As a result of immune responses and tissue injury, certain cells rapidly migrate from the blood to enter the connective tissue, particularly neutrophils and monocytes. Their presence generally indicates an acute inflammatory reaction. In these reactions, neutrophils migrate into the connective tissue in substantial numbers, followed by large numbers of monocytes. As noted, the monocytes then differentiate into macrophages. A description of these cells and their roles is found in Chapter 10. The eosinophil, which functions in allergic reactions and parasitic infections, is also presented in that chapter. Eosinophils may be observed in normal connective tissue, particularly the lamina propria of the intestine, as a result of chronic immunologic responses that occur in these tissues.
FIGURE 6.25 • The plasma cell. a. This photomicrograph shows the typical features of a plasma cell as seen in a routine H&E preparation. Note clumps of peripheral heterochromatin alternating with clear areas of euchromatin in the nucleus. Also note the negative Golgi (arrows) and basophilic cytoplasm. 5,000. b. Electron micrograph shows that an extensive rER occupies most of the cytoplasm of the plasma cell. The Golgi apparatus (G) is also relatively large, a further reflection of the cell’s secretory activity. 15,000.
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Loose and dense connective tissue represents one of the several types of connective tissue. The others are namely cartilage, bone, blood, adipose tissue, and reticular tissue. Loose connective tissue is characterized by a relatively high proportion of cells within a matrix of thin and sparse collagen fibers. In contrast, dense irregular connective tissue contains few cells, almost all of which are fibroblasts that are re-sponsible for the formation and maintenance of the abundant collagen fibers that form the matrix of this tissue. The cells that are typically as-sociated with loose connective tissue are fibroblasts, the collagen-forming cells, and those cells that function in the immune system and those of the body’s general defense system. Thus, in loose connective tissue, there are, to varying degrees, lymphocytes, macrophages, eosinophils, plasma cells, and mast cells.
Loose and dense irregular connective tissue, mammary gland, human, H&E, ×175; insets ×350.
This micrograph shows at low magnification both loose connective tissue (LCT ) and dense irregular connective tissue (DICT ) for comparative purpose. The loose connective tissue surrounds the glandular epithelium (GE). The dense ir192 regular connective tissue consists mainly of thick bundles of collagen fibers with few cells present, whereas the loose connective tissue has a rela tive paucity of fibers and a considerable number of cells.  The upper inset is a higher magnification of the dense connective tissue.  Note that only a few cell nuclei are present relative to the larger expanse of collagen fibers.  The lower inset, revealing the glandular epithelium and surrounding loose connective tissue, shows very few fibers, but large numbers of cells.  Typically, the cellular component of loose connective tissue contains a relatively small proportion of fibroblasts, but large numbers of lymphocytes, plasma cells, and other connective tissue cell types.
Loose connective tissue, colon, monkey, Mallory, ×250.