Exocrine Glands – Definition, Structure, Functions, Types, Examples

What is Exocrine gland?

• Exocrine glands play a vital role in the human body by secreting various substances onto epithelial surfaces through ducts. These glands are responsible for producing important fluids and substances that are necessary for bodily functions and maintaining homeostasis. Unlike endocrine glands, which release their secretions directly into the bloodstream, exocrine glands transport their products to specific locations through a network of ducts.
• There are several types of exocrine glands found in the human body, each serving a unique purpose. Sweat glands, for example, are integral components of the integumentary system, which includes the skin, and are responsible for regulating body temperature. Sweat glands can be further classified into eccrine and apocrine glands. Eccrine glands are found all over the body and secrete sweat, a watery solution that helps cool down the body through evaporation. Apocrine glands, on the other hand, are found primarily in the armpits and groin area and produce a thicker, odorous secretion that becomes noticeable when bacteria on the skin break it down.
• Salivary glands are another type of exocrine gland that secretes saliva into the oral cavity. Saliva aids in the digestion process by moistening food, facilitating chewing and swallowing, and containing enzymes that initiate the breakdown of complex carbohydrates. Mammary glands are specialized exocrine glands present in the breasts of females. These glands produce and secrete milk, providing nourishment for infants.
• Other examples of exocrine glands include ceruminous glands, which secrete earwax (cerumen) to protect the ear canal; lacrimal glands, responsible for producing tears to keep the eyes lubricated and free from debris; sebaceous glands, which produce sebum to moisturize and protect the skin and hair; prostate glands, which secrete seminal fluid to support sperm function; and mucous glands, which produce mucus to lubricate and protect various tissues, such as those lining the respiratory and digestive tracts.
• It is worth noting that some organs in the body, such as the liver and pancreas, have dual functions as both exocrine and endocrine glands. These organs secrete their products, such as bile and pancreatic juice, into the gastrointestinal tract through ducts, making them exocrine glands. Simultaneously, they release other substances, such as hormones, directly into the bloodstream, categorizing them as endocrine glands.
• In summary, exocrine glands are essential cellular structures and organs in the human body that secrete substances onto epithelial surfaces through a ductal system. They differ from endocrine glands by releasing their secretions externally, rather than into the bloodstream. The wide variety of exocrine glands perform crucial functions, including regulating body temperature, aiding in digestion, moisturizing and protecting the skin, producing milk, facilitating vision, supporting reproduction, and providing lubrication to various tissues. Their diverse roles contribute significantly to the overall functioning and well-being of the human body.

Definition of Exocrine gland

An exocrine gland is a gland that releases its secretions through ducts onto epithelial surfaces outside of the body.

Characteristics of Exocrine Glands

Exocrine glands possess several characteristics that distinguish them from other types of glands in the body. Here are some key characteristics of exocrine glands:

• Secretion through Ducts: Exocrine glands secrete their products through ducts that transport the secreted substances to specific locations, either on the body’s surface or into body cavities. These ducts act as conduits, allowing the secretions to reach their target destinations.
• Localized Secretory Structures: Exocrine glands are composed of localized secretory structures that are responsible for the production and release of their specific products. These secretory structures may vary in shape and organization depending on the gland type, but they are distinct and separate from the surrounding tissues.
• Diverse Secretory Products: Exocrine glands produce a wide range of secretory products that serve various functions in the body. These products can include proteins, enzymes, mucus, lipids, hormones, and other specialized substances, depending on the specific gland type and its role in the body.
• Ductal System: Exocrine glands have a ductal system that connects the secretory structures to the surface or internal body cavities where the secretions are released. The ducts may be simple or branched, depending on the complexity of the gland and the volume of secretion required.
• Method of Secretion: Exocrine glands utilize different methods of secretion to release their products. The three main methods are merocrine, apocrine, and holocrine secretion, each involving distinct mechanisms of product release from the glandular cells.
• Regulation and Control: The secretion of exocrine glands is tightly regulated and controlled by various factors, including hormonal signals, neural input, and local tissue conditions. This regulation ensures that the secretion occurs in response to specific physiological needs or stimuli.
• Localized Effects: The secretory products of exocrine glands typically exert their effects locally, in close proximity to the gland itself. For example, sweat glands regulate body temperature through sweat evaporation on the skin’s surface, and digestive glands release enzymes and fluids into the digestive tract to facilitate digestion and nutrient absorption.

