Question

The GI tract is lined by a series of tissues in an organized way. Describe the four layers of the GI tract (5 points). Using the stomach as a model organ, discuss the various cell types that exist, their function, and the communication that exists amongst the various cell types in order to accomplish its digestive function. Be sure to explain the types of communication that are present (e.g. autocrine, paracrine, endocrine etc.) (10 points)

Answer 1

The GI tract contains four layers:

the innermost layer is the mucosa, underneath this is the submucosa, followed by the muscularis propria and finally, the outermost layer - the adventitia.

Mucosa- A lining epithelium, including glandular tissue, an underlying layer of loose connective tissue called the lamina propria, which provides vascular support for the epithelium, and often contains mucosal glands. Products of digestion pass into these capillaries. Lymphoid follicles, and plasma cells are also often found here. Finally, a thin double layer of smooth muscle is often present - the muscularis mucosa for local movement of the mucosa.

Submucosa-A loose connective tissue layer, with larger blood vessels, lymphatics, nerves, and can contain mucous secreting glands.

Muscularis propria (externa): smooth muscle layer.-There are usually two layers; the inner layer is circular, and the outer layer is longitudinal. These layers of smooth muscle are used for peristalsis (rhythmic waves of contraction), to move food down through the gut.

Adventia layer (or serosa)-Outermost layer of loose connective tissue - covered by the visceral peritoneum. Contains blood vessels, lymphatics and nerves.

Four major types of secretory epithelial cells cover the surface of the stomach and extend down into gastric pits and glands:

• Mucous cells: secrete an alkaline mucus that protects the epithelium against shear stress and acid
• Parietal cells: secrete hydrochloric acid
• Chief cells: secrete pepsin, a proteolytic enzyme
• G cells: secrete the hormone gastrin

There are four categories of chemical signaling found in multicellular organisms: paracrine signaling, autocrine signaling, endocrine signaling, and signaling by direct contact. The main difference between the different categories of signaling is the distance that the signal travels through the organism to reach the target cell.

Paracrine signaling

Often, cells that are near one another communicate through the release of chemical messengers (ligands that can diffuse through the space between the cells). This type of signaling, in which cells communicate over relatively short distances, is known as paracrine signaling.

Autocrine signaling

In autocrine signaling, a cell signals to itself, releasing a ligand that binds to receptors on its own surface (or, depending on the type of signal, to receptors inside of the cell). This may seem like an odd thing for a cell to do, but autocrine signaling plays an important role in many processes.

Endocrine signaling

When cells need to transmit signals over long distances, they often use the circulatory system as a distribution network for the messages they send. In long-distance endocrine signaling, signals are produced by specialized cells and released into the bloodstream, which carries them to target cells in distant parts of the body. Signals that are produced in one part of the body and travel through the circulation to reach far-away targets are known as hormones.

Signaling through cell-cell contact

Gap junctions in animals and plasmodesmata in plants are tiny channels that directly connect neighboring cells. These water-filled channels allow small signaling molecules, called intracellular mediators, to diffuse between the two cells. Small molecules, such as calcium ions are able to move between cells, but large molecules like proteins and DNA cannot fit through the channels without special assistance.