S TRUCTURE AND F UNCTION OF E RYTHROPOIETIC T ISSUE The RBCs.

STRUCTURE AND FUNCTION OF ERYTHROPOIETIC TISSUEThe RBCs

ERYTHROPOIESIS (RBC PRODUCTION) Mature erythrocytes are derived

from committed erythroid proginator cells through a series of mitotic divisions and maturation phases.

Erythropoietin, a humoral agent produced

mainly by the kidneys stimulates erythropoiesis by acting on committed stem cells to induce proliferation and differentiation of erythrocytes in the bone marrow.

ERYTHROPOIESISTissue hypoxia (lack of oxygen) is

the main stimulus for erythropoietin production.

Nucleated red cell precursors in the bone marrow are collectively called normoblasts or erythroblasts.

RBCs that have matured to the non-nucleated stage gain entry to the peripheral blood.

Once the cells have lost their nuclei, they are called erythrocytes.

ERYTHROPOIESIS

Young erythrocytes that contain residual RNA are called

reticulocytes. Bone marrow normoblast

proliferation and maturation occurs in an orderly and well defined sequence.The process involves a gradual decrease in cell size, condensation and eventual expulsion of the nucleus, and an increase in hemoglobin production.

BASIC BLOOD CELL MATURATION

Nearly all hematopoietic cells mature in the manner shown below. For RBCs the nucleus is eventually extruded and the cytoplasm increase correlates with hemoglobin increase.

ERYTHROPOIESIS

For red cell production to be efficient , 85% or more of the erythroid activity must have a balanced incorporation of heme and globin to form hemoglobin. The immature, nucleated RBC must have an

adequate supply of iron‚ as well as normal production of porphyrin and globin polypeptide chains‚ for adequate synthesis of hemoglobin.

Folic acid and vitamin B12‚ are also needed in adequate amounts to maintain proliferation and differentiation.

Defects may occur at any stage of development and these defects will lead to the death of the cell.

ERYTHROPOIESIS

Normally 1-15% of the RBCs die during maturation.

Ineffective erythropoiesis occurs when there is a failure to deliver the appropriate number of erythrocytes to the peripheral blood.

Normoblasts normally spend 4-7 days proliferating and maturing in the bone marrow.

The stages of maturation from the most immature to the most mature are:

PRONORMOBLAST OR RUBRIBLAST

Pronormoblast (A)

BASOPHILIC NORMOBLAST OR PRORUBICYTE

Basophilic normoblast

POLYCHROMATOPHILIC NORMOBLAST OR RUBICYTE

Polychromatophilic normoblasts

ORTHOCHROMIC NORMOBLAST OR METARUBICYTE

Orthochromic normoblast

RETICULOCYTE OR POLYCHROMATOPHILIC ERYTHROCYTE

Reticulocyte

MATURE ERYTHROCYTE

ERYTHROPOIESIS

ERYTHROPOIESIS Reticulocytes are released from the bone marrow into

the peripheral blood where they mature into erythrocytes , usually within 24 hours.

It is rare to see more than 1% reticulocytes in the peripheral smear from an adult, but common in healthy newborns. They can be visualized more easily by staining with

new methylene blue which allows for visualization of the remnants of the ribosomes on the endoplasmic reticulum.

ERYTHROPOIESIS

Mature RBCs have a lifespan of 100-120 days and senescent RBCs are removed by the spleen.

3 areas of RBC structure/metabolism are crucial for normal erythrocyte maturation, survival and function: The RBC membrane Hemoglobin structure and function Cellular energetics

ERYTHROPOIESIS Defects or problems associated with any of these

will result in impaired RBC survival. The RBC must be flexible in order to squeeze

through the capillaries of the spleen. Flexibility is a property of the membrane and the fluidity

of the cell’s content. Any decrease in flexibility results in a decrease in

RBC deformability and a decrease in RBC survival in passage through the spleen.

THE RBC MEMBRANE

The RBC membrane is a semi-permeable lipid bilayer supported by a protein cytoskeleton (contains both integral and peripheral proteins).

Since the mature cells lack enzymes and cellular organelles necessary to synthesize new lipid or protein, extensive damage cannot be repaired and the cell will be culled in the spleen.

THE RBC MEMBRANE

The constituents of the RBC membrane include:Phospholipids- exchange between

phospholipids in the membrane and the plasma may occur. Since the fatty acid content of the diet and the

plasma are correlated, changes in the diet may have an effect on the fatty acid composition of the phospholipids in the RBC membrane which can adversely effect the flexibility of the RBC and may result in an RBC with a decreased survival time.

THE RBC MEMBRANE

Cholesterol- membrane cholesterol exists in free equilibrium with plasma cholesterol. Therefore, an increase in free plasma cholesterol

results in an accumulation of cholesterol in the RBC membrane. RBCs with increased cholesterol appear distorted

and the increased cholesterol results in the formation of target cells, and acanthocytes.

