Small intestine big

When you’re studying pathology, it’s a good idea to review normal stuff quickly before you get into all the diseases. It’s easy to forget histology, for example – and a couple minutes of review can be enough to bring it all back.

So I thought I’d do a little review here and there of the histology of different organ systems, starting with the gastrointestinal tract. This is a super-short crash course, with just the very most important stuff – so for a more in-depth review, you may want to go back to your notes.

Main structure of the GI tract

The GI tract is composed of four main structural layers. From inside to outside, they are:

  1. Mucosa (consisting of epithelium, lamina propria, and muscularis mucosa)
  2. Submucosa
  3. Muscularis externa
  4. Adventitia/serosa

Of these layers, the most high-yield to focus on is the mucosa. It varies according to where you are (esophagus vs. stomach vs. small intestine vs. colon), and it’s easy to forget after you’ve been away from it for a while. If you remember the main function of each area, it will help you remember what cells are present in the epithelium.

  • Esophagus: carries undigested food down to stomach (nice, strong squamous epithelium)
  • Stomach: begins digestive process (parietal cells secrete HCl and intrinsic factor; chief cells secrete pepsinogen)
  • Small intestine: absorption of most nutrients (cells have microvilli)
  • Large intestine: absorption of water (tons of goblet cells secrete mucus to help the dried-out poopy slide through)

Here is a quick summary of mucosa in each main area.

Esophagus

  • Lined with non-keratinized stratified squamous epithelium
  • Mucosa is flat with no villi or pits or crypts (compared to the other layers below)

Stomach

  • Pits (lined with simple columnar epithelium) dip down from the surface into the lamina propria
  • Glands (lined with simple columnar epithelium with parietal cells and chief cells) in the lamina propria dump into the bottom of each pit

Small intestine

  • Plicae circulares are large infoldings into the lumen of the intestine (contain mucosa and submucosa – see image of a single plicum below)
  • Villi are smaller infoldings into the lumen of the intestine (contain epithelium and lamina propria – see image below)
  • Villi are lined with simple columnar absorptive cells with occasional goblet cells
  • Crypts of Lieberkuhn are long, straight glands in lamina propria that are continuous with the surface epithelium
  • Crypts are lined with simple columnar absorptive cells with Paneth cells, microfold (M) cells, and occasional goblet cells

small intestine

Large intestine

  • Crypts of Lieberkuhn are long, straight glands in the lamina propria that are continuous with the surface epithelium
  • Crypts are lined with simple columnar absorptive cells with tons of goblet cells