Erythrocyte

Cell Morphology

Erythrocytes are small red biconcave discoid cells. Erythrocytes are not true cells in that they lack a nucleus, organelles, and cannot divide. Erythrocytes have a diameter of 7–8 μm and are smaller than leukocytes but larger than platelets. The edge of an erythrocyte has a thickness of 2.6 μm, while the center has a thickness of 0.8 μm, thus making them biconcave. Erythrocytes have a longer lifespan than the majority of blood cells and can survive for 100–120 days. Erythrocytes are the most numerous cells in the blood and have a concentration of 4.3-5.7 x 1012 cells/L and 3.9-5.0 x 1012 cells/L in males and females, respectively (Pawlina, 2016).

In the cytoplasm of an erythrocyte contains hemoglobin, which is an oxygen and carbon dioxide-binding protein. It’s a tetrameric protein meaning that it has four subunits and each subunit contains an iron-containing molecule called heme. An iron atom is located in the center of each heme group and binds to oxygen (Silverthorn et al., 2013; Marieb, Wilhelm and Mallatt, 2012). Therefore, one hemoglobin binds four molecules of oxygen. Each erythrocyte contains 250 million molecules of hemoglobin, meaning each erythrocyte can transport around 1 billion oxygen molecules (Reece et al., 2014).

Erythrocytes have proteins in their cell membrane called glycophorins. Additionally, erythrocytes possess antigens which attach to the glycophorins. There are three main antigens A, B and O that make the ABO blood group system. All three antigens share the same molecular structure but their difference is the presence of an additional sugar molecule. The presence of these antigens indicates an individual’s blood type (Reece et al., 2014; Pawlina, 2016).

—Erythrocytes that possess the O antigen make the O blood type.

—Erythrocytes that possess the O and A antigens make the A blood type.

—Erythrocytes that possess the O and B antigens make the B blood type.

—Erythrocytes that possess the O, A, and B antigens make the AB blood type.

Production

Erythrocytes are produced in the red bone marrow by pluripotent hematopoietic stem cells and usually takes 5–9 days. A hematopoietic stem cell gives rise to a myeloid stem cell, which undergoes various stages of differentiation until it becomes an erythrocyte. This differentiation is driven by growth factors such as cytokines and hormones. Erythropoietin is a hormone that regulates the synthesis and differentiation of erythrocytes. It is produced by the kidneys when tissues are deprived of oxygen, resulting in an increase in hemoglobin and oxygen transport in the blood circulation. Colony stimulating factors are involved in regulating the production of erythrocytes (Silverthorn et al., 2013).

When erythrocytes approach their maximum lifespan (around 120 days), they become senescent. Subsequently, some erythrocytes rupture while others are phagocytosed by macrophages in the spleen, liver, or bone marrow (Pawlina, 2016).

Function

Erythrocytes function only in the bloodstream. Erythrocytes are elastic and able to fold on themselves, thus allowing them to change their shape and pass through narrow capillaries or adapt during osmotic changes. Their biconcave shape increases their surface area, therefore, enhancing the transportation of oxygen.

Erythrocytes transport oxygen from the lungs to cells and remove carbon dioxide from cells and transport it to the lungs (Silverthorn et al., 2013).

List of Clinical Correlates

—Iron-deficiency anemia

—Hemolytic Anemia (sickle-cell anemia, thalassemia)

—Polycythemia

—Hereditary spherocytosis

—Hereditary elliptocytosis

References

Dorland, W. (2011) Dorland's Illustrated Medical Dictionary. 32nd edn. Philadelphia, USA: Elsevier Saunders.

Eroschenko, V. P. (2008) DiFiore's Atlas of Histology with Functional Correlations. Wolters Kluwer Health/Lippincott Williams & Wilkins.

Marieb, E. N., Wilhelm, P. B. and Mallatt, J. (2012) Human Anatomy. 14th edn.: Benjamin Cummings.

Pawlina, W. 2016. Histology: A text and atlas with correlated cell and molecular biology. 7th ed. Philadelphia: Wolters Kluwer.

Reece, J. B., Meyers, N., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., Jackson, R. B., Cooke, B. and Campbell, N. A. (2014) Campbell Biology. Pearson.

Silverthorn, D. U., Johnson, B. R., Ober, W. C., Garrison, C. W. and Silverthorn, A. C. (2013) Human Physiology: An Integrated Approach. Pearson Education.