ERYTHROPOIESIS OR BLOOD FORMATION

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ERYTHROPOIESIS

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 3rd 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. They become free in blood-stream, and finally about the mid-foetal life lose their nuclei to become non-nucleated discs.

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 cells. These young cells betray their youth by a fine reticulum which is probably remnant of the basophil cytoplasm of the immature cell. So they are called Reticulocytes.
  2. When these cells become, and they are now know as Erythrocytes of Red blood cells.
  3. The endothelial lining of the collapsed capillaries becomes swollen and divides first into cells called Megaloblasts with large vesicular nucleus, well-marked nucleolus and scanty chromatin. Their cytoplasm stains with basic dyes.
  4. Magaloblas divide and form early Erythroblasts with a large unleus and a fine chromatin network.
  5. The early Erythroblasts divide to from late Erythroblasts with an eccentric nucleus, dense chromatin network and more haemoglobin.
  6. The late Erythroblasts divide and give rise to Normoblasts which are normal in size & haemoglobin content, but still retain their nuclei which show condensation of their chromatin material (pyknosis) staining deep with basic dyes.

Ques. 2        What are the factors influencing erythropoiesis?

Ans.             (1) Factors influencing (responsible) erythropoiesis are: General stimulants 0

  1. Thyroid – thyroxine acts as a general metabolic stimulant, and so there is increased marrow activity.
  2. Vitamins – Vitamins C, B, & B12, folic acid, riboflavin, pantothenic acid and nicotinic acid are probably a marrow stimulant.
  • Anoxia (hypoxia) – The exact nature is unknown. But the stimulus becomes more effective of is supplemented when there is low 02 tension in tissues. When air with low 02 tension (as in high altitude) is breathed for some length of time, red cell count rises due to liberation of erythropoietin or haemopoietin or erythrocyte-stimulating factor (ESF). It stimulates bone marrow and increases the rate and maturation of red cells formation.

(2)  Factors affecting Hb formation – Raw materials, such as iron, copper, proteins and porphyrins must be available in adequate amount. HC1 of gastric juice helps to liberate food iron in an inorganic form. Copper & iron are helpful in the ratio of 1 : 100. Copper acts as an adjuvant, and is probably related to building up of iron haemoglobin.

(3)  Maturation factors help to turn –

  1. Megaloblasts – Normobasts. Haematinic principle, formed in the stomach by action of gastric enzyme (intrinsic factor of Castle) with a food constituent (extrinsic factor of Castle), is necessary for this stage of development.
  2. Endothelial cells- Megaloblasts.

Ques. 3        What is the normal span of life of a Red Blood Corpuscle?

Ans.              The average span of life of a mature R.B.C. is about 120 days. (It is interesting to remember that number of red blood cells formed per second is about 3.5 to 4.5 millions).

Ques. 4        What are Red cells?

Ans.              Fate of Red cells: When the red cells are worn out they put out long processes, and are then known as Poikilocytes. The processes vibrate with increasing rapidity, and are finally shed. This process of fragmentations continues till cells are reduced to a series of smaller fragments which are ingested by the phagocytic cells of the Reticuloendothelial system. The Hb is disintegrated iron in simple inorganic from is liberated and stored up in liver, spleen and bone-marrow for futures use, and the iron-free porphyrin is converted into bile pigment bilirubin. The latter is used in the formation of fresh Hb. 

Ques. 5        What is the character of R.B.C.?

Ans.             Character of R.B.C. : The size is smaller than W.B.C., it is non-nucleated and it contains haemoglobin. The 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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