Mucous Membrane (Mucosa): Structure, Locations, Functions and Disorders

Introduction to Mucous Membrane

Mucosa, or mucous membrane, is thick, soft tissue lining that forms protective layer for internal organs of body, body canals including orifices such as nostrils, ears, lips, urethral opening, anus, etc.

They are mainly associated with secretion of fluid-like, thick, lubricating substance known as mucus, a mucopolysaccharide consisting of mucin, which is secreted by specialized mucus-secreting cells in body.

Structure of Mucous Membrane

Mucosal membrane is characterized by presence of several layers of cells and tissue varying with location but exhibits general configuration applicable to most locations in body.

Typical structure includes three basic structural sub-layers:

Epithelial Layers

• Outermost non-keratinized covering of mucous layer, consisting of stratified or even simple squamous epithelial cells varying with location.
• Serves as protective barrier against pathogens, irritants, and foreign particles.
• Arrangement may vary with location of body.
• Some epithelial cells are equipped with cilia responsible for removal of dust and other foreign particles, process known as mucociliary clearance.

Lamina Propria

• Thin layer of connective tissue located immediately beneath epithelial layers.
• Comprising blood vessels, lymphatic vessels, and mucus-secreting cells.
• Responsible for structural integrity and offers wide range of immune cells for destruction of invading pathogens.

Muscularis Mucosae

• Smooth muscle layer situated beneath lamina propria responsible for movements of mucosal layer, aiding in secretion and process of absorption.

Goblet Cells

• Along with three major layers, goblet cells, special secretory cells are also present in mucosal layer.
• Protecting and lubricating mucous membrane.
• Also responsible for trapping and washing away particulate matter such as dust particles and other foreign substances.

Locations of Mucous Membrane in Body

They are found lining body cavities, which majorly include:

Respiratory System

• Includes nasal, olfactory, and bronchial mucosa.
• Abundant in regions of nostrils and throat.

Gastrointestinal Tract

• Includes oral, esophageal, gastric, and intestinal mucosa found in mouth, tongue, esophagus, and intestinal region.

Urogenital System

• Includes urethral and vaginal mucosa found in genitals.

Ocular Mucosa

• Found in region of eyes.

Functions of Mucous Membrane

General Functions

• Secretion
• Protection
• Absorption
• Primary line of defense: Primary and foremost function of mucosal layer is to protect body as it forms first line of defense of immune system.
  • Forms barrier against foreign particles clearing them out by trapping them in sticky secretion.
  • Lubrication prevents mechanical injury.
  • Also, immune cells and naturally present antibiotics defend against invading pathogens.

Specific Regional Functions

• Oral mucosa: Forms lining over soft palate and floor of mouth. Also provides surface for chewing.
• Esophageal mucosa: Lines inner cavity of esophagus preventing it from abrasion.
• Gastric mucosa: Produces mucus, digestive enzymes, and cells that stimulate acid production to break down food in stomach.
  • Also protects stomach and bladder from abrasive effects of highly acidic in stomach or alkaline environments.
• Intestinal mucosa: Forms gut lining aiding in absorption of nutrients and water from foods.

Mucosal Immunity

Mucosal immunity, also known as mucosal-associated lymphoid tissue (MALT), is part of immune system of body that provides defense at surface of mucous membrane protecting host from potentially harmful pathogens in environment.

• Associated with mucosal sites such as gut mucosa comprising Peyer's Patches (PPs), intestinal lamina propria, cryptopatches, and lymphoid follicles.
• Can be broadly categorized based on location as:
  • Gut-associated lymphoid tissue (GALT)
  • Nasopharynx-associated lymphoid tissue (NALT)
  • Bronchus-associated lymphoid tissue (BALT)
  • Conjunctiva-associated lymphoid tissue (CALT)
  • Lachrymal duct associated (LDALT)
• IgA cells are most common B cells involved in immunity exhibited by mucosal layer.
• MALT majorly functions to produce and secrete IgA antibodies across surface of mucosa.
• Also, T cells of thymic origin play role in immunity majorly associated with regulation of differentiation of IgA precursors.
• IgA can neutralize food antigens along with inhibition of commensal and bacterial pathogens.

