Embryonic connective tissue, a. Photomicrograph of mesenchymal tissue from a developing fetus stained with H&E. Although morphologically the mesenchymal cells appear as a homogeneous population, they will give rise to cells that differentiate into various cell types. Their cytoplasmic processes often give the cell a tapering or spindle appearance. The extracellular component of the tissue contains a sparse arrangement of reticular fibers and abundant ground substance. x480. b. Photomicrograph of Wharton's jelly from the umbilical cord stained with H&E. Wharton's jelly consists of a specialized, almost gelatin-like ground substance that occupies large intercellular spaces located between the spindle-shaped mesenchymal cells. X480.

• Dense connective tissue, which can be further subcate-gorized 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

Loose connective tissue is a cellular connective tissue with thlrT and relatively sparse collagen fibers (Fig. 5.3).

The 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 those epithelia that cover the body surfaces and line the internal surfaces of the [>ody. ItTsUlso associated with the epithelium of glands and surrounds the smallest blood vessels. This tissue is thus the initial site where pathogenic agents, such as bacteria, that have breached an epithelial surface can be challenged and destroyed by cells of the immune system. Most_C£iiJXpes in loose connective tissue are transient wandering cells that migrate from local blood vessels in response to specific stimuli. This tissue is, therefore, the site of inflammatory and immune reactions. During these reactions, loose connective tissue can undergo considerable swelling. In areas of the body where foreign substances are continually present, large populations of defending cells are maintained. For example, the lamina propria, the loose connective tissue of mucous

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