Exocrine Glands Definition

Exocrine glands are cellular sub-structures, organs, in a body that provide a system to secrete substances out and external to the body. They are distinct from the other type of gland, endocrine, in that exocrine gland secretions end up external to the body, while endocrine secretions go into the bloodstream/internal. Exocrine glands secrete their substances through a ductal system.

Function of Exocrine Glands

Depending on the exocrine gland, they can function to regulate body temperature, lubricate, nurture newborns (lactation), aid in digestion, and aid in reproduction.

Exocrine Gland Structure

Exocrine gland structure is broken down into the ductal portion and the glandular portion. The glandular portion is either a round (also called acinus or acini (plural)) or elongated cluster of cells that produce the secreted substance. There are different types of cells found in the glandular portion depending on the substance secreted. Some common cell types include serous cells (protein excretion) and mucous cells (mucus/fluid excretion).

The tubular portion is often a single, cuboidal cell thick wall that aids in movement of the secretion. The tubular duct can be simple in structure (unbranched) or complex with many duct branches. The tubular duct can also be observed in a simple coiled structure.

Types of Exocrine Glands

Exocrine glands are classified by the way secretion is accomplished in each organ:

  • Holocrine glands will release whole broken open cells into their ductal system. These cells contain the stores of substance to be released by the gland. This method of secretion requires frequent cell turnover and replacement.
  • Merocrine or Eccrine glands release their substances directly out into the duct, through cellular channels or pores, with no loss of cell structure or membrane. This is the most common type of exocrine gland.
  • Apocrine glands release their secretion by budding off a part of their cellular cytoplasm and membrane. This bud contains the secreted substance and is released into the ductal system.

Examples of Exocrine Glands (and Secreted Product)

  1. Lacrimal gland (Tear ducts and glands near each eye)
  2. Mammary gland (Breast milk)
  3. Eccrine sweat glands (Perspiration or salty water release)
  4. Salivary glands (Saliva consisting of fluid with digestive enzymes)
  5. Pancreas (Pancreatic juice with digestive enzymes into the stomach)
  6. Liver (Bile, green-brown fluid that contains salts and digestion substances)

Note: Examples 5 and 6 are also considered endocrine glands because they also secrete other substances into the bloodstream

Quiz

1. What is the main difference between an exocrine and endocrine gland?
A. The type of secreted substance
B. The way secretion is accomplished
C. Where the secretion ends up
D. How the secretion gets processed

Answer to Question #1
C is correct. Endocrine glands secrete products to the bloodstream while exocrine secrete products that end up external to the body.

2. List the component that would NOT be found in an exocrine gland.
A. Mucous
B. Bile
C. Hormones
D. Cuboidal cell

Answer to Question #2
C is correct. Hormones are released into the bloodstream and therefore be released by an endocrine gland not an exocrine gland.

3. Sebaceous glands secrete the oily, waxy substance called sebum. This is done by fat producing cells that break open along the base of the gland and release their contents into the duct. This is an example of what type of exocrine gland?
A. Merocrine gland
B. Apocrine gland
C. Salivary gland
D. Holocrine gland

Answer to Question #3
D is correct. Holocrine glands secrete by the total cellular breakdown of the substance producing cell.

References

  • Marieb, E.N and Hoehn, K. (2015). Human Anatomy & Physiology, 10th Ed. Ch. 12. Boston: Pearson. ISBN: 978-0133997040
  • Kimball, John W. (2015-03-02). “Exocrine Glands” Kimball’s Biology Pages Retrieved 2017-4-26 from http://www.biology-pages.info/.