CONNECTIVE TISSUES I. Introduction A ... - Faculty.rmc.edu

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. Matrix-removing cells (1) Osteoclasts (Figs. 8.12 - 8.15) (a) Osteoclasts occur on and near surfaces where bone matrix is being resorbed. (b) Osteoclasts are large multinucleate cells with acidophilic cytoplasm containing numerous acid phosphatase- rich lysosomes. The surface of the osteoclast next to the bone matrix has numerous infoldings in the plasma membrane which form a ruffled border. (c) Osteoclasts resorb bone matrix by releasing lysosomes and organic acids onto the bone matrix surface. Osteoclasts can also break down cartilage matrix trapped in bone matrix during endochondrial bone formation. Osteoclasts can be formed from monocytes and are therefore related to macrophages. c. Storage cells (1) White fat cells (white adipocytes) (Figs 9.1 - 9.3) (a) White adipocytes occur in loose connective tissues and are the dominant cells in white adipose connective tissue. (b) White adipocytes are large spherical cells with a flattened eccentric nucleus in a thin rim of cytoplasm surrounding a single large central fat droplet. (c) White adipocytes are the major fat storage cells in adult humans. (d) White adipocytes develop from the same precursor cells that give rise to fibroblasts, but the control of their development is unclear. (2) Brown fat cells (brown adipocytes) (Figs 9.1, 9.4) (a) Brown adipocytes occur in brown fat deposits in embryonic and neonatal humans but are rare in adult humans. (b) Brown adipocytes are spherical cells (smaller than white fat cells) containing spherical eccentric nuclei, multiple small fat droplets, and numerous mitochondria. (c) Brown adipocytes are capable of rapid heat generation. 6
2. Transient or migratory cells a. Macrophages (Fig. 6.22) (1) Macrophages occur in most loose and moderately dense fibrous connective tissues. Some macrophages may be relatively long-term residents of connective tissues while other macrophages are clearly migratory. (2) Two types of macrophages are frequently described, but the relationships between these morphological categories and functional categories are unclear. Most macrophages are probably quite variable in morphology and are capable of rapid changes in function in response to chemical signals. (a) "Fixed" macrophages are associated with CT fibers and therefore tend to have a more elongated shape. (b) Free macrophages are not associated with CT fibers and tend to be more oval in shape. (3) Macrophages usually appear in routine LM preparations as flattened cells (slightly smaller than fibroblasts) with football- shaped to kidney bean-shaped nuclei containing both euchromatin and heterochromatin. Macrophage nuclei are usually slightly smaller and slightly darker staining than fibroblast nuclei in the same preparation. Macrophages are difficult to distinguish from fibroblasts, so positive identification depends on detection of specific cell surface markers which are unique to macrophages. Newly developed macrophages have weakly acidophilic cytoplasm containing lysosomal granules (acid phosphatase positive). Activated macrophages contain cytoplasm filled with obvious granules of undigested material. Most macrophages have elongated processes which are not obvious by LM but which are seen with TEM and SEM. (4) Macrophages are phagocytic cells derived from monocytes. Macrophages are important in removal of debris from connective tissues, in stimulating immune responses, and in defense against some types of bacteria and some types of tumor cells. 7
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CONNECTIVE TISSUES I. Introduction A ... - Faculty.rmc.edu

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