Tongue and Dental Anatomy and Physiology

Tongue Development and Anatomy

• Embryological Development

  • Tongue development begins at approximately $4.5$ weeks in utero.
  • It starts as a swelling arising from the back of the pharyngeal arches, specifically the first pharyngeal arch (mandibular arch).
  • This swelling originates from the future floor of the mouth.
  • Key structures associated with the point of origin include the forebrain, Rathke's pouch, stomodeum, buccopharyngeal membrane, mandibular arch, heart, foregut, and notochord.

• Musculature

  • The muscles of the tongue are controlled willfully and are classified as skeletal muscle or voluntary striated muscle.
  • They are categorized into two primary groups:
    1. Intrinsic muscles.
    2. Extrinsic muscles.

• Epithelial Covering

  • The tongue is covered with stratified squamous epithelium.
  • Ventral (Lower) Surface: Characterized by thin epithelium.
  • Dorsal (Upper) Surface: Characterized by thick epithelium, which is parakeratinized to keratinized.

Lingual Papillae

• The dorsum of the tongue contains four types of elevated structures called papillae:

• 1. Circumvallate (Vallate) Papillae

  • There are approximately $13$ of these elevations.
  • They are arranged in a V-shaped row facing posteriorly, dividing the posterior $\frac{1}{3}$ of the tongue from the anterior $\frac{2}{3}$.
  • Microscopically, they rest in troughs.
  • They contain many taste buds around their lateral surfaces.
  • Small salivary glands known as the glands of Von Ebner lie beneath these papillae to wash them clean.

• 2. Fungiform Papillae

  • Located on the anterior $\frac{2}{3}$ of the tongue.
  • Appear as tiny, round, red, raised spots.
  • Contain taste buds.
  • Located only on the upper portion of the tongue and not on the sides (lateral surfaces).
  • In younger children, they often appear redder.

• 3. Filiform Papillae

  • Cover the remainder of the anterior $\frac{2}{3}$ of the tongue.
  • Appear as slender, threadlike, pointed projections.
  • These do not contain taste buds.
  • They provide tactile sensation, allowing one to know something is present (similar to a cat's tongue).
  • Hairy Tongue: A condition where the epithelia on these papillae grow very long.
  • Glossitis: A condition where the epithelia are lost, causing the surface to become very smooth; often associated with vitamin deficiencies.

• 4. Foliate Papillae

  • Located on the posterior $\frac{1}{3}$ of the tongue and on the lateral (side) surfaces, close to the vallate papillae.
  • Visible as a roughened surface when the tongue is pulled out and to the side.
  • Contains few taste buds that are not well developed.
  • This is a critical area for oral cancer screenings, as tissue folds can make it difficult to visualize accurately.

Lingual Tonsils and Systemic Health

• Lingual Tonsils

  • Located near the midline on the dorsum, just behind the circumvallate papillae at the base of the tongue.
  • Composed of lymphoid tissue, similar to palatine tonsils.
  • Provides a defense mechanism against infection.
  • They become reddened and enlarged during episodes of infection.

• Clinical Significance

  • The tongue is considered a strong indicator of a person's overall systemic health.

Properties and Functions of Teeth

• Core Functions

  • Digestion: Teeth break down food physically to aid in the digestive process.
  • Protection: They form a hard physical barrier to protect soft oral cavity tissues.
  • Mastication: Biting and tearing food.
  • Speech: Aids in phonetics and clear communication.
  • Aesthetics: Appearance is a strong factor in physical attraction.

• Anatomical vs. Clinical Definitions

  • Anatomic Crown: The entire portion of the tooth covered by enamel, regardless of whether it has erupted through the gums.
  • Clinical Crown: The part of the crown visible above the gingiva; if a tooth is unerupted, it has no clinical crown.
  • Anatomic Root: The part of the tooth covered by cementum.
  • Clinical Root: The part of the tooth that is not visible in the mouth (underneath the gingival line).

• Junctions

  • Cementoenamel Junction (CEJ): Also called the cervical line, it is the line formed by the junction of the cementum and the enamel.
  • Dentinocemental Junction (DCJ): The union of the cementum and the dentin.

