LEUCOPOIESIS ( FORMATION OF WHITE BLOOD CELLS

LEUCOPOIESIS

 

Under normal conditions, five types of mature white blood cells (WBCs) or leucocytes are found in the peripheral blood. Three of these types contain granules in their cytoplasm, and are therefore called granulocytes.

 Depending on the nature of the granules, they are called neutrophils , eosinophils and basophils. The other two types of cells, called agranulocytes, are monocytes and lymphocytes.

Out of these cells, granulocytes and monocytes are formed in the bone marrow from a pluripotential cell by differentiation, proliferation and maturation. 

Lymphocytes, however, have a very complex method of maturation and this process takes place in the reticulo-endothelial system. The blood granulocytes and monocytes, on the other hand, are controlled by a feedback system similar to that of RBCs. 

There are colony stimulating factors (CSFs) or leucopoietins which stimulate and produce colonies of a particular white cell line.

There are certain criteria for identifying the cells: 
(1) size of the cell, (2) ratio of size of nucleus to that of the cytoplasm, (3) appearance of nuclear membrane, (4) presence of nucleoli, (5) staining reaction of the cytoplasm, (6) presence of cytoplasmic granules, and (7) the type, size and distribution of granules.

Development of Granulocyte Series (Myeloid series)

The description of blood and bone marrow cells stained with a Romanowsky stain can be summarised as follows: 

1. Pluripotential stem cell 

Size Usually 25-40 microns, having an irregular margin with blunt protoplasmic projections.

Nucleus Round, large with finely reticulated purple-red stained nuclear chromatin. Nucleus to cytoplasm ratio is 1:1. 

Nucleoli 1-4, pale blue, well defined and irregular in shape. Cytoplasm Bluish, non granular, less deeply stained near the nucleus.

2 Myeloblast 

Size 10-18 microns in diameter, ovoid. 

Nucleus Occupies most of the cell, round or slightly ovoid, nucleus to cytoplasm ratio approximately 6:1. It has a smooth, thin nuclear membrane with fine, reticular, evenly distributed, purplish nuclear chromatin. 

Nucleoli 2 or more, distinct, ovoid, pale blue. Cytoplasm Very thin, blue basophilic rim though less basophilic than pronormoblast. 

3. Promyelocyte 

Size 12-20 microns, round or oval. 

Nucleus Ovoid, large; with slightly coarse clumping, light purple chromatin. The nucleus to cytoplasm ratio is 5:1. 

Nucleoli 2 or more, less distinct than in the blast cell, ovoid and pale blue. Cytoplasm Light purple, basophilic with few relatively large dark blue granules occasionally overlapping the nucleus. 

4. Myelocyte 

Size 12-18 microns. Round or oval. 

Nucleus Round or oval, indistinct with more coarse clumping of purple chromatin. The nucleus to cystoplasm ratio is approximately 2:1. 

Nucleoli Usually absent.

Cytoplasm Bluish pink with early undifferentiating azurophilic granules, more numerous and smaller than in promyelocyte. They may start differentiating into secondary specific granules such as chunky orange red (eosinophilic), blue black (basophilic), or lilac (neutrophilic) granules. 

5. Metamyelocyte 

Size 10-18 microns, round or oval, slightly smaller than the myelocyte. It can be differentiated from the myelocyte by the shape of the nucleus. 

Nucleus Indented and kidney shaped. 

The nucleus to cytoplasm ratio is approximately 1.5:1. The nuclear chromatin is dark, purplish and coarse or in strands. 

Cytoplasm Abundant, pinkish blue and filled with numerous small granules (neutrophilic, eosinophilic or basophilic). 

6. Stab cell (Band cell) 

Size 10-16 microns, round or oval.

Nucleus Sausage or horse shoe shape with occa sional areas of constriction. The nucleus to cytoplasm ratio is approximately 1:2. Nuclear chromatin is coarse and deep purple blue. 

Nucleoli Absent. 

Cytoplasm A large amount is present, and it is pale blue or pink, with fine lilac (neutrophilic), large chunky orange (eosinophilic) or coarse blue black (basophilic) granules. 

7. Polymorphonuclear granulocytes (Neutrophil, Eosinophil and Basophil) 

Size 10-15 microns. 

Nucleus The nucleus in a younger cell consists of 2 purplish lobes separated by a strand of chromatin. The older cells have 3 or more lobes separated by very thin chromatin strands. The nucleus cytoplasm ratio is approximately 3:1. 

Nucleoli Absent. 

Cytoplasm Light pink to blue. The neutrophil has many small violet pink granules, the eosinophil has large bright orange granules and the basophil has large coarse blue black granules which completely fill the cytoplasm and obscure the nucleus.

Note

Stages 1-4 is the proliferation phase of development while stages 5-7 is the differentiation and maturation phase (Fig. 2.1).

Figure 2.3 shows stages of maturation of a granulocyte.

Development of The Lymphocyte Series

1. Lymphoblast 

Present in bone marrow on occasions but in ex: tremely small numbers; it is difficult to differentiate it from myeloblast.

Size 10-18 microns. 

Nucleus Occupies most of the cell, round or oval. The chromatin is dark purple and aggregates along the nuclear membrane. It is coarser than in myeloblast. The nucleus to cytoplasm ratio is 6:1. Nucleoli 1-2, indistinct light blue. 

Cytoplasm Deep blue with no granules, most cells show paler perinuclear area.

2. Prolymphocyte 

Size 9-17 microns. 

Nucleus Ovoid, occasionally indented border. Chromatin is coarser, darker and more aggregated than in the lymphoblast. The nucleus to cytoplasm ratio is 4.5:1. 

