Animal Digestion and Teeth

Introduction to Digestion

  • All animals need energy and nutrients for daily activities, cellular work, growth, and development.
  • This is achieved by breaking down and absorbing nutrients from consumed food.
  • The process starts with consuming a meal.

Dietary Categories:

  • Herbivores: Eat plants.
    • Rely on microbial fermentation in their gastrointestinal (GI) tract to convert plant material into usable nutrients and energy.
    • Examples: horses and cattle.
  • Carnivores: Eat meat.
    • The GI tract itself converts meals into nutrients and energy without microbial fermentation.
    • Example: cats.
  • Omnivores: Eat both plants and meat.
    • Example: humans and pigs.
  • Dogs:
    • Dental anatomy similar to cats (sharp canine teeth for tearing flesh and premolars/molars for cutting meat)
    • Evolved to be omnivores, utilizing both plant and meat material.

Digestive System Differences in Domestic Animals:

  • Ruminants (cattle, sheep, goats):
    • Herbivores with large microbial fermentation chambers to partially break down plant materials before reaching the true stomach.
  • Nonruminant herbivores (horses):
    • Hindgut Fermenters: Have a well-developed cecum at the junction of the small and large intestines for microbial breakdown of plant materials.
  • Carnivores:
    • Have a small, inconspicuous cecum due to the insignificant role of microbes in their digestion.

Digestion as an External Process

  • Digestion occurs inside the gastrointestinal tract.
  • Anything within the GI tract is technically considered "outside" the body.
  • During embryonic development, a flat sheet of cells folds to form a tube, which becomes the lumen of the GI tract.

Basic Structure of the Gastrointestinal Tract

  • The gastrointestinal (GI) tract runs from the oral cavity to the anus.
  • Includes structures such as the oral cavity, esophagus, stomach (gastric), small intestine, and large intestine (enteric).
  • Most of the GI tract wall consists of four layers of tissue (from the lumen outward):
    • Mucosa
    • Submucosa
    • Muscular layers (circular and longitudinal)
    • Serosa
  • Variations exist within these layers depending on the specific part of the GI tract.

Mucosa

  • Innermost layer with three parts:
    • Epithelium
      • Lines the lumen.
      • Varies based on location.
      • Stratified squamous epithelium near the mouth and anus for protection (many layers of cells).
      • Simple columnar epithelium in the rest of the GI tract. These epithelial cells are connected to one another with tight junctions, which helps to protect the animal from unwanted substances from entering the body.
    • Lamina propria
      • Loose areolar connective tissue.
      • Contains blood and lymph vessels and glands.
    • Muscularis mucosae
      • Thin layer of smooth muscle.
      • Forms folds in the mucosa to increase surface area for absorption.

Submucosa

  • Lies underneath the mucosa.
  • Consists of dense connective tissue.

Muscular Layer

  • Third layer.
  • Typically consists of two smooth muscle layers:
    • Inner circular layer
    • Outer longitudinal layer
  • Skeletal muscle is present in the oral cavity, pharynx, and esophagus (in some species).
  • The external anal sphincter is made of skeletal muscle for controlling defecation.

Serosa or Adventitia

  • Outermost layer.
  • Made of loose connective tissue.
  • Serosa: when the GI tract is suspended from the body cavity.
  • Adventitia: when the GI tract is surrounded by other tissue.

Regulation of Gastrointestinal Function

  • Regulated by two control systems:
    • Central nervous system and the endocrine system
    • Enteric or intrinsic nervous system with an intrinsic endocrine/paracrine component (unique to the GI tract).

Enteric Nervous System

  • Commonly known as the "brain of the gut."
  • Consists of receptors, sensory neurons, interneurons, and motor neurons.
  • Controls motor and secretory functions.
  • Contains pacemaker cells.
  • Influenced by the autonomic nervous system.
    • Parasympathetic branch: enhances digestive processes.
    • Sympathetic branch: inhibits digestion.
  • Afferent neurons:
    • Travel to the central nervous system (CNS) from receptors in the gut.
    • Monitor GI tract tension and chemical conditions.
    • Associated with the autonomic nervous system, providing sensory information to the CNS.

Enteric Nervous System Plexuses

  • Two plexuses make up the enteric nervous system:
    • Submucosal plexus (Meissner's plexus)
      • Located in the submucosa.
      • Controls secretions and blood flow in the GI tract.
    • Myenteric plexus (Auerbach's plexus)
      • Runs between the circular and longitudinal layers of smooth muscle.
      • Controls movements of the GI tract through local reflexes.

Intrinsic Endocrine and Paracrine Systems

  • Regulatory function rather than a digestive one.
  • Examples:
    • Cholecystokinin: inhibits gastric emptying.
    • Gastrin: stimulates stomach motility.
  • Endocrine cells:
    • Secrete hormones directly into the bloodstream.
    • Have effects at distant sites.
  • Paracrine cells:
    • Secrete substances into the interstitial fluid.
    • Travel by diffusion and affect nearby cells.

