Introduction

The organization of the digestive tract is similar from the esophagus to the rectum. In cross sections of each of the parts of the digestive tract to be studied, three divisions should be apparent. The mucosa encloses the lumen; it is lined by epithelial cells and contains glands, blood vessels, lymphatic vessels, and nerves. A thin muscular layer, the muscularis mucosa, is also part of the mucosa and separates it from the submucosa. The submucosa is a layer of connective tissue which binds the mucosa to the muscular layer. The muscular layer (muscularis) consists of two layers of smooth muscles, an inner circular and an outer longitudinal layer. There is a fourth layer, the serosa, which will be difficult to see on our sections. The serosa is composed of an inner layer of flexible connective tissue covering the gut and an outer layer of membranous peritoneum. The digestive system is thus composed of different kinds of tissues, most of which you studied individually earlier in the semester. The figure to the right diagrammatically illustrates the different layers of the digestive tract. Use it to orient yourself to the slide material.

Begin the examination of each slide with the scanning objective. Orient yourself first at low power by identifying the lumen, the outer surface of the section and the four basic layers. Then use the higher powers to observe the detailed structure of each layer.