Tissues & Organs

DIGESTIVE SYSTEM

Tissues & Organs

DIGESTIVE SYSTEM

General Histology of Digestive System

1. Mucosa

Lined by epithelium

Loose connective tissue

Lamina propria (BV, LV, glands, Peyer’s Patches (Ileum))

Muscularis mucosae (IC/OL SMC layer)

2. Submucosa

Surrounded by dense irregular (fibroelastic) connective tissue

Glands, BV, LV

Parasympathetic nerve tissue (Meissner’s submucosal plexus)

3. Muscularis Externa

IC/OL SMC layer

Parasympathetic nerve tissue (Auerbach’s myenteric plexus)

4. Serosa/Adventitia

May be surrounded by simple squamous epithelium of visceral peritoneum

Structure of the Esophagus Diagram

Esophagus

1. Mucosa

Epithelium | stratified squamous non-keratinized epithelium

Cellular component | Langerhans cells (macrophages, APC)

Lamina Propria | esophageal cardiac glands (compound tubular)

Muscularis Mucosae | single layer of longitudinal SMC

2. Submucosa

Connective Tissue | dense irregular fibroelastic connective tissue

Key ID | esophageal glands proper (compound) – mucous cells & serous cells

3. Muscularis Externa

Upper third – skeletal muscle

Mid third – skeletal and smooth muscle

Lower third – smooth muscle

Esophagus

Cross section of the esophagus. In a relaxed state, the esophagus is deeply folded and becomes extended when food is present. The lumen is lined by stratified squamous epithelium. It is hard to tell what section of the esophagus this is from, try looking at the following picture for a better muscularis externa.

Esophagus

Closer look at the esophagus. The muscularis externa transitions from skeletal muscle (key ID – nuclei found at the periphery) to smooth muscle (key ID – may have 2 or more nuclei and are located in the center). Therefore, this is the mid third section of the esophagus.

Esophagus

Another look at the esophagus. Try determining what section of the esophagus this is from based on the muscle types you see here.

Fibroblasts

White Adipose Cells

Stomach

1. Mucosa

Epithelium | simple columnar epithelium

Cellular component | surface lining cells, parietal cells, regenerative cells, mucous neck cells, chief cells, enteroendocrine cells, NO goblet cells

Lamina Propria | loose connective tissue, gastric glands (simple tubular)

Muscularis Mucosae | IC/OL SMC layer & third circular SMC layer

2. Submucosa

Connective Tissue | dense irregular collagenous connective tissue

3. Muscularis Externa

Inner oblique SMC

Middle circular SMC

OL SMC

Stomach

A view of the mucosa in a folded state showing the gastric glands extending from the lumen into the muscularis mucosae.

Stomach

Another look at the gastric glands which are straight tubular glands synthesizing gastric juice. The next photo shows a closer look at the epithelium layer.

Stomach

A closer view of the simple columnar epithelium lining the mucosa. Note the nuclei are all close to the basement membrane.

Stomach

This shows the thickness of the gastric body in the mucosa and a small part of the submucosa. The gastric pits are evident. Look at the next photo for a closer view of the mucous neck cells and parietal cells. Although very small, the chief cells can be found near the base of the gastric glands, which extend down into the muscularis mucosae. Notice how the normal gastric mucosa has no lymphoid cells.

Stomach

Here, you can see the parietal cells (magenta colour with central nuclei) lining the lower part of the gastric pits. Again, the simple columnar epithelium is evident near the top. You may also spot some mucous neck cells which are more pale in appearance. Not seen here, are the chief cells which are located close to the bottoms of the gastric pits.

Pyloric Stomach

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Fundic Stomach

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Cardiac Stomach

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Stomach Cross Section

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Small Intestine Common Features

1. Mucosa

Valves of Kerckring/plicae circulares

Villi | differs between the regions

Epithelium | simple columnar epithelium, Crypts of Lieberkhun (simple tubular or branched tubular glands)

Cellular component | surface absorptive cells (brush border), goblet cells, enteroendocrine cells, regenerative cells, EED cells, Paneth cells

Lamina Propria | reticular tissue, BV, LV, SMC

Muscularis Mucosae | IC/OL SMC layer

2. Submucosa

Valves of Kerckring/plicae circulares

Connective Tissue | dense irregular fibroelastic connective tissue

3. Muscularis Externa

Connective Tissue | thin layer of fibrous connective tissue

Nerve Plexi

IC/OL SMC layer

Crypts of Lieberkuhn

This is a section of the jejunum where the Crypts of Lieberkuhn are evident. They are present in the duodenum and jejunum as well. They reside just above the connective tissue layer, extending down into the muscularis mucosae.

