 Mucous connective tissue is present in the umbilical cord. It consists of a specialized, almost gelatinlike ECM; its ground substance is frequently referred to as Wharton’s jelly. The spindle-shaped cells are widely separated and appear much like fibroblasts in the near-term umbilical cord (e.g., the cytoplasmic processes are thin and difficult to visualize in routine hematoxylin and eosin [H&E] preparation). Wharton’s jelly occupies large inter-cellular spaces located between thin, wispy collagen fibers (Fig. 6.2b).
Connective tissues that belong to this category are divided into two general subtypes:  Loose connective tissue, sometimes called areolar tissue, and  dense connective tissue, which can be further subcategorized into two basic types based on the organization of its collagen fibers: dense irregular connective tissue and dense regular connective tissue.
Loose connective tissue is characterized by loosely arranged fibers and abundant cells of various types.
Loose connective tissue is a cellular connective tissue with thin and relatively sparse collagen fibers (Fig. 6.3). The
FIGURE 6.3 • Loose and dense irregular connective tissue.
Photomicrograph comparing loose and dense irregular connective tissue from the mammary gland stained with Masson’s trichrome. In the center, loose connective tissue surrounds the glandular epithelium. The loose connective tissue is composed of a wispy arrangement of collagen fibers with many cells. Note the large number of nuclei visible at this low magnification. On the upper left and lower right of the figure is dense irregular connective tissue. In contrast, few nuclei are revealed in the dense connective tissue. However, collagen is considerably more abundant and is composed of very thick fibers. 100.
ground substance, however, is abundant; in fact, it occupies more volume than the fibers do. It has a viscous to gel-like consistency and plays an important role in the diffusion of oxygen and nutrients from the small vessels that course through this connective tissue as well as in the diffusion of carbon dioxide and metabolic wastes back to the vessels.
Loose connective tissue is primarily located beneath the epithelia that cover the body surfaces and line the internal surfaces of the body. It is also associated with the epithelium of glands and surrounds the smallest blood vessels (Plate 4, page 192). This tissue is thus the initial site where pathogenic agents such as bacteria that have breached an epithelial surface are challenged and destroyed by cells of the immune system. Most cell types in loose connective tissue are transient wandering cells that migrate from local blood vessels in response to specific stimuli. Loose connective tissue is, therefore, the site of inﬂammatory and immune reactions. During these reactions, loose connective tissue can swell considerably. In areas of the body where foreign substances are continually present, large populations of immune cells are maintained. For example, the lamina propria, the loose connective tissue of mucous membranes, such as those of the respiratory and alimentary systems, contains large numbers of these cells.
Dense irregular connective tissue is characterized by abundant fibers and few cells.
Dense irregular connective tissue contains mostly collagen fibers. Cells are sparse and are typically of a single type, the fibroblast. This tissue also contains relatively little ground substance (Plate 4, page 192). Because of its high pro portion of collagen fibers, dense irregular connective tissue provides significant strength. Typically, the fibers are arranged in bundles oriented in various directions (thus, the term irregular) that can withstand stresses on organs or structures. Hollow organs (e.g., the intestinal tract) possess a distinct layer of dense irregular connective tissue called the submucosa in which the fiber bundles course in varying planes. This arrangement allows the organ to resist excessive stretching and distension. Similarly, skin contains a relatively thick layer of dense irregular connective tissue called the reticular layer (or deep layer) of the dermis. The reticular layer provides resistance to tearing as a consequence of stretching forces from different directions.