A polymorphonuclear cell is a group of cells that played a major role in the defense of the organisms against a great variety of extraneous invaders.
It was first recognized by the great Russian Zoologist Metchnikoff, he named it the white cells of the blood macrophages (now we known as the polymorphonuclear cells), believing them to be concerned solely with the defense of the organism against small microorganisms, i. e., bacteria, and designated as macrophages certain cells in the tissues because they phagocytosed large preys like parasites and other cells.
Aside from polymorphonuclear cells referring to a group of named cells, the term also refers to their appearance. Poly + morph relates to “different shapes” and nuclear refers to the cell nuclei of polymorphonuclear cells when stained and viewed under the microscope.
Polymorphonuclear cells are thus known for the odd assortment of shapes that their nuclei can take on. Nuclei of polymorphonuclear cells white blood cells have deep partial divisions so that instead of looking like a nice, round circle in cross-section, they may appear to have two or three lobes.
These lobed nuclei cause the “fried egg” appearance, so common with many other cell types, to be noticeably absent—instead, the cell looks like an egg that had its yolk broken while being cooked.
The distinction is now known to be erroneous since both cell types are capable of phagocytosing large and small particles. The polymorphonuclear leukocytes constitute about 60%-70% of the total circulating leukocytes and are subdivided into three types, based on staining characteristics.
- neutrophils possess granules that do not stain intensively when viewed under the light microscope.
- eosinophils have granules that stain a bright orange-red; and
- basophils having granules that stain a dark blue-black. The normal range of total polymorphonuclear leukocytes is between 4,000 and 8,000 per mm3 of blood; The vast majority (> 90%) consist of the neutrophilic chain.
All three lineages derive from a common pluripotent stem cell in the bone marrow (of adults) under the influence of granulopoietin probably identical to the “colony-stimulating factor” (CSF); CSF Characterized partially in serum and urine as glycoproteins with approximately 45,000 molecular weights.
The most initially differentiated neutrophil precursor is myeloblast, which has a large nucleus and very little cytoplasm. The granule begins to appear in the next, or promyelocyte stage and is very evident in myelocytes; Due to their histochemical staining characteristics, granules are called azurophilic.
They originate from the concave side of the Golgi-system, are relatively large in size, and are electron-dense. These include acid hydrolysis, lysozyme, myeloperoxidase, neutral proteases, cationic proteins with bactericidal activity, and NADPH-oxidase. As the cells continue to develop, the beginning of the segmentation of the nucleus occurs in the metamyelocyte stage.
At this stage, secondary or specific granules appear, which are smaller and less dense than azurophilic granules. The specific granules contain alkaline phosphatase, lysozyme, lactoferrin, and collagenase.
The band form and mature neutrophils originate from the metamyelocyte stage and enter the circulation from the bone marrow. The mature cells are virtually devoid of mitochondria. There is a large reserve of granulocyte precursors in the bone marrow; the complete maturation process requires approximately 9-11 days.
Once in the circulation, however, the half-life of the mature neutrophil in the blood is only 6-8 h. This gives rise to an estimated neutrophil turnover of approximately 126 billion cells per day in a normal 70-kg individual.
The largest number of granulocytes seem to be lost from the blood through the gastrointestinal tract. Granulocytes also pass from the blood vessels into the tissue, attracted by bacterial and other chemotactic substances, and die there quite rapidly. Death can occur by fragmentation or the cells may be phagocytized and rapidly digested by macrophages.
Neutrophils have a well-developed capacity for locomotion when they are attached to a solid surface (endothelia). They extend a clear cytoplasmatic projection (protopod) in the direction of locomotion, while the opposite end of the cell (uropod) is attached to the support by a number of filaments.
They easily adhere to surfaces, migrate into the tissue by diapedesis, and are able to phagocytose microorganisms but also cells and inorganic substances of considerable size (erythrocytes, leukocytes, crystals).
Maturation of eosinophils parallels that of neutrophils, except that large eosinophilic granules take the place of neutrophilic granules in the myelocyte, in which the production of the secondary, specific granules starts.
The granules are formed at the Golgi complex in the same fashion as those of the neutrophils. Very recent findings suggest that the differentiation and maturation of eosinophils might be under the control of their “own CSF”, distinct from neutrophil CSF. The granules contain acid phosphatase, glycuronidase, cathepsin, ribonuclease, arylsulphatase, and other enzymes.
Peroxidase is present but different from the mye1operoxidase of neutrophils. The eosin granules contain phospholipids as well as basic proteins.
The mature eosinophils possess more and larger mitochondria than neutrophils, and their Golgi-apparatus is well developed. They have numerous glycogen particles. The fate of eosinophils is unknown; some of them are phagocytized, others are probably eliminated through the intestinal tract and the lungs.
The eosinophils respond to the same chemotactic stimuli as neutrophils, but particularly to soluble bacterial factors and antigen-antibody-complexes. They play a particular role in allergies and helminthic infections.
The maturation of basophils runs a course very similar to one of neutrophils and eosinophils, except that metachromatic granules are produced. The earliest identifiable granules are formed in the Golgi complex during the promyelocyte stage and contain abundant glycogen deposits in their cytoplasm. The mature basophil has numerous large and opaque granules masking the nucleus.
The granules contain acid mucopolysaccharides, histamine, and heparin in large quantities, and numerous enzymes, e.g. dehydrogenases, diaphorase, histidine-carboxylase, and peroxidases. The movement of basophils is ameboid and is similar to the movement of eosinophils though less active.
The basophils have the little phagocytic capability. They possess on their surface receptors for IgG and IgE as well as C3b. Nothing is known about life span and death.