Histology of Hollow Tubular Organs
Any organ that is tubular and hollow shares a common structural organization. Examples include the trachea, esophagus, and ureters. The wall of these organs is composed of four distinct layers, known as tunics. These layers, from the innermost to the outermost, are as follows:
- Tunica Mucosa – Closest to the lumen of the organ.
- Tunica Submucosa – Located outer to the tunica mucosa.
- Tunica Muscularis – Lies outside the tunica submucosa.
- Tunica Serosa or Tunica Adventitia – The outermost layer, depending on the organ’s location.
1. Tunica Mucosa
This layer consists of three sublayers:
- Epithelium: The innermost layer, lining the lumen. It varies based on the function of the organ (e.g., stratified squamous in the esophagus, simple columnar in the intestines).
- Lamina Propria: A layer of loose connective tissue containing mucosal glands, blood vessels, and lymphatics. It is located just beneath the epithelium.
- Lamina Muscularis Mucosae: A thin layer of smooth muscle, typically arranged in a circular fashion. In some organs, it is discontinuous or absent. When absent, the lamina propria blends with the next layer, in which case it is referred to as the propria-submucosa.
2. Tunica Submucosa
This layer is composed of loose and dense irregular connective tissue. It may contain:
- Submucosal glands (e.g., duodenal and esophageal glands).
- Blood vessels and lymphatics.
- A network of nerve fibers known as Meissner’s plexus, which regulates glandular secretions and local blood flow.
3. Tunica Muscularis
This layer consists of smooth muscle, typically arranged in two layers:
- Inner circular layer
- Outer longitudinal layer
In some organs, a third oblique layer may also be present (e.g., the stomach). Between the circular and longitudinal layers lies the myenteric plexus (Auerbach’s plexus), which controls peristalsis and muscle contractions.
4. Tunica Serosa or Tunica Adventitia
The outermost layer of the hollow tubular organ varies depending on its anatomical location:
- Tunica Serosa: Found in organs within body cavities (e.g., the intestines). It consists of:
- A thin layer of loose connective tissue (inner layer).
- A layer of simple squamous epithelium (outermost layer), known as the mesothelium.
- Tunica Adventitia: Present in organs that are not located within body cavities (e.g., the esophagus). Unlike serosa, it lacks mesothelium and consists only of loose connective tissue, which merges with surrounding structures.
This structural organization provides the necessary support, flexibility, and function required for various tubular organs in the body.
Histology of the Oral Cavity (Lips, Cheeks, and Palate)
1. Epithelium of Lips and Cheeks
- Carnivores and Pigs: Stratified squamous non-keratinized epithelium.
- Ruminants and Horses: Stratified squamous keratinized epithelium.
2. Propria-Submucosa
- Composition: Dense irregular connective tissue.
- Contains:
- Labial glands in lips.
- Buccal glands in cheeks.
3. Tunica Muscularis
- Lips: Contains the orbicularis oris muscle (circular muscle around the mouth).
- Cheeks: Contains the buccal muscle (buccinator muscle).
- Ruminants: Thickened buccal muscle adapted for chewing.
4. Dermis and Epidermis of Lips and Cheeks
- Dermis: Dense irregular connective tissue. May contain sweat and sebaceous glands as well as hair follicles.
- Epidermis: Stratified squamous keratinized epithelium in chicks.
5. Hard Palate
- Epithelium: Stratified squamous keratinized epithelium (very thick in ruminants).
- Lamina Propria: Dense irregular connective tissue, fused with the underlying periosteum.
- Palatine Ridges: Present on the oral surface, aiding in food manipulation.
- Palatine Glands:
- Similar to labial and buccal glands (compound or branched).
- Contains a dense venous network that functions as a cushion.
6. Dental Pad (Ruminants Only)
- Location: Anterior margin of the hard palate.
- Function: Replaces the upper incisors.
- Structure:
- Stratified squamous keratinized epithelium.
- Core made up of dense irregular connective tissue.
7. Soft Palate
- Structure: Muscular, membranous fold separating the oral and nasal cavities.
- Propria-Submucosa: Loose irregular connective tissue with lymphoid tissue.
- Palatine Tonsils: Present within the soft palate.
- Epithelium:
- Oral surface: Stratified squamous epithelium.
- Nasal surface: Pseudostratified ciliated columnar epithelium.
8. Tongue Structure
- Layers (from dorsal to ventral surface):
- Epithelium.
