Granular Leukocytes

Granular Leukocytes

Granular leukocytes are all approximately the same size - ~ 12-15 µm in diameter. Their nuclei form lobes, and nucleoli cannot be seen. The number of nuclear lobes varies according to cell type. All granulocytes are motile.
The term granulocytes refers to the presence of granules in the cytoplasm of these cells. The granules correspond to secretory vesicles and lysosomes. Specific granules are the granules which are only found in one particular type of granulocytes.

 
Neutrophil granulocytes (or neutrophils)

have a very characteristic nucleus. It is divided into 3-5 lobes which are connected by thin strands of chromatin. The number of lobes increases with cell age. Up to 7 lobes can be found in very old neutrophils (hypersegmented cells).

Neutrophils (like all other granulocytes, monocytes and lymphocytes) contain all the organelles that make up a typical cell. In addition to the usual complement of organelles, they also contain two types of granules. Primary granules (or A granules) contain lysosomal enzymes and are likely to be primary lysosomes, although they are larger (0.4 µm) than the "ordinary" primary lysosome. Secondary granules (or B granules), the specific granules of the neutrophils, contain enzymes with strong bactericidal actions. The specific granules of neutrophils stain only weakly if they are at all visible - they are "neutral", hence the term neutrophil.

Functions

Neutrophils play a central role in inflammatory processes. Large numbers invade sites of infection in response to factors (e.g. cytokines) released by cells which reside at an infection site. Neutrophils are the first wave of cells invading infection sires. Receptors in their plama membrane allow them to recognise foreign bodies, e.g. bacteria, and tissue debris, which they begin to phagocytose and destroy. The phagocytotic activity of neurophils is further stimulated if invading microorganisms are "tagged" with antibodies (or opsonised). Neutrophils cannot replenish their store of granules. The cells die once their supply of granules has been exhausted. Dead neutrophils and tissue debris are the major components of pus. Their lifespan is only about one week.

Lost neutrophils are quickly replenished from a reserve population in the bone marrow. Because they are younger, their nuclei have fewer lobes than the "average" neutrophil. A high proportion of neutrophils, with few nuclear lobes indicates a recent surge in their release from the bone marrow.

 
Suitable Slides

Blood Smear, human - Leishman stain Neutrophil granulocytes are easy to find. They are the most frequent type of white blood cells, and the complex shape of their nucleus identifies them unequivocally. In darkly stained smears it is possible to see some faintly purple, very small granules in the cytoplasm. These granules represent the primary, lysosom-like granules of neutrophils. Have a close look at the nucleus of a number of neutrophils, and make a qualified guess at the gender of the individual, which donated blood for the slides. Identify and draw one or two neutrophil granulocytes. How the neutrophils and other leukocytes exactly will look like depends somewhat on how the stain turned out. In some batches (labeled B2 in the images) nuclei are dark and crisp, and the cytoplasm is well demarcated. In other batches (labeled B1 in the images) nuclei and cytoplasm are lighter and their boundaries are less well defined. While the morphology appears clearer in the darker stained smears, it will usually be more difficult to identify eosinophils and basophils.

 
Eosinophil granulocytes (or eosinophils)

Their nucleus usually has only two lobes. Almost all of the cytoplasm appears filled with the specific granules of the eosinophils. As the term "eosinophil" indicates, these granules are not neutral but stain red or pink when eosin or a similar dye is used in the staining process. Aside from the usual complement of organelles eosinophils contain some large rounded vesicles (up to 1 µm in diameter) in their cytoplasm. These granules correspond to the eosinophilic grains that we see in the light microscope. These specific granules contain, in addition to enzymes that otherwise are found in lysosomes, an electron-dense, proteinaceous crystal. This crystal is composed of major basic protein (MBP).

Functions

The presence of antibody-antigen complexes stimulates the immune system. Eosinophils phagocytose these complexes, and this may prevent the immune system from "overreacting". Their granules also contain the enzymes histaminase and arylsufatase. These enzymes break down histamine and leukotrienes, which again may dampen the effects of their release by basophils or mast cells. MBP, which can also function as a cytotoxin, and its release by eosinophils may be involved in the response of the body against parasitic infections, which are accompanied by an increase in the number of eosinophils.

Suitable Slides

Blood Smear, human - Leishman stain
Eosinophils and basophils are the only cell types present in normal blood which initially may be difficult to distinguish - in particular in darker smears. If you see them side by side in your drawing, the difference between them should become apparent. Chances are 6:1 that the you find an eosinophil before you find a basophil. The two lobes of the nucleus of eosinophils are usually well-defined and of about equal size. The nucleus is embedded in a cytoplasm crowded with granules, which seem to form a solid mass in the cell. The 2-3 nuclear lobes of basophils are not as well defined as those of eosinophils, granules are not as numerous as in eosinophils, and pretty much all of them can be identified "as individuals" rather than the dense mass they form in eosinophils.
Note that eosinophils and basophils are much easier to distinguish in B1. In B2, the difference in the staining of their of granules is not as pronounced, and the nuclei do not stand out as clear as in B1.
Identify, draw and label an eosinophil and a basophil.

Basophil granulocytes (or basophils)

Basophilic granulocytes have a 2 or 3 lobed nucleus. The lobes are usually not as well defined as in neutrophilic granulocytes and the nucleus may appear S-shaped. The specific granules of basophils are stained deeply bluish or reddish-violet. In some smears, their colour corresponds closely to the colour of the nucleus which sometimes is difficult to distinguish amongst or behind the granules. The granules are not as numerous as those in eosinophils. The specific granules of basophils (~ 0.5 µm in diameter) appear quite dark in EM pictures. They contain heparin, histamine, lysosomal enzymes and leukotrienes. The later correspond to the slow-reacting substance of anaphylaxis (SRS-A).

Functions

Heparin and histamine are vasoactive substances. They dilate the blood vessels, make vessel walls more permeable and prevent blood coagulation. As a consequence, they facilitate the access of other lymphocytes and of plasma-borne substances of importance for the immune response (e.g. antibodies) to, e.g., a site of infection. The release of the contents of the granules of basophils is receptor-mediated. Antibodies produced by plasma cells (activated B-lymphocytes) bind to Fc-receptors on the plasma membrane of basophils. If these antibodies come into contact with their antigens, they induce the release of the contents of all of the basophil granules.