Histology of Leucocytes

Leucocytes (White blood cells) are nucleated, (true cells) and may be divided into non-granular  (agranulocytes) and granular (granulocytes) varieties.

The cytoplasm of granular leucocytes shows numerous granules. In many, these granules are fine and slightly refractive but in others they are coarse and highly refractive.

Granulocytes

• These are divided into three classes according to the nature of the staining reaction of the granules present as neutrophils, eosinophils and basophils.
• Neutrophils
  • They are large cells 8 to 12 microns in size. The nucleus shows 3 to 5 lobes connected by fine stands and is in the form of 2 or horse shoe. Granules are neutrophilic as they have affinity towards the neutral stain of the Romanowsky compound and take a light purple stain. In the rabbit, guinea pig and chicken they take acid stain. In different species, based on staining reaction, these cells are also called heterophils.
  • They are highly phagocytic and show great amoeboid motility. After the migration from blood stream, the neutrophils phagocytose bacteria or other small particles. They have been cells that engulf larger particles. The neutrophils also elaborate powerful proteolytic enzymes which may act within the cell as in the digestion of phagocytosed bodies liberated and act outside the cell body.
  • Chemotactically attracted by devitalized tissue bacteria or other foreign bodies the cells reach these places by amoeboid movement and they liberate the proteolytic enzymes either by secretion or most commonly by disintegration and rupture of the leucocytes themselves. These cells form the so called pus corpuscles.
  • Number of neutrophils in percentage of total number of leucocytes. In man, dog cat, horse and pig, the neutrophils are in majority. Whereas in ruminants, chicken and laboratory rodent, the lymphocytes are in majority.

• Eosinophils
  • These are larger than neutrophils and the size is about 10 to 14 microns. It contains spherical and highly refractive granules which stain with eosin and other acid dyes. The nucleus generally has only two lobes. The granules are especially large in horse. They show amoeboid movement but are not actively phagocytic. Their number is greatly increased in allergic states, bronchial asthma, skin diseases and parasitic infestations.
  • Number of eosinophils in percentage of total number of leucocytes.

• Basophils
  • These are rare in adult blood being only 0.5 percent of leucocytes except in the chicken (3 percent). Their size is 8 to 10 microns. The nucleus is large irregularly polymorphous. The coarse granules are basophilic, intermediate in size between eosinophil and neutrophil types and are soluble in water. They resemble the mast cells of connective tissue.

Non granular leucocytes or agranulocytes

• These include the lymphocytes which are small about the size of erythrocytes, and a group of largest cells, the monocytes which have more cytoplasm and a more indented nucleus.
• Lymphocytes
  • The size of the small lymphocyte varies from 6 to microns majority being 7 to 8 microns. The nucleus is large and stains intensely. The basophilic cytoplasm forms a narrow rim around the nucleus. The large lymphocytes measures from 10 to 12 microns. The larger lymphocytes also have a deeply stained spherical nucleus and a basophilic cytoplasm, but there is greater proportion of cytoplasm than in the small lymphocyte. The basophilia of the cytoplasm in both is due to diffuse distribution of free ribosome or RNA granules.
  • The lymphocytes migrate through walls of capillaries into the connective tissue where they are believed to perform a number of functions. They are believed to act as precursors of monocytes and macrophages in the connective tissue. Following a primary antigenic stimulation, the lymphocytes give rise to a line of cells which differentiate into plasma cells, which produce the specific antibodies. They probably have a detoxifying action and are concerned in protection against foreign substance including response against grafts of foreign tissue.
  • Number of lymphocytes in percentage of the total number of leucocytes.

• Monocytes
  • These are large cells, size varying from 12 to 15 microns, but they may occasionally be up to 20 microns. The nucleus is ovoid, kidney or horse-shoe shaped and eccentrically placed. It stains less intensely than lymphocytes. Cytoplasm is basophilic and abundant. They migrate through the capillary wall into the surrounding connective tissue, become actively phagocytic and transform into macrophages, indistinguishable from the macrophages present in the connective tissue. They serve to combat certain types of bacterial infection like tuberculosis.
  • Number of monocytes in percentage of total number of leucocytes.

Blood platelets

• They are colorless about 3 microns biconvex discs but appear spindle shaped when seen sideways.
• They have no nucleus. They show a central granular mass which stains deeply with basic stains called the chromatomere and a peripheral hyaline lightly stained zone called the hyalomere.
• In mammals they are not cells but are only fragments of cells derived from the giant cells in red marrow.
• Birds and other lower vertebrates have true thrombocytes which are nucleated and have basophilic cytoplasm.
• The platelets play an important role in coagulation of blood.