- Propria-submucosa in the form of papillae (present on the dorsal surface but absent on the ventral surface).
- Epithelium: Stratified squamous keratinized epithelium (very thick in herbivores).
- Propria-Submucosa: Dense irregular connective tissue.
- Contents:
- Lingual glands.
- Lingual tonsils.
- Blood vessels.
9. Lingual Papillae
Two types based on function:
A. Mechanical Papillae (for food movement and manipulation):
- Filiform Papillae
- Conical Papillae
- Lenticular Papillae
- Marginal Papillae (well-developed in newborns for suckling).
B. Gustatory Papillae (for taste perception):
- Fungiform Papillae
- Foliate Papillae
- Circumvallate Papillae
Histology of the Tongue and Lingual Papillae
The tongue is a muscular organ with specialized structures to aid in food manipulation, taste perception, and swallowing. It consists of different types of lingual papillae, taste buds, and other unique structures that vary between species.
I. Lingual Papillae
1. Mechanical Papillae (For Food Manipulation)
These papillae primarily increase the friction between the tongue and food, making it easier for animals to chew and manipulate food within the oral cavity.
A. Filiform Papillae
- Appearance: Narrow, slender, and pointed, giving the tongue a rough texture.
- Distribution: Cover the entire surface of the tongue.
- Function:
- Most numerous type of papillae.
- Provide mechanical support (not involved in taste).
- Help in grasping food and directing it toward the pharynx.
- Species Differences:
- Dogs and Cats: Best developed, making their tongues rough.
- Herbivores (Cattle, Horses): Highly keratinized for processing fibrous food.
B. Conical Papillae
- Appearance: Large, cone-shaped structures.
- Location:
- Found mainly at the root of the tongue.
- Function: Assist in food manipulation and swallowing.
- Species Differences:
- Well-developed in ruminants, dogs, and cats.
C. Lenticular Papillae (Only in Bovines)
- Appearance: Lens-shaped, flat and superficial.
- Location: Found only in bovines, specifically on the caudo-dorsal third of the tongue.
- Function: Assist in the grinding of fibrous food in ruminants.
D. Marginal Papillae (Temporary Papillae in Newborns)
- Appearance: Long, flat, and soft.
- Location: Found on the lateral margins of the tongue.
- Function:
- Form a tight seal around the teeth to aid in suckling.
- Help newborns (especially pigs and dogs) efficiently consume milk.
- Species Differences:
- Present only in newborns and disappear as the animal matures.
2. Gustatory (Taste) Papillae
These papillae contain taste buds that help in identifying different tastes.
A. Fungiform Papillae
- Appearance:
- Mushroom-shaped, projecting slightly above the tongue surface.
- Found between filiform papillae.
- Location:
- Most numerous at the tip and lateral parts of the tongue.
- Function:
- Taste perception (contain taste buds).
- Species Differences:
- More prominent in herbivores than in carnivores.
B. Circumvallate (Vallate) Papillae
- Appearance: Large, dome-shaped structures surrounded by a deep groove (moat-like structure).
- Location:
- Found on the root of the tongue.
- Few in number, but their size compensates for their low numbers.
- Function:
- Taste perception (contain taste buds on their lateral surfaces).
- Gustatory serous glands (Von Ebner’s glands) open into the groove, helping to flush out food particles.
- Species Differences:
- Number varies by species (fewer in some animals, more in others).
C. Foliate Papillae
- Appearance: Leaf-like structures, arranged in vertical folds.
- Location:
- Found in the caudo-lateral part of the tongue.
- Function:
- Contain taste buds on their lateral margins.
- Species Differences:
- Absent in ruminants.
- Least developed in cats (may lack taste buds).
II. Taste Buds: Structure and Function
Taste buds are sensory structures responsible for detecting different tastes.
1. Location of Taste Buds
- Found in fungiform, circumvallate, and foliate papillae.
- Also present in the soft palate and epiglottis.
2. Structure of Taste Buds
- Shape: Elliptical or ellipsoid.
- Opening: Taste buds communicate with the surface through a taste pore.
- Cell Types:
- Sensory (Neuroepithelial) Cells:
- Spindle-shaped, with apical microvilli extending through the taste pore.
- Contain nerve fibers that transmit taste signals to the brain.
- Supporting (Sustentacular) Cells:
- Narrow and elongated, located between sensory cells.
- Provide structural support.
- Basal (Stem) Cells:
- Cuboidal or polyhedral cells located near the basement membrane.
