Structure and function of blood-forming tissues, reticulo-endothelial system, and blood components and process of haematopoiesis

Overview

The blood-forming (haematopoietic) system - predominantly the bone marrow in adults - produces all blood cell lineages from a common haematopoietic stem cell, via a tightly regulated process of proliferation and differentiation
Haematopoiesis is a continuous, high-turnover process throughout life - the bone marrow produces billions of new blood cells daily to replace those lost to normal senescence, reflecting blood cells' generally short lifespans relative to most other tissues

A single pluripotent haematopoietic stem cell gives rise to myeloid (red cells, platelets, granulocytes, monocytes) and lymphoid (T cells, B cells, NK cells) progenitor lineages, each undergoing progressive commitment and maturation under the influence of specific growth factors/cytokines (erythropoietin for red cell production, thrombopoietin for platelets, G-CSF for granulocytes, among others)
In adults, haematopoiesis is normally confined to the axial skeleton and proximal long bones (vertebrae, pelvis, sternum, proximal femur/humerus) - the marrow of distal long bones becomes fatty/inactive after childhood, though it retains the capacity to resume haematopoiesis under sufficient stress (extramedullary/marrow reactivation)

Blood components - red cells (oxygen transport, ~120-day lifespan), platelets (haemostasis, ~7-10 day lifespan), white cells (immune function, variable lifespans from hours for neutrophils to years for some memory lymphocytes) - each with a distinct production rate and lifespan explaining the differing time course over which each cytopenia develops and resolves following marrow injury

Understanding normal haematopoietic hierarchy and site explains disease patterns: marrow infiltration/failure disorders (leukaemia, aplastic anaemia, myelofibrosis) disrupt production at the stem/progenitor cell level, affecting multiple lineages simultaneously, while disorders of a single mature lineage (e.g. isolated immune thrombocytopenia) reflect peripheral destruction rather than a production-level problem

Aplastic anaemia (stem cell failure), myelofibrosis (marrow fibrosis displacing normal haematopoiesis, with extramedullary haematopoiesis in the spleen/liver as compensation), leukaemia (clonal proliferation arresting normal differentiation)

The differing lifespans of each blood cell lineage (platelets ~7-10 days, red cells ~120 days) explain why a sudden marrow insult causes thrombocytopenia and leucopenia before a significant drop in haemoglobin becomes apparent - a clinically useful principle when interpreting the time course of cytopenias after a marrow-toxic exposure

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