Histology: Digestive System

• Module Histology/Embryology: Digestive System for the academic year 2024-2025.

Objectives:

• Identify the different segments and organs
• Describe the histological organization of the oral cavity
• Describe the basic histological characteristics of the tunics constituting the wall of each organ in the digestive tract
• Define the structural and histological features of different organs and establish a link with their functions
• Describe the histological structure of the digestive annex glands

Plan of the Digestive System

• Oral cavity
• Digestive tract
• Associated glands

Oral Cavity Segments

• Buccal cavity
• Lips
• Tongue

Digestive Tract Segments

• Esophagus
• Stomach
• Small intestine
• Large intestine
• Anal canal

Glands Associated with the Digestive System

• Salivary glands
• Liver
• Pancreas
• Gallbladder

Histological Description of the Buccal Cavity wall

• Composed of:
  • Mucosa:
  • Lining epithelium
  • Underlying chorion covering muscular or osseous surfaces
  • Submucosa:
  • Houses the minor salivary glands,
  • Located in the chorion.
  • Striated muscle fibers
• The lining epithelium is stratified squamous non-keratinized
• The chorion equals loose connective tissue
• There is no clear demarcation between the chorion and the submucosa
• The mucosa is of the buccal type
  • This includes the epithelium, chorion, sero-mucous salivary glands and excretory duct.

Histological Description of Lips

• The outer cutaneous side is defined
• The inner buccal side is defined
• The vermilion border is also important to define

Outer Cutaneous Side of Lips:

• Epidermis: Consisting of a thin keratinized stratified squamous epithelium
• Dermis: Consisting of pilosebaceous follicles and sweat glands

Vermilion Border of Lips:

• Mucosa shows a thin keratinized malpighian epithelium
• Presence of the lamina propria (chorion)
• Notable for the absence of sebaceous or sudoriparous glands

Inner Buccal Side of Lips:

• Consists of buccal-type mucosa: Consisting of malpighian non-keratinized epithelium, basement membrane, chorion, and capillary vessels.
• Includes serous salivary glands consisting of excretory duct and muscle bundles of striated muscle.

Histological Description of the Tongue

• It is a musculo-conjunctive organ covered with buccal-type mucosa
• Has a center is formed of bundles of striated skeletal fibers oriented in the 3 planes of space
• Muscle is of the lingual type
• Has two sides to define

Tongue Description

Ventral Side of Tongue:

• Smooth and lined with a typical buccal mucosa

Dorsal Side of Tongue:

• Rough surface due to vertical projections: Lingual papillae
• Consists of a non-keratinized malpighian epithelium except at the level of the filiform papillae and has an underlying chorion.

Types of Lingual Papillae on Dorsal Side of Tongue:

• Filiform type
  • Elongated and numerous anterior to the lingual V
• Fungiform type
  • Project above the epithelial plane (mushroom shape), located on the edges and tip of the tongue.
• Caliciform type
  • Surrounded by a groove (or vallum), located at the level of the lingual V.

Taste bud

• The taste bud is an intra-epithelial ovoid structure opening onto the surface of the epithelium through a taste pore.

• Consists of:

  • Sensory taste cells
  • Support cells
  • Basal cells

Basic Histological Structure of the Digestive Tract

• The wall is formed of four tunics:
  • Mucosa
  • Submucosa
  • Muscularis externa
  • Serosa or adventitia (outermost layer)

Mucosa (formed of):

• Lining epithelium, which varies depending on the function of each organ in digestion like protection, secretion, and absorption

• The Chorion contains loose connective tissue rich in blood vessels, lymphatics, lymphoid elements, and may contain exocrine glands.

• The Muscularis mucosae (or muscularis mucosae): a thin layer of smooth muscle absent at the extremities of the digestive tract (upper 1/3 of the esophagus and anal canal)

Submucosa

• Formed of loose connective tissue containing blood and lymphatic vessels, and a submucosal nerve plexus of Meissner
• Contains exocrine glands in the esophagus and duodenum
• May contain significant lymphoid tissue

Muscularis

• Formed of smooth muscle cells in two to three sub layers for peristaltic movement
  • 1/3 striated muscle(upper esophagus and middle)
  • Two layers of smooth muscle
  • Auerbach's myenteric nerve plexus is present between the muscle layers.

Serosa or Adventitia

• Serosa: a thin layer of loose connective tissue rich in blood and lymphatic vessels, adipose tissue and covered by a mesothelium (simple squamous epithelium)
• Absence of mesothelium is termed adventic found at both ends of the digestive tube
• The two are present along the digestive tract.

Esophagus

• Is a hollow muscular tube that propels food from the oral cavity to the stomach
• The lumen is narrow and star-shaped to due to deep folds
• The tunics are mucosal, submucosal, muscularis and adventicia

Esophageal Mucosa

• Consists of a stratified pavementous non-keratinized epithelium that protects from friction and is formed of basal, intermediate, and superficial cells
• The chorion is rich in vessels, nerves, lymphoid elements, and houses glands
  • Found in the upper and lower extremities of the esophagus

Esophageal Submucosa

• Rich in connective tissue, elastic fibers (distensibility of the wall), and blood vessels
• It contains exocrine glands are esophageal glands

Esophageal Muscularis Mucosa

• Has two muscular layers of circular internal and longitudinal external
  • The superior 1/3 is striated muscle
  • the mediail 1/3 has striated and smooth msucle
  • the inferior 1/3 has exclusively smooth muscle.