Nucleolus One, bluish. 

Cytoplasm More than in the lymphoblast, with light to dark blue colour, occasionally with azurophilic granules. 

3. Lymphocyte

Size Two types: large. 8-16 microns and small, 7-9 microns. 

Nucleus Usually round in shape and frequently eccentrically located. Chromatin is dark purple blue, very coarse and clumped. The nucleus to cytoplasm ratio is 1.5:1 in large lymphocyte and 1.25:1 in small lymphocyte. The nuclear membrane is sharply defined. 

Nucleoli Absent. 

Cytoplasm Light blue and varies from only a ring around the nucleus in small lymphocytes to relatively abundant in large lymphocytes. Occasional azurophilic granules may be seen in large lymphocytes. 

Development of The Monocyte Series

1. Monoblast 

Size 12-20 microns.

Nucleus Large, round or ovoid. Light purple pink reticular chromatin, very fine with a distinct nuclear membrane. The nucleus to cytoplasm ratio is about 2:1. Nucleoli 1-2. Cytoplasm Light blue without granules. 

2. Promonocyte 

Size 12-18 microns.

 Nucleus Large lobulated, kidney shaped with fine, light purple, thread like (reticular) chromatin. The nucleus to cytoplasm ratio is approximately 2.5:1. 

Nucleoli 0-1. Cytoplasm Grey blue with large and small lilac azurophilic dust-like granules.

3. Monocyte 

This is the largest of normally occurring peripheral blood cells. 

Size 12-16 microns. 

Nucleus Oval, kidney or horse shoe shaped. Fine lacy delicate chromatin stained light purple pink. Nucleus to cytoplasm ratio is approximately 2.5:1. 

Nucleoli Absent. 

Cytoplasm Abundant, slate grey with many fine lilac coloured granules and vacuoles.

Note

The monocyte can be differentiated from metamyelocyte and prolymphocyte by the lighter staining nucleus and fine lilac granules.

Development of the Plasmocyte Series

Plasmocyte is formed by transformation of antibody producing lymphocyte (B-lymphocyte) When stimulated by an antigen, the B-lymphocyte transforms into a plasmoblast which proliferates and matures to form a plasmocyte or plasma cell. These plasma cells produce antibody which is specifically directed against the stimulating antigen. 

1. Plasmoblast 

Normally not present in blood and only rarely seen in normal bone marrow. 

Size 14-24 microns. 

Nucleus Eccentrically placed, somewhat eggshaped, usually in the narrower pole of the cell. The chromatin is reticulated, purplish and coarse. The nucleus to cytoplasm ratio is about 1-2:1. 

Nucleoli 1-3 large nucleoli. 

Cytoplasm Relatively abundant, deeply basophilic with occasional areas of light staining. 

2. Proplasmocyte 

Size 14-22 microns. 

Nucleus Eccentric, oval shaped, with peripheral coarse chromatin. The nucleus to cytoplasm ratio is 2:1. 

Nucleoli 1-2 and large. 

Cytoplasm Abundant, light blue with lighter perinuclear halo.

3. Plasmocyte (Plasma cell)

Egg shaped, narrower at one end than the other. 

Size 8-18 microns. 

Nucleus Ovoid, eccentrically located usually in the narrower end of the cell. The chromatin is purplish and extremely coarse giving a cart-wheel appearance. The nucleus to cytoplasm ratio is approximately 1:2. 

Nucleoli Absent 

Cytoplasm Deep blue with pale peri-nuclear halo near one side of the nucleus. Vacuoles may be present near the cell border.

FUNCTIONS OF WHITE BLOOD CELLS

Granulocyte Function

A large number of white cells, especially of the granulocyte or myeloid series, are held in the marrow as a 'reserve pool'. A normal bone marrow contains more myeloid cells than erythroid cells in the ratio 2:1-12:1. The largest population is that of neutrophils and metamyelocytes. When released into the peripheral blood, granulocytes spend about 10 hours in the circulating blood. From circulation, they move into the tissues where they perform their phagocytic function. 

They clear the body of unwanted particulate material such as dead or injured tissue cells, and also engulf and destroy foreign bodies such as bacteria. They have a comparatively short life-span of about 4-5 days in the tissues before they are destroyed during defensive action or are removed because they are worn-out. At any given time, normally, the number of granulocytes in the circulating blood is equal to that of granulocytes in tissues, and this is 5 - 10 x109/L (5,000 to 10,000/mm).

Monocyte Function

Like granulocytes, monocytes also act as phagocytes and scavengers. They are released from the marrow into the circulating blood. They remain in blood for about 20-40 hours and enter the tissues. In tissues, they are called macrophages. Their life span in tissues may be as long as several months or even years. They may assume specific functions in different tissues such as skin, liver and intestines.

Lymphocyte Function

The immunologically competent cells or immunocytes comprise the lymphocytes, their precursors and plasma cells. These cells assist the phagocytes in the defence of the body against infection. Unlike the phagocytic function, the protection by lymphocytes is specific. The immune response depends on two types of lymphocytes, B-cells and T-cells. In man, B-cells are derived from the bone marrow. After stimulation by antigens, B-cells proliferate, transform and mature into plasma cells which secrete specific immunoglobulin antibody.

The T-lymphocyte is derived from the thymus gland and is concerned with cell-mediated immunity (CMI). It is a very complex process involving the cells surrounding the antigenic material and destroying it. The T-lymphocytes have various sub-populations, each with a specific immune function, e.g., helper T cells (T.), suppressor Tcells (Tg), memory cells, etc. They have a very long life span, possibly up to 20 years.