by disulfide bonds at a carboxy terminus to form 50-nmlong arms (see Fig. 6.18). Each molecule contains several binding domains that interact with different ECM molecules (e.g., heparan sulfate; collagen types I, II, and III; fibrin; hyaluronan; and fibronectin) and integrin, a cell-surface receptor. Binding to a cell-surface receptor activates fibronectin, which then assembles into fibrils. Fibronectin plays an important role in cell attachment to the ECM. At least 20 different fibronectin molecules have been identified to date.
 Laminin (140 to 400 kilodaltons) is present in basal and external laminae. It possesses binding sites for collagen type IV molecules, heparan sulfate, heparin, entactin, laminin, and the laminin receptor on the cell surface. The process of basal lamina assembly and the role of the laminin in this process are described in Chapter 5 (see page 138).
 Tenascin (280 kilodaltons/monomer) appears during embryogenesis, but its synthesis is switched off in mature tissues. It reappears during wound healing and is also found within musculotendinous junctions and malignant tumors. Tenascin is a disulfide-linked dimer molecule that consists of six chains joined at their amino-terminus (see Fig. 6.18). It has binding sites for fibrinogen, heparin, and EGF-like growth factors; thus, it participates in cell attachment to the ECM.
 Osteopontin (44 kilodaltons) is present in the ECM of bone. It binds to osteoclasts and attaches them to the underlying bone surface. Osteopontin plays an important role in sequestering calcium and promoting calcification of the ECM.
Important multiadhesive glycoproteins found in the ECM of connective tissue are summarized in Table 6.5.
Connective tissue cells can be resident or wandering.
The cells that make up the resident cell population are relatively stable; they typically exhibit little movement and can be regarded as permanent residents of the tissue. These resident cells include  fbroblasts and a closely related cell type, the myof broblast, macrophages, adipocytes, mast cells,and  adult stem cells.
The wandering cell population or transient cell population consists primarily of cells that have migrated into the tissue from the blood in response to specific stimuli. These include  lymphocytes, plasma cells, neutrophils, eosinophils, basophils,and  monocytes.
The fibroblast is the principal cell of connective tissue.
Fibroblasts are responsible for the synthesis of collagen, elastic and reticular fibers, and the complex carbohydrates of the ground substance. Research suggests that a single fibro blast is capable of producing all of the ECM components.
Fibroblasts reside in close proximity to collagen fibers. In routine H&E preparations, however, often only the  nucleus is visible. It appears as an elongated or disclike structure, sometimes with a nucleolus evident. The thin, pale-staining, flattened processes that form the bulk of the cytoplasm are usually not visible, largely because they blend with the collagen fibers. In some specially prepared specimens, it is possible to distinguish the cytoplasm of the cell from the fibrous components (Fig. 6.19a). When ECM material is produced during active growth or in wound repair (in activated fbroblasts), the cytoplasm of the fibroblast is more extensive and may display basophilia as a result of increased amounts of rER associated with protein synthesis (Fig. 6.19b). When examined with the TEM, the fibroblast cytoplasm exhibits profiles of rER and a prominent Golgi apparatus (Fig. 6.20).