TRANSIENT CELLS OF THE CONNECTIVE TISSUE – 2
LEUKOCYTES
In addition to the resident cells the connective tissue usually contains several types of transient cells. Most of them are leukocytes (white blood cells). They leave the blood capillaries and venules by diapedesis, the passage of cells between the endothelial cells that line the blood vessels, followed by their establishment in the connective tissue.
The main criteria used for classifying leukocytes in blood smears and in tissue sections are: their size, the shape of their nuclei, the presence or not of specific granules in the cytoplasm and the features of these granules.
Polymorphonuclear and mononuclear leukocytes
Polymorphonuclear leukocytes have lobulated nuclei meaning nuclei subdivided in two or more parts called lobules joined by thin chromatin filaments. Neutrophils, eosinophils, and basophils are polymorphonuclear leukocytes.
Mononuclear leukocytes are blood cells whose nuclei are round or oval and non-lobulated. They are the lymphocytes and monocytes.
Granulocyte and agranulocyte leukocytes
The cytoplasm of the leukocytes may contain two groups of different-sized granules with different contents, the non-specific and the specific granules.
Combinations of stains (mainly mixtures of azure with eosin, called Romanowsky-type dyes) were created to help to differentiate the leukocytes of blood smears according to the staining of their granules. Examples of some of these mixtures: Leishman, May-Grünwald, Wright stains.
Small nonspecific granules are present in different amounts in cytoplasm of the diverse types of leukocytes. They are also called azurophilic granules, as they stain mainly by azure in a bluish-purple color.
The cytoplasm of lymphocytes and monocytes only contains nonspecific granules and for this reason these cells are also called agranulocyte leukocytes, or simply agranulocytes.
Polymorphonuclear leukocytes, also called polymorphonuclears, have in their cytoplasm specific and nonspecific granules. These leukocytes are distinguished by their size, by the features of their nuclear lobulation and by the size and staining of their specific granules. This group constitutes the granulocyte leukocytes or granulocytes.