These characteristics collectively contribute to the function and importance of exocrine glands in maintaining homeostasis, supporting digestion, providing lubrication, regulating body temperature, and performing various other vital physiological processes in the body.

Secretions of exocrine glands

Exocrine Gland	Secretion
Sweat glands	Sweat
Sebaceous glands	Sebum
Salivary glands	Saliva
Lacrimal glands	Tears
Mammary glands	Milk
Gastric glands	Enzymes for digestion
Intestinal glands	Intestinal juices
Pancreas	Pancreatic juices
Ovary	Ova
Testis	Sperm

Structure Of The Exocrine Glands

• The structure of exocrine glands consists of two main components: the ductal portion and the glandular portion.
• The ductal portion of the exocrine gland is tubular in shape. It is composed of a single layer of thick, cuboidal cells that facilitate the movement of the secretions. The duct can be either unbranched (simple) or branched (compound). In some cases, the duct may also exhibit a coiled structure.
• The glandular portion is responsible for the production of the secreted substance. It is composed of circular or elongated clusters of cells. The specific cell types present in the glandular portion depend on the type of substance being secreted. For instance, serous cells are responsible for secreting proteins, while mucous cells produce fluids or mucus.
• Based on the arrangement of the ductal and glandular portions, exocrine glands can be classified into different types. If the glandular portion is tubular, acinar (also known as acinus or acini), or a combination of both (tubuloacinar), the gland is categorized accordingly. If the ductal portion branches, the gland is referred to as a branched gland.
• In summary, the structure of exocrine glands comprises a ductal portion and a glandular portion. The ductal portion can be simple or complex, while the glandular portion consists of cells responsible for producing the secreted substance. The structural characteristics of exocrine glands contribute to their specific functions and modes of secretion.

Types of Exocrine Glands

Exocrine glands exhibit a variety of types based on their mode of secretion and the nature of their secretory product. Here are the main types of exocrine glands:

1. Apocrine Glands: Apocrine glands are characterized by the loss of a portion of the cell’s body during secretion. While the term “apocrine glands” is commonly associated with apocrine sweat glands, it is debated whether they truly follow the apocrine secretion method. Examples of apocrine glands include mammary glands (which produce milk) and certain sweat glands found in areas such as the armpits, pubic region, skin around the anus, lips, and nipples.
2. Holocrine Glands: Holocrine glands release their secretions by disintegrating the entire cell. The entire cell breaks down to release its substances. Sebaceous glands, such as the meibomian glands of the eyelids and the zeis glands of hair follicles, are examples of holocrine glands. These glands secrete an oily substance called sebum, which helps lubricate the skin and hair.
3. Merocrine Glands: Merocrine glands, also known as eccrine glands, release their secretions via exocytosis. In this process, the secretory cells package their substances into vesicles that fuse with the cell membrane, releasing the contents outside the cell. Examples of merocrine glands include major sweat glands (eccrine sweat glands) found throughout the human body, salivary glands, tear glands, goblet cells in the respiratory and digestive tracts, and intestinal glands.

In addition to the different types of exocrine glands, the secretory product of these glands can be categorized into three main groups:

a. Serous Glands: Serous glands secrete a watery fluid that often contains proteins. Examples include sweat glands, which produce a watery solution to regulate body temperature.

b. Mucous Glands: Mucous glands secrete a viscous, carbohydrate-rich product known as mucin. Goblet cells are an example of mucous glands found in the respiratory and digestive tracts. They produce mucus, which helps lubricate and protect the lining of these organs.

c. Sebaceous Glands: Sebaceous glands secrete a lipid-based substance called sebum. Sebum helps moisturize and protect the skin and hair. Examples of sebaceous glands include Fordyce spots and the meibomian glands of the eyelids.

In summary, exocrine glands can be classified into apocrine, holocrine, and merocrine glands based on their mode of secretion. The secretory product of exocrine glands can be serous (watery), mucous (viscous and carbohydrate-rich), or sebaceous (lipid-based). Each type of gland and its secretory product serve specific functions in the body’s physiology and overall well-being.

