Atlas of plant and animal histology

Blood smear and general staining is the best way of observing and studying the various cellular components of blood. A blood smear is a drop of blood spread over a glass slide, and then fixed and stained.

Erythrocytes are a biconcave disc of about 7.5 µm in diameter. They are colored reddish with general staining techniques. In fresh blood, the red color is provided by erythrocytes because of their high content of hemoglobin, a protein which contains iron in its structure. Erythrocytes are the most abundant cells in blood, about 45% of blood volume. Erythrocyte, in mammals, can be regarded as a modified cell because it has no nucleus and lacks mitochondria and other cellular organelles.

Platelets are small portions of cytoplasm without nucleus. In blood smears, they appear as small bluish or pale purple aggregates. They are present in mammals, but not in lower vertebrates. Platelets are generated by fragmentation of the cytoplasm of megakaryocytes, a cell type found in the bone marrow.

Leukocytes (white blood cells) contain a nucleus and are colorless in fresh blood. That is why we need staining in order to observe leukocytes. There are granular and agranular leukocyte. Granular leukocytes contain two types of granules: azurophilic or primary granules, which are lysosomes, and specific or secondary granules, containing diverse types of substances. Agranular leukocytes lack specific granules.

Granular leukocytes encompass 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. Although they show cytoplasmic azurophilic granules, the specific granules are more abundant. Specific granules contain lysozymes, complement activators, collagenases, etcetera. Eosinophils account for 2 to 5% of the leukocyte population. The nucleus is bilobed and the cytoplasmic specific granules show a strong appetence for acid dyes such as eosin. Basophils are the less abundant and smaller granular leukocytes, and are up to 0.5% of the total leukocytes. Their nucleus is slightly lobed. They contain specific granules which are stained by basic dyes such as hematoxylin. That is why they show blue-purple dark granules.

Agranular leukocytes lack specific granules in their cytoplasm, but they have a small population of nonspecific grains. After neutrophils, lymphocytes are the second most abundant type of leukocyte, accounting for 20 to 35% of the blood cells. These cells are small, showing some variability in size, which appears not related to the different types of lymphocytes. The nucleus is rounded and sometimes occupies the majority of the cell volume. The other agranular leukocytes are monocytes. Monocytes show large size in blood smears and have a kidney-shaped nucleus.