FIGURE F6.3.1 • Fibroblasts and myofbroblasts in the culture. This immunofluorescence image shows wild-type 3T3 fibroblasts cultured on the collagen lattice. Under the stimulation of certain growth factors such as TGF-1, some fibroblast differentiate to myofibroblasts, expressing -SMA, the marker of myofibroblasts differentiation. Cells were stained with fluorescein-labeled phalloidin to visualize F-actin filaments (green), and -SMA were labeled with primary antibodies against -SMA and visualized with secondary goat anti-mouse antibodies conjugated with FITC (red). Co-localization of -SMA with F-actin is indicated by yellow color. Note that some cells have completed their differentiation, and others are in the early stages. 1,000. (Courtesy of Dr. Boris Hinz.)
FIGURE 6.22 • Photomicrograph and electron micrograph of a macrophage. a. This photomicrograph shows several macrophages (M) in the connective tissue from the area of wound healing. They can be distinguished from other cells by the presence of an indented or kidney-shaped nucleus. Note several mature neutrophils (N) with segmented nuclei located in the connective tissue that surround blood vessel filled with red and white blood cells in the center of the image. 480. b. The most distinctive EM feature of the macrophage is its population of endocytotic vesicles, early and late endosomes, lysosomes, and phagolysosomes. The surface of the cell reveals a number of fingerlike projections, some of which may be sections of surface folds. 10,000.
 FOLDER 6.4 Functional Considerations: The Mononuclear Phagocytotic System
The cells that are included in the mononuclear phagocy-totic system (MPS) are derived from monocytes and denote a population of antigen-presenting cells involved in the processing of foreign substances. These cells are able to phagocytose avidly vital dyes such as trypan blue and India ink, which makes them visible and easy to identify in the light microscope. The common origin of MPS cells from monocytes serves as the major distinguishing feature of the system as it is currently perceived and is the basis for the system’s name. In addition, cells of the MPS display receptors for complement and Fc fragments of im-munoglobulins. The various cells of the MPS are listed in the following table. Most cells of the MPS become fixed in specific tissues and may adopt a variety of morphologic appearances as they differentiate. The main functions of MPS cells are phagocytosis, secretion (lymphokines), antigen process-ing, and antigen presentation to other cells of the immune system. Some functionally important phagocytotic cells are not derived from monocytes. For example, microglia are small, stellate cells located primarily along capillaries of the central nervous system that function as phagocy-totic cells. They are generally thought to arise from the me-sectoderm of the neural crest and not from monocytes; nevertheless, they are included in the MPS. Similarly, fibroblasts of the subepithelial sheath of the lamina propria of the intestine and uterine endometrium have been shown to differentiate into cells with morphologic, enzy-matic, and functional characteristics of connective tissue macrophages. Cells of the Mononuclear Phagocytotic System Name of Cell Location Macrophage (histiocyte) Connective tissue Perisinusoidal macrophage (Kupffer cell) Liver Alveolar macrophage Lungs Fetal placental antigen-presenting cell (Hofbauer cell) Placenta Macrophage Spleen, lymph nodes, bone marrow, and thymus Pleural and peritoneal macrophage Serous cavities Osteoclast Bone Microglia Central nervous system Langerhans’ cell Epidermis Fibroblast-derived macrophage Lamina propria of intestine, endometrium of uterus Dendritic cell Lymph nodes, spleen