In humans, the small intestine is usually consists of three sections; the first, as food passes from the stomach, is the duodenum. The jejunum and ileum are other parts, typically differentiated by the types of cells that line the inside of the tract. This first section is a tube-like structure that is generally 9.8 to 11.8 inches (25 to 30 centimeters) long. It is usually composed of several layers. The histology of the duodenum is also characterized by microscopic structures called Brunner’s glands, which can secrete mucus that neutralizes stomach acids.
Food often enters the organ after up to four hours after eating. Small ducts can allow pancreatic enzymes and bile to flow in, which also neutralize acids as well as help with digestion. The histology of the duodenum typically represents a folded structure, especially in the mucosa layer, which is typically made up circular or transverse folds. There are also projections called villi, which are up to 0.04 inches (1 millimeter) long. At their core are lymph and blood vessels as well as nerves.
Usually inside the villi is a microscopic layer called a brush border, which is made up of hair-like parts of cells called microvilli. There are also goblet cells, which secrete mucous to help food pass through the tube. Microfold cells also make up the histology of the duodenum; they normally function by presenting antigens and line a series of lymphatic nodules. Cells called lymphocytes are often found in a deeper layer called the lamina propria, which also consists of connective tissues. The lymphatic structures can be concentrated in certain areas, while glands typical of the intestines are found in this layer as well.
Intestinal glands, called Crypts of Liberkuhn, are generally tube-shaped and open at the inner surface of the duodenum. There are usually mucous and other endocrine-secreting cells as well. The submucosa layer is typically the location of the Brunner glands, which are only found in the histology of the duodenum. Generally identified by a coiled, branched tubular shape, these are lined by mucous secreting cells and typically connect with ducts that lead to the intestinal glands.
A circular layer and longitudinal layer of muscle usually makes up a significant part of the histology of the duodenum called the muscularis. The outer layer is typical of that found throughout the small intestine, while the serosa generally forms the outer part of the duodenum. This layer is also a component of the peritoneum, or the tissue that surrounds the abdominal organs.