Types of Connective Tissue

Adult connective tissue proper can be divided into four types: loose or areolar, reticular, adipose, and dense con­nective tissue. Dense connective tissue is further subdivided into irregular and regular connective tissue. Specialized con­nective tissues, such as blood and blood-forming tissues, are discussed in Chapter 14; cartilage and bone are discussed in Chapter 56.

Loose Connective and Adipose Tissue. Loose connective tissue, also known as areolar tissue, is soft and pliable. Although it is more cellular than dense connective tissue, it contains large amounts of intercellular substance (Fig. 4-23). It fills spaces between muscle sheaths and forms a layer that encases blood and lymphatic vessels. Areolar connective tissue supports the epithelial tissues and provides the means by which these tissues are nourished. In an organ containing functioning epithelial tissue and supporting connective tissue, the term parenchymal tissue is used to de­scribe the functioning epithelium as opposed to the con­nective tissue framework or stroma.

Cells of loose connective tissue include fibroblasts, mast cells, adipose or fat cells, macrophages, plasma cells, and leukocytes. Loose connective tissue cells secrete sub­stances that form the extracellular matrix that supports and connects body cells. Fibroblasts are the most abun­dant of these cells. They are responsible for the synthesis of the fibrous and gel-like substance that fills the intercel­lular spaces of the body and for the production of colla­gen, elastic, and reticular fibers.

The basal lamina is a special type of intercellular ma­trix that is present where connective tissue contacts the tissue it supports. It is visible only with an electron micro­scope and is produced by the epithelial cells. In many lo­cations, reticular fibers, produced by the connective tissue cells, are associated with the basal lamina. Together, the basal lamina and the reticular layer form the basement

membrane seen by light microscopy. A basement mem­brane is found along the interface between connective tis­sue and muscle fibers, on Schwann cells of the peripheral nervous system, on the basal surface of endothelial cells, and on fat cells. These basement membranes bond cells to the underlying or surrounding connective tissues, serve as selective filters for particles that pass between connective tissue and other cells, and contribute to cell regeneration and repair.

Adipose tissue is a special form of connective tissue in which adipocytes predominate. Adipocytes do not gener­ate an extracellular matrix but maintain a large intracellu-lar 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 helps to keep organs in place. The subcutaneous layers of fat help 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 (white) adipose tissue is composed 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. These two types of fat are discussed in Chapter 11.

Reticular Connective Tissue. Reticular tissue is character­ized by a network of reticular fibers associated with reticu­lar cells. These reticular cells are believed to retain

 

IGURE 4-23 Diagrammatic representa­tion of cells that may be seen in loose connective tissue. The cells lie in the ex­tracellular matrix that is bathed in tissue fluid that originates in capillaries. (From Cormack D.H. [1987]. Ham's histology [9th ed.]. Philadelphia: J.B. Lippincott)

 

multipotential capabilities similar to undifferentiated mes-enchymal cells. Reticular tissues comprise the framework of the liver, bone marrow, and lymphoid tissues such as the spleen.

Dense Connective Tissue. Dense connective tissue exists in two forms: dense irregular and dense regular. Dense ir­regular connective tissue consists of the same compo­nents found in loose connective tissue, but exhibiting a predominance of collagen fibers and fewer cells. This type of tissue can be found in the dermis of the skin (i.e., retic­ular layer), the fibrous capsules of many organs, and the fibrous sheaths of cartilage (i.e., perichondrium) and bone (i.e., periosteum). It also forms the fascia that invests mus­cles and organs. Dense regular connective tissues are rich in collagen fibers and form the tendons and aponeuroses that join muscles to bone or other muscles and the liga­ments that join bones to bone. Tendons and ligaments are white fibers because of an abundance of collagen. Liga­ments such as the ligamenta flava of the vertebral column and the true vocal folds are called yellow fibers because of the abundance of elastic fibers.