Figure 913

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 lysosonies (L) of the cell. The slightly larger and less dense profiles are mitochondria (M). x22,000. (Courtesy of Dr. Dorothea Zucker-Franklin.) Inset. Light microscopic appearance of a monocyte from a blood smear, x 1,800.

Monocytes transform into macrophages, which function as antigen-presenting cells in the immune system

During inflammation, as indicated, 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 to helper CD4+T lymphocytes for recognition.

s? platelets

Platelets are small, membrane-bounded, anucleate cytoplasmic fragments derived from megakaryocytes

Platelets are derived from large polyploid cells (cells whose nuclei contain multiple sets of chromosomes) in the bone marrow, called megakaryocytes (Fig. 9.14). In platelet formation, small bits of cytoplasm are separated from the peripheral regions of the megakaryocyte by extensive platelet demarcation channels. The membrane that

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