Method of secretion

Exocrine glands employ different methods of secretion to release their products. Here are the main methods of secretion:

1. Merocrine Secretion: Merocrine secretion involves the excretion of substances by exocytosis into a duct. The secretory cells of the gland package their products into vesicles, which are then released through the cell membrane into the duct. Examples of exocrine glands that utilize merocrine secretion include pancreatic acinar cells, sweat glands (eccrine glands), salivary glands, goblet cells in the respiratory and digestive tracts, tear glands, and intestinal glands.
2. Apocrine Secretion: Apocrine secretion occurs when the apical portion of the cell membrane, containing the secretory product, buds off from the cell. This process involves the release of a portion of the cell along with the secretory material. Examples of exocrine glands that use apocrine secretion include the sweat glands in areas such as the armpits, pubic region, skin around the anus, lips, and nipples. Mammary glands, which produce breast milk, also employ apocrine secretion.
3. Holocrine Secretion: Holocrine secretion involves the entire cell disintegrating to release its substance. The secretory cells accumulate the product within their cytoplasm until they reach a certain level of maturity. At this point, the cell ruptures, releasing the accumulated substance along with cellular debris. Sebaceous glands found in the skin and nose, meibomian glands of the eyelids, and zeis glands are examples of exocrine glands that utilize holocrine secretion.

These different methods of secretion allow exocrine glands to effectively release their products to carry out their respective functions in the body. Merocrine secretion involves vesicular transport, apocrine secretion involves the budding off of the apical portion of the cell, and holocrine secretion involves the disintegration of the entire cell for product release. Each method is adapted to the specific needs and functions of the exocrine gland and its secretory product.

Product secreted

Exocrine glands secrete various products that serve specific functions in the body. Here are some examples of the products secreted by different types of exocrine glands:

1. Serous Cells: Serous cells primarily secrete proteins, often enzymes. Examples of exocrine glands with serous cells include gastric chief cells in the stomach, which secrete pepsinogen, an enzyme involved in protein digestion. Paneth cells in the small intestine secrete antimicrobial peptides and enzymes that help protect the intestinal lining.
2. Mucous Cells: Mucous cells secrete mucus, a viscous fluid rich in carbohydrates, such as glycoproteins. Mucus acts as a lubricant and protective barrier. Examples of exocrine glands with mucous cells include Brunner’s glands in the duodenum, which secrete mucus to protect the intestinal lining, esophageal glands in the esophagus that produce mucus to aid in swallowing, and pyloric glands in the stomach, which secrete mucus to protect the stomach lining from gastric acid.
3. Seromucous Glands: Seromucous glands are a combination of serous and mucous cells, and they secrete both proteins and mucus. An example is the salivary glands. The parotid gland is predominantly serous and secretes saliva containing enzymes like amylase for initial digestion of carbohydrates. The sublingual gland is mainly mucous and produces mucus that lubricates the oral cavity. The submandibular gland is a mixed gland, with a higher proportion of serous cells, and secretes saliva that contains both proteins and mucus.
4. Sebaceous Glands: Sebaceous glands secrete sebum, which is a lipid-based product. Sebum helps moisturize and protect the skin and hair. Examples of sebaceous glands include Fordyce spots, which are small sebaceous glands found on the lips and genitals, and Meibomian glands in the eyelids, which secrete an oily substance that helps prevent tears from evaporating too quickly.

These examples highlight the diverse range of products secreted by exocrine glands, including proteins, enzymes, mucus, and lipids. Each secretory product plays a crucial role in various physiological processes, such as digestion, protection, lubrication, and maintaining the health and integrity of different tissues and organs in the body.

