Porth's Essentials of Pathophysiology, 4e

23

Cell Structure and Function

C h a p t e r 1

All glandular cells arise from surface epithelia by means of cell proliferation and invasion of the underlying connective tissue. Epithelial glands can be divided into two groups: exocrine and endocrine glands. Exocrine glands, such as the sweat glands and lactating mam- mary glands, retain their connection with the surface epithelium from which they originated. This connection takes the form of epithelium-lined tubular ducts through which the secretions pass to reach the surface. Endocrine glands are epithelial structures that have had their con- nection with the surface obliterated during development. These glands are ductless and produce secretions (i.e., hormones) that move directly into the bloodstream. Epithelial Cell Renewal Cells making up the epithelial tissues generally exhibit a high rate of turnover, which is related to their loca- tion and function. The rate of cell turnover is character- istic of specific epithelium. For example, the epithelial cells of the small intestine are renewed every 4 to 6 days by regenerative cells in the lower portion of the intesti- nal glands (crypts). Similarly, the cells of the stratified squamous epithelium of the skin are constantly being renewed at the basal layer by cell division. ConnectiveTissue Connective or supportive tissue is the most abundant tissue in the body. As its name suggests, it connects and binds or supports the various tissues. Connective tis- sue is unique in that its cells produce the extracellular matrix that supports and holds tissues together. The proximity of the extracellular matrix to blood vessels allows it to function as an exchange medium through which nutrients and metabolic wastes pass. The functions of the various connective tissues are reflected by the types of cells and fibers present in the tissue and the characteristics of the extracellular matrix (Fig. 1-16). The capsules that surround organs of the body are composed of connective tissue. Bone, adipose tissue, and cartilage are specialized types of connective tissue that function to support the soft tissues of the body and store fat. One type of cell, the fibroblast, is respon- sible for synthesis of collagen, elastic, reticular fibers, and ground substance of the extracellular matrix. Other cells, such as lymphocytes, plasma cells, macrophages, and eosinophils, are associated with body defense systems. Adult connective tissue can be divided into two types: connective tissue proper, which is the focus of the dis- cussion in this chapter, and specialized connective tissue (cartilage, bone, and blood cells), which is discussed in other chapters. There are four recognized types of con- nective tissue proper: loose (areolar), adipose, reticular,

Ground substance

Collagen fibers

Capillary

Mast cell

Macrophage

Adipocyte Plasma cell

Elastic fiber

White blood cell

Fibroblast

FIGURE 1-16. Diagrammatic representation of cells that may be seen in loose connective tissue.The cells lie in an intercellular matrix that is bathed in tissue fluid that originates in the capillaries.

by which these tissues are nourished. Loose connec- tive tissue is characterized by an abundance of ground substance and tissue fluid housing the fixed connective tissue cells: fibroblasts, mast cells, adipose or fat cells, macrophages, and leukocytes. Fibroblasts are the most abundant of these cells. They produce all three fiber types—collagen, elastic, and reticular fibers—found in loose connective tissue, and synthesize the ground sub- stance that fills the intercellular tissue spaces. AdiposeTissue Adipose tissue is a special form of connective tissue in which adipocytes predominate. Adipocytes do not generate an extracellular matrix but maintain a large intracellular space. These cells store large quantities of triglycerides and are the largest repository of energy in the body. Adipose tissue helps fill spaces between tissues and to keep organs in place. The subcutaneous fat helps to shape the body. Because fat is a poor conductor of heat, adipose tissue serves as thermal insulation for the body. Adipose tissue exists in two forms: unilocular and multilocular. Unilocular (white) adipose tissue is com- posed of cells in which the fat is contained in a single, large droplet in the cytoplasm. Multilocular (brown) adipose tissue is composed of cells that contain multiple droplets of fat and numerous mitochondria. Reticular and Dense ConnectiveTissue Reticular and dense connective tissue is characterized by a networkof fibers interspersedwithmacrophages andfibro- blasts that synthesize collagen fibers. Reticular fibers pro- vide the framework for capillaries, nerves, and muscle

and dense connective tissue. Loose ConnectiveTissue

Loose connective tissue, also known as areolar tissue, is soft and pliable. It fills spaces between muscle sheaths, forms a layer that encases blood and lymphatic vessels, and provides support for epithelial tissues and the means