Formation of blood. This is a fairly well understood process with most of the regulatory hormones and cell precursors well characterised. Blood cells are divided into either myeloid or lymphoid cells, which in turn are subdivided into various different precursors. Lymphoid cells eventually differentiate into lymphocytes which includes B, T and NK cells. The myeloid lineage divides into:
- erythroid lineage - forms erythrocytes
- granulocyte lineage (to confuse matters, sometimes called myeloid) - forms neutrophils, eosinophils, basophils and monocytes/macrophages.
- megakaryocytes - from which platelets derive
Each lineage occupies a particular topographical niche within the bone marrow (where most of haematopoiesis occurs in adults).
At the heart of haematopoiesis are haematopoietic stem cells (HSC). These are the source of all blood cells and reside mainly in the bone marrow, though some circulate within the blood itself. Large numbers can be forced into the circulation by hyperstimulation, as is done in bone marrow harvests from healthy donors or in the patient themselves in autologous transplants.
Like all stem cells, these cells, firstly, can renew themselves, and secondly, are pluripotent, i.e. have the ability to differentiate into a range of different cell types. In the context of blood, they are able to produce any type of blood cells. Interestingly, there are several classes of HSCs with different dynamics, but all of which can be categorised into 3 types based on the proportion of the two major lineages that they produce. This is a stable, epigenetically-determined characteristic of HSCs. Myeloid-biased HSCs produce predominantly myeloid cells, lymphoid-biased HSCs produce predominantly lymphoid cells, while balanced HSCs produce both types in equal proportions.
A large number of pathological processes interfere, varying from the genetically obscure such as cyclic neutropenia caused by mutations in the neutrophil elastase gene to activation of the immune response whose effects on the blood are now regarded as basic medical knowledge.
Accumulated mutations in stem cells also give rise to various disorders of blood cell production. Excess functional cells are seen in myeloproliferative, whereas cytopenias are seen in myelodysplastic conditions. Autoimmune conditions may also give rise to various cytopenia, e.g. aplastic anaemia, paroxysmal nocturnal haemoglobinuria and idiopathic thrombocytopenic purpura.