Digestive System Part 1: Histology and Physiology

Functions and Anatomy of the Gastrointestinal System

• The gastrointestinal system (GIT) serves several critical biological roles:

  • Prehension: This involves the ingestion of food followed by fragmentation, which is the process of breaking material into smaller parts.
  • Digestion: This is the mechanical and enzymatic breakdown of nutrients, including fermentation.
  • Absorption: The process of taking in nutrients into the body's systems.
  • Synthesis and Secretion: The production and release of substances, such as hormones.
  • Excretion: The removal of waste material from the body.

• The GIT is composed of the following anatomical structures:

  • Mouth and Pharynx: Includes the oral cavity.
  • Oesophagus: The tubular passage connecting the pharynx to the stomach.
  • Stomach: The primary site for enzymatic digestion.
  • Liver and Pancreas: Accessory organs that assist in digestion.
  • Small Intestine: Divided into the duodenum, jejunum, and ileum.
  • Large Intestine: Comprises the caecum, colon, rectum, and anal canal.

• Comparative anatomy involves observing the size differences of organs across species:

  • In the equine, ruminant, and canine GIT, the proportions of the stomach and colon vary significantly.
  • The interior of the ox rumen features several specialized structures: the ventricular diverticulum, dorsal sac, oesophagus, reticular groove, cranial pillar, ruminoreticular fold, reticulum, ruminal atrium, ruminal recess, ventral sac, right accessory pillar, dorsal coronary pillar, caudodorsal blind sac, right longitudinal pillar, caudal pillar, caudoventral blind sac, and ventral coronary pillar.

Embryological Development of the Digestive System

• The origins of the digestive system are derived from three primary germ layers:
  • Endoderm-derived: Gives rise to the epithelium (mucosa), parenchymatous organs (such as the liver and pancreas), and their associated glands.
  • Mesoderm-derived: Gives rise to the submucosa, muscle layers, serosa, and mesentery.
  • Ectoderm-derived: Gives rise to the stratified squamous epithelium found in the oral cavity and anus, as well as the nerve plexi.

Epithelial Distribution in the Digestive Tract

• The lining of the gastrointestinal tract changes based on functional requirements:
  • Stratified squamous epithelium: Lines the tract from the lips to the non-glandular stomach. It also lines the anus.
  • Simple columnar epithelium: Lines the glandular stomach and the entire intestine.

The Oral Cavity and Palate

• The oral cavity is formed by the lips, cheeks, palate, pharynx, and tongue.

• The mucosa is lined by stratified squamous epithelium, which can be either keratinized or non-keratinized depending on the species and mechanical stress.

• The submucosa consists of connective tissue containing salivary glands and tonsils.

• A distinctive feature in ruminants is the presence of a dental pad.

• Histology of Lips and Cheeks:

  • 1. Alveolar bone
  • 2. Connective tissue papilla
  • 3. Dental lamina
  • 4. Dental papilla
  • 5. Dental sac
  • 6. Duct
  • 7. Hair follicle
  • 8. Inner enamel epithelium
  • 9. Intralobular duct
  • 10. Labial gland
  • 11. Mucous acinus
  • 12. Outer enamel epithelium
  • 13. Sebaceous gland
  • 14. Serous acinus
  • 15. Serous demilune
  • 16. Skeletal muscle
  • 17. Stellate reticulum
  • 18. Stratified squamous epithelium (keratinized)
  • 19. Stratum granulosum

• Soft and Hard Palate Structure:

  • 1. Adipose tissue
  • 2. Connective tissue papilla
  • 3. Horn tubule-like structure
  • 4. Lamina propria
  • 5. Loose connective tissue
  • 6. Mixed gland
  • 7. Mucous gland
  • 8. Palatine bone
  • 9. Stratified squamous epithelium (keratinized)
  • 10. Stratum basale
  • 11. Stratum corneum
  • 12. Stratum spinosum
  • 13. Submucosa

Anatomy and Histology of the Tongue

• The tongue (Latin: Lingua; Greek: Glossa) is covered by a mucosa of keratinized stratified squamous epithelium on the dorsal side.

• The dorsal surface is irregular due to small outgrowths called papillae, which serve mechanical and gustatory (taste) functions.

• The tongue contains skeletal muscle arranged in three distinct directions: longitudinal, transverse, and vertical.

• Types of Tongue Papillae:

  • Filiform (papillae filiformes): Slender, conical, and numerous; located on the dorsal surface (body and apex) for mechanical purposes.
  • Fungiform (papillae fungiformes): Small, mushroom-shaped, and slightly raised; located on the dorsum, margins, and parts of the ventral tongue. These contain taste buds.
  • Circumvallate (papillae vallatae): Large, flat, and surrounded by an annular sulcus; found at the transition from the body to the root of the tongue. These contain taste buds.
  • Foliate (papillae foliatae): Folds of mucosa with transverse furrows; located laterally near the root of the tongue. These contain taste buds.

Dentition and Tooth Development

• Tooth structure is categorized into hard and soft tissues:

  • Hard Tissues:
    • Enamel: Covers the external surface above the gumline.
    • Cementum: Covers the external surface below the gumline.
    • Dentin: Situated beneath both the enamel and the cementum.
  • Soft Tissues:
    • Pulp: Contains nerves and loose connective tissue.
    • Periodontal ligament: Connective tissue anchoring the tooth.