- Function as stem cells that regenerate new taste cells.
- Sensory (Neuroepithelial) Cells:
III. Special Structures of the Tongue
A. Lyssa (Present in Carnivores and Pigs)
- Structure:
- Elongated, cord-like structure found along the midline of the tongue.
- Enclosed by a fibrous capsule.
- Contents:
- Dogs: Mostly skeletal muscle fibers.
- Cats & Pigs: Mostly fat cells (adipocytes).
- Function:
- Provides flexibility and support.
- Aids in the lapping of liquids (e.g., milk, water).
B. Dorsal Lingual Cartilage (Horse-Specific)
- Location:
- Found mid-dorsally in horses.
- Composition:
- Hyaline cartilage.
- Skeletal muscle.
- Fat cells.
- Collagen and elastic fibers.
C. Torus Linguae (Only in Ruminants)
- Structure:
- Thick, elliptical fold on the dorsal surface of the tongue.
- Traversed by a transverse groove.
- Function:
- Helps in grinding food, especially fibrous material.
- Species Differences:
- Only found in ruminants (e.g., cattle, sheep, goats).
Summary of Species-Specific Differences
| Feature | Dogs & Cats | Horses | Ruminants | Pigs |
|---|---|---|---|---|
| Filiform Papillae | Best developed, rough tongue | Present | Heavily keratinized | Present |
| Conical Papillae | Present | Present | Well developed | Present |
| Lenticular Papillae | Absent | Absent | Only in bovines | Absent |
| Marginal Papillae | Present in newborns | Absent | Absent | Present in newborns |
| Foliate Papillae | Least developed (no taste buds) | Present | Absent | Present |
| Lyssa | Present | Absent | Absent | Present |
| Dorsal Lingual Cartilage | Absent | Present | Absent | Absent |
| Torus Linguae | Absent | Absent | Present | Absent |
Histology of the Small Intestine and Large Intestine
I. Small Intestine
The small intestine is specialized for maximum absorption of nutrients. Three structural modifications increase its surface area:
1. Plicae Circulares (Circular Folds)
- The entire intestinal mucosa forms a series of circular folds, known as plicae circulares, which increase the surface area for absorption.
2. Villi
- Finger-like projections of the mucosa that further increase the absorptive surface of the intestine.
3. Microvilli
- Microscopic, finger-like projections of the cytoplasm present on columnar absorptive cells, forming the brush border, which enhances absorption.
Divisions of the Intestine
The intestine is divided into two major parts:
A. Small Intestine
- Composed of three segments:
- Duodenum
- Jejunum
- Ileum
B. Large Intestine
- Larger in diameter than the small intestine.
- Composed of three segments:
- Cecum
- Colon
- Rectum
II. Structure of the Small Intestine
1. Tunica Mucosa
A. Epithelium
- Composed of simple tall columnar epithelium.
- Contains:
- Columnar absorptive cells (enterocytes).
- Goblet cells (mucus-secreting cells).
- Endocrine cells with eosinophilic granules at the base of intestinal glands.
- Paneth cells that secrete antimicrobial enzymes.
- The tall columnar cells exhibit apical microvilli, forming the brush border.
B. Lamina Propria
- Made up of loose connective tissue containing:
- Connective tissue cells (mast cells, plasma cells, eosinophils).
- Lymphoid aggregations, including lymphatic nodules and diffuse lymphoid tissue (DLT) (e.g., solitary lymphoid nodules in the intestine).
- Intestinal glands (Crypts of Lieberkühn).
C. Muscularis Mucosae
- Composed of inner circular and outer longitudinal smooth muscle fibers.
2. Intestinal Glands (Crypts of Lieberkühn)
- Simple, straight, tubular glands located at the base of villi.
- Lined by:
- Columnar absorptive cells.
- Few goblet cells.
- Endocrine cells.
- Paneth cells at the base.
- The lamina propria (LP) contains capillary plexuses and a central lymph capillary, which extends into the villi.
Villi Structure
- Finger-like projections of the mucosa.
- Core is made up of:
- Lamina propria (LP).
- Blood capillaries.
- Lymph capillaries.
- Actin and myosin filaments and smooth muscle, which help in villi movement.
3. Tunica Submucosa
- Composed of loose connective tissue.
- Contains submucosal glands, grouped as:
- Simple tubular serous glands.
- Mucous or seromucous glands.
- Brunner’s glands (duodenal glands):
- Occur in the submucosa of the duodenum and some parts of the jejunum.
