The digestive system is composed of two separate parts: (1) a long muscular tube, stretching from the lips to the anus and (2) a number of glands located outside of the tube proper. The digestive tube is lined with epithelium, which is continuous with the skin at the mouth and anus. The type of epithelium differs, in some places serving a protective role and in others, an absorptive role. Further protection of the epithelial membranes is provided by the secretion of mucus by individual cells or glands. The digestive process, which converts food into substances which can be absorbed, is performed by glands located inside and outside of the tube.
Studying the epithelial covering of the lips, one can visualize the continuous nature of the epithelial lining of the digestive tube. The outer surface of the lips are covered by skin, a stratified squamous keratinized epithelium, containing hair follicles and glands (Lip1). Beginning at the red free margin of the lips and on their inner surfaces, they are covered with a stratified squamous non-keratinized epithelium, containing no hair follicles (Lip 2). This epithelial type provides protection from the abrasions and is also found lining the cheeks and pharynx. Although absent at the free margin, mucus glands can be found in the connective tissue underlying the inner epithelium. The core of the lips is filled with skeletal muscle and fibroelastic connective tissue (Lip 3).
The tongue is a complex tissue composed of skeletal muscle, glands, lymphatic tissue, and an epithelial covering modified to form papillae of three types on the anterior two-thirds of the tongue (oral tongue) (Tongue). The posterior one-third is the root or pharyngeal tongue. It lacks papillae, but does contain lingual tonsils. Papillae are composed of an epithelial covering stuffed with connective tissue known as the lamina propria and three different types are observed. Filiform papillae are the most numerous and are conical in structure consisting of a surface covering of stratified squamous keratinized epithelium with a lamina propria core (Filiform 1) . They are found in parallel rows across tongue and function for licking (rough tongue) and contain nerve endings for touch (Filiform 2). Fungiform or Foliate papillae are wider and "fungi" shaped with a covering of stratified squamous non-keratinized epithelium (Fungiform 1). They are found scattered between the filiform types and in rows along the edge of the tongue (Fungiform 2). Vallate papillae are found at the border of the oral and pharyngeal portions, are the largest of the papillae. They consist of a stratified squamous non-keratinized epithelial covering, surrounded by deep clefts or moats. These moats are cleansed by secretions from Von Ebner's glands (Vallate 1). Located along the sides of the Fungiform and Vallate papillae are the gustatory receptors or taste buds. They are barrel shaped and contain three different cell types: (1) sustentacular cells (supportive); (2) taste receptor cells; and (3) basal stem cells (Taste Buds).
The tooth is composed of a number of layers of calcified connective tissue. The bulk of the tooth is dentin. The protruding portion (crown) is covered by enamel; whereas the embedded root is covered by cementum. Within the tooth is the pulp cavity, which contains loose connective tissue, nerves, and blood vessels. As the tooth forms the dentin layer is deposited first (Tooth 1). The dentin is composed of elongated cells called odontoblasts which possess long cytoplasmic projections around which the calcified dentin is deposited. At this stage, the cells that will form the enamel layer condense outside the dentin forming the enamel organ (Tooth 2). With continued development, the enamel layer begins to form (Tooth 3). The long processes of the odontoblasts become house in long lacunae called dentinal tubules with the nucleus of the cell located at the border of the pulp cavity. The enamel is composed of ameloblasts, a similarly polarized cell with long projections (Tomes Process) abutting the dentin layer (Tooth 4).
The digestive tube consists of four layers: (1) Mucus Membrane; (2) Submucosa; (3) Muscularis externa; (4) Serosa (Digestive Tube). The mucus membrane is composed of three layers: (1) epithelial lining; (2) lamina propria containing loose connective tissue and lymphatics; and (3) muscularis interna consisting of two thin layers of smooth muscle (Mucus Membrane). The submucosa, containing loose connective tissue and blood vessels, connects the mucus membrane to the muscularis externa. The muscularis externa consists of two large layers of smooth muscle, one oriented circularly and one longitudinally. The entire tube is wrapped by an outer layer of connective tissue and mesothelium, the serosa. When the tube is connected to an adjacent structure, it will lack the mesothelial layer and its connective tissue wrapping is referred to as the adventitia.
