1. Haematopoiesis
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Haemopoiesis is a production of blood cells.
Haemopoiesis is a tightly regulated process that forms blood cells and haematology is a
study of blood, and it involves the diagnosis and monitoring of diseases that are associated
with blood and blood forming organs.
Blood cells are manufactured in the bone marrow and released into the peripheral blood
once they mature.
Blood is a dynamic and crucial fluid providing molecular and cellular transport and has lots of
regulatory functions.
Approximately 45% of blood is made up of blood cells and they are classified into three main
types. The first are your red blood cells, these are also called erythrocytes and they are
produced by a process called erythropoiesis. Their main function is to transport oxygen from
the lungs to the tissues. The second type of cell prominently in the middle of this blood film
is the white cells. They defend the body against infection, and they are produced by a
process called leukopoiesis. The final type of cell are these tiny fragments called platelets
and they are produced by a process called thrombopoiesis and their primary function is to
prevent blood loss at the site of an injury.
The remaining 55% of the blood is made up of a component called plasma. It acts as a
transport medium for these blood cells, it also contains dissolved nutrients and proteins
In a healthy individual between 5-10*10^11 blood cells are produced by the bone marrow
each day and they are highly regulated by growth factors, cytokines, environmental factors
and apoptosis.
In a healthy individual the number of cells produced is actually equivalent to the number of
cells reaching the end of life, any disruption to these balance leads to a disease state.
because blood interfaces with all the organs and tissues of the body it carries oxygen, and it
removes waste products from the cells and moves them to the excretory organs.
It is crucial in ensuring adequate whole-body physiology and homeostasis.
Ontogeny is where things take place so ontogeny of haemopoiesis is where haemopoiesis
takes place. Effective haemopoiesis is crucial to health, in an adult life haemopoiesis
predominantly takes place in the bone marrow, the soft tissues within the centre of the
bones where blood cells develop.
Haemopoiesis that occurs in the bone marrow is termed intramedullary, this can take place
in the bone marrow of the vertebra, the ribs, sternum, skull, sacrum, and pelvis.
In neonates and infants is occurring in all the bones.
The difference is in the foetus.
If they find that the haemopoiesis has gone back to the foetal sites such as a liver spleen or
the lymph nodes it is indicating a pathological condition, so we call this extramedullary
The reason this occurs is that the bone marrow is unable to meet the demands of the body
and we often see this in tumours or haemolytic anaemias so it would go back to the liver and
the spleen.
The bone marrow accounts for about 5% of the weight of an adult and it consists of two
compartments, you've got the red marrow and that contains all the hematopoietic tissue
and then you got the yellow marrow which is predominantly fat. In a neonate almost all the
bone marrow is red and as we age it gets replaced by the yellow marrow.
, 1. Haematopoiesis
Haemopoiesis begins with the pluripotent stem cell and there are about 11,000 to 22,000 of
these per individual.
It has the capacity to independently replicate, proliferate, and differentiate into any of the
different lineages and it ultimately gives rise to these mature blood cells at the bottom of
this diagram.
It's impossible to see this cell morphologically down the microscope however it has been
shown to be CD34 positive so it can be identified by flow cytometry, the pluripotent stem
cell gives rise to two stem cell colony forming units so you got the common myeloid
progenitor cell which gives rise to your red cells, platelets and all your white cells and then
we got the common lymphoid progenitor cell and this gives rise to your B and T cells and
your natural killer cells.
The major structural function of the bone is to provide support and pivotal points for the
muscles, ligaments and tendons, this includes your lungs, your heart, your brain and your
spinal cord. It also gives support for body organs such as your liver and your spleen and
although most of the physical demands are met by the strength of the bone there are also
some areas of the bone that are hollow, and they contain connective tissue.
Spaces within the bone that are not there for structural reasons are where haemopoiesis
takes place so the haemopoietic tissues is sometimes described as cords and it consists of
stem cells and their progeny. Immature developing blood cells are also there such as
myeloblasts and erythroblasts, it contains sinuses which are vascular spaces or pools of
blood that are lined within the endothelial cells, and they can regulate the release of mature
and immature cells in the blood.
It also hosts non-haemopoietic cells that support the bone marrow and often produce
growth factors, and these can include the stromal cells such as your fibroblast and they
produce scaffolding like collagen that supports the other cells, it contains macrophages and
they produce growth factors, they promote red cell production iron storage and can also get
rid of debris and adipocytes which store energy as fat.
There are also adhesion molecules that are present on this extracellular matrix, and you can
see them in blue on this diagram and they have an important role in retaining immature cells
in the bone marrow to make sure they're not released prematurely.
The extracellular matrix can also control of stem cell mobilisation and homing, stem cell
homing attracts any kind of circulating haemopoietic stem cells and keeps them in the bone
marrow until optimal proliferation can take place and mobilisation is the opposite, it is
where the extracellular matrix can release haemopoietic stem cells e.g., in response to an
injury you’d want lots of platelets and it will start platelet production.
The bone marrow environment is crucial for haemopoiesis, and cells need to be able to talk
or communicate with one another in order to allow this to happen and this is where your
growth factors come in, they can be both proteins and glycoproteins and they're actually
essential in the development and control of cells from each stage.
They are released by one cell such as a macrophage or a stromal cell and they can act on
different stem cells, blast cells and promote the process of cell development one single cell
can secrete many different growth factors in response to stimuli and each can have an
impact on the type of cell that's produced and it can have a different effect depending on
whether it is acting alone or in combination with other growth factors they will stimulate the
activity of one another.
