Physiology, Exocrine Gland


Article Author:
S. Caleb Freeman
Ahmad Malik


Article Editor:
Hajira Basit


Editors In Chief:
Michelle Miranda


Managing Editors:
Avais Raja
Orawan Chaigasame
Carrie Smith
Abdul Waheed
Khalid Alsayouri
Frank Smeeks
Kristina Soman-Faulkner
Radia Jamil
Patrick Le
Sobhan Daneshfar
Anoosh Zafar Gondal
Saad Nazir
William Gossman
Pritesh Sheth
Hassam Zulfiqar
Navid Mahabadi
Steve Bhimji
John Shell
Matthew Varacallo
Heba Mahdy
Ahmad Malik
Mark Pellegrini
James Hughes
Beata Beatty
Nazia Sadiq
Hajira Basit
Phillip Hynes
Tehmina Warsi


Updated:
5/14/2019 2:57:57 PM

Introduction

A gland is a functional unit of cells that works together to create and release a product into a duct or directly to the bloodstream.  Two major types of glands exist, exocrine and endocrine.  An important distinction exists between exocrine and endocrine glands.  Whereas exocrine glands secrete substances into a ductal system to an epithelial surface, endocrine glands secrete products directly into the bloodstream.[1] Exocrine secretions form in the acinus, a small cluster of cells at the origination of glandular ducts. Exocrine glands subclassify into subtypes based on the method of secretion, the compound produced, or the shape of the gland.

Issues of Concern

This article will discuss:

  • Various cell types found within the exocrine gland and their functions
  • Embryologic development of exocrine glands
  • Organ systems impacted by exocrine physiology
  • Functions of exocrine glands
  • Related clinical testing
  • Pathophysiology of exocrine glands
  • Significant clinical aspects

Cellular

Exocrine glands are comprised of an acinus and a duct with different cell types respectively.  These glands are found in many organs within the body and demonstrate a large variety in the function of their secretions.  As such, a wide range of cell types exist in exocrine glands.

While the duct functions primarily to transport glandular secretions, the acinus is responsible for the production of glandular secretions, and as such shows more variety in cellular composition.  Typical cell types within the acinus include serous, mucinous, or sebaceous.  Serous cells secrete an isotonic fluid that contains proteins such as enzymes.  Salivary glands are made up of serous cells to a large extent.[2] Mucinous glands secrete mucus.  A typical example is Brunner glands in the duodenum.  Sebaceous glands secrete sebum, an oily compound.   Sebaceous glands are most prevalent in the face, scalp, groin, and armpits. Cell types can be differentiated histologically as well.  Mucus cells typically stain lighter than their serous counterparts when stained with hematoxylin and eosin.

As ducts move from the acinus toward the final target, secretions initially enter the intralobular duct.  Intralobular ducts have a simple cuboidal epithelium commonly surrounded by parenchyma.  Intralobular ducts drain into interlobular ducts which are a simple columnar epithelium.  The final ductal unit is the interlobar duct recognized by a stratified columnar epithelium.  Connective tissue surrounds both interlobular and interlobar ducts.

Development

The initial manifestation of exocrine gland formation is epithelial budding resulting from a complex interaction between mesenchymal and epithelial cell populations.[3] This initial period of ingrowth gets influenced by fibroblast growth factors, most notably FGF10, and cadherin-2.[4] Other transcription factors that have been shown to contribute to epithelial budding include HlxB9, Isl1, LEF-1, Msx1/2, Pbx1, Pdx1, and Tbx3.[5]

Following the initial formation of the epithelial bud, ductal elongation occurs.  This process undergoes mediation by a large group of molecular signals such as Netrin-1, TIMP1, amphiregulin, IGF1, and leukemia inhibitory factor.[5] Several matrix metalloproteinases (MMPs) contribute assistance with basement membrane renewal and facilitate ductal elongation.[6][7] After an initial period of ductal elongation, the exocrine gland begins to form ductal branches.  NF-kappa-B is thought to play a role,[8] as well as sonic hedgehog and Wnts.[3] As the duct begins to elongate, the acinus undergoes a period of cell proliferation and differentiation.  Due to the large variety in exocrine gland function, the exact number of cellular signals and interactions is immense.  In general, however, a large role exists for cell adhesion molecules such as laminin and cadherins.[9]