Functions Of Exocrine Glands

Exocrine glands serve various important functions in the human body. Here are some key functions performed by exocrine glands:

• Regulation of Body Temperature: Certain exocrine glands, such as sweat glands, play a crucial role in regulating body temperature. Sweat glands release sweat onto the skin’s surface, which evaporates and helps dissipate heat, cooling down the body.
• Lubrication: Exocrine glands contribute to lubrication in different parts of the body. For example, sebaceous glands secrete sebum, an oily substance that moisturizes and lubricates the skin and hair. This helps keep the skin supple and prevents it from drying out.
• Lactation: Mammary glands are specialized exocrine glands responsible for lactation, the production and secretion of milk. During pregnancy and after childbirth, mammary glands produce and release milk to nourish infants, providing essential nutrients and immune factors for their growth and development.
• Digestion: Several exocrine glands are involved in the process of digestion. Salivary glands secrete saliva, which contains enzymes that initiate the breakdown of food in the mouth, facilitating the process of digestion. Additionally, the pancreas and liver, which have dual functions as both exocrine and endocrine glands, secrete digestive enzymes and bile, respectively. These substances aid in the breakdown and absorption of nutrients in the gastrointestinal tract.
• Reproduction: Exocrine glands also play a role in reproduction. For example, the prostate gland, an exocrine gland, produces seminal fluid that helps support and nourish sperm, enabling their motility and survival. This fluid is released during ejaculation and is essential for successful reproduction.

These are just a few examples of the diverse functions performed by exocrine glands in the human body. They contribute to the regulation of body temperature, lubrication of various tissues, secretion of milk for infant nourishment, digestion of food, and assistance in reproductive processes. Without the proper functioning of exocrine glands, these crucial physiological processes would be compromised.

Examples of Exocrine Glands

Exocrine glands are responsible for secreting various substances onto different surfaces of the body. Here are some examples of exocrine glands:

• Lacrimal Glands: Lacrimal glands are located near each eye and are responsible for producing tears. Tears help keep the eyes lubricated, cleanse the surface of the eyes, and provide protection against foreign particles or irritants.
• Pancreas: The pancreas is an organ that serves both endocrine and exocrine functions. Its exocrine function involves secreting pancreatic juice into the stomach. Pancreatic juice contains digestive enzymes such as amylase, lipase, and proteases, which aid in the digestion and breakdown of carbohydrates, fats, and proteins.
• Salivary Glands: Salivary glands are found in the mouth and produce saliva. Saliva is a fluid that helps moisten food, making it easier to chew and swallow. It also contains digestive enzymes, such as amylase, which initiate the breakdown of starches.
• Liver: The liver has a dual role as both an endocrine and exocrine gland. Its exocrine function involves the secretion of bile. Bile is a greenish-brown fluid that contains salts, bile pigments, and cholesterol. It aids in the digestion and absorption of fats in the small intestine.
• Mammary Glands: Mammary glands are specialized exocrine glands found in the breasts. They produce and secrete breast milk, which is rich in nutrients and antibodies. Breast milk provides essential nourishment for infants and helps support their growth and development.
• Eccrine Sweat Glands: Eccrine sweat glands are widespread throughout the body, particularly in the skin. They are responsible for the production and release of sweat, which is a watery fluid that helps regulate body temperature. Sweating helps cool down the body through evaporative cooling.

These examples illustrate the diversity of exocrine glands and their specific functions in the body. Each gland produces and secretes substances that are essential for various physiological processes, such as tear production, digestion, temperature regulation, and infant nutrition.

What is the difference between exocrine and endocrine glands? – endocrine glands vs exocrine

Features	Endocrine Glands	Exocrine Glands
Mode of Secretions	Pour their secretions directly into the blood	Have ducts to pour their secretions
Examples	Hypothalamus, Pituitary, Pineal, Thyroid, Parathyroid, Adrenal, Thymus glands, Pancreas (Islet of Langerhans), Gonads (Ovary and Testis)	Sweat glands, Salivary glands, Sebaceous glands, Lacrimal glands, Mammary glands, Gastric glands, Intestinal glands, Pancreas (exocrine part)
Secretions	Hormones	Saliva, Sweat, Sebum, Tears, Milk, Gastric juices, Intestinal juices, Pancreatic juices
Function	Regulation of various physiological processes	Facilitate digestion, lubrication, protection, temperature regulation, and other specific functions
Location	Distributed throughout the body	Located in specific regions or organs

FAQ

References

• Freeman SC, Malik A, Basit H. Physiology, Exocrine Gland. [Updated 2022 Sep 26]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK542322/
• https://biologydictionary.net/exocrine-glands/
• http://histologyguide.com/slidebox/12-exocrine-glands.html
• https://www.geeksforgeeks.org/difference-between-endocrine-and-exocrine-glands/