Erythropoietin (Epo)

• A growth factor secreted by cells primarily located in the interstitium of the kidney, is a critical growth factor for early erythroid progenitors. Epo expression in the kidney is regulated by oxygen tension through the transcription factor hypoxia-inducible factor (HIF), described further in Chapters 3 and 12, such that when delivery of oxygen to tissues falls, HIF activity and Epo production rise.

Thrombopoietin (Tpo)

• An important growth factor for megakaryocytes, the large polyploid marrow cells that shed platelets into the circulation. Tpo is secreted constitutively by hepatic parenchymal and endothelial cells as well as by bone marrow stromal cells. Unlike Epo, the activity of Tpo is not regulated by changes in its expression but rather through a competition between platelets and megakaryocyte progenitors for Tpo.
• Platelets and marrow progenitors both express Tpo receptors, such that as platelet levels fall, the level of free Tpo available to stimulate megakaryocytic precursors in the marrow rises. Thus, platelet production is regulated by the total body platelet mass. This relationship explains why the sequestration of platelets in enlarged spleens fails to stimulate increased platelet production; these platelets still bind Tpo, keeping free Tpo levels relatively low even in the face of thrombocytopenia in the peripheral blood.

Granulocyte colony-stimulating factor (G-CSF)

• An important growth factor for neutrophil progenitors. Like Tpo, G-CSF is secreted constitutively by several different cell types, including macrophages, endothelial cells, and fibroblasts. The production of G-CSF increases over basal levels in response to inflammatory cytokines such as IL-1 and tumor necrosis factor. By stimulating granulocytic progenitors in the marrow, G-CSF can markedly increase neutrophil production.

The Cell Cycle

• Divided into the mitotic phase (M phase), during which the cell physically divides, and interphase, during which the chromosomes are duplicated and cell growth occurs prior to division.
  • G1 phase: the cell begins to commit to replication
  • S phase: DNA content doubles and the chromosomes replicate
  • G2 phase: the cell organelles are copied and cytoplasmic volume is increased
  • G0 stage: the cells rest prior to division and can remain for long periods of time
• Controlled by two checkpoints which act as brakes to coordinate the division process at the end of the G1 and G2 phases. Two major classes of molecules control these checkpoints, cyclin‐dependent protein kinases (Cdk), which phosophorylate downstream protein targets, and cyclins, which bind to Cdks and regulate their activity.

Red Blood Cells

• The erythroid‐ specific genes for globin and haem synthesis have binding motifs for GATA‐1.
• Erythropoiesis passes from the stem cell through the progenitor cells, colony‐forming unit granulocyte, erythroid, monocyte and megakaryocyte (CFUGEMM), burst‐forming unit erythroid (BFUE) and erythroid CFU (CFUE), to the first recognizable erythrocyte precursor in the bone marrow, the pronormoblast. This process occurs in an erythroid niche in which about 30 erythroid cells at various stages of development surround a central macrophage.
• The marrow requires many other precursors for effective erythropoiesis. These include metals such as iron and cobalt, vitamins (especially vitamin B12, folate, vitamin C, vitamin E, vitamin B6, thiamine and riboflavin) and hormones such as androgens and thyroxine. Deficiency in any of these may be associated with anaemia.

Granulocytes

Neutrophil kinetics

CSF, colony‐stimulating factor; G, granulocyte; IL, interleukin; M, monocyte; SCF, stem cell factor.

• Primary (azurophilic) granules are acquired at the promyelocyte stage and contain a wide array of proteins, including myeloperoxidase, defensins, cathepsins, and ELANE.
• Secondary granules are secretory granules acquired at the transition to the myelocyte stage. Neutrophil secondary granules contain LF, the vitamin B 12 binding protein transcobalamin I, and the metalloproteinases (neutrophil collagenase and gelatinase), as well as NGAL.
  • With the exception of gelatinase, which is also expressed by monocytes, expression of the secondary granule proteins is restricted within the hematopoietic lineage to neutrophils. Secondary granules and the synthesis of their contents therefore constitute a definitive marker of commitment to terminal neutrophil maturation.
  • Characteristic secondary granules are acquired at the same stage by eosinophils and basophils.
• Tertiary granules, containing primarily gelatinase, are formed during later stages of neutrophil maturation.

Regulation of hemopoiesis

Pathways of stimulation of leucopoiesis by endotoxin, for example from infection. It is likely that endothelial and fibroblast cells release basal quantities of granulocyte–macrophage colony‐stimulating factor (GM‐CSF) and granulocyte colony‐stimulating factor (G‐CSF) in the normal resting state and that this is enhanced substantially by tumour necrosis factor (TNF) and interleukin‐1 (IL‐1).

a. These granule constituents are conventionally assigned to the secondary granule, but their exact compartment remains controversial. Some may be located in the tertiary granule or possibly in one of the other, heterogeneous small-granule populations.