Exocrine morphogenesis is a rapid process. Ductal elongation and branching typically occur in less than a week, with acini formation occurring 5 to 9 days later.[10][11]  In a relatively short developmental period, exocrine glands form and can begin secreting a functional product. 

Organ Systems Involved

Due to the diverse number and function of epithelial surfaces in the body, many organ systems utilize exocrine glands to carry out their respective actions.  Several examples will be included here including skin, mouth, stomach, pancreas, duodenum, and breasts. 

Skin

The skin has a variety of exocrine glands including the eccrine sweat glands and sebaceous glands.  Eccrine sweat glands are the most widespread sweat gland in the body and are present on nearly every external body surface.  The sweat produced is clear with little to no oil, which in contrast to sebaceous glands, also found on the skin, which secrete the more oily substance sebum.  

Salivary Glands

The salivary glands in the mouth are another example of exocrine glands and include the parotid glands, submandibular glands, and sublingual glands.  While each gland has a unique mixture of serous and mucous cells, together the salivary glands act to begin the process of food digestion while also lubricating and protecting the mucosal surfaces.

Stomach

The stomach holds multiple forms of exocrine glands that include pyloric glands, cardiac glands, and fundic glands.  These glands incorporate many different cell types including parietal cells, chief cells, and G cells.  Together they regulate the gastric pH, release enzymes to breakdown food products to a digestible form, and assist with absorption of necessary vitamins and minerals.  

Pancreas

The pancreas has both an endocrine and an exocrine function.  The exocrine pancreas assists in food digestion by releasing a secretion rich in bicarbonate, which helps to neutralize the acidic environment created in the stomach.  The secretion also includes digestive enzymes.

Duodenum

Brunner glands are present in the duodenum of the small intestine.  These exocrine glands are submucosal and produce a mucous product that protects the duodenum from acid released from the stomach.  The alkaline nature of the secretion also activates intestinal enzymes to assist with food breakdown and absorption.

Breast

The mammary gland is one of the most well-known examples of an exocrine gland found in the breast.  Mammary glands produce milk rich in nutrients that also provides passive immunity to a baby’s immune system. 

Function

The specific function of exocrine glands within the body varies by location and organ system.  However, the primary role is to create a secretion which subsequently gets released through a ductal system onto an epithelial surface.  Examples include secretions that assist in food digestion, mucosal protection, thermoregulation, lubrication, and nutrition. 

Mechanism

The three mechanisms by which exocrine glands release their secretions include merocrine, apocrine, and holocrine. Merocrine glands are the most common subtype.  By definition, merocrine gland secretions exit the cell via exocytosis.  In this method of secretion, there is no cell damage.  An example of merocrine secretion is the eccrine sweat gland.  Apocrine glands, in contrast, form buds of the membrane which break off into the duct, losing part of the cellular membrane in the process.  A well-known apocrine gland is the breastmilk-producing mammary gland.  The final subtype of excretion is holocrine, in which the cellular membrane ruptures to release its product into the duct.  Sebaceous glands are a representation of holocrine secretion.

Related Testing

In general, testing for an individual exocrine grand function is not performed.  However, dysfunction of exocrine glands can create a wide range of clinical manifestations.

