Atlas of plant and animal histology

The digestive system shows a variety of organizations in both invertebrates and vertebrates. In vertebrates, the digestive system is a duct connecting the mouth, in the rostral part of the body, to the anus, in the caudal part of the body. Although there are major differences when comparing the digestive systems across species, mostly due to the type of food they eat, we are going to outline the overall organization of the digestive system in mammals taht have an omnivorous diet.

The digestive system can be divided into a cephalic region, an axial part, and two large glands: the liver and pancreas (Figure 1).

The cephalic area includes the oral cavity, salivary glands, and teeth, as well as the tongue and the pharynx, which connects the digestive and respiratory systems. The oral and nasal cavities are separated by the palate, except at the level of the pharynx. Both the hard palate (anterior) and the soft palate (posterior) are covered by the same type of epithelium as the rest of the oral cavity: stratified squamous epithelium. When comparing different animal species, the cephalic part shows the higher structural variability of the digestive system. The functions of the cephalic area are mechanical digestion, secretion of digestive enzymes like amylase, and swallowing, as well as taste sensing.

The oral cavity is connected to the esophagus through the pharynx. The pharynx is involved in both digestion and breathing. When food enters the pharynx, the ducts that drive the air to the lungs are closed. The pharynx can be divided into an upper part related to the nasal cavity, the middle oropharynx region, and the caudal nasopharynx, which is related to both digestion and breathing.

The part of the digestive system outside the cephalic region comprises the esophagus, stomach, and intestines (small, large, and rectum). Ingested food goes through the pharynx into the esophagus on its way to the stomach. In the stomach, the low pH and digestive enzymes produce partial digestion. Subsequently, food moves to the small intestine for further breakdown of the meal. Here, water and nutrients are absorbed through the epithelium and transferred to the blood and lymphatic vessels. Finally, leftover products are pushed into the large intestine and expelled through the anal canal.

The histological organization of the esophagus, stomach, and intestine consists of four layers (Figure 2). The mucosa is the innermost layer, which directly faces the lumen. It is formed of an epithelium that lines the inner surface of the digestive tract, showing different morphological features involved in functions like protection, secretion, and absorption throughout the digestive system. This epithelium rests on the basal lamina, which in turn is situated above the lamina propria. The lamina propria is highly irrigated, loose connective tissue that also houses many cells of the immune system, including macrophages, plasma cells, and lymphocytes. Together, the epithelium, basal lamina, and lamina propria form the mucosa. Within the lamina propria, there is a discontinuous layer of smooth muscle cells known as the muscularis mucosae, found in the innermost part of the mucosa (away from the epithelium). The muscularis mucosae makes possible the movement of the mucosa.

The submucosa is a layer of irregular, dense connective tissue located beneath the mucosa. This layer contains many exocrine glands, known as submucosal exocrine glands. It is also highly irrigated by blood vessels and is innervated by a dense network of nerves that form the Meissner's plexus, which controls the motility of the mucosa and the secretory activity of the exocrine glands. 

The muscularis, or muscle layer, covers the submucosa. It is made up of smooth muscle cells, except for the upper part of the esophagus, which shows skeletal striated muscle fibers. This muscle layer is organized into two sublayers, an inner layer with circularly arranged cells and an outer layer with longitudinally arranged cells. Between both layers, there is a neurovegetative neural plexus, known as Auerbach's plexus, that drives the muscle contraction, resulting in peristaltic movements along the digestive tube.

The serose is the innermost layer. It is made up of loose connective tissue with many adipose cells. This layer sets the boundary between the digestive tube and the coelomic cavities, and it is attached dorsally to the mesentery.

Two major glands release their secretory products into the digestive tract: the liver and pancreas. Both share the common bile duct as an excretory duct that leads to the small intestine. The liver releases bile, a fluid that contains biliary acids needed for digestion and the absorption of fat. The exocrine component of the pancreas releases many enzymes for digestion. Additionally, both glands also have endocrine functions by releasing hormones into the bloodstream. The hepatocytes in the liver are able to synthesize and release both exocrine and endocrine compounds, whereas the endocrine and exocrine functions are performed by different cell types in the pancreas. The Langerhans islets are the endocrine structures of the pancreas.