Connective tissues are aptly named—they connect epithelia to underlying body structures, link muscles to bones, and hold joints together. However, connective tissues provide the body with more than structural connections and form—they play vital roles in defense, repair, storage, and nutrition. The extent and composition of extracellular matrix (ECM) is a key feature of connective tissues. Unlike epithelia, muscle, and nerve tissue, which consist mostly of cells, connective tissues contain significant amounts of extracellular material, which may be synthesized by only a small number of resident cells. This chapter focuses on what are termed connective tissues proper; that is, loose and dense connective tissues and adipose tissue. A few other rare types of connective tissue are mentioned briefly in this chapter; however, several specialized connective tissues (i.e., cartilage, bone, blood, and hematopoietic tissues) require detailed discussion and will be considered in Chapters 3, 4, and 8.
CHARACTERISTICS AND FUNCTIONS OF CONNECTIVE TISSUES
LOCATION OF CONNECTIVE TISSUES
Connective tissues are found throughout the body, characteristically in the following locations:
Beneath basal lamina of epithelia. For example, in skin, the connective tissue of the dermis lies below the stratified squamous keratinized epithelium (see Figure 1-1, Chapter 1).
Stroma. Connective tissues within organs often serve as the stroma, a support tissue, rather than as the parenchyma, a functional tissue. For example, connective tissues form organ capsules and surround blood vessels, nerves, and accessory structures within organs. The portal areas in the liver, as seen in Figure 2-1A, are an example of a stromal connective tissue area that provides support to the parenchymal epithelial liver cells.
Between organs. Loose connective tissue fills spaces between organs.
Connective tissue; locations, organization, and production. A. Stromal connective tissue in the liver. Dense connective tissue (CT) surrounds the veins (V), arteries (A), lymphatics (L), and bile ducts (BD) in a portal triad in the liver. The arrow indicates the nucleus of a fibroblast cell of the connective tissue. H, hepatocytes. B. Loose and dense connective tissues in the duodenum. The loose connective tissue (L) of the lamina propria underlies the simple columnar epithelium and consists of many closely packed cells. Below that is the layer of dense connective tissue (D) of the submucosa, which is composed of numerous large extracellular fibers (arrows) and few cells. The lumen of the duodenum is above the view shown here. Mucus-secreting cells of the epithelium are stained red in this preparation. (H&E stain with periodic acid–Schiff reaction). C. Production of connective tissue cells. Stem cells derived from embryonic mesoderm give rise to all the differentiated cells listed in the table. Stem cells with similar capabilities persist in adults.
ORGANIZATION OF CONNECTIVE TISSUES