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Blood Histology                                          Back to 106 Lab Schedule Back to 106 Lab Schedule         

Formed Elements of the Blood

The cells found in the circulating blood arise from two sources in the body. Myeloid tissue or red marrow is the source of erythrocytes, the granulocytes and the platelets. These cell types arise from a common ancestral cell, the hemocytoblast. Two other populations of cells making up the formed elements are the lymphocytes and monocytes. These cells develop from a precursor of the hemocytoblast, the reticular cell. Lymphocytes and monocytes are formed in the bone marrow and in the lymphoid tissues.

Occurrence – red marrow is the soft vascular tissue found in the marrow cavities of the diaphyses of long bones and in the spaces of the spongy bone located in the epiphyses of long bones and the middle portion of flat bones. These cavities are lined with an endosteum.

Marrow tissue consists of:

1. Blood cells in various stages of maturation from primitive stem cells to mature blood cells.

2. Adipose cells are common. During puberty and adolescence, adipose tissue replaces the blood-forming red marrow converting it to yellow marrow. This fatty tissue is primarily a filler tissue which can be compressed when the inactivated marrow proliferates during periods of accelerated hemopoiesis.

3. Blood vessels can be seen. Nutrient marrow arterioles branch into capillaries which empy into large, abundant sinusoids lines by flattened, phagocytic reticular cells. Sinusoids then converge into venules. Lymphatic vessels have not been demonstrated in bone marrow.

4. The structural framework of marrow tissue consists of a syncitium of reticular fibers associated with reticular cells. This network is connected to the endosteum and closely associated with blood vessels. These reticular cells are primitive connective tissue cells. They are the progenitors of macrophage (either fixed or wandering).

5. Nerves are also found in marrow. They are associated with blood vessels and follow their pathways. These nerves may have a vasomotor function.

Proliferative Function of Bone Marrow

There is about 0.56 grams of marrow for each gram of blood, about 3.5 to 6% of body weight. This corresponds to 1600 to 3700 grams, the weight of the liver. About one half of this material is fat.

The development of the formed elements begins with the division and differentiation of stem cells in the marrow. These cells are distinguished from one another using polychrome Romanowsky stains (eosin, azure and methylene blue dissolved in absolute alcohol). The three most commonly used Romanowsky stains are Wright’s, Giemsa and May Grunwald stains.

I. Granulocytes – General Characteristics
A. Cell is polymorphonucleated with a two to eight lobed nucleus.

B. Lobes of the nucleus are connected by thin strands of chromatin.

C. Cytoplasm is acidophilic and contains granules. These granules have a size and stain affinity which allows the granulocytes to be placed into three categories.

1. Neutrophiles
a. Have a 3 to 8 lobed nucleus.

b. Granules are pale staining (neutral).

c. 12 to 14 um in diameter.

d. They make up about 60% of the circulating   leukocytes.

e. A phagocyte, it plays a primary role in inflammation..

2. Eosinophiles
a. Have a bilobed nucleus

b. The granules are richly eosinophilic (acidophilic).

c. About 12 to 17um in diameter.

d. They make up about 3% of the circulating leukocytes.

e. Their functions are associated with allergic reactions and parasitic infections.

3. Basophiles
a. The nucleus has 3 to 4 lobes.

b. The large, deeply basophilic granules may obscure the nucleus.

c. About 12 to 15 um in diameter.

d. They make up less than 1% of the circulating leukocytes.

e. They have a purely secretory function. The contents of their granules function

in inflammatory, immune and hypersensitivity reactions.

4. Platelets – form from giant myeloid cells called megakaryocytes
        (60 to 100 um  in diameter). Cytoplasmic processes of cell extend into marrow sinuses and fragment away. These cellular fragments are the platelets or thrombocytes.

a. They are membrane-bound sacs.
b. They are 2 to 5 um in diameter.
c. The periphery of the platelet is a
colorless and homogeneous region called the Hyalomere.
d. The central area or chromomere contains azure-colored granules. The granules contain several different substances:

i.   heparin – an anticoagulant
ii.  histamine – triggers inflamation
iii. serotonin – elicits pain
iv. platelet-derived growth factor (PDGF) - stimulates tissue regeneration to promote healing
v. thromboplastin – promotes blood clotting

5. Erythrocytes – are formed by a complex process which occurs through several stages beginning with the hemocytoblast.

    
The image to the left shows a group of mature erythrocytes which exhibit the typical biconcave shape due to the loss of their nucleus just before being released from the bone marrow.

Stages of Erythrocyte Maturation in the Bone Marrow.

a. Pronormoblast – arises by division and differentiation of the hemocytoblasts.

         i.  20 to 25 um in diameter.

         ii. Large nucleus (80% of cell volume).

         iii. Cytoplasm is deeply basophilic.

     b. Basophilic Normoblast – arises by division and differentiation of the
         pronormoblasts.

           i.  16 to 18 um in diameter.

           ii.  Nuclear size is reduced.

           iii.  Chromatin forms a "spoke wheel" configuration.

           iv.  Cytoplasm is still basophilic.

     c. Polychromatic Normoblast – arises by division and differentiation of the
         Basophilic normoblast.

i.  9 to 12 um in diameter.

ii.  Nucleus is further reduced in size.

iii.  Nucleus is deep staining, i.e., less active in transcription.

iv.  Hemoglobin synthesis is apparent (the cytoplasm is becoming acidophilic).

v.  These cells have lost the capacity to divide.

     d. Acidophilic Normoblast – arises by differentiation from the polychromatic
         normoblasts.

i.  8 to 9 um in diameter

ii.  "Spoke wheel" chromatin is gone.

iii.  Cytoplasm is acidophilic.

iv.  Nucleus is extruded at the end of this stage.

     e. Reticulocyte

i.  8 um across

ii.  Anuclear.

iii.  Cytoplasm contains mitochondria.

iv.  Cytoplasm is stippled with basophilic material giving a reticulated         appearance.

     f. Erythrocyte

i.  7 to 8 um in diameter.

ii.  Biconcave in outline due to the loss of the nucleus.

iii.  Contain abundant hemoglobin in an acidophilic cytoplasm.

6. Lymphocyte

a. Form in the red marrow and lymphoid tissue.

b. They are the second most numerous leukocyte in the circulation (25%).

c. They are grouped into different size categories, i.e., small, medium and large lymphocytes. Functionally, these types cannot be distinguished from each other.The small lymphocytes have the least amount of cytoplasm. The nucleus takes up most of the cell’s volume.

d. Functionally, the lymphocytes fall into two categories:

i.  B lymphocytes are responsible for antibody formation. When they are activated by contact with foreign antigen, B cells divide and differentiate into antibody secreting plasma cells.

ii. T lymphocytes are responsible for contact destruction of foreign tissues, hypersensitivity reactions and cancer cell detection and destruction.

7. Monocytes

a. Form from reticular cells called histiocytes.

b. Histiocytes remaining in myeloid or lymphoid tissues are called macrophage. These may be fixed or wandering.

c. Histiocytes that enter the circulation are referred to as monocytes.

d. Monocytes are large cells, about 20 um in diameter.

e. The nucleus is off center and indented or C-shaped.

          f. They are voracious phagocytes and become highly activated after making                contact with foreign antigen.

         g. They make up about 5% of the circulating leukocytes