Oral Cavity, Pharynx, and Esophagus

  • The entrance to the gastrointestinal tract is the mouth, also known as the oral cavity or buccal cavity.
  • Contains the teeth, tongue, and everything else required to ingest food.

Oral Cavity Parts

  • Two parts:
    • Vestibule: space between the outer surface of the teeth and the surrounding lips and cheeks.
    • Oral cavity proper: space bordered by the inner surface of the teeth laterally and rostrally and by the hard and soft palate dorsally.
  • Oral fissure: the opening into the oral cavity.
  • Lips: mark the boundary of the oral fissure.
    • Contain both long, tactile hairs and regular hairs at the margins.
    • Maneuverability varies by species.
    • Assist in prehension (bringing food into the oral cavity).
  • Cheeks: form the lateral walls of the vestibule.
  • Lips and cheeks consist of:
    • Outer layer of haired skin
    • Middle layer of muscles and fibroelastic tissue
    • Inner layer of mucosa that lines the vestibule and oral cavity
  • Middle muscular layer:
    • Consists of the muscles of mastication (chewing).
    • Contributes to the biting strength of the mouth.
  • Philtrum: the cleft that divides the two halves of the upper lip.
    • Depth and prominence vary by species (deeper in carnivores, shallow in horses).

Palate

  • Dorsal border of the oral cavity (roof of the mouth).
  • Two distinct parts:
    • Hard palate
      • More rostral part.
      • Made up of the palatine, maxillary, and incisive bones.
      • Covered by a mucous membrane.
      • Several transverse elevations create ridges.
    • Soft palate
      • The caudal part of the palate.
      • Made up of muscle and connective tissue.
      • Divides the pharynx (throat) into the oropharynx (connects with the mouth) and the nasopharynx (leads into the nasal passageway).
      • Raised to close off the nasal passage during swallowing to prevent food from entering.

Teeth

  • Embedded in the upper maxilla bone and lower mandible bone.
  • Found in sockets or cavities called alveoli.
  • Held in place by the periodontal ligament.
  • Assist with the mechanical breakdown of food.
  • Food is broken into smaller pieces by the tearing, cutting, and crushing action of the teeth during mastication (chewing).
  • Crown: the part of the tooth that projects above the gingiva (gums).
  • Root: embedded in the alveoli below the gingiva.
  • Apex: the tip of the root where blood vessels and nerves enter the tooth.
  • Neck: the area where the crown and the root meet.

Tooth Surfaces

  • Important to know for dental cleanings to properly document lesions.
    • Buccal: outer surface facing the cheeks.
    • Labial: outer surface facing the lips.
    • Lingual: inner surface facing the tongue.
    • Palatal: inner surface facing the soft palate.
    • Mesial: the edge of the tooth facing the midline of the dental arch.
    • Distal: the edge facing away from the center of the dental arch.
    • Occlusal/masticatory: surfaces on the upper and lower teeth that come together when the mouth is closed.
    • Coronal: toward the crown of the tooth.
    • Apical: toward the root of the tooth.

Tooth Structure

*Enamel:
*Hardest substance in the body.
*Covers the crown
*Dentin:
*Forms the bulk of the tooth
*As hard as bone, but not nearly as hard as enamel.
*Surrounds an inner area called the pulp cavity.
*Pulp cavity:
*Contains the blood supply and nerves which supply the tooth

Classification of Teeth

  • Brachyodont teeth:
    • Relatively small crowns and well-developed roots.
      *Found in carnivores, humans, pigs (except for tusks, and in ruminant incisors
      *Apices of roots are open for only a finite period of time, hence do not continually grow
  • Hypsodont teeth:
    • Grow continuously during most of the animal's life due to a large reserve of crown beneath the gingiva.
    • Found in horse incisors and cheek teeth, boar's canine teeth (tusks), ruminant cheek teeth, and some teeth of rodents and lagomorphs.
    • Divided into two types:
      • Radicular hypsodont:
        *Apices of roots remain open for a significant part of the horse's life, leading to continued growth but eventually the apices close and the teeth stop growing.
        *Cheek teeth of the horse.
        *Wear is offset by continued eruption until growth ceases.
        *Points and hooks can develop on the teeth due to uneven wear on the occlusal surfaces, requiring filing down in a process called floating to create a level occlusal surface.
      • Aradicular hypsodont:
        *Incisor and cheek teeth of lagomorphs and some rodents.

Clinical Application: Tooth Resorption

  • Feline odontoclastic resorptive lesions (tooth resorption):
    • Initially discovered in the necks of teeth (hence,