Chapter 5 - Erythrocyte Maturation PDF

6/28/2024 ERYTHROCYTES: ERYTHROPOIESIS, MATURATION, MEMBRANE CHARACTERISTICS, AND METABOLIC ACTIVITIES CHAPTER 5 PREAMBLE PowerPoints are a general overvi...

6/28/2024 ERYTHROCYTES: ERYTHROPOIESIS, MATURATION, MEMBRANE CHARACTERISTICS, AND METABOLIC ACTIVITIES CHAPTER 5 PREAMBLE PowerPoints are a general overview and are provided to help students take notes over the video lecture ONLY. PowerPoints DO NOT cover the details needed for the Unit exam Each student is responsible for READING the TEXTBOOK for details to answer the UNIT OBJECTIVES Unit Objectives are your study guide (not this PowerPoint) Test questions cover the details of UNIT OBJECTIVES found only in your Textbook! 1  6/28/2024 INTRODUCTION The mature erythrocyte is a biconcave disc with a central pallor that occupies the middle one third of the cell. In the mature cell, the respiratory protein, hemoglobin, performs the function of oxygen–carbon dioxide transport. Throughout the life span of the mature cell, an average of 120 days, this soft and pliable cell moves with ease through the tissue capillaries and splenic circulation. As the cell ages, cytoplasmic enzymes are catabolized, leading to increased membrane rigidity (density), phagocytosis, and destruction. ERYTHROPOIESIS The term used to describe the process of erythrocyte production is erythropoiesis. Erythropoiesis encompasses differentiation from the hematopoietic stem cell (HSC) through the mature erythrocyte. Erythropoiesis takes place in erythroblastic islands. These islands consist of normoblasts (erythroblasts) clustered around an iron-laden macrophage. The macrophage not only provides the iron needed for hemoglobin maturation but also aids by providing cytokines to the developing normoblasts to mature into functional erythrocytes. Transport of oxygen to the tissues and transport of carbon dioxide from the tissues is accomplished by the heme pigment in hemoglobin, which is synthesized as the erythrocyte matures. The basic substances needed for normal erythrocyte and hemoglobin production are amino acids (proteins), iron, vitamin B12, vitamin B6, folic acid (a member of the vitamin B2 complex), and the trace minerals cobalt and nickel. Abnormal erythropoiesis can result from deficiencies of any of these necessary substances. 2  6/28/2024 ERYTHROPOIETIN Erythropoietin (EPO) is produced primarily (80% to 90%) by the peritubular cells of the kidneys. The remaining 10% to 20% is produced in the liver, which is the primary site of EPO production in the developing fetus. Blood levels of EPO are inversely related to tissue oxygenation. The greater the hypoxia, the higher the EPO levels. EPO is considered an early-acting and late-acting cytokine, acting on BFU-E and CFU-E progenitors as well as the erythroblastic precursors. It also interacts with IL-3, GM-CSF, IL-1, and TSF to promote maturation and differentiation of other cell types. Other functions of EPO include the following: Accelerates the rate of mRNA and protein (hemoglobin synthesis) Decreases the time for the maturation of metarubricyte (orthochromatophilic normoblasts; nRBCs) Stimulates the premature release of immature RBCs, retics, from the bone marrow Increases the rate of extrusion of an RBC nucleus (enucleation) MATURATION AND DEVELOPMENT #1 Once the stem cell differentiates into the erythroid cell line, a cell matures through the nucleated cell stages in 4 or 5 days. Bone marrow reticulocytes have an average maturation period of 2.5 days. Once young reticulocytes enter the circulating blood, they remain in the reticulocyte stage for an average of 1 day. Reticulocytes represent approximately 0.5% to 1.5% of the circulating erythrocytes. 