Erythropoiesis Flashcards
Haematopoiesis
Development of the haematopoietic elements, comprising blood cells, protein components and supporting structures (stroma).
Burst Forming Unit – Erythroid (BFU-E)
Earliest characterized progenitor committed to the erythroid lineage.
Defined in vitro by its ability to create a “burst” in semi solid media.
Form smaller clusters around a larger central colony, giving the appearance of a “sunburst”.
A “burst” is a colony consisting of several hundred to thousand cells formed in 10-14 days.
It requires IL-3, GM-CSF, Erythropeitin and other factors for proliferation, prevention of apoptosis and differentiation.
Colony Forming Unit - Erythroid (CFU-E)
As maturation progresses, this late progenitor can be defined in vitro.
It forms a small colony of morphologically recognizable erythroid precursors (50 – 200 cells) in 2-5 days.
Can undergo only a few divisions.
Adhesion between erythroid cells and macrophages occurs at this stage of maturation (in preparation to form erythroblastic islands)
Which Forming unit is most sensitive to erythropoietin.
CFU-E
The Erythroblasts
These are the PRECURSOR cells of erythropoiesis.
They constitute the visible cells of the erythroid series.
They are arranged against the outside surface of the vascular sinuses in distinctive clusters.
These clusters are called ERYTHROBLASTIC ISLANDS
What are erythroblastic Islands?
This is the anatomical unit of erythropoiesis in the normal adult.
Consists of one/two centrally located macrophages surrounded by maturing erythroid cells.
The central macrophage sends out extensive slender membranous processes that envelop each erythroblast.
These cytoplasmic processes may phagocytize defective erythroblasts and extruded nuclei.
When the erythroblasts are sufficiently mature for nuclear expulsion, it makes contact with an endothelial cell.
The cell then passes through a pore in the cytoplasm of the endothelial cell and enters the circulation.
This island is a fragile structure and is usually disrupted in the process of obtaining a bone marrow specimen by needle aspiration.
They are only found occassionally in bone marrow aspirate.
They are however commonly seen in clinical situations with accelerated erythroblastic activity eg Acute haemolytic anaemias.
Proerythroblasts
Large cells (12-20μm in diameter).
Irregularly rounded or slightly oval.
Nucleus occupies approximately 80% of the cell.
Contains fine chromatin delicately distributed in small clumps.
One/several well defined nucleoli are present.
Basophilic Erythroblasts
Also called Early Normoblasts
Smaller than proerythroblasts
Measures 12-16μm in diameter.
Nucleus occupies 75% of cell area.
The nucleus is composed of heterochromatin and euchromatin linked by irregular strands (“Wheel spikes” OR “Clockface appearance”).
Polyribosomes are still present, giving rise to cytoplasmic basophilia.
Polychromatic Erythroblasts
Also called Intermediate normoblast.
Smaller cells measuring approximately 10-12μm in diameter.
Nucleus occupies less than 50% of the red cell area.
Cytoplasm changes from blue to pink as haemoglobin dilutes the polyribosome content.
Heterochromatin is located in well defined clumps spaced regularly about the nucleus.
This gives rise to a “checkerboard pattern”.
The golgi apparatus becomes quite small and may contain lysosomes.
Orthochromatic Erythtoblast
Also called Late normoblasts or Acidophilic erythroblasts
This results after the final mitotic division.
Smallest of the series, measuring about 8-10μm in diameter.
Nucleus occupies approximately 25% of the cell and is eccentric.
Still has some residual monoribosomes and polyribosomes (thus is still somewhat polychromatophilic)
Concentration of haemoglobin rises within the erythroblast, thus it stains like a mature erythrocyte more than its precursors.
Extrusion of the nucleus
Prior to enucleation:
Intermediate filaments and marginal bands of microtubules disappear.
Tubulin and actin become concentrated at the point where the nucleus will exit.
Nucleus may be lost within the erythroblastic island or during passage through the wall of the marrow sinus.
The nucleus which cannot traverse the small opening remains in the marrow
The expelled nucleus is rapidly ingested by a macrophage.
At this point, the erythroblast is now called a reticulocyte.
Reticulocyte and it’s clinical significance
This is an orthochromatic erythroblast post enucleation.
Small cell 8-16μm in diameter.
Loss of fibronectin heralds its migration into the bloodstream.
Retains mitochondria, small number of ribosomes, centrioles and remnants of golgi apparatus as it enters the cytoplasm.
On supravital staining, these aggregates stain deep blue and are arranged in RETICULAR strands, hence the name RETICULOCYTE.
Maturation of the circulating reticulocytes require about 48-72 hours.
Spends 24 – 48 hours in the bone marrow, another 24 – 48 hours in the peripheral circulation before maturing into a red blood cell.
About 20% of ultimate haemoglobin content is synthesized during this period.
CLINICAL SIGNIFICANCE: Used to estimate the erythropoietic activity of the Bone Marrow.
Erythrocytes
Final product of erythropoiesis
Committed stem cells develop to mature erythrocytes in about 7 days.
They are produced in the red marrow of long bones at a rate of about 2 million cells /second in the healthy adult.
Typical human erythrocyte has a disc diameter of 6-8 μm and a thickness of 2μm.
Volume is about 90fl.
Surface area is about 136μm2.
Can swell up to a sphere shape containing 150fl without membrane distension.
Adult humans have about 2-3 x 1013 (20-30 trillion)RBCs at any given time.
This comprises ¼ of the total human body cell number.
Human RBCs take on average 20 seconds to complete one cycle of circulation.
The erythrocytes live in blood circulation for about 100-120 days.
At the end of their lifespan, they become senescent and are removed from the circulation.
Transcription factors
Play a role in regulating the formation, survival, proliferation and differentiation of multipotent stem cells and progenitors.
May operate on their own or as members of multicomponent complexes.
Many of these factors were originally identified because they are associated with chromosomal translocations found in leukaemia.
Dysregulation of normal transcription programmes plays a causal role in heamatologic malignancies.
Transcription factors that function on early definitive progenitors
Those that function on early definitive progenitors:
Runx1(Runt-related transcription factor 1 also known as acute myeloid leukemia 1)
SCL (stem cell leukaemia)
I.MO2(Rhombotin)
Tel (ETV6)
MLL
GATA-2.