An increase in the cholesterol to phospholipid ratio results in a cell membrane that is less deformable and therefore, the RBC has a decreased survival time.

ACANTHOCYTES

TARGET CELLS

THE RBC MEMBRANERBC membrane proteins- 10 major and over

200 minor proteins are asymmetrically organized in the RBC membrane. Integral proteins- many carry RBC antigens and act

as receptors or are transport proteins. Glycophorins are the major integral membrane proteins in the RBC. Located in the membrane are proteins that function as

cationic pumps. The RBC maintains its volume and water homeostasis

by controlling the intracellular concentrations of Na+ and K+ via these cationic pumps which require ATP.

ATP is also required in the Ca++ pump system that prevents excessive intracellular build-up of Ca++.

In ATP depleted cells there is an intracellular build-up of Na+ and Ca++ and a loss of K+ and water. This leads to dehydrated, rigid cells that are culled by the spleen.

THE RBC MEMBRANE Any abnormality that increases membrane

permeability or alters cationic transport may lead to decreased RBC survival.

The major peripheral protein is spectrin and it binds with other peripheral proteins such as actin to form a skeleton of microfilaments on the inner surface of the membrane. This strengthens the membrane and gives it its elastic properties. For spectrin to participate in this interaction, it must

be phosphorylated by a protein kinase that requires ATP.

Thus, a decrease in ATP leads to decreased phosphorylation of spectrin.

Unphosphorylated spectrin can no longer bind to actin to give the membrane its elastic properties.

This then leads to a loss in membrane deformability and a decreased RBC survival time.

RBC MEMBRANE STRUCTURE

HEMOGLOBIN STRUCTURE AND FUNCTION Hemoglobin occupies 33% of the RBC volume

and 90-95% of the dry weight. 65% of the hemoglobin synthesis occurs in the

nucleated stages of RBC maturation and 35% during the reticulocyte stage.

Normal hemoglobin consists of 4 heme groups, which contain a protoporphyrin ring plus iron, and globin, which is a tetramer of 2 pairs of polypeptide chains.

STRUCTURE OF HEMOGLOBIN

HEMOGLOBIN STRUCTURE AND FUNCTION

Normal hemoglobin production is dependent upon 3 processes: Adequate iron delivery and supply, adequate synthesis of protoporphyrins and adequate globin synthesis. Iron delivery and supply:

Iron is delivered to the RBC precursor by transferrin. It goes to the mitochondria where it is inserted into protoporphyrin to form heme.

Synthesis of protoporphyrin: Begins in the mitochondria where glycine + succinyl

CoA delta aminolevulenic acid ( ALA). This is the rate limiting step.

In the cytoplasm 2 ALA prophobilinogen (PBG)


4 prophobilinogen (PBG) uroporphyrinogen I and III (UPG I and III). Only type III is used. Type I represents a dead-end pathway. PBG deaminase and UPG cosynthase are both required for UPG III synthesis. UPG I synthesis requires only PBG deaminase. In the absence of UPG cosynthase large amounts of UPG I accumulate in the RBCs , bone marrow, and urine causing a condition called congenital erythropoietic porphyria (more on this later).

Decarboxylation of UPG III coproporphyrinogen III (CPG III). This moves to the mitochondria.

In the mitochondria CPG III protoporphyrin IX Fe is added to form ferroprotoporphyrin IX= HEME

SUMMARY OF HEME SYNTHESIS

HEME SYNTHESIS

STRUCTURE OF HEME

HEMOGLOBIN SYNTHESIS


Since porphyrinogens are readily oxidized to form porphyrins, excess formation of porphyrins can occur if any of the normal enzymatic steps in heme synthesis is blocked. Metabolic disorders in which this occurs are called porphyrias. There are 2 categories of porphyrias: inherited and acquired Inherited

Erythropoietic porphyria - results from excessive production of porphyrins in the bone marrow.

Hepatic porphyria - results from excessive production of porphyrins in the liver.

Acquired Lead intoxication - interferes with

protoporphyrin synthesis

Chronic alcoholic liver disease


Globin Synthesis In the yolk sac, the embryonic hemoglobins epsilon

and zeta are produced. In the fetus and the adult, 4 types of hemoglobin

chains may be formed: alpha ( α), beta (β ), gamma ( γ), and delta ( δ).

Normal hemoglobin's contain 4 globin chains. Hemoglobin (hgb) F= α2 γ2 and is the predominant

hgb formed during liver and bone marrow erythropoiesis in the fetus. A normal, full term baby has 50-85% hgbF.

