The monocyte is a spherical cell with prominent surface
ruffles and blebs when examined by scanning electron microscopy.
When reconstructed from sections examined under transmission electron microscopy,
the monocyte has a reniform nucleus containing a small nucleolus.
The cytoplasm has many mitochondria, microtubules, and microfilaments.
The Golgi apparatus is well developed and has neighboring centrioles.
Numerous microvilli and microcytotic vesicles are evident at or
near the cell surface. The cytoplasm contains scattered granules,
akin to lysosomes. The granule contents share features with the
primary granules of neutrophils, although, in contrast to the neutrophil,
the monocyte granule is characterized by fluoride-inhibitable esterases.
As the monocyte enters the tissue and differentiates into a macrophage,
the cell volume and number of cytoplasmic granules increase. Cell
shape varies, depending on the tissue type in which the macrophage
resides (e.g., lung, liver, spleen, brain). A characteristic feature
of macrophages is their prominent electron-dense membrane-bound
lysosomes, which can be seen fusing with phagosomes to form secondary
lysosomes. The latter contain ingested cellular and noncellular
material in different stages of degradation. A broad range of surface
receptors for many ligands, including the Fc portion of immunoglobulin,
complement proteins, cytokines, chemokines, lipoproteins, and others,
are on the cell surface. Macrophages differ in appearance, biochemistry,
and function based on the environment in which they mature from monocytes.
These differences are exemplified by the diversity among dendritic
cells of lymph nodes, histiocytes of connective tissue, osteoclasts
of bone, Kupffer cells of liver, microglia of the central nervous
system, and macrophages of the serosal surfaces, each fashioned
to meet the local needs of the mononuclear phagocyte system, which
plays a role in inflammation and host defense against microbes.
Acronyms and Abbreviations
Acronyms and abbreviations
that appear in this chapter include: CR1, complement receptor 1;
CR3, complement receptor 3; CSF, colony-stimulating factor; FcR,
Fc receptor; GM-CSF, granulocyte-monocyte colony-stimulating factor;
HIV, human immunodeficiency virus; HLA, human leukocyte antigen;
IgG, immunoglobulin G; IL-4, interleukin-4; IMP, intramembrane particle;
LPS, lipopolysaccharide; M-CSF, macrophage colony-stimulating factor;
MHC, major histocompatibility complex; TLR, toll-like receptor.
Modern study of mammalian phagocytes began with Metchnikoff in
the 19th century. An understanding of the ontogeny, kinetics, and
function of phagocytic cells in animals led to the concept of the
mononuclear phagocyte system.1,2 The system consists
of marrow monoblasts and promonocytes, blood monocytes, and both
free and fixed-tissue macrophages. Vascular endothelium, reticular
cells, and dendritic cells of lymphoid germinal centers usually
are not included in the mononuclear phagocyte system, although the
now obsolete term reticuloendothelial system3 denoted
these cells as playing some complementary part with mononuclear
phagocytes. Monocytes can differentiate into dendritic cells in
vitro.4 Monocytes and macrophages comprise
the functional system formerly thought to be the reticuloendothelial
system. Tissue macrophages share many functional characteristics,
such as phagocytic and microbial killing capabilities and adherence
to glass or plastic surfaces in vitro. Kinetic studies
indicate macrophages are transformed monocytes and that the ...