Duodenum

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Jejunum

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Ileum

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Small Intestine Differing Features

Duodenum

1. Mucosa

Villi | leaf shape

Cellular component | goblet cells

2. Submucosa

Key ID | Brunner’s glands (compound tubular/branched)

Jejunum

1. Mucosa

Villi | finger-like

Cellular component | more goblet cells

Ileum

1. Mucosa

Villi | club-like

Cellular component | even more goblet cells

Lamina Propria | Peyer’s Patches

2. Submucosa

Key ID |more lymphoid tissue

Duodenum

The duodenum has characteristic long villi mimicking leaf shapes. The Crypts of Lieberkuhn are present which extend down towards the muscularis mucosae. A unique feature and key ID are the Brunner’s Glands found in the submucosa (the whitish coloured branched tubular glands) just above the connective tissue layer. When distinguishing from the Jejunum, use the shapes of the villi and presence of Brunner’s glands to guide you.

Jejunum

Notice how the villi are becoming less elongated and more stubby, taking on a finger-like shape. The crypts of Lieberkuhn are present in the mucosa just above the connective tissue layer. Notice the lack of Brunner’s Glands and Peyer’s patches, making this the Jejunum.

Ileum

The most evident feature that separates the ileum from the other parts of the small intestine are the Peyer’s Patches dominating the mucosa. The lymphoid tissue stains dark purple and is a key ID when identifying the Ileum. The villi become even shorter here too. This one is easier to identify, the duodenum and jejunum are a bit trickier.

Large Intestine

Colon

1. Mucosa

NO Valves of Kerckring/plicae circulares

Villi | NO villi

Epithelium | simple columnar epithelium, crypts of Lieberkhun

Cellular component | lots of goblet cells, absorptive cells, NO Paneth cells

Lamina Propria | more lymphatic tissue (lymph nodules), long simple tubular glands

Muscularis Mucosae | IC/OL SMC layer

2. Submucosa

Lymphatic tissue

3. Muscularis Externa

Teniae coli

Nerve Plexi

4. Serosa

Adipose tissue

Colon

In the mucosa, goblet cells dominate secreting mucous for faecal lubrication. You can see the closely packed tubular glands which create crypts that sit on the muscularis mucosae. Although not visible here, lymphoid aggregates are also found in the colon, however, they are smaller than the Peyer’s Patches found in the ileum.

Colon

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Appendix

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Appendix

Structure closely resembles that of the colon

1. Mucosa

NO Valves of Kerckring/plicae circulares

Villi | NO villi

Epithelium | simple columnar epithelium, crypts of Lieberkhun

Cellular component | lots of goblet cells, absorptive cells, NO Paneth cells

Lamina Propria | more lymphatic tissue (lymph nodules), long simple tubular glands

Muscularis Mucosae | IC/OL SMC layer reduced

2. Submucosa

Lymphatic tissue extending to submucosa

3. Muscularis Externa

Teniae coli

Nerve Plexi

4. Serosa

Adipose tissue

Rabbit Appendix

Human Appendix

Rectum & Anus

Resembles colon except:

Mucosa in the Anal Canal transitions from simple columnar epithelium to stratified squamous non-keratinized epithelium to stratified squamous keratinized epithelium at the anus

Increase in goblet cells, adipose tissue, and thicker teniae coli

Recto-Anal Junction

An abrupt transition is illustrated here, where the simple cuboidal epithelium transitions into stratified squamous epithelium in the anal canal.

Recto-Anal Junction

A satisfying closer look at the transitioning epithelium! You can even see the numerous goblet cells.

Anal Canal

The anal canal is the last section of the gastrointestinal tract and is surrounded by voluntary muscle which forms the anal sphincter (not visible here). You can see the stratified squamous keratinized epithelium. You may also find sebaceous glands and large apocrine sweat glands in this area.

Pancreas

Exocrine

Stroma | thin dense irregular connective tissue

Parenchyma | acinar cells

Glands | compound tubuloacinar gland

Duct system | centroacinar cells, simple columnar cells, goblet cells, EED cells

Endocrine

Islets of Langerhans | alpha cells (glucagon), beta cells (insulin), D cells (somatostatin), G cells, PP cells

Pancreas

A thin collagenous capsule surrounds the pancreas which is a lobulated organ. It is composed of both exocrine and endocrine tissue. Look at the following two photos to see the difference between the cell types.

Exocrine Pancreas

The exocrine component consists of the acinar cells which release their secretions into the main pancreatic duct which joins the common bile duct, draining into the duodenum via the ampulla of Vater.

Endocrine Pancreas

The endocrine component of the pancreas consists of the alpha and beta cells (evident here) forming the Islets of Langerhans.