Lecture 6 Flashcards
what are Erythrocytes
- Anucleate cells when mature
- Comprised of 95% hemoglobin
- Function
‒Primary carry O2 from lungs to tissues
‒Secondary – return CO2 from tissues to lungs and buffer pH of blood
What does Normal Erythropoiesis look like
- Mature RBCs circulate for 120
days - BM always over-produces RBCs:
‒Excess precursors ready for quick
response if needed
‒‘Unnecessary’ RBC precursors die by ‘programmed cell death’ or
apoptosis (natural process) if no stimulus occurs - 1% RBCs replaced daily
what are the
*Progenitors and
precursor cells?
*Controlled by?
*Maturation
sequence?
of Erythropoiesis
-the process of how RBCs are produced in the bone marrow in erythroid island next to the venus sinusoid or around a central macrophage with Iron
start with the common myeloid progenitor - megakaryocyte erythrocyte progenitor - mature in the perpherial blood after release
-controlled by EPO - main hormone to control Bone marrow production of red cells - growth factor cytokine
Erythropoietin (EPO) what is it
- Glycoprotein hormone (or growth factor)
- Synthesized mainly in the kidneys (some in liver)
- Released into the bloodstream in response to hypoxia
‒E.g., not enough RBC or abnormal RBCs, defective HGB, or poor lung function - Peritubular cells (fibroblasts) in kidney detect insufficient O2 this triggers EPO production mediated by increased EPO gene transcription by Hypoxia Inducible Factors (HIFs)
HIF proteins build up and promote erythropoiesis
in low PO2 , HIF proteins promote activation of genes that can adapt to hypoxia conditions. HIF binds to kidney and increases EPO and increases O2 in blood by increasing RBC production
Erythropoietin (EPO) what does it do
- stimulate and regulate the production of erythrocytes
- Binds to EPO-receptors on RBC progenitors and precursors causing:
1. Allows early release of immature RBCs (reticulocytes) from BM
2. Preventing RBC apoptosis (CFU-E progenitors protected)
3. Reduce BM transit time (shortens time between and reduces the number of mitoses of precursors) - Recombinant version available for use in treating anemia
‒ Especially due to renal disease (no EPO)
‒ Used by athletes in ‘doping’ scandals
when epo binds to its receptor EPOR and stimulates JAK2 pathway to promote RBC production at a faster rate
Nomenclature for Erythroid
Precursors
Pronormoblast
Basophilic normoblast
Polychromatic (or Polychromatophilic) normoblast
Orthochromic normoblast
All above nucleated precursors found ONLY in the bone marrow
Reticulocyte- no nuc- in BM then PB
Erythrocyte - slowly loses nucleus as it matures
How does Erythropoiesis occur
CFU-GEMM acts on the HSC forming the BFU-E: Burst-Forming Unit-Erythroid - produces daughter cells with few EPO receptors with the help of EPO form CFU -E
colonies CFU-E: Colony-Forming Unit Erythroid have many EPO receptors allowing for their differentiation of the below precursors
Pronormoblasts:
* Earliest recognizable erythrocyteprecursor in the BM
* Production stimulated by EPO
* Able to divide
* Each daughter cell matures to the next stage
Basophilic & Polychromatic Normoblasts:
* Able to divide
* Each daughter cell matures to the next stage
Orthochromatic normoblast:
* Does not divide (nucleated)
* Matures to a Reticulocyte in BM then to PB (no nucleus)
Mature Erythrocyte:
* 18-21 days from BFU-E to RBC
* 8-32 RBCs produced from 1 pronormoblast
Typical Production of
Erythrocytes
- 1 Pronormoblast produces 8 RBCs
- Last division at Polychromatic
normoblast stage - Lose nucleus in BM, released to PB before complete maturation
Criteria for differentiating the
stages of RBC development:
A - Cell size decreases, and cytoplasm turns blue to salmon pink
B- Nucleus size decreases and color changes from purplish-red to dark blue – N:C Ratio decreases
C - Nuclear chromatin becomes coarser, clumped & condensed (pyknotic) and nucleoli disappear
D- Composite of changes during developmental process
RBC Maturation Chart:
18-24 days= mature RBC from BFU E for 1 week and one week as a CFU E and 1 week to mature from the pronormoblast to mature RBC.
cell diameter and DNA reduces as cell matures
what happens the RBC as the DNA/RNA content reduce and increased acidophilia occurs in a Wrights stain?
RBC cytoplasm from blue to pink increases in hemoglobin content.
Basophilia is high in the prophase - high DNA/RNA/Protein content.
Green arrow increase in Hemoglobin or eosinophile staining. Acidophila corresponds with RBC maturation
hemoglobin makes up most of the protein in the later stages
HEMOGLOBIN BIOSYNTHESIS
-made in mitochondria and cytoplasm
-65% of Hemoglobin Synthesis
occurs in nucleated stages-
-35% Hgb synthesis occurs in the
Reticulocytes
Why can’t mature RBCs make hemoglobin? Mature RBCs lack nucleus, mitochondria, and other organelles, and are therefore, incapable of protein synthesis
The immature forms have these cell tools to make hemoglobin
Orthochromic Normoblast
How does cell lose its nucleus?
