Saturday, August 30, 2008
1. Fixed cells are native to the tissue in which they are found. a. Mesenchymal cells are the precursors of most connective tissue cells. Embryonic mesenchyme comprises a loose network of stellate cells and abundant intercellular fluid. Some mesenchymal cells remain undifferentiated in adult connective tissue and constitute a reserve population of stem cells called adventitial cells, which are difficult to distinguish from some fibroblasts. b. Fibroblasts are the predominant cells in connective tissue proper. They synthesize, secrete, and maintain all the major components of the extracellular matrix. Structurally, fibroblasts are of 2 types, one of which resembles mesenchymal cells. This type is stellate, with long cytoplasmic processes and a large, ovoid, pale-staining nucleus. The cytoplasm is mitotically active and contains abundant RER and Golgi complexes. This cell type is important in producing collagen and other matrix components. Cells of the second type are less active and ate sometimes termed fibrocytes, because they are believed to be more mature. Fibrocytes are smaller and spindle-shaped, with a dark, elongated nucleus and fewer organelles. They may revert to the fibroblast state and participate in tissue repair. c. Reticular cells make up a functionally diverse yet morphologically similar group. They produce the reticular fibers that form the netlike stroma of hematopoietic and lymphoid tissues. Some apparently can phagocytose antigenic material and cellular debris. Others (antigen-presenting cells) collect antigens on their surfaces and help activate immunocompetent cells to mount an immune response. Reticular cells are typically stellate with long, thin cytoplasmic processes. Each has a central, pale, irregularly rounded nucleus and a prominent nucleolus. In the cytoplasm, the number of mitochondria and the degree of development of the Golgi complex and RER are variable. Some reticular cells, particularly those with less developed organelles, may be stem cells of various blood types. d. Adipose cells or adipocytes are mesenchymal derivatives specialized as storage depots for lipids.