|| ERYTHROPOIESIS ||

Erythropoiesis

The term erythropoiesis means the genaration of erythrocytes. Upto the 3rd month of the intrauterine life erythropoiesis occurs in the mesoderm of yolk sac. Between the 3rd month and the 5th month of entrautrine life, erythropoiesis occurs in the liver and spleen. This stage is called the "hepatic stage"of erythropoiesis. From the 5th month red bone marrow begins to produce erythrocytes. This phase is called 'myloid phase'.

Note: At birth, all the marrow are of red variety. As age advances, in same bones, the red variety is gradually replaced by yellow marrow. After 20 years in a healthy adults red bone marrow is found in the "flat bones" ( ribs, sternum, cranial bone, vertebrae, pelvic bones etc.). The shafts of the long bones contain only, the yellow bone marrow. However, in hypoxia conditions, the yellow bone marrow is converted into the red variety and under some very extreme demands the liver and spleen may begin to generate red blood cells in adults life.

Stages of Development

The development of a mature erythrocytes is through several stages of division and differentiation. There are controversies regarding the details but following details represent the consensus thinking.

1. A pluripotent stem cell is the most primitative cell which gives rise to myeloid as well as lymphocytic series cells.
2. The pluripotent stem cell then gives rise to committed stem cells. One type of commited stem cell produces RBCs, neutrophils, eosinophils, monocytes, and platelets ( cell of the myeloid series). The other types of commited stem cell produce the lymphocytes.
3. Cells commited to produce the myeloid series, further differentiate and from "progenitor cells". There are distinctly separate proginator cells for RBC, granulocytes, monocytes and platelets. Progenitor cells are generally called colony

INTRODUCTION TO HAEMATOLOGY

HEMATOLOGY

Hematology encompasses the study the of blood components and coagulation. It includes,

1. Analysis of the concentration, structure and functions of the cells and their precursors in the bone marrow.
2. Analysis of chemical constituents of plasma or serum intimately, linked with blood cell structure and functions.
3. Study of functions of the platelets and proteins involved in blood coagulation.

Changes in one or more of the characteristics mentioned above, may produce hematological disease or manifestations. The hematology laboratory deals with routine determination of total number of cells in circulation, hemoglobin concentration, and differential count of leukocytes based on the study of the stained of the blood smear helps in detecting morphological abnormalities of a various cells seen in the peripheral  blood circulation.

BLOOD

Blood may be described as a specialized connective tissues, which circulates in a closed system of blood vessels. The circulating blood consists of suspension of formed elements such as erythrocytes, leukocytes and platelets in a pale yellow colored fluid called plasma. In adults the total volume of blood comprises about 8% of the body weight or about 5,600ml in a 70kg man. The formed elements account for about 46% of total blood volume.

The functions of blood are as follows:

1. Respiration: Transport of oxygen from the lungs to the tissues and of carbon dioxide from tissues to the lungs.
2. Excretion: Transport of the metabolic wastes to the lungs, kidney, skin and intestines for removal.
3. Maintenance of normal acid - base balance.
4. Nutrition: Transport of absorbed fatty acids, monosaccharides and amino acids.
5. Regulation of water balance.
6. Regulation of body temperature.
7. Transport of hormones, vitamins and salts which contain cations such as sodium, potassium, calcium etc, and anions such as chlorides, phosphates, sulfates and carbonates. 
8. Transport of metabolites. 
9. Defence against infection by the white cells and the antibodies. 
10. Coagulation of blood: To stop bleeding by clotting. 

HEMATOPOIETIC SYSTEM OF THE BODY 

Hematopoiesis is,

• Production
• Development and
• Maturation of cellular elements of blood (hem =blood, poiesis =synthesis).

It consists of production of ,(1) erythrocytes (erythropoiesis) (2) leukocytes (leukopoiesis) and (3) thrombocytes (thrombopoiesis).

Normal sites.                               Site for poiesis

1. Fetus: less than 2 months.   : Yolk sac
2. From 2 to 7 months.              : Liver and                                                        Spleen                                                             (partially)
3. After 3 months.                      : Bone                                                                   marrow
4. Full term infant.                :Bone marrow
5. Children and the adults. :Bone marrow

In normal health only mature cells are seen in the peripheral blood. Development of blood cells takes place through 3 stages. 

1. Multiplication of precursor cells
2. Structural and functional maturation and 
3. Release into the peripheral circulation.

Under normal conditions, the production of blood cells is carried out by the bone marrow (medullary hematopoiesis),while in the fetus, until the bone marrow is matured the extra medullary hematopoiesis is normal. The development of extra medullary  hematopoiesis in children and adults is a pathological phenomenon.

A totipotent stem cell is a common precursor for all series of cells i.e. erythrocytes , megakaryocytes, lymphocytes and granulocytes. The blast cells through several stages of division and differentiation develop into mature cells of each series. Vitamin B¹² and folic acid are necessary  for DNA synthesis and hence for cell division. The deficiency of these two nutrients results in a decrease in the number of mature cells in the the circulation.