Glandular Epithelium

Glandular epithelium is composed of cells that are specialized to produce and secrete substances into ducts or into body fluids. Such cells are usually found within columnar or cuboidal epithelium, and one or more of these cells constitutes a gland. Glands that secrete their products into ducts that open onto some internal or external surface are called exocrine glands. Glands that secrete their products into tissue fluid or blood are called endocrine glands. (Endocrine glands are discussed in chapter 13.)

An exocrine gland may consist of a single epithelial cell (unicellular gland), such as a mucus-secreting goblet cell, or it may be composed of many cells (multicellular gland). In turn, the multicellular forms can be structurally subdivided into two groups—simple and compound glands.

A simple gland communicates with the surface by means of an unbranched duct, and a compound gland has a branched duct. These two types of glands can be further classified according to the shapes of their secretory portions. Glands that consist of epithelial-lined tubes are called tubular glands; those whose terminal portions form saclike dilations are called alveolar glands (acinar glands). Branching and coiling of the secretory portions may occur as well. Figure 5.10 illustrates several types of exocrine glands classified by structure. Table 5.3 summarizes the types of exocrine glands, lists their characteristics, and provides an example of each type.

Exocrine glands are also classified according to the ways these glands secrete their products. Glands that release fluid products by exocytosis are called merocrine glands. Glands that lose small portions of their glandular cell bodies during secretion are called apocrine glands. Glands that release entire cells are called holocrine glands. After release, the cells containing accumulated secretory products disintegrate, liberating their secretions (figs. 5.11 and 5.12). Table 5.4 summarizes these glands and their secretions.

Most exocrine secretory cells are merocrine, and they can be further subdivided as either serous cells or mucous cells. The secretion of serous cells is typically

Tissue surface Duct

Secretory portion-

Pictures Merocrine Gland

Secretory portion-

Simple tubular Simple branched tubular

Simple coiled tubular

Simple branched alveolar

Tubular Exocrine Gland

Compound tubular

Compound alveolar

Figure 5.10

Structural types of exocrine glands.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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