Imaging may be performed to confirm a diagnosis in cases of blocked glands.  Sialolithiasis refers to instances where a stone becomes lodged within the salivary gland or duct, and sialoadenitis refers to inflammation of the gland.  CT and ultrasound are effective methods of identifying and localizing stones.[12]

The liver itself acts as an exocrine gland when creating and excreting bile to be stored in the gallbladder awaiting expulsion and release through the pancreatic duct into the duodenum.  Obstruction at any point of this pathway can cause cholecystitis, due to inflammation and dysfunction of the gallbladder.  Ultrasound is the initial diagnostic test to diagnose cholecystitis.[13]

In cystic fibrosis, sodium and chloride are not reabsorbed within the sweat duct due to a dysfunctional CFTR protein, resulting in abnormally salty skin.  The sweat chloride test is the primary test for the diagnosis of cystic fibrosis.[14]

Pancreatic insufficiency occurs when the exocrine glands of the pancreas are no longer able to produce the digestive enzymes necessary for food breakdown in the small intestine.  Common etiologies include chronic pancreatitis, cystic fibrosis, and hereditary hemochromatosis.  Several methods can be used to evaluate the function of the exocrine pancreas.  Fat malabsorption can lead to deficiencies in the fat-soluble vitamins A, D, E, and K. Thus vitamin levels can be used to estimate pancreatic function.[15] Fecal elastase-1 testing is another method with relatively high specificity and sensitivity.  Low levels of fecal elastase-1 indicate a poorly functioning exocrine pancreas.[16] The most sensitive diagnostic method for exocrine pancreatic insufficiency, however, is utilizing direct pancreatic function tests such as the cholecystokinin (CCK) or secretin stimulation test.[17]

Pathophysiology

Sjogren Syndrome

Sjogren’s syndrome is commonly associated with rheumatoid arthritis and other rheumatic diseases.  The syndrome is an autoimmune disorder that demonstrates decreased lacrimal and salivary gland function that can also have associated systemic symptoms.[18] The disease is characterized by eye and mouth dryness due to the gland dysfunction.  Due to mouth dryness, patients with Sjogren syndrome show increased rates of oral candidiasis and dental caries.[19][20]

Cystic Fibrosis

Cystic fibrosis is an autosomal recessive disease that causes impaired chloride transport due to a mutation of the CFTR protein.  Because CFTR is involved in the production of sweat, mucus, and digestive fluids, the mutation causes a direct effect on exocrine gland secretions.  Indeed, approximately 90% of infants born with cystic fibrosis will develop pancreatic insufficiency by one year of age.[21]

Acne vulgaris

The prevalence of acne is an estimated 35 to 90% in adolescents.[22] The disorder affects the pilosebaceous unit, of which sebaceous glands are an example.  Pathogenesis is multifactorial and often involves hyperkeratinization of the follicle, increased sebum production, and proliferation of Propionibacterium acnes with associated inflammation.  As sebum accumulates, an open comedo forms, also known as a white head.  Hyperkeratinization and increased sebum production lead to clogging of the pores of the pilosebaceous unit. As the lipids within sebum oxidize, the follicular orifice opens, forming an open comedo, or blackhead.

Treatment for acne largely depends on the severity of inflammatory symptoms, but topical retinoids are usually the first line treatment, although antimicrobial agents are an additional option for refractory cases.[23]  For severe cases of nodulocystic acne or for patients who have failed treatment with systemic antibiotics, oral isotretinoin the therapeutic choice.[24]

Clinical Significance

The exocrine gland can be found in many organs and serves a wide variety of functions within the body.  Due to this fact, an understanding of the physiology of exocrine glands is essential for healthcare workers.  Exocrine glands can be found in everything from the skin to the pancreas, and provide the body with a method to release secretions containing proteins, mucus, and other products to epithelial surfaces around the body.  Dysfunction of exocrine glands is associated with diseases as wide-ranging as acne vulgaris to Sjogren syndrome. 


Interested in Participating?

We are looking for contributors to author, edit, and peer review our vast library of review articles and multiple choice questions. In as little as 2-3 hours you can make a significant contribution to your specialty. In return for a small amount of your time, you will receive free access to all content and you will be published as an author or editor in eBooks, apps, online CME/CE activities, and an online Learning Management System for students, teachers, and program directors that allows access to review materials in over 500 specialties.

