Exocrine glands are glands which produce secretions destined for the surface of an organ, as opposed to endocrine glands, which secrete compounds into the bloodstream. Some examples of these glands include the mammary glands, sweat glands, and saliva glands. Some glands are both endocrine and exocrine in nature, secreting hormones into the bloodstream along with compounds which reach the surface of the organ.
Some exocrine glands secrete directly, but more commonly, their secretions are routed through ducts, which may be simple or compound. Simple ducts consist of a single duct, while compound ducts branch out, providing more coverage. The ducts can also twist and turn in a variety of ways which create a number of subclassifications based on the shape of the duct. The shape of the ducts can be discerned clearly with the use of magnification, and sometimes tracers or dyes can be utilized to make the ducts more clear.
Some exocrine glands are classified as merocrine glands, in which intact cells produce secretions. By contrast, holocrine glands produce compounds by allowing their cells to break up to release the desired secretion, and apocrine glands release their cells along with the secretion, with cells budding off and being replaced as needed. These three types of glands appear in many different areas of the body, with each type having advantages and disadvantages that make it particularly suitable for specific applications.
The secretions produced by these glands can be broken into proteins and mucus. Some exocrine glands produce both proteins and mucus, depending on where they are located and what their function is. Mucus glands are classically used to create a layer of lubrication and protection for the body, while glands which secrete proteins can have a number of functions. For example, exocrine cells in the intestinal tract produce proteins which are used in digestion.
As with endocrine glands, the function of exocrine glands is critical to the overall health of the body. A number of techniques can be used to evaluate the function of these glands to determine whether or not they are working properly, and what the cause of failures might be. Damage to a gland can play a role, as can issues with cell signaling which lead to mixed or missed messages which confuse a gland or cause it to shut down. Synthetic versions of some of the secretions of the exocrine glands are available to make up for production problems, such as artificial tears to address problems with the tear glands.