19. Granulocytes - types, structural, ultrastructural and functional characteristics. Granulocytopoiesis

Granulocytes are a form of leukocytes which contain specific granules in their cytoplasm, and a lobed nucleus (2-5 lobes). There are 3 different types of Granulocytes; neutrophils, eosinophils and basophils. 

Neutrophils are spherical in shape and 10-12μм in size. They are the most numerous leukocytes and constitute 55%-65% of the leukocyte count. The nucleus looks like a clump-like pattern and with 3-5 lobes connected by fine chromatin strands. Older cells have darker-stained lobes. 

Females often have a small drumstick-shaped lobe called the Barr body. It is absent in males and may be useful in determining chromosomal sex.  

Neutrophil cytoplasm contains two types of granules. Azurophilic granules that stain reddish/purple and are primary lysosomes containing peroxidase and hydrolytic enzymes. It contains the organelles; Golgi complex, ER and mitochondria and some glycogen deposits. Neutrophils are actively motile and they function outside the circulation. Their lifetime in the blood is 12 hours and in connective tissue their lifetime is 1-4 days. The main function of neutrophils is phagocytosis where they engulf bacteria, cell debris, and foreign matter and their numbers increase in bacterial infections.

Eosinophilic granulocytes are spherical with size of 12-17μм and their nucleus has 2 lobes shaped like a telephone handle. Eosinophils make up a small proportion of leukocytes: 2%-4% of the leukocyte count. They stain red and these granules contain hydrolytic enzymes and secrete histaminase, which inactivates histamine produced by basophils and mast cells. They contain Golgi complex, ER, small mitochondria and ribosomes. It’s lifetime in the blood is 6-10 hours whereas in connective tissue its lifetime is 8-10 days. The main functions is to destroy parasites via phagocytosis and neutralise histamines in allergic reactions. Their number increases during an allergic reaction/phagocytosis and this is known as eosinophilia. Their numbers decrease when a patient takes a drug steroid, this is eosinopenia. 

Basophils, are the least numerous leukocytes, accounts for 0.1-1% of the leukocyte count. These granulocytes have a diameter of 10-12μm. Their nucleus is irregular in shape and has no lobes. They contain large basophilic granules that contain histamine, serotonin and heparin (prevents blood coagulation). Basophils participate in allergic reactions via their granules and basophils increase in number during skin allergic diseases.

Granulocytopoiesis is hematopoiesis of granulocytes. It occurs in the bone marrow and takes 14-18 days. WBCs originate from hemocytoblasts and the process starts with the nucleus becoming flat and indented, and then smaller and lobulated; the cytoplasm accumulates non-specific and specific granules. 3 cell populations arise, myeloblasts, promyelocytes, and myelocytes, each with proliferative or mitotic potential. 

Myeloblasts are large, spherical cells, 12-18 μm in diameter. Their basophilic cytoplasm lacks specific granules but contains lots of ribosomes. Has a large, round nucleus containing chromatin with several nucleoli. 

Myeloblasts divide and give rise to promyelocytes. They have a flat nucleus and basophilic cytoplasm. Promyelocytes divide and give rise to myelocytes, which are smaller. 

Myelocytes have a pale, lightly basophilic cytoplasm; their nucleus is pushed off to the side of the cell. Neutrophilic, basophilic, and eosinophilic myelocytes appear. 

Myelocytes of these three cells mature into metamyelocytes, which then develop into band cells and then finally into their specific granulocytes.