35. Histology - Microanatomy of the Oral Cavity

Oral Cavity Lining

• The oral cavity is lined by a mucosa, submucosa, and minor salivary glands
• There are 3 types of oral mucosa:
  • Masticatory mucosa: Found on the hard palate and gingiva
  • Lining mucosa: Found on lips, cheeks, floor of mouth, inferior surface of tongue, and soft palate
  • Specialized mucosa: Found on dorsal surface of the tongue
• Masticatory mucosa exhibits stratified squamous keratinized epithelium
• Lining mucosa exhibits stratified squamous non-keratinized epithelium
• Specialized mucosa exhibits stratified squamous keratinized epithelium with taste buds
• The submucosa contains dense irregular connective tissue
• Submucosa provides attachment to muscle (cheeks and lips) or bone (palate, dental arches)
• The submucosa contains minor salivary glands which are compound tubuloalveolar exocrine glands.

Lip

• The lip is the entry point to the alimentary canal
• The lip has a cutaneous surface and a mucosal surface
• The cutaneous surface is covered with stratified squamous keratinized epithelium and hair follicles
• The mucosal surface is covered with stratified squamous non-keratinized epithelium
• The free red (vermilion) border of the lip is highly vascularized and lacks keratinized epithelium

Tongue

• The tongue is a muscular organ that plays a role in speech, mastication, and swallowing
• The dorsal surface of the tongue is covered with lingual papillae, which include:
  • Filiform papillae: Most numerous, conical, keratinized. They provide a rough surface to help in manipulating food
  • Fungiform papillae: Mushroom-shaped, contain taste buds, and are vascularized, giving them a reddish color
  • Circumvallate papillae: Large, circular, and surrounded by a deep groove. Each papilla contains multiple taste buds and is associated with Von Ebner's glands
  • Foliate papillae: Located on the lateral borders of the tongue, contain taste buds, and are associated with mucous glands.
• The ventral surface of the tongue is smooth and lacks papillae

Taste Buds

• Taste buds contain epithelial cells called taste receptor cells, which are responsible for detecting tastants
• Taste receptor cells are renewed every 10 days
• The taste bud is surrounded by supportive cells
• Taste buds are connected to nerve fibers that transmit taste signals to the brain

Major Salivary Glands

• Major salivary glands are large paired multicellular exocrine glands that produce saliva
• The 3 major salivary glands are:
  • Parotid glands: Largest, located below and in front of the ear, ducts enter the oral cavity opposite the 2nd upper molar
  • Submandibular glands: Located under either side of the floor of the mouth, ducts run along the floor of the mouth and empty just lateral to the frenulum of the tongue
  • Sublingual glands: Smallest, located in the floor of the mouth anterior to the submandibular glands. Ducts empty into the submandibular ducts and directly onto the floor of the mouth
• Saliva is a mixture of water, electrolytes, enzymes, and mucus
• The major salivary glands are composed of secretory units called acini
• Acini contain 2 types of cells:
  • Serous cells: Produce a watery secretion containing enzymes like amylase
  • Mucous cells: Produce a viscous secretion containing mucins
• Myoepithelial cells are located in the acini, aid in the expulsion of saliva, and are contractile cells
• Acini are further classified into 3 types:
  • Serous acini: Produce serous secretions
  • Mucous acini: Produce mucous secretions
  • Mixed acini: Contain both serous and mucous cells
• Serous acinar cells, in conjunction with plasma cells, are involved in the secretion of sIgA
• The duct system of the major salivary glands transports saliva to the oral cavity
• The ducts include:
  • Intercalated ducts: Line the acini, lined by simple cuboidal epithelium
  • Striated ducts: Lined by columnar cells with numerous mitochondria and basal infoldings
  • Excretory ducts: Lined by stratified cuboidal epithelium
• Saliva production is regulated by neural and hormonal signals
• Parotid glands are predominantly serous, submandibular glands are mixed serous and mucous, and sublingual glands are predominantly mucous.

Teeth

• Teeth are hard, calcified structures that play a role in mastication
• The tooth consists of a crown and a root
• The crown is the part of the tooth that is covered by enamel
• The root is the part of the tooth that is embedded in the alveolus of the alveolar bone and covered by cementum
• The pulp cavity contains loose connective tissue, blood vessels, and nerve fibers
• The pulp cavity is connected to the apical foramen
• The dentin, the main component of the tooth, is a bone-like material produced by odontoblasts
• Enamel, the hardest substance in the body, is acellular and covers the crown
• Enamel rods are comprised of calcium hydroxyapatite phosphate crystallite
• Cementum, a bone-like material, covers the root of the tooth

Oral Mucosa

• The oral mucosa lines the oral cavity and is divided into three types: lining mucosa, masticatory mucosa, and specialized mucosa.
• Lining mucosa is found on the lips, cheeks, floor of the mouth, inferior surfaces of the tongue, and soft palate. It is generally nonkeratinized, but may be parakeratinized in some areas.
• Masticatory mucosa is found on the hard palate and gingiva. It is keratinized and has a thicker epithelium.
• Specialized mucosa is found on the dorsal surface of the tongue and contains lingual papillae and taste buds.

Lingual Papillae

• Filiform papillae: most numerous, small, conical, and keratinized. They do not contain taste buds.
• Fungiform papillae: mushroom-shaped, contain taste buds on their superior surface.
• Circumvallate papillae: large, dome-shaped, and contain taste buds in the grooves that surround them.
• Foliate papillae: leaf-like projections on the lateral margins of the posterior tongue, containing taste buds.

Taste Buds

• Taste buds are associated with all types of papillae except filiform
• They are specialized sensory organs that detect taste.
• Taste buds contain gustatory cells, basal cells, and supporting cells.

Salivary Glands

• Major salivary glands: parotid, submandibular, and sublingual glands.
• Minor salivary glands: labial, buccal, lingual, and palatine glands.
• Serous glands: produce a thin, watery secretion rich in enzymes.
• Mucous glands: produce a thick, viscous secretion containing mucin.
• Mixed glands: contain both serous and mucous cells.

Lip

• The lip is a mucocutaneous junction, meaning that it is where the skin of the face transitions to the oral mucosa.
• The lip has three distinct zones: outer cutaneous surface, vermilion border, and inner oral mucosal surface.
• The vermilion border is characterized by keratinized epithelium and finger-like connective tissue projections called stromal papillae.

Tongue

• The tongue is a muscular organ that projects into the oral cavity.
• It is composed of skeletal muscle fibers arranged in a complex pattern.
• The tongue is responsible for taste, mastication (chewing), swallowing, and speech.

Parotid Gland

• The parotid gland is the largest salivary gland.
• It is located anterior to the ear.
• It is a serous gland that produces a watery secretion rich in enzymes.
• The parotid gland is drained by the parotid duct (Stensen’s duct), which opens into the oral cavity near the second upper molar.

Submandibular Gland

• The submandibular gland is located beneath the mandible.
• It is a mixed gland, producing both serous and mucous secretions.
• Its secretions are mostly serous.
• The submandibular gland is drained by the submandibular duct (Wharton’s duct), which opens into the oral cavity beneath the tongue.

Sublingual Gland

• The sublingual gland is the smallest of the major salivary glands.
• It is located beneath the tongue.
• It is a mixed gland, producing both serous and mucous secretions.
• Its secretions are mostly mucous.
• The sublingual gland is drained by multiple small ducts (ducts of Rivinus), which open into the oral cavity beneath the tongue.