Haemopoiesis and erythrocytes

What is haemopoiesis?

This is the production of all blood cells. It ensures the delivery of appropriate cell types, in appropriate numbers, and when they are needed. Due to the variety of cell types produced by the same system, it must be highly flexible and responsive.

What are the characteristics of red blood cells?

Red cells carry oxygen in the body. There are around 20-30 trillion cells in the body, each with a lifespan of up to 120 days. 4,000 million are produced per day on average but can vary depending on need.

What are the different types of needs for red blood cells?

Sudden need (transfusion is a recent invention which replaces the natural rapid process), temporary need (may be increased at higher latitudes to improve oxygen absorption), and chronic need (they may be increased throughout life to compensate for disease - may have shorter lifespan so more need to be produced).

What are the characteristics and needs of white blood cells?

White cells fight infection in the body. They survive for around 5-10 days, but may be much less during infection. 10,000 million are made per day as some are always required to fight minor infection.

Too few white cells is a problem which leads to infection, but we may need large numbers very suddenly due to infection.

What are the characteristics and needs of platelets?

Platelets are small cell fragments which promote clotting. They survive from minutes to days. The body produces 400,000 million a day on average. 

Too few is a problem which leads to excessive bleeding. Too many is a problem which causes vessel blockage. We may need large numbers very suddenly (surgery, injury).

What is the general overview of haematopoietic stem cell division?

The multipotent haematopoietic stem cell gives rise to any mature cell type. These differentiate into proliferating precursor cells which allow rapid change in production of each cell type due to changes in need. Small peptide hormones (CYTOKINES) act at various points in the control process, acting at different stages - stem cell factor, interleukin-3, erythropoietin, thrombopoietin, GM-CSF, G-CSF, and M-CSF.

The proliferating pool provides a mechanism for each stem cell to make many mature cells. Each time a cell divides it makes 2 daughter cells. For the proliferating hematopoietic cells, this continues for 19 cycles. This means each stem cell makes around 500,000 mature cells.

What 2 states can stem cells be in and what ways do they differentiate?

Stem cells are undifferentiated or partially differentiated (the haematopoietic cell is partly as it can only produce blood cell types). The key features are that they can differentiate in different ways; into different maturing precursor blood types, or can self-renew to maintain stem cell number.

What is the importance of maintaining a mature cells and a proliferating pool?

When the stem cell divides to make 2 daughter cells, it can undergo a choice - differentiate or create an identical copy.

The differentiated cease to be stem cells and now proliferate and slowly mature over multiple division cycles. If all differentiate, the stem cell population depletes. Identical copies are made to ensure the population are not depleted. If all undergo self-renewal, there would be no mature cells.

On average, if an identical copy should be made, one should differentiate. This varies depending on the needs of the body.

What ways do stem cells prevent damage?

There are very few systems. This is because having cells that can self renew produces a high cancer risk if the control system goes wrong.

The separation between stem cells and proliferating pool offers protection since division mainly occurs after differentiation, the pool of stem cells is relatively small (fewer cells so less chance of mutation), each stem cell makes few divisions (smaller chance of going wrong), and stem cells are maintained within a “niche” - an area where there are local hormones, adhesive proteins and supporting “stromal” cells which helps them survive in the area. They do not survive well outside this niche.

What choices can a differentiated stem cell make and why?

There is a choice for differentiation or cell death. Under normal circumstances, 40% of cells produced in bone marrow spontaneously die. Without the death, the system cannot respond rapidly since stem cells require 19 generations of cells to produce mature cells. If we prevent cell death, then the number of cells produced can be rapidly and greatly increased - cell numbers are controlled by reducing the death rate.

This process can be done selectively or not to either increase particular lineages, or all of them - the cell types formed may be directed by reducing death selectively.

What is the role of erythropoietin and how does it work?

Erythropoietin increases the red cell production. EPO acts to increase the survival of red cell precursors in the proliferating cell pool to expand their numbers. It ensures that the number of red cells is sufficient to deliver oxygen to tissues.

Oxygen level is sensed in the kidney – if levels drop then EPO secretion is increased. When red cell numbers are sufficient then oxygen level is restored and EPO secretion is reduced again. This way the optimal number of red cells is produced. Oxygen levels may drop due to disease, altitude and blood loss.

There is no correct number of red cells. What is important is the amount of oxygen delivered to tissues – the amount of EPO produced therefore depends on the amount of oxygen sensed by kidney cells so production and effect relate only to need.

What is the role of G-CSF and how does it work?

G-CSF increases neutrophil production. It must provide a baseline number of neutrophils to protect the body, but also respond very rapidly during infection to increase numbers. It acts mainly on late granulocyte precursors; it enhances the survival of the precursors and increases maturation rate and granule formation to massively increase output and function.

It is produced when the body detects infection or inflammation - the cytokine is released by endothelial cells. The response depends on how great the stimulus, and once it is resolved, the response ceases.

What is the role of TPO and how does it work?

TPO is produced to increase platelet production. It ensures there are enough platelets to prevent haemorrhage. It acts on the megakaryocytes to increase their numbers and rate of maturation.

Like G-CSF it can be produced in response to inflammation, but mainly the TPO is produced in fairly constant amounts by the liver – control is achieved because mature platelets are able to bind the TPO and destroy it. This means if platelet numbers are high the level of TPO is reduced, but if platelet numbers are reduced e.g. after bleeding, the TPO level rises to stimulate production.

How is the red blood cell specialised?