• Tooth Eruption

  • Eruption is the process of a tooth moving through surrounding tissues.
  • As the tooth erupts through bone and gum tissue, the clinical crown gradually appears longer until the entire anatomic crown is exposed.

Root Structure and Attachment

• Root Types

  • A tooth may have a single root or multiple roots.
  • Bifurcation: Division of a root into two segments.
  • Trifurcation: Division of a root into three segments.
  • Furcal Region: The area located between the roots.

• Anchorage and Support

  • Alveolar Process: The portion of the jaw that supports the teeth.
  • Alveolus: The bony socket in which the tooth fits.
  • Maxillary Teeth: Located in the upper jaw, anchored in the maxillary bone.
  • Mandibular Teeth: Located in the lower jaw, anchored in the mandible.
  • Function of Roots: Roots anchor the tooth in bone. Longer and wider roots offer more resistance to displacement and provide more surface area for periodontal ligaments (PDL) to attach.

Tissues of the Tooth

• 1. Enamel

  • Covers the anatomic crown; hardest tissue in the human body and the most densely mineralized.
  • Composition: $96\%$ inorganic matter, $4\%$ organic matter and water.
  • Thickness: Thickest at the tip (cusps or incisal edge) and thinnest at the cervical line.
  • Color: Thicker enamel appears whiter; thinner enamel is more translucent, allowing dentin to show through.
  • Variations: People with darker skin may have brownish, grayish, or yellowish tones. Individuals with red or auburn hair may have reddish or brown-red tones in their enamel, most visible near the gingival area.
  • Aging: Teeth darken with age as enamel thins and secondary dentin forms.

• 2. Dentin

  • Forms the main portion or body of the tooth, located under enamel and cementum.
  • Composition: $70\%$ inorganic matter, $30\%$ organic matter and water.
  • Properties: Hard, dense, calcified tissue; yellow in color and elastic in nature. It is softer than enamel but harder than cementum and bone.
  • Secondary Dentin: Formed after eruption throughout the normal aging process in response to normal attrition and wear; it grows very slowly within the pulp.
  • Reparative Dentin: Laid down specifically in response to caries (decay) or trauma.

• 3. Cementum

  • Covers the root; thin at the cervical line and thicker at the apex.
  • Composition: $45\%$ inorganic matter, $55\%$ organic matter.
  • Function: Provides a medium for attaching the tooth to the alveolar bone.
  • Cementoblasts: Cells that produce cementum.
  • Cellular Cementum: Found in the apical $\frac{1}{3}$ of the root; can reproduce itself to compensate for attrition.
  • Acellular Cementum: Covers most of the anatomic root; its primary function is part of the attachment system via Sharpey's fibers.

• 4. Dental Pulp

  • The only soft tissue of the tooth, housed in the center and surrounded by dentin.
  • Components: Blood vessels, lymph vessels, connective tissue, nerve tissue, and odontoblasts.
  • Odontoblasts: Dentin-forming cells that line the pulp cavity walls; they lay down primary, secondary, and reparative (tertiary) dentin.
  • Sensory and Maintenance: Blood vessels provide nourishment and white blood cells to fight bacteria. Lymph tissue filters fluids. Nerves in the pulp respond only to pain, not to cold or heat. Pressure is sensed by the periodontal ligament (PDL) outside the tooth.
  • Divisions of the Pulp Cavity:
    1. Pulp Chamber: Coronal portion of the tooth.
    2. Pulp Canals: Root portion of the tooth.
    3. Pulp Horns: Extensions into each cusp of the tooth.
  • Apical Foramen: The opening at the root tip where vessels and nerves enter.