3  6/28/2024 RUBRIBLAST (PRONORMOBLAST) PRORUBRICYTE (BASOPHILIC NORMOBLAST) 4  6/28/2024 RUBRICYTE (POLYCHROMATOPHILIC NORMOBLAST) METARUBRICYTE (ORTHOCHROMATOPHILIC NORMOBLAST) 5  6/28/2024 RETICULOCYTES #1 A supravital stain, such as new methylene blue, precipitates the ribosomal RNA in these cells to form a deep-blue, mesh-like network. The reticulocyte count procedure (see Chapter 26) is frequently performed in the clinical laboratory as an indicator of the rate of erythrocyte production. RETICULOCYTES #2 Usually, the count is expressed as a percentage of total erythrocytes. The normal range is 0.5% to 1.5% in adults. In newborn infants, the range is 2.5% to 6.5%, but this value falls to the adult range by the end of the second week of life. The corrected reticulocyte count be made mathematically by correcting the observed reticulocyte count to a normal packed red blood cell volume. Correct for anemia with following formula: Corrected retic count = retic count (%) x patient’s PCV = % normal PCV based on age & sex 6  6/28/2024 RETICULOCYTES #3 Reticulocyte production index (RPI) A simple percentage calculation of reticulocytes does not account for an existing anemia, which prematurely releases reticulocytes. These require an additional 0.5 to 1.5 days longer to mature. The RPI measures erythropoietic activity when stress reticulocytes are present. The rationale for this is that the life span of the circulating stress reticulocytes is 2 days instead of the normal 1 day. To compensate for the increased maturation time and consequent retention of residual RNA of the prematurely released reticulocytes, the corrected reticulocyte count is divided by a correction factor derived from the maturation timetable. RETICULOCYTES #4 Reticulocyte production index (RPI) (cont.) RPI = corrected reticulocyte count in % _________________________ maturation time in days For clinical purposes, a maturation time typically used is 2 days. 7  6/28/2024 MATURE ERYTHROCYTE After the reticulocyte stage, the mature erythrocyte (RBC; red blood cell) is formed that has the following characteristics: Is anucleated and functions to transport oxygen to the tissues via hemoglobin. Survives in circulation for 120 days. It metabolizes glucose through anaerobic glycolysis. Has an average diameter of 6 to 8 μm. It lacks the ability to make hemoglobin, lacks a nucleus and functional organelles. ERYTHROID MATURATION #1 8  6/28/2024 ERYTHROID MATURATION #2 DISORDERS RELATED TO ERYTHROCYTE MATURATION AND PRODUCTION #1 Disorders of erythropoietin Polycythemia is the term used to refer to an increased concentration of erythrocytes (erythrocytosis) in the circulating blood that is above normal for gender and age. Secondary, or absolute, polycythemias reflect an increase in erythropoietin production and should not be confused with polycythemia vera (see Chapter 21) or relative polycythemias. Mechanisms that can produce secondary polycythemia include the presence of high oxygen affinity hemoglobin, chronic lung disease, smoking, and dwelling in high altitudes. 9  6/28/2024 DISORDERS RELATED TO ERYTHROCYTE MATURATION AND PRODUCTION #2 Red cell increases Increases in erythrocytes can result from conditions that are not related to increased erythropoietin production. These conditions include the relative polycythemias. As the name suggests, relative polycythemia rarely has to do with the cells, but rather the proportion of cells to plasma volume. In essence, when plasma volume is lowered, for example, in dehydration, it will appear as though the hematocrit is higher than expected, but it is not. DEFECTIVE NUCLEAR MATURATION #1 A defect in maturation known as megaloblastic maturation can be seen in certain anemias, such as vitamin B12 or folate deficiencies. The most noticeable characteristic of this type of defect is that nuclear maturation lags behind cytoplasmic maturation. Because of an impaired ability of the cells to synthesize DNA, both the interphase and the phases of mitotic division are prolonged. This asynchronous pattern of maturation can be confusing because the nuclear development of the cell is much younger looking than the actual developmental age, which is expressed by the cytoplasmic development. 10  6/28/2024 METABOLIC ACTIVITIES OF ERYTHROCYTES #1 The mature erythrocyte has no nucleus or other organelles but is capable of existing in the blood circulation for an average of 120 days. An erythrocyte has a limited ability to metabolize fatty acids and amino acids and lacks mitochondria for oxidative metabolism. Energy resources for the RBC are exclusively produced by anaerobic glycolysis, through the Embden-Meyerhof-Parnas pathway, mainly, but also get assistance through: Hexose monophosphate shunt Methemoglobin reductase pathway Luebering-Rapoport pathway METABOLIC ACTIVITIES OF ERYTHROCYTES #2 The overall pathway of erythrocyte glycolysis may be subdivided into the following: The major anaerobic Embden-Meyerhof glycolytic pathway