Digestive System

THE DIGESTIVE SYSTEM

The digestive system consists of the digestive tract and associated glands.

The main components of the digestive tract are the:

Oral cavity
Esophagus
Stomach
Small Intestine
Large Intestine
Rectum and anus

The entire digestive tract can be considered as a hollow tube surrounded by a wall composed of four main layers:

Mucosa
Submucosa
Muscularis
Serosa or Adventitia

1. Mucosa

The mucosa consists of :

epithelium (lining the lumen)
lamina propria (loose connective tissue)
muscularis mucosae (thin layer of smooth muscle cells).

The epithelial lining of the mucosa forms a selective barrier between the external environment (lumen) and the body. All the food products that are digested and absorbed by the body need to pass through the epithelial lining. This epithelial lining may contain goblet cells, that secrete mucus for lubrication. Endocrine cells (part of the diffuse endocrine system) are common in the epithelium and produce polypeptide hormones, that play a role in the regulation of the digestive processes.

The lamina propria, situated just below the epithelium, consists of loose connective tissue, with an abundant blood supply. Lymphatic nodules, lymphocytes and plasma cells, and macrophages are common in the lamina propria and form a first line of immunological defense against bacterial and viral invasion.

The muscularis mucosa causes local muscular contractions in the mucosa.

2. Submucosa

The submucosa consists of dense connective tissue and Meissner's nerve plexus.

3. Muscularis

The muscularis consists of two sub-layers of smooth muscle cells (typically inner circular layer, outer longitudinal layer). These are involved in the peristaltic movements of the intestine. Between these two muscle layers is Auerbach's (myenteric) nerve plexus. The rhythmic peristaltic contractions of the muscularis are responsible for propelling and mixing food in the digestive tract. These movements are mainly generated by Auerbach's myenteric plexus. The neurons of the plexus can be visualized by silver impregnation techniques. It can be shown that the plexuses consist of aggregates of nerve cells in the form of small parasympathetic ganglia.

4. Serosa or Adventitia

The serosa consists of a thin layer of loose connective tissue covered by a simple squamous epithelium (mesothelium). Serosa is present in the parts of the intestinal tract that are present in the peritoneal cavity. The regions of the intestinal tract that are not present in the peritoneal cavity are held in place by an outer layer of loose connective tissue (adventitia).

ESOPHAGUS

The esophagus is a straight muscular tube connecting the oral cavity to the stomach. The esophagus contains the four basic layers common to the rest of the digestive tract.

The esophagus is lined with stratified squamous epithelium (without keratin).

The lamina propria near the stomach contains mucus-secreting esophageal cardiac glands.

Mucus-secreting esophageal glands are present in the submucosa. (This is the only site in the intestinal tract, apart from the duodenum, where exocrine glands are present in the submucosa.)

The muscularis in the upper third of the esophagus is composed of striated muscle cells (non-voluntary muscle). In the mid-region of the esophagus the muscularis has a mixture of striated muscle and smooth muscle. The muscles of the lower third of the esophagus are only smooth muscle.

The outermost layer of the esophagus consists of adventitia, apart from a small portion of the esophagus that extends into the peritoneal cavity (serosa).

STOMACH

The stomach is a very muscular organ in which acid secretions and digestive enzymes contribute to the digestion of food. From a histological viewpoint the stomach can be divided into two major histological regions:

Fundus and Body
Pylorus

FUNDUS and BODY

These are lined with a homogeneous simple columnar epithelium consisting of mucus-secreting cells. Mucus secreted by these cells provides protection from the highly acidic contents of the lumen. The surface epithelium invaginates into the lamina propria to form gastric pits. The gastric glands of the fundus and body are branched tubular glands, which open into the gastric pits. These tubular glands consist of:

Stem cells (in the neck region). These cells proliferate rapidly and migrate to replace the stomach epithelial lining cells (whose turnover time is short, about 4 days, owing to the harsh acidic environment). These stem cells also migrate deeper into the glands and differentiate into the various secretory cells of the glands.
Mucous neck cells. These cells, also present in the neck region, secrete mucus (stained intensely with PAS), with staining characteristics that differ from the surface epithelial lining cells.
Parietal cells (oxynitic cells). These are mainly found in the upper part of the gastric glands. These are rounded or pyramidal cells characterized by their round central nuclei and very acidophilic cytoplasm (owing to the large number of mitochondria). These cells possess intracellular canaliculi with microvilli opening to the lumen (difficult to see in wax-sections). The parietal cells are responsible for the synthesis and secretion of hydrochloric acid. In humans these cells are also responsible for synthesis of intrinsic factor, a glycoprotein essential for intestinal vitamin B₁₂ absorption.
Chief cells (Zymogen cells). These are present mainly in the deeper parts of the glands. These cells are involved in synthesis and secretion of digestive enzymes and consequently display the features of protein synthesizing and exporting cells. This is seen as basophilic staining in the more basal regions of the cells (areas of rough endoplasmic reticulum). The zymogen granules that accumulate in the apical regions of the cells, contain the proenzyme pepsinogen, which is converted into the proteolytic enzyme, pepsin, when secreted into the acid environment of the stomach.
Enteroendocrine cells.

