Monocytes are the precursors of the cells of the mononuclear phagocytotic system.
Monocytes are the largest of the WBCs in a blood smear (average diameter, 18 m). They travel from the bone marrow to the body tissues, where they differentiate into the various phagocytes of the mononuclear phagocytotic system— that is, connective tissue macrophages, osteoclasts, alveolar macrophages, perisinusoidal macrophages in the liver (Kupffer cells), and macrophages of lymph nodes, spleen, and bone marrow among others (see Chapter 6, Connective Tissue). Monocytes remain in the blood for only about 3 days.
The nucleus of the monocyte is typically more indented than that of the lymphocyte (Fig. 10.12 and Plate 18, page 304). The indentation is the site of the cell center where the well-developed Golgi apparatus and centrioles are located. Monocytes also contain smooth endoplasmic reticulum, rough endoplasmic reticulum, and small mitochondria. Although these cells are classified as agranular, they contain small, dense, azurophilic granules. These granules contain typical lysosomal enzymes similar to those found in the azurophilic granules of neutrophils.
Monocytes transform into macrophages, which function as antigen-presenting cells in the immune system.
During inﬂammation, the monocyte leaves the blood vessel at the site of inﬂammation, transforms into a tissue macrophage, and phagocytoses bacteria, other cells, and tissue debris. The monocyte–macrophage is an antigen-presenting cell and plays an important role in immune responses by partially degrading antigens and presenting their fragments on the MHC II molecules located on the macrophage surface of helper CD4 T lymphocytes for recognition.
Thrombocytes are small, membrane-bounded, anucleate cytoplasmic fragments derived from megakaryocytes.
Thrombocytes (platelets) are derived from large polyploid cells (cells whose nuclei contain multiple sets of chromosomes) in the bone marrow called megakaryocytes (Fig. 10.13). In platelet formation, small bits of cytoplasm are separated from the peripheral regions of the megakaryocyte by extensive platelet demarcation channels. The membrane that lines these channels arises by invagination of the plasma membrane; therefore, the channels are in continuity with the extracellular space. The continued development and fusion of the platelet demarcation membranes result in the complete partitioning of cytoplasmic fragments to form individual platelets. After entry into the vascular system from the bone marrow, the platelets circulate as discoid structures about 2 to 3 m in diameter. Their life span is about 10 days.
Structurally, platelets may be divided into four zones based on organization and function.
The TEM reveals a structural organization of the thrombocyte cytoplasm that can be categorized into the following four zones (Fig. 10.14).  The peripheral zone consists of the cell membrane covered by a thick surface coat of glycocalyx. The glyco calyx consists of glycoproteins, glycosaminoglycans, and
FIGURE 10.12 • Electron micrograph of a human mature monocyte. The nucleus is markedly indented, and adjacent to this site, a centriole (C) and several Golgi profiles (G) are evident. The small dark granules are azurophilic granules, the lysosomes (L) of the cell. The slightly larger and less dense profiles are mitochondria (M). 22,000. (Courtesy of Dr. Dorothea Zucker-Franklin.) Inset. Light microscopic appearance of a monocyte from a blood smear. 1,800.