Blood

Blood is considered a connective tissue for two basic reasons: (1) embryologically, it has the same origin (mesodermal) as do the other connective tissue types and (2) blood connects the body systems together bringing the needed oxygen, nutrients, hormones and other signaling molecules, and removing the wastes. In circulating blood two different cell types are found: enucleated erythrocytes or red blood cells and nucleated leukocytes or white blood cells. We will study their histology in blood smears.

_________________________________________Erythrocytes (red blood cells)

RBCs are small (7 um) cells lacking a nucleus. They stain red with eosin and due to their concave shape have a lighter staining center. Numerous examples are found in the images below.

Leukocytes (white blood cells)

The WBCs are divided into two groups: (1) granular leukocytes, containing distinctive cytoplasmic granules, including neutrophils, eosinophils and basophils and (2) agranular leukocytes, without granules, including monocytes and lymphocytes.

Neutrophils (10-12 um diameter), also called polymorphonuclear leukocytes (PMNs) or polymorphs, have nuclei with 2-5 lobes connected by thin chromatin threads. Their cytoplasm contains 2 types of granules; small neutrophilic granules (which give the cell a lavender hue and purplish-red azurophilic granules which are lysosomes (Neutrophil

).

Eosinophils are larger than PMNs (12-17 um diameter), have a bilobed nucleus, and large eosinophilic (red-orange) granules in their cytoplasm (Eosinophil

).

Basophils, which represent only 0.5% of the WBCs are difficult to find. They are 10-12 um diameter, with a large, irrgeularly shaped nucleus (the least condensed chromatin of all granulocytes). The cytoplasm is dominated with large dark blue (basophilic) granules (Basophil

).

Monocytes are the largest WBC (14-20 um diameter) with variable nuclear morphology from ovoid to horseshoe shaped. The cytoplasm stains a light blue and contains a few pale...