PYLORUS

The gastric pits of the pylorus are longer and wider than the pits of the fundus/body, however the gastric glands are shorter and more coiled. These glands consist almost entirely of mucus-secreting cells (there are no parietal or zymogen cells). The gastric gland cells also secrete lysozyme.

The endocrine cells of the pylorus include cells secreting gastrin (which stimulates acid secretion by the parietal cells).

The muscularis of the stomach is composed of three layers of smooth muscle :

external longitudinal layer
middle circular layer
internal oblique layer

These layers are not always easily distinguishable in histological sections. The pyloric sphincter, which controls the discharge of stomach contents to the duodenum, consists of an enlarged middle layer of smooth muscle.

SMALL INTESTINE

The small intestine is the main site of absorption of digested food. The small intestine is specialized for the completion of the digestion processes and the subsequent absorption of the digested produc. The overall length of the small intestine is about 5 meters, and consists of three main segments :

Duodenum
Jejunum
Ileum

Characteristic features of the small intestine include:

(a) Intestinal villi. These are finger-like projections into the lumen (consisting of surface epithelium and underlying lamina propria).

The epithelium lining the lumen consists of a simple columnar heterogeneous epithelium with goblet cells. The apical surface of the absorptive epithelial cells has a "brush border" (resulting from an orderly arrangement of closely-packed microvilli, which may number several hundred per absorptive cell). The microvilli, as seen by transmission electron microscopy, have a central core of actin filaments. The main function of the microvilli is to incthe surface area available for absorption. The absorptive cells have oval nuclei, typically in the basal half of the cells.

The lamina propria of the small intestine is formed from loose connective tissue. This contains blood vessels, nerves, large lymphatic vessels (site of absorption of lipids), and cells of the immune system, often in the form of lymphatic nodules.

(b) Intestinal glands. These are simple tubular glands that open to the intestinal lumen between the base of the villi. The intestinal glands are sometimes called the crypts of Lieberkuhn. Secretory cells (Paneth cells) with large acidophilic granules are found at the base of the intestinal glands. Their function is still not fully understood, but it is known that they secrete lysozyme, which has anti-bacterial properties.

(c) Valves of Kerckring. The lining of the small intestine has permanent folds known as Valves of Kerckring or Plicae circulares. These are most prominent in the jejunum. These folds, seen macroscopically in transverse sections, consist of mucosa and submucosa.

DUODENUM

The main distinguishing feature of the duodenum is the presence of glands in the submucosa. These duodenal or Brunner's glands produce alkaline secretions to counteract the effects of gastric acids that reach the duodenum. They also provide the necessary alkaline environment for the functioning of the exocrine pancreatic secretions.

JEJUNUM

The main distinguishing feature of the jejunum is the presence of prominent Valves of Kerckring (plicae circulares).

ILEUM

The ileum is almost devoid of Valves of Kerckring, however large accumulations of lymphatic tissue, both nodular and dense, are found in the lamina propria. These can often be seen macroscopically as large white patches and are commonly known as Peyer's Patches.

LARGE INTESTINE

The large intestine lacks folds or villi. It is characterized by many tubular intestinal glands with large numbers of goblet cells. This is sometimes described as a glandular epithelium.

The large intestine is the site of water absorption (via columnar absorptive cells) and is also the site of formation of the feces. The secretions of the goblet cells provide lubrication for the luminal surfaces. Abundant lymphatic tissue is common in the lamina propria (owing to the large bacterial population in the lumen of the large intestine).

Whereas the circular smooth muscle layer is continuous, the longitudinal smooth muscle of the muscularis is in the form of three thick bands, known as teniae coli.

The anal region, unlike the rest of the large intestine, has a series of longitudinal folds and the epithelium becomes a stratified squamous epithelium.

APPENDIX

The appendix is a blind-ended tube with the basic histological structure of the large intestine. The lumen is usually fairly narrow and irregular. Lymphatic tissue (nodular and dense) is abundant in the wall of the appendix.