Digestive Tract

DT

Embryology

Eearly trilaminar embryonic disc

The Endoderm is lying over the yolk sac

What does the Primitive gut have

Foregut, midgut, hind gut

How is the primitive gut formed?

Through cranio- caudal and lateral folding of the embryonic disc.

The forgut gives rise to the ?

Pharynx, the respiratory system, the oesophagus, the stomach, liver pancreases and proximal portion of duodenum.

The midgut forms most of the?

Intestine - connected to the yolk sac via the yolk sac stalk in the umbilicus.

Hindgut gives rise to?

Caudal parts of the intestine ( Distal Colon and Rectum)

Epithelial lining of the digestive tract from the forgut to the rectum is derived from?

Endoderm

Connective tissue, muscular components, serosal coverings and mesenteries are derived from?

Mesoderm

Nervous components derived from?

Neural crest

Oral Cavity - Epithelial lining is what?

Stratified squamous epithelium - Which is keratinised in many areas (hard palate, dental pad, surface of the tongue, cheek)

Lips and Cheeks

External Surface of the lip has skin ( ex dermis, folliculs) with a transition:

mucocutaneous junction, to the mucosa on the oral side of the lip.

What do Ruminants have?

Buccal papilla : Highly keritinized conical projections on the inside surface of their cheeks.

Hard and Soft Palate

Hard Palate separates into?

Separates the oral and nasal cavaties

Hard palate has a **CORE OF BONE** covered by what type of Connective tissue?

**CORE OF BONE** - covered by dense irregular connective tissue

Oral surface is lined by?

**Keritinized** stratified squamous epithelium

Soft Palate Separates the oropharynx and nasopharynx and has a Core what?

**CORE SKELETAL MUSCLE**

The oral surface is lined by?

**Non-keritinised** stratified squamous epithelium.

Tongue- what is its purpose?

Manipulates food during mastication, swallowing and has sensory awareness or taste and touch.

The tongue has what kind of muscle?

**SKELETAL MUSCLE -** fibers running in all 3D planes

The Tongue is covered by a mucosa with what type of epithelium?

**STRATIFIED SQUAMOUS EPITHELIUM**

The **mucosal surface** of the tongue has _____ of various sizes and morphology ( some have taste buds- mushroom shaped).

**Papillae**

__**Teeth & Gingiva**__

How many types of teeth do domestic animals have ?

**TWO distinctive type** - that differ in pattern pf growth and morphology

Low crowned teeth are also called?

**Brachydont - teeth seen in carnivores and swine**

What is so unique about the **bracydont** teeth?

Enamel covers the tooth above gum-line and the teeth stop growing after fully erupted.

What types of teeth do herbivores have?

**Hypsodont** (High crowned) teeth of herbivores

Examples ( All permanent teeth of horses and cheek teeth of ruminants. Rabbits also have these types of teeth. It is so important for them to get their teeth trimmed as they are continuously growing.

**Oral Cavity Embryonic Development of the Teeth**

Teeth are derived from?

**The tooth germ**

The tooth germ is an aggregate of cells derived from the ?

**Ectoderm** -of the overlying gingiva and ectomesnchyme derived from the neural crest

What does the **tooth germ** consist of?

The **enamel** organ, the **dental papilla** and the **dental sac** (or follicle).

**Ameloblasts** are derived from what type of tooth epithelium and what do they produce?

**The inner enamel epithelium- produce enamel and odontoblasts.** They are also derived from the **immediately adjacent** dental papillae, **produce dentin.**

__**IMPORTANT COMPONENTS OF TEETH**__

What important components do the teeth consist of ?

* Enamel
* Dentin
* Dental pulp cavity
* Cementum
* Periodontium

What is **Enamel** and what does it consisit of?

* **Highly mineralised**
* **Extremely hard material** **produced by ameloblast**

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Side note: ( brachydont teeth make enamel only before eruption, after which the ameloblast die).

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What is %%Dentin%% and what does it consisit of?

Bone like %%**connective tissue**%% that is secreted by %%**odontoblasts**%%

What is @@**Dental pulp cavity**@@ and what does it consist of?

* Lies within the @@**dentil and contains vessels and nerves** @@

What is ^^**Cementum**^^ and what does it consist of?

* ^^**Calcified connective tissue**^^ secreted by ^^**cementoblasts**^^

What is **Periodontium** and what does it consist of?

* Attaches the **tooth to the jaws and is composed of the cementum, periodontal ligament, alveolar bone (socket) and associated gingiva.**

Gingiva (Gum) - What is it composed of (three things)

* **Keritinised oral mucosa**
* **Dentogingival junction - Zone of attachment between ggingiva and enamel - ceiling of tooth prevents periodontal diseases**
* **Ggingival sulcus**

**Salivary Glands**

See Image

**Tubular Portion**

Grossly- What should the normal mucosal and serosal look like?