The biconcave disc optimises O2 uptake and delivery; optimal shape to collect oxygen in lungs, move easily to tissues and then effectively give up oxygen and transport CO2 out. Its shape shortens diffusion distance of O2 throughout the cell, and increases surface area to volume ratio so there is more O2 uptake. 

The cytoskeleton structure supports circulation and safety - moves safely through blood vessels without being stuck or slowed.

Haemoglobin structure is specialised so there is rapid loading and unloading of O2 as required. This is flexible - a baby competes for oxygen with the mother so has a higher affinity haemoglobin to absorb sufficient oxygen

Why is the red blood cell vulnerable?

The simplicity (lacks cell structures) of the cell leaves it vulnerable to disease. It has given up the nucleus, mitochondria and ribosomes for its ability to have room for Hb, unattractive for infecting organisms (high O2 is bad for many bacteria, no nucleus for viruses), and it makes the cell stable and efficient - nothing unnecessary.

However, no nucleus means no capacity for making mRNA so no new protein. No mitochondria so no TCA cycle which limits the capacity to generate ATP or reducing power, making it vulnerable. No ribosomes so no protein synthesis. The lack of a nucleus and ribosomes effectively mean no cellular repair if damaged – damaged cells need to be removed by spleen. The lack of protein biosynthesis and lack of mitochondria mean that the cell cannot undergo apoptosis – this means that infected cells cannot “self-destroy”.

What are the proteins that make up the cytoskeleton in a red blood cell?

Anchoring proteins (band 3 and glycophorin C) connect the cytoskeleton via a vertical interaction to the membrane of an erythrocyte. These connect to ankyrin 4.2 and 4.1 which have horizontal interactions with the cytoskeleton. This is flexible; forms and reforms. Spectrin allows horizontal interactions for the biconcave shape to stretch or compress due to external pressure changes. VERTICAL ANCHORS, HORIZONTAL LINKS.

What is the shape that red blood cells spend most of their time in?

During the typical “lamina flow” within high speed vessels such as arteries the pressure changes cause the red cell to elongate and line up in the centre of the vessel – the torpedo form. Red cells do not spend most of their time in the biconcave shape, but rather alternate forms due to pressure.

Why is red blood cell membrane damage dangerous?

The lack of ability to make new proteins limits repair capacity and the damaged cells cannot undergo apoptosis so temporary fixing is needed to prevent toxicity – the membrane structure is self-repairing to limit immediate damage.

Membrane damage by slicing or diffuse damage will potentially allow haemoglobin to be released into circulation. This is highly toxic (it tends to aggregate blocking kidneys and also carries oxygen which can add to damage). The cytoskeleton structure allows a fix to prevent Hb “leakage”.

What ways is damage self-repaired by the membrane?

Slicing damage (can be caused by fibrin strands in circulation caused by local clotting activation in sepsis) is continually repaired by the cytoskeleton. First, a vacuole is formed where the slice occurred, which pops to leave behind the sealed cell. Further sealed fragments may be formed.

Damage can also be caused by diffusion whereby small blobs of membrane are lost which get sealed. The cell gradually shrinks to become a rigid sphere - this forms a damaged spherocyte which is removed in the spleen. Again, this is continuously repaired by the cytoskeleton.

What is hereditary spherocytosis?

Hereditary spherocytosis is caused by a defect of red cell membrane proteins (such as ankyrin or Band 3). The red cell membrane is unstable and is progressively lost. This causes spherocytes to form. The more rapid breakdown of red cells causes anaemia. The release of red cell breakdown products leads to mild jaundice and gallstones. The increased red cell destruction leads to the spleen being enlarged.

What is the shape of the oxygen dissociation curve?

The oxygen dissociation curve of haemoglobin shows a sigmoid curve. This shows that within the range of oxygen encountered in the lung, the haemoglobin molecule will have maximal binding of oxygen. It also shows that in the tissues, for the expected oxygen levels encountered the release is approximately linear i.e. most oxygen is released when needs are highest.

What is the structure of haem and globin?

Haem has a porphyrin structure which holds iron in a flat two dimensional structure; two interaction sites remain above and below the plane: one fixes the molecule to the globin protein and one is available to bind oxygen.

Globin keeps haem contained (not free) and therefore safe. The oxygen binding is reversible so that if PaO2 is high oxygen is bound, but if PaO2 is low then oxygen is released.

What is the structure of haemoglobin and how does it work?

Haemoglobin comprises four chains – two alpha and two beta which exhibit cooperativity. The globin molecule is not rigid; it can have different conformations, either “tight” or “loose”. The loose conformation has plenty of room for O2 so binds it tightly. The tight formation has less room for O2 so is more likely to give it up. The interconnection of chains means that a change to any one chain will affect others and can change them between relaxed or tight conformation.

The “tight” (gives up oxygen easily) and “relaxed” (tightly binds oxygen) are also influenced by other factors; pH, CO2, metabolic products. These are produced in active tissues and further promote the “tight formation” causing better oxygen release when needed. This is the Bohr shift.

What is foetal haemoglobin?

Foetal haemoglobin – this form does not bind 2,3 DPG so never becomes as tight as regular haemoglobin. This allows the foetal haemoglobin (HbF) to compete with the mothers haemoglobin for Oxygen – transferring the oxygen from the mother to the developing foetus.

What would the oxygen dissociation curve look like if haemoglobin was single chained?

If haemoglobin were only a single chained molecule it would produce a hyperbolic release curve that would not suit the needs of the body. It would release O2 most to the nearest tissues etc.