- Nucleus is condensed or ‘pyknotic’ meaning the cell cant divide. Losing the nuc will mean that the RBC will be able to carry more hemoglobin
- Membrane ‘projection’ forms as nucleus squeezes out to edge of cell
- Entire projection pinches off and nucleus is ejected
- Bone marrow macrophages engulf and breakdown expelled nucleus
- This stage is often seen in PB during disease states
‒ Rarely are earlier stages seen except in very severe disease - We call this a ‘Nucleated RBC’ or NRBC if seen in PB
Polychromatic Erythrocyte
(Reticulocyte) - after nucleus is lost
- Large, oval or irregularly-shaped (lumpy) cell with no nucleus
- Still producing hemoglobin- through RNA and Ribosomes that are leftover in the cytoplasm
- Cytoplasm contains precipitated RNA (seen supravitally)
- Location
‒ In BM for ~ 1 to 2 days
‒ In PB for ~ 1 day then mature into Erythrocytes - Up to 2.5% of PB RBC are ‘Retics’ – this is normal
‒ > 2.5% in PB is reported as ‘Increased Polychromasia’ - disease process - Wrights GIemsa with methylene blue azure
the polychromatic erythrocyte is retained in the spleen for polishing by splenic macrophages, which
results in the biconcave discoid mature RBC
Mature Erythrocyte
Biconcave disc- shape is good for hemoglobin are close to the RBC membrane
* Central pale area- Central pallor 1/3 of cell diameter~
‒ Corresponds to concavity
- Unable to synthesize HGB
‒ No nucleus, ribosomes or mitochondria, no protein synthesis - Carries oxygen from lungs to tissues and exchanges O2 for CO2
- Recall, mature RBC lives ~ 120 days
Three aspects of RBC physiology
are crucial for normal erythrocyte
maturation, survival and function:
- RBC membrane‒ Deformability depends on
membrane shape, viscosity, and elasticity - Hemoglobin structure and function
- Cellular energetics
RBCs job is gas exchange using hemoglobin and biconcave shape
RBC Membrane contains:
- to maintain shape stable membrane structure
- transport of nutrients, ions, etc.
- Considered a lipid-bilayer tied to an cytoskeleton
‒ Phospholipids (fluidity) and cholesterol (tensile strength) in equal parts elasticity and strength
Separating plasma from cytoplasm
* Maintain an osmotic differential
* Permeable to H2O, bicarbonate and chloride (anions)
* Impermeable to Ca2+, K+, and Na+ (cations)
‒ Passive transportation of anions, gases, water and glucose
‒ Active transportation of cations
RBC Membrane Deformability
*RBCs must squeeze through capillaries to exchange O2 & CO2
*Depends on RBC SHAPE, VISCOSITY, & MEMBRANE
ELASTICITY
*Biconcave SHAPE better than sphere – more surface area
Cytoplasm Viscosity
- Refers to concentration of HGB in RBC cytoplasm
- We measure:
‒ Amount of HGB (g/L) in total blood
‒ By weight per RBC- MCH (pg)
‒ Concentration of HGB per RBC – MCHC (g/L) – Normal 32-36% - Low viscosity allows the RBC to deform and squeeze through narrow spaces
- RBCs with MCHC >36% are too viscous & less deformable = RBC life span ↓
‒ Cells become damaged as they pass through narrow capillaries or small splenic pores - cold agglutination or spherocytes
RBC Membrane part Membrane Carbohydrates
- glyco- protein and -lipid chains
- Function in cell-cell recognition and adhesions (such as, ABH blood)
- anchor protective coating called Glycocalyx
‒ Adsorbs substances from extracellular matrix
‒ Gives net negative charge to RBC membrane
-RBCs able to repel one another
MEMBRANE CHARGE & ZETA
POTENTIAL
- Negative charges of membrane keep RBCs apart – stops them from
‘bunching’ up in circulation - Anything that will reduce the Zeta potential will allow RBCs to come
closer together
-the negative charge of RBC is produced by high salic acid content on the membrane
-the positive charge form an ionic cloud around the negatively charged cells
-the difference in the neg and positive is the zeta potential
RBC Membrane * Transmembrane Proteins –
function as transport and adhesion sites, surface antigens, and signaling receptors ‒ Rh Blood group system, cation pumps (Na/K-ATPase and Ca2+ ATPase), Aquaporin 1, Glut-1, etc.
‒ Possible immunogenicity to
proteins that act as ‘antigens’
- Anchor to underlying cytoskeleton (Ankyrin or Actin junctional complex) which Provide vertical membrane structure and stability helps with integrity and prevent membrane loss
RBC Membrane * Cytoskeletal Proteins
*complex lattice and/or anchorage structure found on inner (cytoplasmic) lipid bilayer ‒ Major proteins are α- and β-spectrin, Ankyrin, Protein 4.1, Protein 4.2,Tropomodulin, Tropomyosin, -
-do not penetrate the bilayer
‒ Provides horizontal or lateral support to RBC
membrane and Gives mechanical stability and supports membrane elasticity
what are Spectrins
- form a hexagonal cytoskeletal lattice on inner lipid bilayer
(they are antiparallel heterodimers) - Ends linked to actin junctional complex
- Bonds break and form to RBC membrane deformability
- Loss or mutations of these proteins can affect the membrane integrity and eventually the shape of the RBC
What is
Hemoglobin?
- Globular protein
- Consists of- 4 Globin chains
and 4 Heme groups