Blood can be considered a liquid tissue. Many authors consider blood as a type of connective tissue in which the intercellular material is replaced by a liquid of a very specific composition.
strong>Erythrocytes or red blood cells are anucleated cells. They lost their nuclei during their differentiation in the hematopoietic bone marrow. They contain a large amount of the protein hemoglobin. Red blood cells measure about 7 μm in diameter. This number is useful to be used as a size standard to diagnose other blood cells.
Leukocytes or white blood cells are a group consisting of several cell types. Based on the features of their nuclei and cytoplasm they are classified into granulocytes ot agranulocytes. Each type of of the types of the granulocytes has specific and characteristic granules in their cytoplasm a feature that is not present in the agranulocytes, as they lack these specific granules.
Granulocytes. In blood smears they present dense, well-stained chromatin. Their nuclei are segmented into lobes, meaning that the nuclei have several portions that are joined by thin nuclear strands.
The number of lobes is variable, and for this reason, leukocytes in this group are also called polymorphonuclear leukocytes.
Types of granulocytes
Neutrophils – they are the most common white blood cells present in a blood smear. Seen in smears they measure about 12 to 13 μm in diameter, therefore being larger than red blood cells. Their nuclei have dense chromatin. When young, the nuclei are rod-shaped, resembling the letter C. This stage of neutrophils is called “band neutrophil”. In mature cells, their nuclei are subdivided into several lobes (usually two to five). This lobed neutrophil form is also called “segmented neutrophil” or polymorphonuclear leukocyts.
Their cytoplasm contains specific small granules, which stain with the various dyes in the mixtures used to analyze blood cells and take on a light pink color. Because they stain with various dyes, the granules are called neutrophil granules.
Eosinophils – measure approximately 12 to 15 μm in diameter, similar in size to neutrophils. Their nuclei usually have two lobes, although one may find eosinophils with more lobes. An important characteristic of eosinophils lies in the granules present in their cytoplasm. They present in the cytoplasm a large number of specific, relatively large granules, stained orange or pink. This staining is due to eosin, hence the name of this cell.
Basophils – they measure approximately 12 to 15 μm in diameter. Their nuclei are segmented and occupy most of the cytoplasm. Often the nuclei cannot be seen in blood smears because they are obscured by clusters of strongly stained blue/purple specific granules, characteristic of this cell type. Basophils are the least frequent cells among leukocytes and, therefore, the rarest to be found in blood smears.
Agranulocytic or mononuclear leukocytes.
These cells have spherical, oval, or indented nuclei (in the shape of the letter C or U) but their nuclei are not segmented, hence their name mononuclear leukocytes. Their cytoplasm does not contain specific granules (hence the name agranulocytes), but may contain nonspecific granules (azurophilic), which are also present in granulocytes. These granules are very small, appearing as tiny dots in the cytoplasm, and stain blue/purple.
Monocytes – are large cells, the largest of the leukocytes. In smears they measure 15 to 17 μm in diameter. Their nuclei, with loose and delicate chromatin, are lightly stained, indented, and eccentric (located off-center). They have a large weakly stained cytoplasm, slightly basophilic, and uysually weakly stained.
The platelets are fragments of hematopoietic bone marrow cells called megakaryocytes. The fragments detach from these cells and pass into the blood vessels of the bone marrow. They measure 1 to 2 μm in diameter. In smears, they are frequently seen forming small clusters among the blood cells.