Describe the stages of erythropoiesis

Answer 1

ERYTHROPOIESIS

Definition: It is the process of formation of red blood corpuscles or erythrocytes in the human body

Site of formation:

Foetus: (early stages) Mesodermal cells of yolk sac

Adult: Mainly in bone marrow

Stages: The different stages of erythropoiesis are as follows:

a) Production of RBC at developmental stage:

I) Mesoblast Stage: From 3rd week of intrauterine life, RBC is produced by mesodermal cells of the yolk sac

II) Hepatic Stage: From 5th month of intrauterine life, Rbc production is taken over by liver and spleen. This stage lasts upto last trimester.

III) Myeloid Stage: After birth, red bone marrow of all bones start RBC production. After 20 years of age, RBC production is restricted to ends of long bones and membranous bones since red marrow is replaced by yellow marrow.

A) Genesis of Blood cells:

1. Pluripotent hematopoietic stem cells(PHSC): These are primitive stem cells present in the bone marrow which can produce all types of blood cells - RBC, WBC and platelets as well as PHSC themselves.
2. Commited Stem cells: These are also known as progenitor stem cells, formed from PHSC and are of two types----

i) Committed stem cells of myeloid series (CFU-S & CFU-B)-They differeniate into RBC, neutrophil, monocyte, eosinophl, platelets

ii) Committed stem cells of lymphocyte series (LSC)- They differentiate into T- and B- lymphocytes.

Factors controlling:

a) Growth inducers: Interleukin-3

b) Differentiation inducers

B) Stages of Differentiation of RBC (Schematic):

1. Proerythroblast: Large cells, 15-20 micron in diameter, large nucleus with 2-3 nucleolus, cytoplasm very less and basophilic with perinuclear halo and sometimes forms 'ear-sahped" bulges, nuclear chromatin is non-homogeneous
2. Early Normoblast: Slightly smaller cells, 12-17 micron in diameter, reduction of nuclear size, disappearence of nucleolus, cytoplasm is moderately basophilicwith high protein and RNA content, condensation of chromosomes occur.
3. Intermediate normoblast:Diameter of cells is 12-15 micron, nuclear size is further reduced having a "cart-wheel" appearance, chromosomes are further condensed, cell division stops totally, cytoplasm is polychromatophillic, appearance of hemoglobin takes place.
4. Late normoblast: Cells are reduced in size with 8-12 micron diameter, "ink-spot" nucleus and finally disappears,increase of hemoglobin amount, cytoplasm or pyknotic becomes acidophilic.
5. Reticulocyte: Size of cells are 8 micron in diameter, formation of highly branched pattern by remains of mitochondria, ER, ribosomal RNA -hence named as reticulocyte.
6. Erythrocyte:Mature RBC with 7.2 micron in diameter,biconcave shape,absence of nucleus,high hemoglobin content makes the cells typicaly pink red in color.