Blood is a tissue in which highly specialized cells are suspended in a fluid matrix called plasma. Blood serves to transport many diverse substances, from gases, nutrients, and wastes to information bearing molecules such as hormones and antibodies. Histologically, blood is sometimes classified as a specialized form of connective tissue. In stained blood smears, one can quickly distinguish between the oxygen bearing erythrocytes without nuclei and the basophilic, nucleated leukocytes which perform diverse roles in bodily defense. Recognition of the cell types, particularly leukocytes, and the significance of their relative numbers are frequently important in medical diagnoses. Blood in adults is produced in bone marrow and an examination of marrow reveals immature forms of the various blood cells and platelet-producing cells.

The learning objectives for this module are:

• Distinguish the histological organization of a red bone marrow core biopsy, bone marrow smear, and peripheral blood smear.
• Describe and identify erythrocytes, megakaryocytes, and platelets in light and electron micrographs.
• Recognize and describe the structure of neutrophils, eosinophils, basophils, and monocytes in light and electron micrographs.
• Describe and identify the structure of lymphocytes in light and electron micrographs.
   

Consult this diagram of the developmental sequence to help understand and recognize the intermediate cells in the formation of red blood cells and the three types of granulocytes.

Let's go straight to a blood smear.