Tooth Classifications and Morphology

• Functional Categories

  1. Incisors (8 total): Anterior teeth designed to cut. They have an incisal edge and a shovel-shaped lingual surface to guide food.
  2. Canines (4 total): Anterior teeth designed to hold, grasp, and tear. They are the longest teeth with the longest roots (maxillary canines are the longest in the dentition). They have triangular roots in cross-section to resist displacement and protect the TMJ during lateral jaw movements.
  3. Premolars (8 total): Posterior teeth (also called bicuspids) designed to hold and grind food. They usually have two or three cusps; buccal cusps hold food, and lingual cusps grind it.
  4. Molars (12 total): Posterior teeth designed to chew and grind. They have four or five cusps that interlock for crushing. They are larger than premolars and differ between the maxillary and mandibular arches.

• Summary of Basic Food Processing Functions

  • Cutting: Incisors.
  • Holding/Grasping: Canines and Premolars.
  • Grinding/Chewing: Premolars and Molars.

Dental Surfaces and Divisions

• Terminology of Surfaces

  • Facial: Faces the cheek (Buccal on posterior teeth) or lip (Labial on anterior teeth).
  • Lingual: Faces the tongue.
  • Proximal: Faces the neighboring tooth in the same arch.
    • Mesial: Surface closest to the midline of the face.
    • Distal: Surface away from the midline.
  • Occlusal: Biting/chewing surface of posterior teeth.
  • Incisal: Biting/chewing surface of anterior teeth.

• Division into Thirds

  • Proximal surfaces can be divided into facial, middle, and lingual thirds.
  • Facial and lingual surfaces can be divided vertically into mesial, middle, and distal thirds.
  • Horizontal divisions for both facial/lingual and proximal surfaces: Incisal (anterior) or Occlusal (posterior), middle, and cervical thirds.
  • Root divisions: Apical, middle, and cervical thirds.

Morphological Landmarks

• Elevations and Ridges

  • Lobes: Growth centers where calcification begins. Fusion of lobes creates developmental grooves.
  • Cusps: Mounds on the crown that make up major divisions of the occlusal surface (found on canines, premolars, and molars).
  • Tubercle: A small elevation of enamel on the crown (can be lingual, labial, or occlusal).
  • Cingulum: The lingual lobe of maxillary anterior teeth.
  • Ridges: Elevated portions running in a line. All cusps have four ridges: buccal (or labial), lingual, mesial, and distal.
  • Marginal Ridges: Rounded enamel borders forming the mesial and distal shoulders of posterior occlusal surfaces and anterior lingual surfaces.
  • Triangular Ridges: Main ridges on each cusp running from the cusp tip to the central occlusal surface.
  • Transverse Ridges: Reaching across the occlusal surface; the union of two triangular ridges (a buccal and a lingual).
  • Oblique Ridge: A specific ridge found on the occlusal surface of certain teeth.

• Depressions and Openings

  • Fossa: A depression or concavity.
  • Pits: Small pinpoint holes in the enamel, usually at the junction of developmental grooves or within fossae.
  • Developmental Grooves: Lines formed by the fusion of lobes.
  • Supplemental Grooves: Smaller, less distinct grooves.
  • Concavity: A carved-out, cave-like area.
  • Convexity: A bulging-out area (e.g., ridges and cusp tips).

• Lobe Counts by Tooth Type

  • Incisors: $3$ facial, $1$ lingual.
  • Canines: $3$ facial, $1$ lingual.
  • Premolars: $3$ facial, $1$ lingual.
  • Maxillary 1st Molars: $2$ facial, $3$ lingual.
  • Mandibular 1st Molars: $3$ facial, $2$ lingual.
  • 2nd Molars: $2$ facial, $2$ lingual.

Line and Point Angles

• Line Angle: The junction where two surfaces meet.

• Anterior Line Angles: Distolabial, distolingual, linguoincisal, mesiolabial, mesiolingual, labioincisal.

• Posterior Line Angles: Mesio-occlusal, mesiolingual, mesiobuccal, bucco-occlusal, linguo-occlusal, distolingual, distobuccal, disto-occlusal.

• Point Angle: The point where three surfaces meet.

• Anterior Point Angles: Mesiolabioincisal, distolabioincisal, mesiolinguoincisal, distolinguoincisal.

• Posterior Point Angles: Mesiobucco-occlusal, distobucco-occlusal, mesiolinguo-occlusal, distolinguo-occlusal.