- Also called submucosal glands or duodenal glands.
- Submucosa may also contain lymphoid tissues (e.g., Peyer’s patches in the ileum).
4. Tunica Muscularis
- Composed of two smooth muscle layers:
- Inner circular layer.
- Outer longitudinal layer.
5. Tunica Serosa
- Outermost layer, composed of loose connective tissue covered by mesothelium.
III. Regional Characteristics of the Small Intestine
1. Duodenum
- Villi: Regular in shape, blunt, and wide leaf-like.
- Brunner’s glands: Present in high quantity.
- Lymphoid tissue: Gradually increases in the caudal direction.
- Goblet cells: Increase in number caudally.
2. Jejunum
- Villi: Slender, cylindrical, and fewer in number.
- Brunner’s glands: May be present only in the first part.
3. Ileum
- Villi: Club-shaped.
- Brunner’s glands: Absent.
- Lymphoid tissue: Abundant (Peyer’s patches).
- Goblet cells: Numerous, especially in terminal parts.
Histology of the Large Intestine
I. Differences from Small Intestine
- Villi: Absent in mammals but present in poultry.
- Intestinal glands: Longer, straight, and compact.
- Paneth cells: Absent.
- Lymphoid tissue: More frequent and prominent.
- Plicae circulares: Absent; instead, the mucosa contains longitudinal folds, which are more prominent cranially.
- Mucocutaneous junction: Abrupt transition from intestinal to skin epithelium.
- Submucosa contains anal glands in dogs, which are modified tubular sweat glands that secrete lipid-rich secretions.
- In carnivores, these glands produce a pungent smell in feces.
Liver: The Largest Gland of the Body
I. Functions of the Liver
- Metabolism of macromolecules:
- Synthesis of sugars, urea, ketone bodies, proteins, and plasma proteins.
- Bile secretion and excretion of bile pigments.
- Detoxification and degradation of toxins, hormones, and drugs.
- Phagocytosis by stellate macrophages (Kupffer cells).
II. Structure of the Liver
1. Stroma and Vasculature
- Capsule:
- Made of dense irregular connective tissue.
- Externally lined by mesothelium (serous membrane or visceral peritoneum).
- Also called Glisson’s capsule.
- Septa and Trabeculae:
- Loose collagenous connective tissue that divides the liver into lobules.
- Supports vascular and biliary systems.
- Hepatic Lobules:
- Poorly distinct in most animals due to scant connective tissue.
- Pigs have well-defined hepatic lobules due to thick septa.
- Interlobular connective tissue:
- Made up of fine reticular fibers, surrounding hepatic sinusoids and hepatocytes.
III. Special Features in Different Species
| Feature | Mammals | Poultry | Dogs | Pigs |
|---|---|---|---|---|
| Villi in Large Intestine | Absent | Present | Absent | Absent |
| Brunner’s Glands | Present in duodenum | Present | Present | Present |
| Peyer’s Patches (Ileum) | Present | Present | Present | Present |
| Paneth Cells | Present in small intestine | Present | Absent | Absent |
| Hepatic Lobules | Poorly distinct | Distinct | Poorly distinct | Highly distinct |
Liver Histology
I. Structure of the Liver
1. Stroma and Vasculature
A. Capsule
- The liver is enclosed in a dense irregular connective tissue capsule, externally lined by mesothelium (serous membrane or visceral peritoneum).
- This capsule is called Glisson’s capsule.
B. Septa and Trabeculae
- Loose collagenous connective tissue septa divide the liver into hepatic lobules.
- The septa support the vascular and biliary systems.
- In most animals, septa are scanty, making hepatic lobules appear indistinct.
- In pigs, the septa are very thick, making hepatic lobules highly distinct.
- Septa are also prominent in the portal area and in cirrhotic livers.
C. Interlobular Connective Tissue
- Made up of fine reticular fibers.
- Found around hepatic sinusoids and hepatocytes.
- Forms portal canals (portal areas), which are located between hepatic lobules, particularly at each angle of hexagonal hepatic lobules.
II. Hepatic Lobules (Structural and Functional Units of the Liver)
A. Hepatic Lobules (Morphological and Structural Unit)
- Shape: Hexagonal or polygonal prismatic column.
- Separated by interlobular connective tissue.
- Formed by radiating plates of hepatocytes around a central vein.
- Hepatic sinusoids: Anastomosing channels between hepatocyte plates, allowing blood flow.
