Electron micrograph of a myofibroblast. The cell exhibits some features of a fibroblast, such as areas with a moderate amount of rER. Compare with Figure 5.15. Other areas, however, contain aggregates of thin filaments and cytoplasmic densities (arrows), features characteristic of smooth muscle cells. The arrowheads indicate longitudinal profiles of collagen fibrils, x 11,000.
Macrophages are phagocytotic cells derived from monocytes
Connective tissue macrophages, also known as tissue histiocytes, are derived from blood cells called mono-
cytes. Monocytes jnigrate from the bloodstream into the ¿o lîîre rîxvejïiss^ "phages. "
In the light microscope and with conventional stains, tissue macrophages are difficult to identify unless they display obvious evidence of phagocytotic activity, i.e., visible ingested material within their cytoplasm. Another feature that assists in identifying macrophages is an indented or kidney-shaped nucleus. Lysosomes are abundant in the cytoplasm and can be revealed by staining for acid phosphatase activity (both in the light microscope and with the TEM); a positive reaction is a further aid in identification of the macrophage. With the TEM, the surface of the macrophage exhibits numerous folds and finger-like projections (Fig. 5.17a). The surface folds engulf the substances to be phagocytosed.
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