that generates ATP and maintains the function of hemoglobin Three supplementary pathways The methemoglobin reductase pathway The Rapoport-Luebering pathway The phosphogluconate pathway 11  6/28/2024 MEMBRANE CHARACTERISTICS OF ERYTHROCYTES #1 Reticulocyte membrane In the maturing reticulocyte, membrane vesiculation leads to loss of surface area. Reticulocytes possess a significant amount of tubulin and actin in the membrane that is not needed by a mature RBC and therefore gets lost. Three major changes occur as a reticulocyte matures into an erythrocyte: Increase in shear resistance Loss of surface area because of membrane lipid loss Acquisition of a biconcave shape MEMBRANE CHARACTERISTICS OF ERYTHROCYTES #2 Mature RBC membrane characteristics The shape of the erythrocyte constantly changes as it moves through the circulation and performs extremely complex maneuvers. The cellular membrane is composed of a protein lattice-lipid bilayer to which the membrane skeleton is attached by transbilayer (peripheral) proteins. The cell membrane is deformable and tolerant against mechanical stress and various pH and salt concentrations in vivo and in vitro. Cell shape changes reversibly depending on ATP level in the cell and intracellular calcium ion concentration. Cytoskeleton is composed of peripheral proteins that control cell shape, attachment to other cells, and maintain organization of specialized membrane domains. The major components of the cytoskeleton are alpha- and beta-spectrin, ankyrin, band 3, band 4.2, and the glycophorins. Together these components form a complex meshwork tethered to the RBC membrane. Transmembrane proteins carry out many different functions (see table 5.3). 12  6/28/2024 MEMBRANE CHARACTERISTICS OF ERYTHROCYTES #3 Characteristics of the aging red cell membrane Age-related changes can be monitored using plasma membrane calcium (PMCA) and glycated hemoglobin (Hgb A1C). PMCA strength declines as the RBC ages Hgb A1C increases as the RBC ages These cause densification of the RBC membrane that contributes to the membrane instability seen in senescent RBCs Cytoplasmic characteristics In addition to hemoglobin, the enzymes synthesized during early cell development have to be sufficient to provide the energy needed for these processes: Maintaining hemoglobin iron in an active ferrous (Fe2+) state Driving the cation pump needed to maintain intracellular sodium ion (Na+) and potassium ion (K+) concentrations despite the presence of a concentration gradient Maintaining the sulfhydryl groups of globins, enzymes, and membranes in an active reduced state Preserving the integrity of the membrane METABOLIC ACTIVITIES OF ERYTHROCYTES #4 If metabolic pathways are blocked or inadequate, the life span of the erythrocyte is reduced and hemolysis results. Defects in metabolism can include the following: Failure to provide sufficient reduced glutathione, which protects other elements in the cell from oxidation Insufficient energy-providing coenzymes such as reduced nicotinamide-adenine dinucleotide (NADH), nicotinamide-adenine dinucleotide phosphate hydrogenase (NADPH), and ATP The two most common erythrocytic enzyme deficiencies, which involves the Embden- Meyerhof-Parnas glycolytic pathway, are deficiencies of: Glucose-6-Phosphate Dehydrogenase (G6PD) Responsible for converting glucose-6-phosphate (G6P) to 6-phosphogluconate (6PG) Pyruvate Kinase (PK) Responsible for converting pyruvate (pyruvic acid) to lactic acid 13  6/28/2024 METABOLIC ACTIVITIES OF ERYTHROCYTES #5 Embden-Meyerhof-Parnas pathway Generates two ATP from anaerobic glycolysis Oxidative pathway or hexose monophosphate shunt Couples oxidative catabolism of glucose with NADP reduction, which is required for glutathione reduction Failure to reduce glutathione causes denaturation of globin and the formation of Heinz bodies Methemoglobin reductase pathway Keeps hemoglobin in the reduced state, which prevents oxidation of hemoglobin Luebering-Rapoport pathway Allows for optimal oxygen transport in hypoxic conditions and acid-base disturbances through the accumulation of 2,3- DPG POSTAMBLE READ the TEXTBOOK for the details to answer the UNIT OBJECTIVES. USE THE UNIT OBJECTIVES AS A STUDY GUIDE All test questions come from detailed material found in the TEXTBOOK (Not this PowerPoint) and relate back to the Unit Objectives 14