Human Digestive System: Oral Cavity

Oral Cavity

• Functions: ingestion, mastication, lubrication by saliva
• Consists of: oral cavity, alimentary canal, glandular portion
• Lined by oral mucosa: stratified squamous epithelium, irregular dense fibrous connective tissue (DWFCT)

Classification of Oral Mucosa

• Masticatory mucosa: keratinized stratified squamous epithelium, resists abrasion (e.g., tongue)
• Lining mucosa: non-keratinized stratified squamous epithelium, overlies thick submucosa with minor salivary glands (e.g., lips)
• Specialized mucosa: has intraepithelial "taste buds" to perceive taste (e.g., tongue)

Lips

• Junction of integument and digestive system
• Three regions: skin part, vermilion zone, mucous (internal) part
• Ruminants and horses: keratinized; carnivores and pigs: non-keratinized

Lingual Papillae

• Mucosal projections on the tongue
• Four types:
  • Filiform papillae: slender, conical, keratinized stratified squamous epithelium
  • Fungiform papillae: mushroom-shaped, non-keratinized stratified squamous epithelium with taste buds
  • Foliate papillae: along the posterolateral aspect of the tongue, with glands of von Ebner
  • Circumvallate papillae: largest, wall-like, with a deep moat-like furrow and glands of von Ebner

Teeth

• Highly mineralized structure in the oral cavity
• Important for food procurement, cutting, and crushing
• Anatomically divided into:
  • Crown: visible part covered by enamel, the hardest substance in the body
  • Neck: junction between crown and root
  • Root: part embedded in the alveolus or socket of the mandible and maxilla
• Mineralized components:
  • Enamel: hardest substance in the body, 96% calcium hydroxyapatite
  • Dentin: forms the bulk of the tooth, 65-70% calcium hydroxyapatite
  • Cementum: overlies dentin of the root, approximately 45-50% calcium hydroxyapatite

Digestive System: Alimentary Canal

• General structure:
  • Tunica mucosa: epithelium, lamina propria, muscularis mucosae
  • Tunica submucosa: houses esophageal glands
  • Tunica muscularis: inner circular, outer longitudinal
  • Tunica adventitia/serosa
• Regions:
  • Esophagus: muscular tube, modified for involuntary movement of bolus to stomach
  • Stomach: enlarged part of the alimentary canal, initiates enzymatic and hydrolytic breakdown of food
  • Small intestine: where most nutrient absorption occurs
  • Large intestine: absorbs water and electrolytes, stores and eliminates waste

Esophagus

• Modified for involuntary movement of bolus to stomach
• Histophysiology:
  • No anatomical sphincter, but two physiological sphincters (pharyngoesophageal and gastroesophageal)
  • Tunica mucosa: stratified squamous non-keratinized epithelium
  • Tunica submucosa: houses esophageal glands proper
  • Tunica muscularis: inner circular, outer longitudinal
  • Tunica adventitia/serosa: houses esophageal glands in the cranial part

Stomach

• Enlarged part of the alimentary canal

• Initiates enzymatic and hydrolytic breakdown of food

• Produces chyme (gastric juices + partially digested food) and intermittently empties into the duodenum

• Regions:

  • Non-glandular region of tunica mucosa
  • Glandular region of tunica mucosa:
    • Cardiac gland region
    • Fundic gland region
    • Pyloric gland region### Stomach

• Entele stomach: primary fold/papillae separate the mucosal surface into shallow compartments, further divided by shorter secondary and tertiary folds.

• Muscularis mucosae: only in upper part of larger primary folds, absent in small folds/papillae.

C omasum

• Large longitudinal primary papillae.
• Epithelium: keratinized stratified squamous.
• Muscularis mucosae: present and continuous, with interdigitating smooth muscle bundles in papillae.

D abomasum

• Glandular portion of the ruminant stomach.
• Similar in structure to the simple stomach.

Small Intestines

• Longest section of the digestive tract.
• Main function: digestion of gastric contents and absorption of nutrients.
• 3 parts: duodenum, jejunum, and ileum.
• Surface modifications:
  • Plicae circulares (valves of Kerckring): transverse folds of submucosa and mucosa.
  • Villi: finger-like or oak leaf-like protrusions in the lamina propria.
  • Microvilli: modifications of the apical surface of epithelial cells.

Large Intestines

• Site for microbial action on the ingesta.
• Absorption of water, vitamins, and electrolytes.
• Secretion of mucus.
• Characteristics:
  • Absence of villi.
  • Increasing goblet cells and intestinal glands towards the caudal end.
  • Plicae circulares absent (replaced by longitudinal folds).
  • Conspicuous diffuse and dense lymphoid tissue.

Cecum

• Varies in size among different species.
• Similar in structure to the small intestine, except for the absence of villi.

Colon

• Thicker mucosa than the small intestine.
• Smooth mucosal surface.
• Increased number of goblet cells.
• Submucosa often distended by lymphatic tissue.

Rectum

• Smooth mucosa.
• Similar to the colon, but with more goblet cells.

Anal Canal

• Terminal segment of the digestive tract.
• Simple columnar epithelium of the rectum changes abruptly to non-keratinized stratified squamous.

Avian Digestive System

• Crop: sac-like diverticulum of the esophagus, storage organ, histologically similar to the esophagus.
• Proventriculus: "glandular stomach", characterized by macroscopic papillae with numerous microscopic folds, and proventricular glands.
• Ventriculus (Gizzard): "muscular stomach", lining is referred to as the cuticle or koilin membrane.
• Cloaca: 3 regions - coprodeum, urodeum, and proctodeum, all with similar structures, and villi.

