Atlas of Plant and Animal Histology
Animall tissues: Connective

Blood: blood cells.
Species: human (Homo sapiens; mammal).
Technique: haematoxylin & eosin, blood smear.

Blood smear and general staining is the best way of observing and studying the various cellular components of blood.

Erythrocytes give the red color to the blood for its high content of hemoglobin, a protein which contains iron in its structure. The main role of erythrocytes is to transport O2 and CO2. Erythrocyte, in mammals, can be considered as a modified cell for these functions because it has no nucleus and lacks mitochondria and other cellular organelles. It is shaped as a biconcave disc of about 7.5 µm in diameter, which gives greater exchange surface with the blood plasma. Erythrocytes are approximately 45% of blood volume.

Platelets are small portions of cytoplasm without nucleus. Its main function is to cooperate on agglutination and blood coagulation. They are present in mammals, but not in lower vertebrates. They are formed by fragmentation of the cytoplasm of megakaryocytes, a cell type found in the bone marrow.

Leukocytes (white blood cells) contain nucleus and are colorless in fresh blood. Their main function is to defend the body against external aggressions such as pathogens, or aberrant internal alterations. This function is performed outside the blood, as they have the ability to cross the vascular wall and act on the damaged tissue. Actually, they use the circulatory system to move through the body. Leukocyte cytoplasm contain two types of granules: azurophilic or primary granules, which are lysosomes, and specific or secondary granules, containing diverse types of substances. White blood cells are classified into granular and agranular. They all have azurophilic granules, but the specific granules only appear in the granular leukocytes.

Granular leukocytes are neutrophils, eosinophils and basophils, whereas agranular leukocytes include lymphocytes and monocytes. Neutrophils are the most abundant granular leukocytes and account for 60-70% of all leukocytes. They are easily recognized by their multilobed nucleus. They show cytoplasmic azurophilic granules, but the specific granules are more abundant. These last granules contain lysozyme, complement activators, collagenases, etcetera. They are very important in the defense against bacterial infections. Eosinophils represent from 2 to 5% of the leukocyte population. Its nucleus is bilobed and its cytoplasmic specific granules show a strong appetite for acid dyes such as eosin. These granules have basic proteins such as major basic protein and eosinophil cationic protein, which are involved in the control of parasitic infections, and histaminases that neutralize the action of histamine in allergic reactions. Basophils are the less abundant and smaller granular leukocytes, representing 0.5% of the total leukocytes. Its nucleus is slightly lobed. They contain specific granules which are stained by basic dyes such as hematoxylin. The cell membrane of basophils has receptors for immunoglobulins E and the specific cytoplasmic granules contain histamine and heparin. Thus, it is suggested that these cells act on the connective tissue in cooperation with mast cells.

Agranular leukocytes lack specific granules in their cytoplasm but have a small population of nonspecific grains. Lymphocytes are the second most abundant type of leukocyte, after neutrophils, and account for 20 to 35% of blood cells. These cells are small, although they show some variability in size, which appears not to be related to the different types of lymphocytes. The two main groups of lymphocytes are B and T. Both are major players in the immune defense of the organism. The other agranular leukocytes are monocytes. They show a large size in blood smears and have a kidney-shaped nucleus. Monocytes help to the defense of the body by leaving the blood and moving to the site of infection or injury, where they become macrophages

Actualizado: 18-12-2012