**Normal** mucosal and serosal surfaces should be **smooth and shiny**.

**EXCEPT FOR - the Rumen** whose mucosal surface/ papillae have a roughened, dull appearance.

__**Tubular portion**__

Microscopically: What does the digestive tract look like?

The digestive tract from the oesophagus to the anus is a hollow tube

The digestive tract has mucosa that is lined by?

* **Epithelial lining; underlying lamina propria**.
* **Loose connective tissue, lymphocytes, +/- glands**
* Muscularis mucosae

__**Tubular portion**__

Submucosa

* Submucosa has denser connective tissue with larger blood lymph vessels, submucosal autonomic nerve plexus

Msucularis (Muscularis Externa)

**Smooth muscle** (mostly 2 layers; an inner circular and an outer longitudinal layer)

* Can find blood, lymph vessels and myenteric autonomic nerve plexus

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Serosa/ adventitia is found where?

**Abdominal Cavity**

**Serosa is a thin layer of loose connective tissue with an outer covering of mesothelium.**

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What type of epithelium is serosa in the abdominal cavity covered by?

Mesothelium ( Simple squamous epithelium)

Does the Oesophagus of an Adventitia? True or False?

**True** - The Oesophagus has an **adventitia**= a layer of connective tissue which blends with surrounding **connective tissue** of the neck or **mediastinum** of the **thorax**

**What does the oesophagus do and what is it made of?**

**Muscular tube, which transports swallowed material from the pharynx to the stomach**

Mucosa is lined with?

**Stratified squamous epithelium (keritinized in some species) and Lamina propria**

Does the Muscularis Externa have an inner and outer layer?

Yes, **an inner circular and outer longitudinal layer.**

* It is skeletal proximally and

in most species transitions to **smooth muscle** distally; (*note, it is all skeletal in dogs & ruminants*

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Ruminant fore-stomachs is lined by?

Mucosal lining of all the ruminant forestomachs is **keratinized stratified squamous epithelium** \n

Ruminants have **Rumen Papillae-** what are they ?

• **Rumen papillae** are projections of the tunica mucosa: they have a **core of lamina propria connective** t**issue with no muscularis mucosae**

Reticulum in Ruminants

Has a **honeycomb-like arrangement** with a m**uscularis mucosae** running along the top ridges of the walls of the honeycomb pockets (contracts the honeycomb openings like a purse- string)

Omasum

Has **numerous large folds (leaves)**; each **fold has a central strand of muscularis externa** which lies between thin layers of lamina propria / muscularis mucosa on each side

__**The Stomach**__

The stomach adds acidic fluid and digestive enzymes (esp pepsin)

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True or False?

The mucosa is entirely glandular in many animals?

**True**

• The glandular mucosa has gastric pits (superficial invaginations) connected to gastric glands (branching tubules) via an isthmus / neck region

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The Stomach can be divided into various regions.

What are they?

* Fundic glands
* Cardiac and pyloric glands

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**Fundic glands - why are they essential in the stomach?**

→Fundic glands (in fundus & body) – gastric glands are mostly a mix of parietal cells (**secrete HCl)** & chief cells (**secrete pepsinogen)**

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**Cardiac and pyloric glands**

**-Gastric glands are mostly mucous cells**

Fundic stomach in dogs is liend by what type of epithelia?

Superficially, the gastric mucosa contains **gastric pits,** which form superficial invaginations and are lined by mucus-secreting columnar epithelial cells.

What are the three cells in the gastric mucosa called?

1\. Mucus cells

2\. Enteroendocrine cells s

3\. Stem cells

Mucus cells - where are they found?

* Found on the surface, gastric pit and neck region; protection from acid and enzymes

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Enteroendocrine cells

* Secrete hormones; ex gastrin, which stimulates secretion of HCl & pepsinogen

^^-Hormone-producing cells that are sprinkled throughout the gut epithelium.^^

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Stem Cells

* Present in the neck region; can replenish all the other cell types

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What species specifically has the **lamina subdlandularis**?

**CATS**

* Cats have dense layer of connective tissue, called the **lamina subglandularis.**


* Is situated between the base of the gastric glands and the thick **muscularis mucosa (helps prevent perforations)**

__**Small Intestine**__

In all regions of the small intestine the **mucosa has what two important factors?**

\n **→Villi**

**→Crypts**

**Villi**

\n **→Villi:** luminal leaf-like projections composed of cores of **lamina propria** that are **lined by a simple columnar epithelium** of **absorptive cells** called **enterocytes** and **interspersed goblet cells**

→Crypts are lined by?

