The macrophage contains a large Golgi apparatus, rER and sER, mitochondria, secretory vesicles, and lysosomes.
The lysosomes of the macrophage, along with the surface cytoplasmic projections, are the structures most indicative of the specialized phagocytotic capability of the cell. The
FIGURE 6.21 • Electron micrograph of a myofbroblast. The cell exhibits some features of a fibroblast, such as areas with a moderate amount of rER. Compare with Figure 6.20. Other areas, however, contain aggregates of thin filaments and cytoplasmic densities (arrows), features that are characteristic of smooth muscle cells. The arrowheads indicate longitudinal profiles of collagen fibrils. 11,000.
macrophage may also contain endocytotic vesicles, phagolysosomes, and other evidence of phagocytosis (i.e., residual bodies). The rER, sER, and Golgi apparatus support the synthesis of proteins involved in the cell’s phagocytotic and digestive functions, as well as in its secretory functions. The secretory products leave the cell by both the constitutive and regulated exocytotic pathways. Regulated secretion can be activated by phagocytosis, immune complexes, complement, and signals from lymphocytes (including the release of lymphokines, biologically active molecules that influence the activity of other cells). The secretory products released by the macrophage include a wide variety of substances related to the immune response, anaphylaxis, and inflammation. The release of neutral proteases and GAGases (enzymes that break down GAGs) facilitates the migration of the macrophages through the connective tissue.
Although the main function of the macrophage is phagocytosis, either as a defense activity (e.g., phagocytosis of bacteria) or as a cleanup operation (e.g., phagocytosis of cell debris), it also plays an important role in immune response reactions. Macrophages have specific proteins on their surfaces known as major histocompatibility complex II (MHC II) molecules that allow them to interact with helper CD4 T lymphocytes. When macrophages engulf a foreign cell, antigens—short polypeptides, 7 to 10 amino acids long, from the foreign cell—are displayed on the surface of MHC II molecules. If a CD4 T lymphocyte recognizes the displayed antigen, it becomes activated, triggering an immune response (see Chapter 14). Because macrophages “present” antigen to helper CD4 T lymphocytes, they are called antigen-presenting cells (APCs).
When macrophages encounter large foreign bodies, they may fuse to form a large cell with as many as 100 nuclei that engulfs the foreign body. These multinucleated cells are called foreign body giant cells (Langhans cells).
Mast cells develop in bone marrow and differentiate in connective tissue.
Mast cells are large, ovoid, connective tissue cells (20 to 30 m in diameter) with a spherical nucleus and cytoplasm filled with large, intensely basophilic granules. They are not easily identified in human tissue sections unless special fixatives are used to preserve the granules. After glutaraldehyde fixation, mast cell granules can be displayed with basic dyes such as toluidine blue. It stains the granules intensely and metachromatically because they contain heparin, a highly sulfated proteoglycan (Fig. 6.23a). The cytoplasm displays small amounts of rER, mitochondria, and a Golgi apparatus. The cell surface contains numerous microvilli and folds.