Common Disorders Affecting Mucous Membranes

• Oral and genital herpes
• Oral thrush and genital yeast infections
• Genital and oral lichen planus
• Oral bullous pemphigoid
• Oral pemphigus
• Mouth sores
• Mononucleosis
• Oropharyngeal HPV infection
• Hand, foot and mouth disease
• H. pylori infection
• Infections and inflammations such as gingivitis, bronchitis
• Allergic reactions involving mucous membrane
• Autoimmune diseases affect mucosa

Mucosal Health

Since mucosal membrane is responsible for providing you with first line of defense against various types of infection, mucosal health is highly essential to maintaining healthy status of body.

• Can be well-maintained with proper hydration and nutrition and avoiding potential irritants.

Comparisons with Other Tissues

Comparison with Serous Membranes

Mucous membrane vs serous membrane:

• Location:

  • Mucous membrane lines cavities and structure that open directly to environment.
  • Serous membrane lines closed cavities in body that do not open directly to outside environment.

• Epithelial layer:

  • Mucous membrane stratified or simple squamous epithelium.
  • Serous membrane usually simple squamous epithelium.

• Secretions:

  • Mucous membrane goblet cells secrete mucus.
  • Serous membrane epithelium secretes serous fluid.

• Function:

  • Mucous membrane acts as protective layer against pathogens, irritants, foreign particles; also responsible for mucosal immunity.
  • Serous membrane serous fluid helps in lubrication lowering friction and abrasion during movement of organs in abdominal or thoracic cavity.

Comparison with Cutaneous (Skin) Tissues

Mucous membrane vs cutaneous membrane:

• Location:

  • Mucous membrane lines cavities that are open to outside environment.
  • Cutaneous membrane forms outer surface of body forming skin.

• Epithelial layer:

  • Mucous membrane stratified or squamous epithelium depending upon location.
  • Cutaneous membrane stratified squamous epithelium in epidermis; apical surface keratinized; and layer of connective tissue in dermis layer of skin.

• Secretions:

  • Mucous membrane mucus secretion by goblet cells.
  • Cutaneous membrane sweat gland and sebaceous gland secrete sweat and sebum respectively.

• Function:

  • Mucous membrane protection, immunity.
  • Cutaneous membrane physical barrier, sensory receptors responsible for sensations of touch, pressure, temperature, pain.

Summary

Mucosa or mucous membrane is thick soft tissue lining internal organs and orifices nostrils, ears, lips, urethral opening, anus secreting mucus mucopolysaccharide of mucin from specialized cells. Typical structure includes three sub-layers: outermost non-keratinized epithelial layer of stratified or simple squamous cells forming protective barrier with cilia for mucociliary clearance removing dust; lamina propria thin connective tissue beneath epithelium with blood vessels, lymphatic vessels, mucus-secreting cells and immune cells providing structural integrity; muscularis mucosae smooth muscle beneath lamina propria aiding movements secretion absorption; plus goblet cells trapping dust. Locations include respiratory nasal olfactory bronchial in nostrils throat, gastrointestinal oral esophageal gastric intestinal in mouth tongue esophagus intestines, urogenital urethral vaginal, and ocular. General functions are secretion, protection, absorption and first line defense trapping particles, preventing mechanical injury via lubrication and immune cells antibiotics; specific functions include oral mucosa lining soft palate and chewing surface, esophageal preventing abrasion, gastric producing mucus enzymes acid and protecting from acidic alkaline, intestinal absorbing nutrients water. Mucosal immunity MALT including GALT, NALT, BALT, CALT, LDALT with Peyer's patches, lamina propria, cryptopatches, lymphoid follicles producing IgA B cells and thymic T cells regulating IgA precursors neutralizing food antigens and pathogens. Disorders include herpes, thrush, lichen planus, pemphigoid, pemphigus, mononucleosis, HPV, hand foot mouth, H pylori, gingivitis bronchitis, allergic and autoimmune reactions. Health maintained by hydration nutrition avoiding irritants. Compared to serous membranes lining closed cavities secreting serous fluid for lubrication and cutaneous skin keratinized with sweat sebum for physical barrier and sensation.

References

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