**Simple tubular intestinal glands; simple columnar enterocytes, goblet cells + Paneth cells**, enteroendocrine cells & stem cells (produce all the cells that line the crypts & villi)

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**The Small Intestine is divided into three segments, what are they?**

\n **→Duodenum**

**→Jejunum**

**→Ileum**

**Duodenum**

- Short proximal segment with submucosal

mucus glands (aka duodenal or Brunner glands)

**Jejunum**

\- The large middle segment, attached by the mesentery, and has scattered GALT

**Ileum**

The short distal segment, which connects with the cecum / large intestine, and has abundant GALT

True or False

Intestinal epithelial cells (IECs) of the mammalian small and large intestine are **regenerated continuously?**

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**True** - throughout adulthood from **stem cells** that reside near the base of crypt

What do Stem cells in the crypts produce?

All the epithelial cell types that line the crypts, villi & GALT

**→Enterocytes:** tall columnar cells with basally located nucleus & a prominent apical brush border (= microvilli on EM)

**→Sugars and amino acids**

**→Lipid subunits**

**Where can you find Crypts?**

(Mitotic figures, indicative of replication, are often frequently found within crypts, Red arrow).

**Base of the intestinal glands**

What type of cells can we find in the small intestine?

→**Goblet cells**

→**Paneth cells**

→**Enteroendocrine cells**

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Where are **Goblet cells** in the small intestine?

Interspersed **between enterocytes** and **Secrete mucus**

**They lubricate and protect intestinal lining**

Where are Paneth cells located in the small intestine?

Located in **crypts**

Have large eosinophilic secretory granules in their apical cytoplasm.

Play an important role in the **innate immunity**

Fun fact: Granules contain antimicrobial peptides

Where are the **Enteroendocrine** cells Located in the small intestine?

Scattered throughout lining of crypts and villi

(they sample taste intestinal content, release hormones “Sprinkle gut”- coordinate gut function motility etc…).

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__**Small Intestine**__

Where are the M (Microfold cells ) Located?

Located mainly in the follicle- associated epithelial (FAE) layer that overlies GALT ; esp **Peyers patches**

What is so distinctive about M cells?

have **apical microfolds** and **Pocket -like invaginations** of basal membbrane that contains immune cells.

What are M cells specialised for?

The **uptake and transepthelial transport of microbes and particles.**

What can we find in the

Gut-associated lymphoid tissue (GALT) of the small intestine? Two things

→Organized as diffuse lymphoid tissue



→Peyer’s patches

**True or False**

**Peyer’s patches** are aggregated lymphoid nodules in submucosa / mucosa of small intestine ?

**True**

From looking at this picture of a section of the small intestine.

What four important structures can you see?

* **Lamina propria**
* **Submucosa connective tissue**
* **Muscularis externa**
* **Serosa**

* **Lamina propria - what type of connective tissue and where can you find it?**

**Loose connective tissue in the core villi and between the crypts**

* **Submucosa- what type of connective tissue and where can you find it?**

* **Loose in most species**
* Moderately **dense in carnivores (larger blood and lymphatic vessels)**

**Muscularis externa**

Internal circular layer and external longitudinal layer

**Serosa covered by?**

**Mesothelium** and is continous with supporting mesentery

__**Large Intestine**__

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What are the **three major** regions?

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* Cecum
* Colon
* Rectum

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* Cecum
* Colon
* Rectum

DO they vary in size?

Yes, greatly in size among mammals

* Extremely large cecum and cololn in hindgut fermenterssuch as horese and rabbits

Large intestine also has?

**Mucosa**

**Proliferatin stem cells**

**GALT**

**Flat bands of longitudinally arranged smooth muscle and elastic fibers**

__**Anal Canal**__

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What lines the Anal Canal?

Rectum terminates at the anal canal with an abrupt change to **stratified squamous epithelium; it** is **nonkeratinized** in **the proximal portion and** switches **to keratinized in the distal** portion which then becomes continuous with the outside hairy skin

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Extra info

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**Look at the photo to understand this;**

• Circular layer of the rectum / anal muscularis forms the internal anal sphincter and the surrounding striated muscle forms the external anal sphincter

__**Glands of the anal sacs**__

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True or False

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Many carnivores have paired anal sacs with lands (paranal sinus glands)

True - lined by keratinized stratified squamous epithelium

Sacs store secretions of surrounding glands

Are they different in both cats and dogs or the same?



Excretory duct of each gland opens into the **keratinized portion of the anal canal**

**Different**

In Cats the surrounding glands are

**Apocrine and sebaceous in cats**

In Dogs the surrounding glands are

**Apocrine tubular in dogs**

**Circumanal Glands**



Subcutis around the around the anus of dogs

* Upper portion of gland is sebaceous, while the deeper regions are non-sebaceous and the cells have an appearance similar to hepatocytes (hence hepatoid glands)