At each of these stages of development, the erythroblast divides several times. It takes about a week for the progeny of a newly formed basophilic erythroblast to reach the circulation. Nearly all erythrocytes are released into the circulation as soon as they are formed; bone marrow is not a storage site for erythrocytes. Erythrocyte formation and release are regulated by erythropoietin, a 34-kilodalton glycoprotein hormone synthesized and secreted by the kidney in response to decreased blood oxygen concentration. Erythropoietin acts on the specific receptors expressed on the surface of ErP.
Erythrocytes have a life span of about 120 days in humans.
When erythrocytes are about 4 months old, they become senescent. The macrophage system of the spleen, bone marrow, and liver phagocytoses and degrades the senescent erythrocytes. The heme and globin dissociate, and the globin is hydrolyzed to amino acids, which enter the metabolic pool for reuse. The iron on the heme is released, enters the iron-storage pool in the spleen in the form of hemosiderin or ferritin, and is stored for reuse in hemoglobin synthesis. The rest of the heme moiety of the hemoglobin molecule is partially degraded to bilirubin, bound to albumin, released into the bloodstream, and transported to the liver, where it is conjugated and excreted via the gallbladder as the bilirubin glucuronide of bile.
Development of Thrombocytes (Thrombopoiesis)
Each day bone marrow of a healthy adult produces about 1  1011 platelets, a number that can increase 10-fold in time of increased demand. The thrombocytopoiesis from the bone marrow progenitors is a complex process of cell divisions and differentiation that requires the support of interleukins, colony-stimulating factors, and hormones.
Thrombocytes (platelets) develop from a bipotent megakaryocyte/erythrocyte progenitor (MEP) cell that differentiates into megakariocyte-committed progenitor (MKP) cell and finally into a megakaryocyte.
Platelets are produced in the bone marrow from the same common myeloid progenitor (CMP) cells as the erythroid and myeloid series. Under the influence of granucolyte-macrophage colony-stimulating factor (GMCSF) and IL-3, CMP stem cell differentiates into a bipotent megakaryocyte/erythrocyte progenitor (MEP) cell.
Further development proceeds toward a unipotent megakariocytecommitted progenitor (MKP) cell (or CFU-Meg), which further develops into the megakaryoblast. The megakaryoblast that develops from this MKP is a large cell (about 30 m in diameter) with a nonlobed nucleus. No evidence of platelet formation is seen at this stage. Successive endomitoses occur in the megakaryoblast (i.e., chromosomes replicate), but neither karyokinesis nor cytokinesis occurs. Under stimulation by thrombopoietin, a 30-kilodalton glycoprotein hormone produced by liver and kidney, ploidy increases from 8n to 64n before chromosomal replication ceases. The cell then becomes a platelet-producing megakaryocyte, a cell measuring 50 to 70 m in diameter with a complex multilobed nucleus and scattered azurophilic granules. Both the nucleus and the cell increase in size in proportion to the ploidy of the cell. With the TEM, multiple centrioles and multiple Golgi apparatuses are also seen in these cells.