Near the end of the first year of life, normal adult hgb levels are reached. All adult normal hgbs are formed as tetramers containing 2 α chains + 2 non-α chains. Normal adult RBCs contain:


92-95% hgb A=α2β2

3-5% hgb Ac= glycosylated α2β2

2-3% hgb A2= α2δ2

1-2% hgb F (fetal hgb)= α2γ2

Each globin chain links with heme to form hgb= 4 globin + 4 heme.

The precise order of the amino acids is critical for hgb structure and function.

An adequate amount of globin synthesis is also important. A decreased production in 1 chain results in thalassemia (discussed later).

GLOBIN SYNTHESIS

ASSEMBLY OF HEMOGLOBIN


Hemoglobin formation is regulated by several mechanisms: The regulation of globin chain synthesis. The rate

of globin synthesis is directly related to the rate of heme synthesis because heme stimulates globin synthesis by inactivating an inhibitor of globin translation.

Negative feedback of heme. High concentrations of heme prevent the mitochondrial import of the first enzyme in heme synthesis, ALA synthase ( ALAS).

The concentration of iron. An iron responsive element-binding protein (IRE-BP) binds to mRNA iron response elements (IRE) to affect the translation of the mRNA for ALAS, ferritin (discussed later), and transferrin receptors (discussed later).


The affinity of IRE-BP for IRE is determined by the amount of cellular iron. When iron levels are low, there is a high binding

affinity which acts to inhibit the translation of ALAS mRNA resulting in a decrease in heme synthesis.

When iron levels are sufficient, the binding affinity is low, thus allowing translation of ALAS mRNA and stimulation of heme synthesis.

HOW IRON LEVELS AFFECT HEME SYNTHESIS


If either heme or globin synthesis is impaired, iron accumulates in the RBC. An RBC with accumlated iron is then called a

siderocyte and the iron can be visualized using a Prussian blue stain.

When protoporphyrin synthesis is impaired, mitochondria become encrusted with iron. This is visible as a ring around the nucleus of the RBC precursor when stained with Prussian blue. A precursor cell with a ring of iron around the

nucleus is called a ringed sideroblast.

RINGED SIDEROBLASTS AND SIDEROCYTE

RINGED SIDEROBLAST

HEMOGLOBIN STRUCTURE AND FUNCTION Hemoglobin function

The primary function of hgb is gas transport. The hgb molecule is capable of a considerable amount of allosteric movement as it loads and unloads O2. This is due to the multichain structure of the molecule. In unloading of O2, the space between the chains

widens and 2,3 diphosphoglycerate (DPG) binds. This is the T (tense) form of hgb and it is called deoxyhgb. It has a lower affinity for O2, so O2 unloads from the hbg.

When hgb loads O2 and becomes oxyhgb, the chains are pulled together, expelling 2,3 DPG. This is the R (relaxed) form of hgb. It has a higher affinity for O2, so O2 binds to or loads onto the hgb.

OXY VERSUS DEOXY HEMOGLOBIN


Binding and dissociation of O2 are not directly proportional to the O2 concentration. Note the hgb-O2 dissociation curve below:


The sigmoid shape of the curve shows that a significant amount of O2 delivery will occur with a small drop in O2 tension.

O2 affinity of hgb is expressed as the partial (P) O2 pressure (in mm Hg) at which hgb is 50% saturated with O2. Increased O2 affinity means that hgb does not

readily give up its O2. Decreased O2 affinity means that hgb releases

the O2 more readily.Normally the partial O2 pressure in the lungs is 100 mm. and the hgb is 100 saturated with O2. In tissues the partial pressure is 40mm. and the hgb is 75% saturated with O2. Therefore 25% of the O2 is delivered to the tissues


In hypoxia there is a compensatory shift to the right in the dissociation curve. This is mediated by an increase in 2,3 DPG and results in decreased hgb affinity for O2 and increased O2 delivery to the tissues. Therefore the RBCs are more efficient in O2 delivery. A patient suffering from anemia due to blood loss may compensate by shifting the O2 dissociation curve to the right.

A shift to the right also occurs in acidosis and when the body temperature is increased.

RIGHT SHIFT IN O2 DISSOCIATION CURVE


A shift to the left in the O2 dissociation curve results in decreased O2 delivery to the tissues. This occurs:In alkalosisWhen there are increased quantities of abnormal hemoglobins such as methgb and carboxyhgb

When there is an increase in hgb F which has a higher affinity for O2 than does hgb A or

When a patient has received multiple transfusions with 2,3 DPG depleted blood.

LEFT SHIFT IN O2 DISSOCIATION CURVE

COMPARISON OF AN O2 DISSOCIATION CURVE AT NORMAL PH AND WITH ACIDOSIS OR ALKALOSIS


Inherited abnormalities in hgb may result in either type of shift and can have profound effects on the RBCs ability to provide the tissues with O2. Acquired abnormal hgbs of clinical importance are those that have been altered post- translationally to produce hgbs that are unable to transport or deliver O2 and they include: Carboxyhgb - CO replaces O2 and binds 200X tighter

than O2.

