Hemopoiesis

Hemopoiesis refers to the formation of blood cells (erythropoiesis, leukopoiesis, thrombopoiesis). Production and destruction of blood are ongoing processes with constant levels of cell types maintained in circulation. RBCs and platelets spend their entire life in circulation - functional in blood. Leukocytes are functional outside of blood (in tissues).

In the adult:

It occurs in the red bone marrow of the skull, sternum, vertebrate, ribes, pelvis, and the proximal epiphyses of the femur and humerus. Lymphocytes are formed in the red marrow and in the lymphatic tissue.

In the fetus:

It occurs in the yolk sac, beginning in the 3rd week of gestation. Blood islands form in the walls of the yolk sac of the embryo. Production changes to the liver during the second trimester, from hemopoietic centers. Production changes to the bone marrow beginning late 2nd trimester, and by the 3rd trimester and at birth it is fully in from the bone marrow.

Erythropoiesis:

Myeloid Lineage of Cells ⇒ Megakaryocyte/Erythrocyte Progenitor Cells ⇒ Erythrocyte Committed Progenitor Cells ⇒ Proerythroblasts ⇒ Basophilic Erythroblasts ⇒ Polychromatophilic Erythroblast ⇒ Orthochromatic Erythroblast or Normoblast ⇒ Reticulocyte ⇒ Erythrocyte

RBC formation and release is controlled by the glycoprotein hormone erythropoietin, which is synthesized and secreted by the kidney.

Thrombopoiesis:

Myeloid Lineage of Cells ⇒ Megakaryocyte/Erythrocyte Progenitor Cells ⇒ Megakaryocyte Committed Progenitor Cells ⇒ Megakaryoblasts ⇒ Megakaryocytes ⇒ Platelets

Granulopoiesis:

Myeloblasts ⇒ Promyelocytes ⇒ Myelocytes ⇒ Metamyelocytes ⇒ Band cells ⇒ mature neutrophils or eosinophils or basophils

Monocytopoiesis:

Monoblasts are the committed precursor cells for monocyte development. Monoblasts differentiate to form promonocytes, which then divide twice to form monocytes. It takes 55 hours for monoblasts to become monocytes, which then remain in circulation for 16 hours before migrating into different tissues to form macrophages.

Lymphopoiesis:

The bone marrow is the primary site of lymphopoiesis. Continuous proliferation occurs in the peripheral lymphatic organs. Lymphoid progenitor cells that have the ability to express GATA-3 transcription factors develop into T lymphocytes. GATA-3 expression cells leave the bone marrow as pre-T lymphocytes and complete their differentiation in the thymus. Mature T lymphocytes go back into the circulation. Transcription factor PAX5 activates the lymphoid progenitor cells to become B lymphocytes, which further develop in lymphatic tissue and the spleen. NK cells develop under the influence of interleukin 2 (IL-2) and interleukin 15 into immature pre-NK cells. Pre-NK cells mature into the NK cells.

Bone marrow sinusoids provide a barrier between hemopoietic compartments and the peripheral circulation. It is a unique vascular unit, equivalent to a capillary between arterial supply and venous drainage. Wall consists of endothelium, a discontinuous basement membrane and an incomplete covering of adventitial cells (also called reticular cells). The adventitial cells provide support for the hemopoeitic cells by sending sheet-like extensions into the substance of hemopoietic cords. This produces reticular fibers (additional support for the cells). They secrete cytokines for blood cell development.