ERYTHROPOIESIS OR BLOOD FORMATION

Ques. 1        Describe the stages of Erythropoiesis or how erythropiesis occurs? What factors are responsible for Erythropoleses? [discuss the role of anoxia (hypoxia) & vitamins in erythropoiesis].

Ans.             In the early embryo – Red – call formation takes place in the area vasculosa of the yolk-sac. This consists of a syncytium of branched cells. The angioblasts from plexuses. Fluid droplets appear gradually in these syncytial masses & run together. Thus a part is hollowed out to from network of capillaries at abut the 3^rd week of intra-uterine life. The remains of syncytial masses persist as endothelial lining which in places divides to form masses of round cells. These cells are calls Megaloblasts. They start acquiring Hb and are know as Erythroblasts.

Mid-foetal life to about 1 month before birth – As foetal development proceeds, the endothelial lining of vascular channels of the liver and the spleen divides and forms nucleated red cells. In acute haemopoietic crisis they may resume this function even after birth.

After birth – Red-cell formation takes place in red bone marrow. Its vascular bed is a meshwork of small blood sinuses called sinusoids lined by thin endothelium. A large number of these intercommunicating sinusoids are normally collapsed & quite impermeable to blood. Others are widely dilated, but owing to the vessels which lead to from them being contricted, they are isolated from the general circulation. Regions of low oxygen tension are thus produced, which serve as stimuli to formation of red cells. It is these collapsed inter-sinusoidal capillaries of ‘Sabin’ where erythropoiesis occurs after birth. Plasma filters in from adjacent capillaries & sweeps into circulation the newly formed red cells. The collapsed capillaries open up as soon as normoblasts have lost their nuclei.

1. In the final stage of maturation the nuclei break up into fragments and disappear, leaving young red cellsThe cells are larger in number and the diameter of a cell varies from 5.5 to 8.8 (mean diameter is 7.3). Shape may be circular or bi-concave, when it is viewed from side, it looks like a dumb-bell. Average thickness is 2.2. Average surface is 120 sq.

   Functions of R.B.C. : (i) It maintains ionic balance and acid – base balance. (ii) It help to maintain viscosity of blood. (iii) Various pigments are derived from haemoglobin after the disintegration of the red cells, e.g. bilirubin, biliverdin etc. (iv) It helps respiration by carrying O2 and CO2.

   Development of R.B.C. : Development process of R.B.C. is Reticulo-Endothelial Cells – Haemocytoblast – Pro-erythroblast – Early normoblast – Late normoblast – Reticulocyte – Erythrocyte.

   Normal total count of R.B.C. : (i) In infants: 6 to 7 million/cu. mm. (ii) In adult male : 5 to 6 million / cu.mm. (iii) In adult female : 4 to 5 million / cu. mm. (iv) In foetus : 7 to 8 million / cu.mm.

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