecules crucial for immune response.

Self-Renewal and Oncogenesis

Self-renewal in HSCs is vital for maintaining blood cell production. Oncogenic mutations can lead to dysregulated haematopoiesis, potentially causing overcrowding in the bone marrow and impaired production of mature blood cells.