Classification of Exocrine Glands

Glands
are cells or aggregations of cells whose function is secretion.

• Exocrine glands release the secretory product via a system of ducts that opens upon one of the surfaces of the body which are in contact with the external world (skin, gastrointestinal tract etc.).
  
  
• Endocrine glands release their secretory product (typically hormones) into the spaces between the secretory cells (extracellular space) from which it enters the bloodstream.

Both endocrine and exocrine glands are developmentally derived from epithelia, which form a down-growth into the underlying connective tissue. The cells forming this down-growth then develop the special characteristics of the mature gland. Exocrine glands maintain the connection with the surface epithelium, whereas the connection is lost by endocrine glands.

Classification of Exocrine Glands

Exocrine glands may be classified according to cell number, and/or the shape and branching pattern of their secretory portions and ducts.

Unicellular Glands

consist of a single secretory cell. In mammals the only example of unicellular exocrine glands are goblet cells, which occur in the epithelium of many mucous membranes. Goblet cells secrete the glycoprotein mucin, which by the uptake of water is converted into a slimy substance, mucus.

Multicellular glands: The simplest form of a multicellular gland is a secretory epithelial sheath - a surface epithelium consisting entirely of secretory cells (e.g. the epithelium lining the inner surface of the stomach, where the mucous secretion protects the stomach wall from the acidic contents of the stomach). Other multicellular glands have their secretory portion embedded in the connective tissue underlying the epithelium. The secretion is either discharged directly from the secretory portion onto the epithelium or reaches the epithelium via a duct system that consists of non-secretory cells.

The secretory portion may have a variety of shapes. Secretory cells may form tubes in tubular glands, acini in acinar glands or alveoli in alveolar glands.

Combinations exist - the pancreas is a tubulo-acinar gland, in which each section of the secretory system has a specialized function.
The precursors of digestive enzymes are produced by the acinar cells. Tubular cells secrete the alkaline bicarbonate solution which eventually neutralizes the acidic contents of the stomach that are released into the duodenum.
Multicellular glands with an unbranched excretory duct are called simple. We talk about a compound gland when the excretory duct is branched. Finally, the part of the gland consisting of secretory cells is branched in a (surprise!)branched gland.
The classification scheme may appear somewhat elaborate - but there are many exocrine glands around. All of them can be identified and described by this scheme, and some ideas about their function can be derived from this description.

Suitable Slides

unicellular exocrine glands (goblet cells): sections of intestines (duodenum, jejunum, ileum or colon) or trachea - H&E
secretory epithelial sheath: stomach - H&E
straight tubular glands: sections of stomach (principal glands) or colon (intestinal glands) - van Gieson, H&E
coiled tubular glands: sections of skin (sweat glands) - see lab section on the Integumentary System page.

Colon, Human - van Gieson Straight tubular glands extend from the surface of the colon into the underlying connective tissue. Although they are present throughout the intestines they are largest in the colon and, because of the smooth inner surface of the colon, they often show in good longitudinal or transverse sections. The lumen of the glands is narrow and surrounded by secretory cells of several types, which include goblet cells. The connective tissue beneath the epithelium and surrounding the glands in the colon contains more cells than the connective tissue beneath other epithelia that were considered on this page. This is a characteristic feature of the epithelia in the digestive system. Glands cut at slightly oblique angles will connect to the lumen outside of the plane of the section. If possible, draw both longitudinally and transversely sections intestinal glands. Include part of the surrounding connective tissue and surface epithelium.