Improve Content - Become an Author or Editor

This is an academic project designed to provide inexpensive peer-reviewed Apps, eBooks, and very soon an online CME/CE system to help students identify weaknesses and improve knowledge. We would like you to consider being an author or editor. Please click here to learn more. Thank you for you for your interest, the StatPearls Publishing Editorial Team.

Physiology, Exocrine Gland - Questions

Take a quiz of the questions on this article.

Take Quiz
A 15-year-old male presents to the clinic with a 2-year history of inflammatory acne. Which of the following types of glands is most likely dysfunctional?



Click Your Answer Below


Would you like to access teaching points and more information on this topic?

Improve Content - Become an Author or Editor and get free access to the entire database, free eBooks, as well as free CME/CE as it becomes available. If interested, please click on "Sign Up" to register.

Purchase- Want immediate access to questions, answers, and teaching points? They can be purchased above at Apps and eBooks.


Sign Up
A 12-month-old Caucasian girl is brought to the hospital with a productive cough, fever, and chills. Physical exam shows crackles in the bilateral lung fields. She has had 3 similar presentations since birth. Sweat chloride testing shows higher than normal levels of chloride on the skin. Which of the following complications is the patient at the highest risk for?



Click Your Answer Below


Would you like to access teaching points and more information on this topic?

Improve Content - Become an Author or Editor and get free access to the entire database, free eBooks, as well as free CME/CE as it becomes available. If interested, please click on "Sign Up" to register.

Purchase- Want immediate access to questions, answers, and teaching points? They can be purchased above at Apps and eBooks.


Sign Up
A pathology researcher is investigating different glandular tissue. He is provided with a certain type of glands which includes sweat glands, parotid glands, and lacrimal glands. Which of the following best describes the function of these glands?



Click Your Answer Below


Would you like to access teaching points and more information on this topic?

Improve Content - Become an Author or Editor and get free access to the entire database, free eBooks, as well as free CME/CE as it becomes available. If interested, please click on "Sign Up" to register.

Purchase- Want immediate access to questions, answers, and teaching points? They can be purchased above at Apps and eBooks.


Sign Up
A 24-year-old woman is 3 weeks postpartum. She had an uncomplicated vaginal delivery at term and has been breastfeeding her child. Which of the following is the first step of embryologic development for the structure involved in producing and secreting breastmilk?



Click Your Answer Below


Would you like to access teaching points and more information on this topic?

Improve Content - Become an Author or Editor and get free access to the entire database, free eBooks, as well as free CME/CE as it becomes available. If interested, please click on "Sign Up" to register.

Purchase- Want immediate access to questions, answers, and teaching points? They can be purchased above at Apps and eBooks.


Sign Up
A 13-year-old male presents to the clinic with a reported increase in sweating during exercise. The physician explains that this is a normal development. Which of the following glands is most similar to the gland showing increased activity?



Click Your Answer Below


Would you like to access teaching points and more information on this topic?

Improve Content - Become an Author or Editor and get free access to the entire database, free eBooks, as well as free CME/CE as it becomes available. If interested, please click on "Sign Up" to register.

Purchase- Want immediate access to questions, answers, and teaching points? They can be purchased above at Apps and eBooks.


Sign Up
A pathologist is examining the biopsy taken from a parotid gland. The glandular structures are identified showing a large proportion of darkly staining cells, with a smaller subset of lighter staining cells. Which of the following substances is most likely secreted by the lighter cells?



Click Your Answer Below


Would you like to access teaching points and more information on this topic?

Improve Content - Become an Author or Editor and get free access to the entire database, free eBooks, as well as free CME/CE as it becomes available. If interested, please click on "Sign Up" to register.

Purchase- Want immediate access to questions, answers, and teaching points? They can be purchased above at Apps and eBooks.