Esophageal Adventitia

• The adventitia is a vascularized and innervated loose connective tissue
• The 1 to 2 cm nearest the last part of the esophagus penetrates the peritoneal cavity and is serous rather than adventice.

Esophagus - Esophagus/Stomach Junction

• Marked by an abrupt transition of the non-keratinized esophageal epithelium (protection) to simple cylindrical gastric epithelium (secretion)
• Contains cardiac glands

Stomach

• Fibromuscular sac is extensible for provisional storage and allows mechanical, chemical, and enzymatic digestion
• Has three anatomical regions being cardia, fundus and pylorus

Stomach - Wall:

• Mucosa: invaginates to form crypts terminating in tubular gastric glands
• Submucosa: classic structure, devoid of glands
• Muscularis: composed of 3 layers of smooth muscle
• Serosa: loose connective tissue lined with mesothelium

Stomach - Gastric Mucosa:

• Cylindrical lining epithelium which is simple with closed mucous pole.
• Is a Glandular sheet, (Intraepithelial exocrine mucous glands in layer)
• The common to all 3 regions the surface epithelium plays a role in chemical protection
• Contains a chorion which connects to for pits (crypts)

Stomach - Regional Variations

Stomach - Cardiac:

• Has shallow pits and coiled tubular gland

Stomach - Pyloric:

• Deep pits with branched tubular gland

Stomach - Fundic:

• Crypts are small and deep and tubular glands are long and branch

Stomach - Fundic Glands

• Synthesize gastric juices and contain a few types of distinct zones -. Neck
  • Corpus
  • Fundus

Cell Types in Fundal Glands:

• Stem cells (for cellular renewal)

• Mucous neck cell

• Parietal (or border) cells

  • Abundant in the gland body produce hydrochloric acid and the production of intrinsic factor

• Main cells are located at the bottom of the glands.

  • Produce pepsinogen and gastric lipase

• Entero-endocrine cells (argentaffin) are scattered and produce gastrin.

Other Gastric Tunics

• Submucosa:

  • Classical and devoid of gland

• Muscularis:

  • Three layers of smooth muscle
  • Oblique internal
  • Circular average type
  • External longitudinal

• Serous

Stomach - Gastro-duodenal junction (pyloric sphincter)

• The transition between the pylorus (stomach) / duodenum (intestine) is marked by the thickening of the middle circular layer of the muscularis

Small Intestine

• Absorption and digestion with amplified area
  • Intestinal loops/Anses intestinales (length)
  • Valvules conniventes (macroscopic folds)
  • Villi/villosités (MO-visible folds)
  • Microvilli/(Plateau strié)ME-visible extensions to the apical pole of the absorbent cells.

Small intestine - Histological Structure:

• It evaginates into the lumen forming villosities
• Invaginates the chorion to form Lieberkühn crypts

Structure of Small Intestine:

• Epithelium
• Chorion
• Crypts of Lieberkühn
• Muscularis externa

Small Intestine - Axis of the intestinal villus:

• Formed of lamina with vessels, a large lymphatic vessel (chyliferous central) and smooth-muscle cells from the muscularis mucosa

Small Intestine - Epithelial Types:

• At the villi which are simple cylindrical and form:Absorbent cells (enterocytes),Caliciform cells, Entéro-endocrine cells (hormone producing)
• In the Lieberkühn a stem cell zone which are simple cylindrical and form:Enterocytes, Caliciform cells, Entéro-endocrine cells, De Paneth cells

Small Intestine - Segments:

• Divided into 3 segments: duodénum, jéjunum and iléon which present variations in the mucosa and submucosa

Small Intestine-Duodénum:

• Characterized by the presence of Brunner's glands from the sub-mucosa into chorion and the the presence glands Lieberkühn in chorion

Small Intestine-Jéjunum:

• No Brunner glands

Small Intestine-Iléon:

• Characterized by the presence of numerous lymphoid nodules forming the plates of Peyer (20 to 40 follicles) located in the chorion.

Small Intestine-Ileocecal Junction

• Valve: Ileo - valve of the colon
• Histological: the villous small intestine joins the colonic large intestine

Gros Intestin :

It is the colon: Appendix vermiform, with caecum, rectum, anal canal and colon (transverse section) Functions: Reabsorption of water- microvillous cells Motricity: Propulsion of materials Secretion: mucus protects the feces Protection: defence of l'organisme and aide's flore intestinale.

Gros Intestin :

Absence of villosities Very big glands to the mucosae Composition of cells of epithelium Cells of Panneth caliciform cells/cells entérocytes

Gros Intestin

Cells: Epithéliales du colon Chorion Musculeuse sous- muqueuse Gros Intestin

Ténia coli Haustra

Intestin-Muqueuse colique:

Cells de Lieberkühn Caliciformes essential quelques enterocytes Absent cells de Paneth chorion Cell muscularis Aspects de champs.

Appendice colon Présence de tissus lymphoid sur la s-muqueuse (s) et muqueuse.

Gros Intestin -Anal

5/10 plis called Colonnes anales Morgani liées à leur face par des sinus anaux (ligne pectinée, aspect resemble peigne) sous muqueuse: nombreux flexus musculeuse Sphincter anal externe formée par ce muscle (volontaire de defecacion) Advenctice Fonctions tubes digestives: Fonction digestive:

Progression brassage canal alimantaire Dégradation des aliments Absorption des aliments

Fonction immunitaire Fonction andocrine.