The digestive system consists of a long muscular tube, extending from mouth to anus, and a large number of associated glands, which secrete substances that aid food digestion and absorption of nutrients. The digestive tube is generally made up of four structurally and functionally distinct layers: the mucosa, submucosa, muscularis, and serosa (or the adventitia, if not covered by serosa). The mucosa, which is in contact with the luminal contents, is constructed to resist abrasion and to perform secretory and absorptive functions. The muscular layer, which serves to propel the food through the tube, is attached to the mucosa by a dense connective tissue layer, the submucosa. The serosa or adventitia carries blood vessels and nerves to the wall of the digestive tube.

The large number of glands associated with the digestive tract range from unicellular and small tubuloalveolar glands residing in the mucosa to large organs such as the pancreas and liver. Although their secretions can be quite dissimilar, they all function to promote the digestive process by imparting enzymes or mucus into the ingested food or by regulating motility of the tract to propel the ingested food distally.

The learning objectives for this unit are:

• Identify the histologic and cellular features of teeth and their supportive structures.
• Identify the histoarchitecture of the tongue, including the gustatory and support cells in taste buds, and the functional significance of these structures.
• Compare and contrast the secretory functions and relative quantities of serous and mucous cells in parotid, submandibular, and sublingual glands.
• Identify the histological layers and unique cellular characteristics associated with the oral cavity, esophagus, stomach, duodenum, jejunum, ileum, appendix, and colon (mucosa, submucosa, muscularis proper, and serosa/adventitia) and their functions.
  

Let's start with the oral cavity.