Digestive Histology – outline notes

INTRODUCTION:


The focus of this lab is digestive system histology.  Remember histology is the study of tissues.  The goal of histology is to not only understand the microanatomy of cells, tissues, and organs, but also to understand their function in structural terms.  Studying the histology of the digestive system will complement the study of its gross anatomy and provide the structural basis for learning digestive physiology in lecture.  In this lab, you will view the histology of selected regions from the gastrointestinal tract (GI tract) and several digestive system accessory organs.

From the esophagus to the rectum, the GI tract is comprised of four major layers.  We will first look at the general characteristics of each of these layers, and then point out the regional modifications along the tract.  Remember that specific functions are carried out by specific structures (function reflects structure).  The function of the stomach is much different from the function of the esophagus or small intestine, therefore their structures must be different as well.




I. General Histology of the 4 major layers of the Digestive Tract
A. Mucosa – comprised of three sublayers
1. Epithelium
a. type varies depending on location
2. Lamina propria
a. loose connective tissue underlying epithelium
3. Muscularis mucosae
a. smooth muscle
b. boundary between mucosa and submucosa
B. Submucosa
1. consists of moderately dense, irregular connective tissue
2. contains large blood vessels and lymphatic vessels
3. Submucosal Plexus (plexus of Meissner)
a. nerve supply
C. Muscularis Externa
1. typically smooth muscle tissue is organized into:
a. inner circular layer
b. outer longitudinal layer
2. Myenteric Plexus (plexus of Auerbach)
a. nerve supply sandwiched between the inner circular and outer
longitudinal layers
b. coordinate movements along GI tract

D. Serosa
1. same as visceral peritoneum
2. mesothelium (simple squamous epithelium) and underlying loose
connective tissue

II. Esophagus 
A. Mucosa
1. Stratified squamous epithelium
2. Lamina propria – usual
3. Muscularis mucosae – thicker than usual

Question – What function does stratified squamous epithelium serve here?

B. Submucosa
1. Dense irregular connective tissue with mucous glands

Question – What function do mucous glands serve here?

C. Muscularis Externa
1. Inner circular and outer longitudinal
a. upper 1/3 = skeletal muscle continuous with pharynx
b. middle 1/3 = mixed skeletal muscle and smooth muscle
c. lower 1/3 = smooth muscle

Question – Why do you think the proximal esophagus contains skeletal
muscle?  Why is there a transition to smooth muscle?

D. Serosa
1. Only after piercing the diaphragm do you find a serosa covering the     esophagus
2. Adventitia – a dense connective tissue, covering the esophagus above
the diaphragm, that blends with surrounding connective tissue

Question – Why is there not a normal serosa that covers the esophagus
above the diaphragm?

III. Stomach (VIEW SLIDE)
A. Mucosa
1. Simple columnar epithelium
a. each cell possesses ability to produce mucus which protects the
stomach lining from gastric juice
2. Gastric pits
a. deep depressions in stomach lining formed by simple columnar
epithelial cells; lead to gastric glands
3. Gastric glands
a. located at the bottom of the gastric pits
b. produce gastric juice and empty into gastric pits
4. Lamina propria
a. surrounds gastric pits and gastric glands
5. Muscularis mucosae – usual

Question – What are the major differences (modifications) of the mucosa
of the stomach compared to the mucosa of the esophagus?

B. Submucosa – usual
C. Muscularis Externa
1. Three layers of smooth muscle
a. Inner oblique
b. Middle circular
c. Outer longitudinal




Question – Which layer of smooth muscle has been added.  Why do you
think the stomach has this extra layer of smooth muscle?
D. Serosa – usual

IV. Small Intestine (DUODENUM)
A. Mucosa
1. Simple columnar epithelium with goblet cells
a. Enterocytes – columnar epithelial cells
2. Lamina propria
3. Muscularis mucosae
4. Modifications that extensively increase surface area       a. Plicae circulares – folds of submucosa covered by mucosa
b. Villi – finger-like or leaf-like projections of the mucosa
1. Covered with simple columnar epithelium with goblet       cells with underlying lamina propria
2. Contain
a) Lacteal – specialized lymphatic capillary
centrally located within villus
b) Capillaries
c. Microvilli – modification of the apical plasma membrane of
enterocytes; form a Brush (Striated) Border

Question – What is the purpose of increasing the surface area in the small
intestine?

5. Intestinal Glands (Crypts of Lieberkuhn)
a. extend through thickness of mucosa and open at base of villi
b. surrounded by lamina propria
c. stem cell division renews epithelial surface cells
B. Submucosa – usual
1. Brunner’s glands (submucosal glands)
2. Hint – it is easy to distinguish between the mucosa and submucosa by
first locating the muscularis mucosae (a narrow pink river) which
separate the two
C. Muscularis Externa – usual
D. Serosa

Digestive system accessory organs

IV. Liver
 A. Hepatocytes – liver cells
B. Lobules – classical, structural, and functional units of the liver
1. Hexagonal shape
2. Central vein – centrally located within lobule
3. Portal triads – six areas located at each corner of the hexagon
a. branch of hepatic artery (vascular)
b. branch of hepatic portal vein (vascular)
c. branch of bile duct
4. Sinusoids
5. Bile canaliculi




1. Trace the flow of arterial blood from the abdominal aorta  through the liver to the inferior vena cava
2. Trace the flow of nutrient rich blood from the digestive tract through the liver to the inferior vena cava
3. Trace the flow of bile from the hepatocytes to the gall bladder
4. Trace the flow of bile from the gall bladder to the duodenum

V. Pancreas
A. Pancreatic acini
1. Exocrine function
B. Islets of Langerhans
1. Endocrine function