III. Biliary System
A. Biliary Canaliculi
- Tiny canals or tunnels that originate from the apical surface of adjacent hepatocytes.
- Formed by tight junctions between hepatocytes.
- Bile flows from hepatocytes into bile canaliculi, then into intra-lobular bile ducts.
B. Intrahepatic and Extrahepatic Bile Ducts
- Bile canaliculi → merge to form intralobular bile ducts.
- Intralobular bile ducts → merge to form interlobular bile ducts.
- Interlobular bile ducts → merge to form the hepatic duct.
- Hepatic duct continues into the gallbladder.
- Hepatic duct and cystic duct join to form the common bile duct, which opens into the duodenum.
IV. Hepatic Sinusoids and Blood Circulation
A. Hepatic Sinusoids
- Dilated, irregular, anastomosing vessels located between hepatocyte plates.
- Blood flow: Sinusoids receive oxygenated blood from the hepatic artery and nutrient-rich blood from the portal vein.
- Sinusoidal walls consist of:
- Discontinuous, porous endothelial cells (allow exchange of substances).
- Kupffer cells (stellate macrophages) that phagocytose foreign particles.
- Perisinusoidal space (Space of Disse) between sinusoids and hepatocytes.
B. Blood Flow Pathway
- Oxygenated blood from the hepatic artery and nutrient-rich blood from the portal vein enter the hepatic sinusoids.
- Blood circulates around the hepatocytes.
- Blood drains into the central vein.
- Central veins merge to form the hepatic vein.
- The hepatic vein drains into the posterior vena cava.
V. Hepatocytes
A. Structure of Hepatocytes
- Shape: Hexagonal or polygonal cells arranged in radiating cell plates around the central vein.
- Nucleus:
- Central and rounded, with a distinct nucleolus.
- Some hepatocytes have two nuclei.
- Cytoplasm:
- Eosinophilic and granular.
- Stores glycogen and lipids.
B. Hepatocyte Surfaces
Each hepatocyte has six or more surfaces, classified into three types:
- Contact Surface
- Space between adjacent hepatocytes.
- Contains desmosomes and gap junctions.
- Microvillous Surface
- Faces the hepatic sinusoids.
- Contains microvilli to increase absorption.
- Biliary Surface
- From which bile canaliculi originate.
- Formed by tight junctions.
VI. Liver Lobule Concepts
The liver has three types of lobules:
1. Hepatic Lobule (Structural Unit)
- Hexagonal prismatic structure seen under the microscope.
- Blood Flow:
- Supplied by branches of the hepatic artery and portal vein from the portal triad.
- Drained by the central vein.
- Bile Flow:
- Bile flows in the opposite direction to blood, from bile canaliculi to bile ducts.
2. Portal Lobule (Functional Unit for Bile Flow)
- Triangular in shape.
- Centered around the bile duct in the portal area.
- Function: Emphasizes bile secretion and excretion.
3. Hepatic Acinus (Physiological Unit)
- Diamond or oval-shaped unit.
- Divided into three metabolic zones based on blood oxygenation:
- Zone 1: Closest to the portal triad, receives the most oxygen.
- Zone 2: Intermediate oxygen supply.
- Zone 3: Closest to the central vein, least oxygenated, most prone to damage.
- Not visible under the microscope (imaginary functional unit).
VII. Summary Table: Blood and Bile Flow in the Liver
| Structure | Blood Flow | Bile Flow |
|---|---|---|
| Hepatic Artery & Portal Vein | Blood enters liver sinusoids | – |
| Hepatic Sinusoids | Blood circulates around hepatocytes | – |
| Central Vein | Blood drains from sinusoids | – |
| Hepatic Vein | Blood drains into vena cava | – |
| Hepatocytes | – | Bile is produced |
| Bile Canaliculi | – | Bile flows toward bile ducts |
| Bile Ducts (Intrahepatic & Extrahepatic) | – | Bile reaches the duodenum |
VIII. Important Notes for Exams
- Portal Triad: Contains a branch of the hepatic artery, portal vein, and bile duct.
- Kupffer Cells: Liver macrophages that phagocytose debris.
- Space of Disse: Perisinusoidal space where exchange of substances occurs.
- Brunner’s Glands: Found in the duodenum, secrete alkaline mucus to neutralize stomach acid.
- Peyer’s Patches: Lymphoid nodules in the ileum for immune defense.
- Gallbladder Function: Stores and concentrates bile.
