The blood may be considered a liquid tissue. Many authors classify blood as a type of connective tissue whose extracellular material is a liquid of very specific composition.
Blood cells are formed from precursor cells of the hematopoietic bone marrow tissue that undergo complex processes of cellular differentiation.
Mature erythrocytes or red blood cells are anucleated in mammals. They loose their nucleus during their differentiation process in the bone marrow. They contain a large amount of the protein hemoglobin. Red blood cells measure about 7 μm in diameter. This value is very useful to diagnose other blood cells when examining a blood smear.
Leukocytes or white blood cells consist of a diverse group of cells. Among other criteria, they are classified according to the presence or absence of distinctive organelles in their cytoplasm, usually called granules. The granules are of two types: specific or nonspecific. The latter ones are lysosomes present in all types of leukocytes. They are also called azurophilic granules (because they stain violet/purple by the stain azure).
Three types of specific granules with different contents characterize three kinds of leukocytes. They are also called granulocyte leukocytes or simply granulocytes.
Two types of leukocytes lack specific granules, having only nonspecific granules in their cytoplasm and are called agranulocyte leukocytes or simply agranulocytes.
Granulocytes: main features of neutrophils, eosinophils, basophils.
Their nuclei are segmented in lobes, meaning that the nuclei have several portions joined by thin strands of chromatin. The amount of lobes is variable, and for this reason they are also called polymorphonuclear leukocytes (PMNs).
The neutrophils are the majority of white blood cells. In smears they measure about 12 to 13 μm in diameter, therefore they are larger than red blood cells. Their nuclei show a dense, strongly stained chromatin. Young neutrophils have rod-shaped nuclei that are similar to the letter “C”. At this stage they are called “band neutrophils”. After they evolve to mature neutrophils their nuclei have several lobes (usually two to five) joined by delicate chromatin strands. Because of the several nuclear lobes the mature neutrophils are also called segmented neutrophils.
Very small pink specific granules are present in the cytoplasm of neutrophils of blood smears stained with common stain combinations to demonstrate blood cells.
Eosinophils – measure approximately 12 to 15 μm in diameter, hence similar in size to neutrophils. Their nuclei have usually two lobes, rarely three. An important feature of these cells lies in the granules present in their cytoplasm: they are relatively large when compared to neutrophil granules and stain orange or pink by eosin used in stain mixtures for smears, hence the name of this cell.
Basophils – are also large cells that measure approximately 12 to 15 μm in diameter. Their nuclei are segmented and occupy most of the cytoplasm. Their nuclei are sometimes difficult to observe in blood smears because they are obscured by clusters of strongly stained blue/purple specific granules. Basophils are the least common leukocytes and the rarest found in blood smears. They are often difficult to find in blood smears.
Agranulocytes: main features of lymphocytes, monocytes.
Main features of agranulocytes also called mononuclear leukocytes: .
They cells have spherical, oval, or indented nuclei (in the shape of the letter “C” or “U”). The nuclei are not segmented, hence the name mononuclear leukocytes. Their cytoplasm does not contain specific granules, but contain very small stain blue/purple nonspecific granules (azurophilic granules), also present in granulocytes.
Monocytes – are the largest leukocytes measuring in smears 15 to 17 μm in diameter. Their nuclei have a loose and delicate chromatin, are indented and are often eccentric (located off-center). The monocytes have a large weakly stained slightly bluish cytoplasm.
Platelets are fragments of the cytoplasm of hematopoietic bone marrow cells called megakaryocytes. These fragments detach from the cells and pass into bone marrow blood vessels and enter the blood circulation. They measure 1 to 2 μm in diameter. In smears, they frequently form small clusters.