• Cellular Formation of Teeth:

  • Ameloblasts: Tall columnar cells responsible for producing enamel.
  • Odontoblasts: Cells that cover the surface of the mesenchymal papilla and produce dentin.
  • Cementoblasts: Cells that produce cementum.
  • Fibroblasts: Cells that form the periodontal ligament.

• Periodontium: The anatomical structures resisting forces applied to teeth, composed of four tissues: gingivae, periodontal ligament, cementum, and alveolar bone.

• Classification of Teeth:

  • Brachydont: Short teeth that cease to grow after eruption. They have a crown, neck, and roots. Found in carnivores, humans, ruminant incisors, and most pig teeth.
  • Hypsodont: Tall teeth that continue to grow after eruption. They have no distinct crown or neck, possessing an elongated body. Found in horses, ruminant cheek teeth, and pig canine teeth.

Salivary Glands and Secretions

• Saliva (Latin: Saliva; Greek: Sialon) consists of water, salts, mucin, enzymes (amylase, maltase, lipase), $IgA$, lysozyme, lactoferrin, and suspended cells (desquamated epithelium, leukocytes, bacteria).

• Saliva functions include digestion, moisture, lubrication, local immunity, and evaporative cooling.

• Gland Classification:

  • Major Glands: Parotid, sublingual, and mandibular.
  • Minor Glands (Intramural): Lingual, palatal, labial, buccal, zygomatic (carnivores), and molar (cats).

• Secretory Units (Adenomeres):

  • Serous: Watery product containing enzymes. Cells have round nuclei and basophilic basal cytoplasm/acidophilic apical zymogen granules.
  • Mucous: Slick, viscous secretion. Cells have flat, heterochromatic basal nuclei and pale, vacuolated (frothy) cytoplasm.
  • Mixed: Contains both mucous and serous cells. Often features serous demilunes, which are crescent-shaped serous cells associated with mucous acinar units.

• Types of Adenomeres:

  • Tubular: Straight or coiled (e.g., sweat glands, stomach glands, colon glands).
  • Acinar: Pie-shaped with a small lumen (e.g., pancreas, salivary glands).
  • Alveolar: Larger luminal space (e.g., mammary gland, prostate, sebaceous glands).

• Duct System:

  • Includes intercalated, intralobular (striated), and interlobular ducts. Myoepithelial cells are present to assist in secretion.

General Organization of Tubular Digestive Organs

• Tubular organs are organized into four tunics:
  1. Mucosa: Consists of three layers: the inner epithelium, the middle lamina propria (loose connective tissue, vessels, lymphatics), and the thin outer muscularis mucosae (smooth muscle).
  2. Submucosa: Loose connective tissue containing vessels, lymphatic nodules, glands, and the submucosal plexus (Meissner's plexus).
  3. Muscularis: Usually composed of two smooth muscle layers: an inner circular and an outer longitudinal layer. It contains the myenteric nerve plexus (Auerbach's plexus).
  4. Serosa or Adventitia:
     • Serosa: Composed of mesothelium and loose connective tissue. Found on organs bordering pleural, pericardial, and peritoneal cavities.
     • Adventitia: Loose or dense connective tissue only. Blends with surrounding fascia (e.g., cervical esophagus).

The Esophagus: Species Variations

• The esophagus is lined with stratified squamous epithelium and contains submucosal glands.
• Species-Specific Characteristics:
  • Horses: Keratinized epithelium; glands in the cranial part; muscularis is striated in the cranial $2/3$ and smooth in the caudal $1/3$.
  • Pigs: Keratinized epithelium; glands in the cranial half; muscularis is striated cranially, mixed in the middle, and smooth in the caudal part.
  • Cattle, Goats, Sheep: Highly keratinized epithelium; glands absent (except pharyngoesophageal junction); muscularis is striated throughout and extends into the reticular sulcus.
  • Dogs: Nonkeratinized epithelium; glands present throughout (extending into the stomach); muscularis is striated throughout.
  • Cats: Nonkeratinized epithelium; glands in the cranial part; muscularis is striated in the cranial $4/5$ and smooth in the caudal $1/5$.

Classification and Structure of the Stomach

• Simple Stomach: Glandular mucosa only (e.g., carnivores and humans).
• Compound Multi-chambered Stomach: Contains a stratified squamous forestomach and a glandular true stomach (e.g., ruminants).
• Composite Stomach: Contains both glandular and stratified squamous mucosa (e.g., pig and horse).

The Ruminant Forestomach and Abomasum

• Rumen: The largest fermentation vat with a complex microbial community. Lined with low papillae for passive absorption of $VFA$ (volatile fatty acids). Functions in mixing, eructation, and regurgitation.
• Reticulum: Features a honeycomb mucosa with conical papillae and small segments of smooth muscle along the ridges. Acts as a sieve and facilitates rumination (cud formation).
• Omasum: The "butcher's bible," containing many layered leaves (papillae). It removes excess water and reduces particle size before moving ingesta to the abomasum.
• Abomasum: The "true stomach" with a glandular lining. Responsible for acid and enzyme digestion.

Questions & Discussion

• What type of lining does the esophagus have?
  • Answer: Stratified squamous (can be non-keratinized or keratinized depending on diet and species).
• What type of muscle is found in the esophagus?
  • Answer: Varies by species; can be skeletal only, smooth only, or a combination of skeletal and smooth.
• What type of external covering does the esophagus have?
  • Answer: Adventitia for most of its length, though serosa may be present in the thoracic cavity or near the stomach.