C. Function
Normal cellular functions
1.3 Erythrocytes
The production of erythrocytes, also known as red blood cells, occurs through a process called erythropoiesis. It takes place in the bone marrow, specifically in the “spongy” bone. Erythropoiesis is regulated by the hormone erythropoietin (EPO), which is produced primarily by the kidneys in response to low oxygen levels in the body.
The process of erythropoiesis can be divided into several stages:
Hematopoietic Stem Cell (HSC) Differentiation: Hematopoietic stem cells, which are multipotent cells found in the bone marrow, give rise to all blood cell types. Under the influence of various growth factors and cytokines, a small population of HSCs differentiate into a specialized type of stem cell called a burst-forming unit-erythroid (BFU-E) cell.
Proerythroblast Stage: BFU-E cells further differentiate into proerythroblasts, which are committed to becoming erythrocytes. Proerythroblasts are large cells with a prominent nucleus and are characterized by an intense basophilic staining due to the presence of ribosomes.
Erythroblast Stage: Proerythroblasts then undergo several divisions to form a series of erythroblasts. During these divisions, the cells gradually decrease in size and the nucleus becomes smaller and denser. The stages of erythroblasts include basophilic erythroblast, polychromatic erythroblast, and orthochromatic erythroblast.
Reticulocyte Formation: Orthochromatic erythroblasts develop into reticulocytes. At this stage, the nucleus is extruded from the cell, and the remaining cytoplasm becomes densely packed with hemoglobin. Reticulocytes contain residual ribosomal RNA, which gives them a reticular or mesh-like appearance when stained with certain dyes. They are still capable of protein synthesis but lack a nucleus.
Maturation and Release: Reticulocytes are released from the bone marrow into the bloodstream, where they continue to mature into mature erythrocytes or red blood cells. During this process, the reticular material is removed from the cell, and it becomes a fully mature erythrocyte. The entire maturation process from the proerythroblast stage to the release of reticulocytes takes around 7-8 days.
Throughout erythropoiesis, the production and maturation of erythrocytes are tightly regulated by various factors, such as EPO, iron availability, vitamin B12, and folic acid. These factors are essential for the synthesis of hemoglobin, the oxygen-carrying protein found in erythrocytes. Once released into the bloodstream, mature erythrocytes circulate for about 120 days, after which they are removed by the spleen and liver, and new erythrocytes are continually produced to maintain a stable red blood cell count in the body.