Gastrointestinal Histology - Dr. Schreeg

Three embryologic tissue layers:

1.) ectoderm

2.) endoderm

3.) mesoderm

Endoderm

forms mucosa, or the tissues that line the GI tract and lungs (gut, liver, and lungs)

Mesoderm

forms the connective tissues and muscle that form the outer layer of GI tract (forms muscle, bones, kidneys, blood, gonads, and connective tissue)

Three embryological origins of GI tract organs:

1.) foregut

2.) midgut

3.) hindgut

The foregut will develop into (6)

1.) esophagus

2.) stomach

3.) proximal duodenum

4.) liver

5.) pancreas

6. (lungs)

The midgut will develop into (3)

1.) remaining small intestine

2.) cecum

3) proximal colon

The hindgut will develop into (2)

1.) distal colon

2.) rectum

What structure sets the boundary of the cranial aspect of developing embryo?

oropharyngeal membrane

oropharyngeal membrane

dissolves during development to become the mouth

What structure sets the boundary of the caudal aspect of developing embryo?

cloacal membrane

cloacal membrane

dissolves during development to become the urogenital and anal orifices

Main vessel that supplies the foregut

celiac artery

Main vessel that supplies the midgut

cranial mesenteric artery

Main vessel that supplies the hindgut

caudal mesenteric artery

Across multiple species, GI anatomy is the same EXCEPT when it comes to the: (3)

1.) stomach

2.) ascending colon

3.) cecum

Across multiple species, GI histology is the same EXCEPT in:

the stomach

Four upper GI tract sections:

1.) oral cavity

2.) tongue

3.) esophagus

4.) salivary gland

What upper GI tract structures are lined by stratified squamous epithelium? (3)

1.) oral cavity

2.) tongue

3.) esophagus

Function of stratified squamous epithelium of the upper GI tract

protection

Which part of the oral cavity may be keratinized?

gingiva

What regions of the oral cavity may contain skeletal muscle? Why?

soft palate; it is mobile

What lies below the epithelium of the upper GI tract?

lamina propria

Describe the lamina propria under the epithelium of the upper GI tract

Deep connective tissue that’s vascular and collagen-rich

The tongue is lined by what epithelium?

Stratified squamous epithelium that may be keratinized

Two special features of the tongue histology:

1.) dorsal surface forms papillae or exophytic projections

2.) skeletal muscle forms core of tongue

What does the dorsal surface of the tongue form?

Papillae or exophytic projections

What is the purpose of papillae found on the dorsal surface of the tongue?

Serves different functions, including grooming and chemoreception (taste buds!)

What forms the core of the tongue?

Skeletal muscle

What type of epithelium is this?

keratinized stratified squamous epithelium

Where might this epithelium be found?

oral cavity

1

keratin layer

2

stratified squamous epithelium

3

lamina propria

What structure is this epithelium found in?

the tongue

What are the projections coming off this epithelium?

tongue papillae

What structure is this epithelium found in?

the tongue

What species does this epithelium belong to?

the cat (spiky papillae used for grooming)

1 (type of tissue)

keratinized stratified squamous epithelium

2 (type of tissue)

skeletal muscle

What structure is this epithelium found in?

the tongue

What is the arrow pointing to?

chemoreceptor cells (taste buds)

What upper GI tract structures are lined by simple cuboidal epithelium?

salivary glands

Function of simple cuboidal epithelium of the salivary gland in the upper GI tract

secretion

The simple cuboidal epithelium of salivary glands is arranged in two ways:

1.) acini

2.) ducts

Acini of salivary glands

make and secrete saliva into ducts

Two types of epithelium cells found in acini:

1.) mucous

2.) serous

Mucous epithelium cells of acini appearance

pale, basophilic (blue) to clear cytoplasm

serous epithelium cells of acini appearance

pale, eosinophilic (pink) granular cytoplasm

What is the function of mucous epithelial cells of acini?

Make mucus for lubrication

What is the function of serous epithelial cells of acini?

Make proteins that contain digestive enzymes

Ducts of salivary glands

convey saliva to the mouth

What structure is this tissue part of?

salivary gland

1

salivary duct

2

salivary acini

3 (type of epithelial cell in acini)

mucous

4 (type of epithelial cell in acini)

serous

What structure is this tissue part of?

salivary gland

This salivary gland tissue is mostly composed of mucous or serous acini epithelium?

serous (pink color, granular)

Four layers of the tubular GI tract, from deep to superficial:

1.) mucosa

2.) submucosa

3.) muscularis

4.) serosa

Muscosa

innermost lining of the GI tract where epithelium is located

Three layers of the mucosa, from deep to superficial:

1.) epithelium

2.) lamina propria

3.) muscularis mucosae

Submucosa

A mostly connective tissue layer of the tubular GI that contains glands and lymphoid tissue

muscularis

Comprised of smooth muscle in both a circular and longitudinal arrangement

Function of muscularis layer of tubular GI

Together, the two layers of muscle contract in 2 dimensions, helping to propel food along the GI tract

serosa

outermost layer of the GI tract lined by mesothelium

mesothelium

epithelium that lines body cavities

Muscularis mucosa

A very thin muscle layer between the mucosa layer and submucosa layer of the tubular GI

1

serosa

2

muscularis

3

submucosa

4

muscosa

5 (part of mucosa)

epithelium

6 (part of mucosa)

muscularis mucosae

7 (part of muscularis)

circular smooth muscle

8 (part of muscularis)

longitudinal smooth muscle

1

muscosa

2

submucosa

3

muscularis

4

serosa

Mucosa of esophagus structure

stratified squamous epithelium +/- keratinization

In which species will the epithelium of the mucosa of esophagus be keratinized?

herbivores

Submucosa of esophagus structure

glands lined by simple cuboidal/columnar epithelium

Muscularis of esophagus structure

divided into sections based on type of muscle, which varies by species

Layers of muscularis of esophagus in pigs and humans

upper third: skeletal muscle

middle third: skeletal + smooth muscle

lower third: smooth muscle

Upper third layer of muscularis of esophagus in pigs and humans

skeletal muscle

Middle third layer of muscularis of esophagus in pigs and humans

skeletal + smooth muscle

Lower third layer of muscularis of esophagus in pigs and humans

smooth muscle

Layers of muscularis of esophagus in cats and horses

proximal 2/3: skeletal muscle

distal 1/3: smooth muscle

Proximal 2/3 layer of muscularis of esophagus in cats and horses

skeletal muscle

Distal 1/3 layer of muscularis of esophagus in cats and horses

smooth muscle

Layers of muscularis of esophagus in dogs and ruminants

100% skeletal

The outer layer of the esophagus can be called two named based on location:

1.) serosa

2.) adventitia

Where is the outer layer of the esophagus called adventitia?

Thoracic cavity

Where is the outer layer of the esophagus called serosa?

Abdominal cavity

What structure of the GI tract has the most variation histologically across species?

stomach

Two types of mucosa in the stomach:

1.) squamous

2.) glandular

Squamous mucosa of stomach: histology

non glandular; covered by stratified squamous epithelium

Squamous mucosa of stomach: function

mechanical protection; in ruminants, microbial fermentation and absorption of microbial products

Glandular mucosa of stomach: histology

simple columnar epithelium

Glandular mucosa of stomach: function

secretes mucous, hydrochloric acid, pepsin, and gastrin