Study hint:Read chapter 25 in your textbook before you study this lecture. Be able to trace the passage of food analward from the mouth. Know the contribution of each of the accessory organs of digestion.

Key Features of the Digestive System

Major Tissue Types: Epithelial, Connective, Muscular and Nervous tissue arranged in layers
Characteristic Cell Types: Simple columnar epithelial cell, Goblet cells, Smooth Muscle cells
Functional Unit: Simple Columnar Epithelial cell with a Brush Border (microvilli)
Parts of the System: Pathway for Food, Accessory Organs of Digestion
Confusing Terms: Villi-- a group of simple columnar cells-- versus microvilli-- extensions of the plasmalemma of individual cells

As you read this lecture, some facts you learned in the beginning of the course will be repeated. Let's start our discussion with some of these facts.

Histology

With this unit, we begin our study of organ histology. Whereas previously, I asked you to identify individual tissues, now you are required to identify the individual organs formed by arrangements of these tissues.

The Layers of the Gastrointestinal Tract or GIT

• Mucosa
  Simple columnar epithelium with a brush border at the luminal surface arranged in villi
  Loose connective tissue in the Lamina propria which also extends into villi
  The Muscularis mucosa is seen at the base of this layer

  Submucosa

  Loose connective tissue

  Contains the Submucosal plexus also called Meissner's plexus

  Muscularis externa

  Two, or, in the stomach, three layers of smooth muscle

  The Myenteric plexus, also called Auerbach's plexus, is located between muscle layers

  Adventitia or Serosa

Confusing Histology Slides

The histology site at the University of Nebraska at Omaha at http://cid.unomaha.edu/~swick/unit3hist.html#pancreas shows the Islets of Langerhans. Yet, the pancreas is confusing to students because the pancreatic acini (singular: acinus) look like the acini in a salivary gland. There are no pancreatic ducts in this view.

Now look at this photomicrograph of a salivary gland from Duke University at http://pathology.mc.duke.edu/research/histo_course/mixed_saliv.jpg. Differentiate the lighter staining, larger cells of the mucous acini from the smaller, darker staining cells of serous acini. Notice that a duct is present, but the acini are not interrupted by Islets of Langerhans.

Another challenge is to identify the triad in a slide of the liver. The triad consists of the hepatic artery, bile duct and hepatic portal vein which course in the connective tissue between lobules of the liver. The lobule is the functional unit of the liver.

Innervation of the GIT by the Autonomic Nervous System

I am amazed by the fact that motoneurons are found in the walls of the GIT. These visceral motoneurons function as postganglionic parasympathetic motoneurons. Let's review the autonomic innervation of the organs of the GIT.

The villi (singular, villus) are interesting structures. Branches of both the vascular and lymphatic circulatory systems extend into each one. Pay particular attention to the lacteals on the model in lab.