Sign Up
A 32-year-old woman on postpartum day 3 is concerned about not producing enough milk. She reports using an herbal supplement which a friend recommended, to increase milk production. Which of the following best describes the physiology involved in breastmilk glandular secretion?



Click Your Answer Below


Would you like to access teaching points and more information on this topic?

Improve Content - Become an Author or Editor and get free access to the entire database, free eBooks, as well as free CME/CE as it becomes available. If interested, please click on "Sign Up" to register.

Purchase- Want immediate access to questions, answers, and teaching points? They can be purchased above at Apps and eBooks.


Sign Up

Physiology, Exocrine Gland - References

References

Hogan BL, Morphogenesis. Cell. 1999 Jan 22;     [PubMed]
Govindarajan V,Ito M,Makarenkova HP,Lang RA,Overbeek PA, Endogenous and ectopic gland induction by FGF-10. Developmental biology. 2000 Sep 1;     [PubMed]
Wang J,Laurie GW, Organogenesis of the exocrine gland. Developmental biology. 2004 Sep 1;     [PubMed]
Witty JP,Wright JH,Matrisian LM, Matrix metalloproteinases are expressed during ductal and alveolar mammary morphogenesis, and misregulation of stromelysin-1 in transgenic mice induces unscheduled alveolar development. Molecular biology of the cell. 1995 Oct;     [PubMed]
Sympson CJ,Talhouk RS,Alexander CM,Chin JR,Clift SM,Bissell MJ,Werb Z, Targeted expression of stromelysin-1 in mammary gland provides evidence for a role of proteinases in branching morphogenesis and the requirement for an intact basement membrane for tissue-specific gene expression. The Journal of cell biology. 1994 May;     [PubMed]
Brantley DM,Chen CL,Muraoka RS,Bushdid PB,Bradberry JL,Kittrell F,Medina D,Matrisian LM,Kerr LD,Yull FE, Nuclear factor-kappaB (NF-kappaB) regulates proliferation and branching in mouse mammary epithelium. Molecular biology of the cell. 2001 May;     [PubMed]
Yurchenco PD,Amenta PS,Patton BL, Basement membrane assembly, stability and activities observed through a developmental lens. Matrix biology : journal of the International Society for Matrix Biology. 2004 Jan;     [PubMed]
Wessells NK,Evans J, Ultrastructural studies of early morphogenesis and cytodifferentiation in the embryonic mammalian pancreas. Developmental biology. 1968 Apr;     [PubMed]
Wolff MS,Mirels L,Lagner J,Hand AR, Development of the rat sublingual gland: a light and electron microscopic immunocytochemical study. The Anatomical record. 2002 Jan 1;     [PubMed]
Thomas WW,Douglas JE,Rassekh CH, Accuracy of Ultrasonography and Computed Tomography in the Evaluation of Patients Undergoing Sialendoscopy for Sialolithiasis. Otolaryngology--head and neck surgery : official journal of American Academy of Otolaryngology-Head and Neck Surgery. 2017 May;     [PubMed]
Shea JA,Berlin JA,Escarce JJ,Clarke JR,Kinosian BP,Cabana MD,Tsai WW,Horangic N,Malet PF,Schwartz JS, Revised estimates of diagnostic test sensitivity and specificity in suspected biliary tract disease. Archives of internal medicine. 1994 Nov 28;     [PubMed]
Denning CR,Huang NN,Cuasay LR,Shwachman H,Tocci P,Warwick WJ,Gibson LE, Cooperative study comparing three methods of performing sweat tests to diagnose cystic fibrosis. Pediatrics. 1980 Nov;     [PubMed]
Dutta SK,Bustin MP,Russell RM,Costa BS, Deficiency of fat-soluble vitamins in treated patients with pancreatic insufficiency. Annals of internal medicine. 1982 Oct;     [PubMed]
Domínguez-Muñoz JE,Hieronymus C,Sauerbruch T,Malfertheiner P, Fecal elastase test: evaluation of a new noninvasive pancreatic function test. The American journal of gastroenterology. 1995 Oct;     [PubMed]
Heij HA,Obertop H,Schmitz PI,van Blankenstein M,Westbroek DL, Evaluation of the secretin-cholecystokinin test for chronic pancreatitis by discriminant analysis. Scandinavian journal of gastroenterology. 1986 Jan;     [PubMed]
Asmussen K,Andersen V,Bendixen G,Schiødt M,Oxholm P, A new model for classification of disease manifestations in primary Sjögren's syndrome: evaluation in a retrospective long-term study. Journal of internal medicine. 1996 Jun;     [PubMed]
Wu AJ, The oral component of Sjögren's syndrome: pass the scalpel and check the water. Current rheumatology reports. 2003 Aug;     [PubMed]
Soto-Rojas AE,Villa AR,Sifuentes-Osornio J,Alarcón-Segovia D,Kraus A, Oral candidiasis and Sjögren's syndrome. The Journal of rheumatology. 1998 May;     [PubMed]
Bronstein MN,Sokol RJ,Abman SH,Chatfield BA,Hammond KB,Hambidge KM,Stall CD,Accurso FJ, Pancreatic insufficiency, growth, and nutrition in infants identified by newborn screening as having cystic fibrosis. The Journal of pediatrics. 1992 Apr;     [PubMed]
Stathakis V,Kilkenny M,Marks R, Descriptive epidemiology of acne vulgaris in the community. The Australasian journal of dermatology. 1997 Aug;     [PubMed]
Zaenglein AL,Pathy AL,Schlosser BJ,Alikhan A,Baldwin HE,Berson DS,Bowe WP,Graber EM,Harper JC,Kang S,Keri JE,Leyden JJ,Reynolds RV,Silverberg NB,Stein Gold LF,Tollefson MM,Weiss JS,Dolan NC,Sagan AA,Stern M,Boyer KM,Bhushan R, Guidelines of care for the management of acne vulgaris. Journal of the American Academy of Dermatology. 2016 May;     [PubMed]
Liu A,Yang DJ,Gerhardstein PC,Hsu S, Relapse of acne following isotretinoin treatment: a retrospective study of 405 patients. Journal of drugs in dermatology : JDD. 2008 Oct;     [PubMed]
Murphrey MB,Vaidya T, Histology, Apocrine Gland . 2019 Jan     [PubMed]
Holmberg KV,Hoffman MP, Anatomy, biogenesis and regeneration of salivary glands. Monographs in oral science. 2014     [PubMed]