Glands Associated with the Digestive System

• Salivary glands: made up of a series of secretory units, controlled by the autonomic nervous system.
• Major salivary glands:
  • Parotid: paired, located anterior and inferior to the external ear, predominantly serous glands.
  • Mandibular (Submandibular/Maxillary): paired, located inferior to the mandible, predominantly serous type in most animals, but predominantly mucous in carnivores.
  • Sublingual: aggregates of smaller glands located inferior to the tongue, mucous cells predominate.

Liver

• Largest gland in the body.
• Capsule and stroma: each lobe covered by serosa, and stroma consists of connective tissue.
• Liver acinus: classic lobule, portal lobule, and liver acinus.

Gallbladder

• Hollow pear-shaped organ attached to the liver of most animals.
• Epithelium: tall columnar.
• Lamina propria: blends with tunica submucosa.
• Muscularis mucosae: absent.
• Tunica muscularis: circular bundles of smooth muscle fibers.

Pancreas

• Encapsulated, lobulated, compound tubuloacinar gland.
• Exocrine part: produces enzymes such as amylase, lipase, and trypsin.
• Endocrine part: produces mainly insulin and glucagon.### Respiratory Portion
• Completely intrapulmonary
• Caudal: transition zone; stratified cuboidal to non-ciliated pseudostratified columnar
• Special features: Vibrissae (tactile hair)

Respiratory Region

• Pseudostratified ciliated columnar (with goblet cells)
• Caudal 2/3 of nasal cavity proper, except at olfactory region
• Mucosa: More vascular than the mucosae of the cutaneous, transitional, or olfactory regions

Olfactory Region

• Ciliated pseudostratified columnar epithelium
• Dorsocaudal portion of the nasal cavity
• Including some of the surfaces of: Ethmoid conchae, dorsal nasal meatus, and nasal septum
• Difference from adjacent respiratory mucosa:
  • Thicker epithelium
  • Numerous tubular glands
  • Many bundles of nonmyelinated nerve fibers in the lamina propria
• 3 primary cell types:
  • A. Sustentacular cells
  • B. Neurosensory olfactory cells (bipolar neurons)
  • C. Basal cells (horizontal and globose)
• Kept moist by a watery secretion produced by serous tubuloacinar olfactory glands (Bowman’s glands)

Trachea

• The largest in diameter and length of these tubes
• Provides the air passageway between the larynx and the bronchi
• Incomplete C-shaped hyaline cartilage rings
• Bridged by elastic & smooth muscle fibers
• Ends of cartilage rings face posteriorly (adjacent to esophagus)
• The trachea terminates by bifurcating into two primary bronchi

Bronchioles

• Tubes of decreasing diameters; no cartilaginous support
• Epithelial lining:
  • Larger bronchioles: ciliated pseudostratified columnar (respiratory epithelium) with few goblet cells
  • Smaller bronchioles: ciliated simple columnar or cuboidal; occasional non-ciliated club cells (Clara cells) replacing the goblet cells
• Manufacture:
  • Club cell secretory protein: believed to protect the epithelial lining
  • Surfactant-like substance: helps prevent these flimsy conduits from collapsing by reducing surface tension

Bronchi

• Replaced by irregular hyaline cartilage plates
• As the bronchi continue to divide and decrease in size, the cartilage plates also decrease in size and number
• Extrapulmonary bronchi (Primary bronchi) → intrapulmonary bronchi (Secondary bronchi)

Respiratory Portion (continued)

• AVIAN RESPIRATORY SYSTEM
  • Nasal cavity: epithelia similar to mammals
  • Respiratory epithelium: groups of goblet cells form intraepithelial glands
  • Infraorbital sinuses: drain into the nasal cavity; lined by respiratory epithelium
  • Larynx: simple; devoid of vocal folds; produces little sound
  • Syrinx: voice box; specialized region of the tracheobronchial junction
  • Trachea: cartilages form complete rings encircling the airway; the cartilage rings overlap and interlock with adjacent rings
  • Trachealis muscle: absent

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Endocrine System

Pituitary Gland (Hypophysis Cerebri)

• Regulates metabolic activities in certain organs and tissues of the body
• Produce hormones (chemical substances)
• Glands
• Isolated groups of cells within certain organs
• Individual cells scattered among parenchymal cells of the body

Adenohypophysis (Anterior Pituitary)

• Oral ectoderm
• Rathke’s pouch
• A. Pars distalis
  • Chromophils (Acidophils)
  • Chromophils (Basophils)
• B. Pars intermedia
• C. Pars tuberalis

Neurohypophysis (Posterior Pituitary)

• Neural ectoderm
• As downgrowth of diencephalon
• Not a true endocrine gland
• Does not produce its own hormones
• Only stores and secretes hormones from hypothalamus
• Infundibular stalk
• Pars nervosa

Parathyroid Gland

• Regulate blood calcium levels
• Chief (principal) cells
• Oxyphil cells

Thyroid Gland

• Produces hormones: thyroxine (T4), triiodothyronine (T3), calcitonin
• Follicular Cells
• Parafollicular cells (clear cells / C cells)

Adrenal Gland

• Produces two different groups of hormones:
  1. Steroids
  2. Catecholamines
• 2 main regions:
  1. Adrenal Cortex
     • Synthesize and secrete several steroid hormones without storing them
     • 3 histological zones: A. Zona glomerulosa (arcuata) B. Zona fasciculata C. Zona reticularis
  2. Adrenal Medulla
     • Chromaffin cells
     • Medullary parenchymal cells
     • Modified sympathetic postganglionic neurons
     • With secretory function
     • Produce catecholamines