Digestive System I: Upper Alimentary Tract

Layers of the gastrointestinal tract

The wall structure of the tubular (hollow) organs follows a general organization that is uniform along the gastrointestinal tract. The lumen of the organs is covered with tunica mucosa (mucous membrane) that is composed of the superficial epithelial lining , the underlying lamina propria mucosae and the tunica muscularis mucosae . Lamina propria is composed of loose connective tissue that often contains glands and lymphocytic aggregations while the thin muscularis mucosae is usually formed by inner circular and outer longitudinal layers of smooth muscle cells. The mucous membrane is attached to the tunica muscularis externa by the tunica submucosa that is composed of dense irregular connective tissue with occasional glands embedded into the layer. Submucosa also contains autonomic ganglion cells and branches of nerves forming the submucosal or Meissner’s plexus that innervates the muscularis mucosae. Tunica muscularis externa is a thick layer of smooth muscle cells under the submucosa that is organized most commonly into an inner layer of circularly and outer layer of longitudinally arranged cells. Ganglion cells and nerve fibers located between the circularand longitudinal layers form the myenteric or Auerbach’s plexus that is responsible for the innervation of the smooth muscle cells of the layers. If the organ is covered by peritoneum, the muscularis externa is covered by tunica serosa that is composed of a loose connective tissue layer (lamina propria serosae) lined outside with a single layer of squamous mesothelial cells. In the absence of the peritoneal coverage, muscularis externa is ensheathed by a simple loose connective tissue coat, the tunica adventitia. Both lamina propria serosae and tunica adventitia contain nerves and vessels that supply the wall of the organ. Organs that are not tubular (e.g., glands associated with the digestive system), commonly contain a functional part, the parenchyma that is surrounded by a connective tissue stroma that provides the structural support and carries vessels and nerves.

Lip

Oral cavity can be subdivided into the vestibule and the oral cavity proper; the former is located between the lips, cheeks, and the teeth while the latter represents the rest of the oral cavity. Vestibule is bordered anteriorly by the lips that contain a core of striated muscle mostly composed of the orbicularis oris. The muscle core is covered by three distinct layers. Pars cutanea represents the outermost (anterior) zone covered by skin, and it contains hair follicles, sebaceous glands, and eccrine sweat glands. The interior (posterior) surface of the lip is covered by lining mucosa that is composed of stratified squamous nonkeratinized epithelium with occasional parakeratinized areas and the underlying lamina propria that sends few short papillae into the epithelium (pars mucosa). The mucous membrane is attached to the core muscle layer by a dense irregular connective tissue submucosa that contains seromucous glands. Sebaceous glands without hair follicles may also appear in the submucosa around the lip (Fordyce spots). Pars cutanea and pars mucosa is connected by the thinly keratinized vermilion zone (rubor labii) that is characterized with high connective tissue papillae and extensive capillary networks that are responsible for the red coloration of this region. The cheeks (bucca) have a similar structural organization to the lips with the exception of the missing vermillion zone.