This may be seen with heavy smokersThis may be reversed with high concentrations of O2

Methgb - occurs when iron is oxidized to the +3 (ferric) state. In order for hgb to carry O2 the iron must be in the +2 (ferrous) state. In the body, normally~ 2% is formed and reducing systems prevent an increase beyond 2%. Increases above 2% can occur with the ingestion of

strong oxidant drugs or As a result of enzyme deficiency.


Methgb can be reduced by treatment with methylene blue or ascorbic acid.

Sulfhgb - occurs when the sulfur content of the blood increases due to ingestion of sulfur containing drugs or to chronic constipation. Unlike the formation of carboxyhgb and methgb, the

formation of sulfhgb is an irreversible change of hgb.

CELLULAR ENERGETICS

Maintenance of hgb function requires active RBC metabolic pathways for ATP production. ATP is required for:Maintaining hgb in the reduced formMembrane integrity and deformabilityMaintaining the RBC intracellular volumeProducing adequate amounts of NADH,

NADPH, and GSH RBCs generate energy almost

exclusively from the anaerobic breakdown of glucose - 4 metabolic pathways are important for maintaining cellular energetics.

CELLULAR ENERGETICS

Glycolysis- generates 90% of the required ATP- the breakdown of 1 glucose generates 2 ATP and 2 NADH.

Hexose monophosphate shunt (pentose phosphate shunt) - 5- 10% of the glucose is metabolized this way. It produces NADPH and GSH which protect the RBC from oxidative injury. If the concentrations of these are too low, the

globin will denature and precipitate in the cell. This is seen as Heinz bodies which attach to the membrane causing membrane damage and RBC destruction.

CELLULAR ENERGETICS

Inherited defects in the pathway result in the formation of Heinz bodies with subsequent extravascular hemolysis (more on this later). Heinz bodies can only be seen with a supravital

stain such as new ethylene blue. The most common deficiency is Glucose-6-

Phosphate Dehydrogenase deficiency.

Heinz bodies

CELLULAR ENERGETICS

Methgb Reductase Pathway- maintains iron in the reduced functional state. There are 2 pathways, the NADH and the NADPH

reductase pathways. They are dependent upon NADH and NADPH respectively.

In the absence of the enzymes or NADH and NADPH, methgb, which can't transport O2, is formed.

Leubering-Rapoport shunt - causes the accumulation of 2,3 DPG which is important in decreasing the hgb affinity for O2 during O2 unloading.

ERYTHROCYTE KINETICS

The normal erythrocyte concentration varies with age, sex, and geographic location.There is a high RBC count at birth which

decreases until the age of 2-3 months where physiologic anemia is seen due to low levels of erythropoietin production.

The RBC count will then gradually increase until adult levels are reached at about 14 years of age.

Males have higher RBC counts because testosterone stimulates erythropoietin production.

ERYTHROCYTE KINETICS

Individuals living at high altitudes have increased RBC levels because of the decreased partial pressure of O2 at high altitudes which leads to decreased O2 saturation.

A decrease in RBC mass and therefore, a decrease in hemoglobin concentration results in tissue hypoxia and can lead to anemia. Anemia is not necessarily a diagnosis in itself, but is a clinical sign of many different pathologies.

ERYTHROCYTE KINETICS An increase in RBC mass is called

polycythemia and it may lead to an increase in blood viscosity.Polycythemia may be relative or absolute

Relative polycythemia occurs with a decreased plasma volume. This occurs with dehydration.

Absolute polycythemia results from an actual increase in RBC mass. This may occur in disorders that prevent adequate tissue oxygenation such as: High affinity hemoglobins Pulmonary disorders Occasionally this is due to a primary defect

resulting in an unregulated proliferation of RBCs (polycythemia vera)

ERYTHROCYTE DESTRUCTION RBC destruction is normally the result of

senescsence.Each day ~ 1% of the RBCs are removed and

replaced.RBC aging is characterized by decreased

glycolytic enzyme activity which leads to decreased energy production and subsequent loss of deformability and membrane integrity.

90% of aged RBC destruction is extravascular and occurs mainly in the phagocytic cells in the spleen, with a small amount occurring in the liver and bone marrow.

5-10% of RBC destruction is intravascular, occurring in the lumen of the blood vessels

Extravascular destruction of RBCs

EXTRAVASCULAR DESTRUCTION OF RBCS

Intravascular destruction of RBCs

INTRAVASCULAR DESTRUCTION OF RBCS

S TRUCTURE AND F UNCTION OF E RYTHROPOIETIC T ISSUE The RBCs.

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Transcript of S TRUCTURE AND F UNCTION OF E RYTHROPOIETIC T ISSUE The RBCs.