Disclaimer

The intent of StatPearls is to provide practice questions and explanations to assist you in identifying and resolving knowledge deficits. These questions and explanations are not intended to be a source of the knowledge base of all of medicine, nor is it intended to be a board or certification review of Nurse-Maternal Newborn RN. The authors or editors do not warrant the information is complete or accurate. The reader is encouraged to verify each answer and explanation in several references. All drug indications and dosages should be verified before administration.

StatPearls offers the most comprehensive database of free multiple-choice questions with explanations and short review chapters ever developed. This system helps physicians, medical students, dentists, nurses, pharmacists, and allied health professionals identify education deficits and learn new concepts. StatPearls is not a board or certification review system for Nurse-Maternal Newborn RN, it is a learning system that you can use to help improve your knowledge base of medicine for life-long learning. StatPearls will help you identify your weaknesses so that when you are ready to study for a board or certification exam in Nurse-Maternal Newborn RN, you will already be prepared.

Our content is updated continuously through a multi-step peer review process that will help you be prepared and review for a thorough knowledge of Nurse-Maternal Newborn RN. When it is time for the Nurse-Maternal Newborn RN board and certification exam, you will already be ready. Besides online study quizzes, we also publish our peer-reviewed content in eBooks and mobile Apps. We also offer inexpensive CME/CE, so our content can be used to attain education credits while you study Nurse-Maternal Newborn RN.