The esophagus consists of the same four layers (Esophagus 1). The mucus membrane is lined with stratified squamous non-keratinized epithelium. The upper 1/3 of the muscularis externa is composed of skeletal muscle, while the remaining 2/3 is composed of smooth muscle (Esophagus 2). In the submucosa, mucus glands (esophageal glands) are found along the length of the tube (Esophagus 3). Additional mucus glands (cardiac glands) are found in the lamina propria near the esophagus/stomach junction. The esophagus runs in parallel with the trachea and is wrapped in a common adventitia.
The stomach consists of a thick mucus membrane containing many tubular glands often observed as folds, with a core of submucosa, called rugae (Rugae). Both muscularis layers are composed of three layers of smooth muscle and the organ is wrapped by a serosa. Histologically, three different regions are seen. Cardiac stomach is found at the esophageal orifice (Cardiac 1). This portion of the stomach secretes primarily mucus, and both mucus secreting surface epithelium and tubular mucus glands are seen in the mucus membrane (Cardiac 2). Fundic stomach is the main body of the stomach. It is lined with mucus secreting columnar epithelium (simple) that is studded with numerous holes or gastric pits. The pits are continuous with the gastric glands. In the fundic region, the pits are short and the glands long (Fundic 1). The pits are lined with a mucus secreting simple columnar epithelium (Fundic 2). The gastric glands are long tubular glands (Fundic 3), consisting of a deep vase containing zymogenic or chief cells, which secrete digestive enzymes. The middle neck and upper isthmus contain neck mucus cells and parietal cells, which secrete HCL (Fundic 4). The pyloric stomach contains long pits and short glands (Pyloric 1). The pits are lined with mucus secreting epithelium and the glands contain primarily mucus secreting cells (Pyloric 2). These glands lack chief cells, but contain a few parietal cells and gastrin cells which stimulate the parietal cells.
The small intestines contain structures for both absorption and digestion. To increase surface area, the mucus membrane is thrown into spiral folds, with cores of submucosa, called plicae circularis (Plicae). The surface of these folds are covered by intestinal villi consist of a simple columnar epithelium with microvilli to aid absorption. Located between columnar cells are mucus secreting goblet cells (Villi). The structures needed for digestion include the goblet cells and glands found in three sites: (1) outside the tube and connected by ducts (i.e. pancreas); (2) in the submucosa, only in the duodenum, which contains the mucus secreting Glands of Brunner (Duodenum 1); and (3) in the lamina propria, set between the villi, the Crypts of Lieberkuhn (Crypts 1).
Anatomically, the stomach empties into the duodenal portion of the small intestine, marked the presence of the Glands of Brunner (Duodenum 2). The duodenum leads to the jejunum and then the Ileum. The ileum has two distinct features. Within the crypts, paneth cells with the eosinphilic secretory granules of digestive enzymes may be seen at the base of the gland (Ileum 1). Common to all the crypts are the differentiating oligomucus cells and villus absorptive cells. The ileum can also be identified by the presence of lymphatic nodules (Peyer's Patches) in the submucosa (Ileum 2).
The mucus membrane of the colon is often folded into plica as seen in the small intestines (Colon 1). The mucus membrane contains no villi, but consists of long tubular glands (Crypts of Lieberkuhn), extending the entire thickness of the epithelium. The crypts contain no paneth or enzyme secreting cells, but an increased number of goblet cells (Colon 2). The lamina propria, muscularis mucosae, submucosa, muscularis externa, and serosa have a morphology similar to that of the small intestines.
The colon leads to the rectum which in turn leads to the anal canal. Morphologically, the rectum is comparable to the colon, containing a mucus membrane lined with simple columnar epithelium with goblet cells, as well as the submucosa, muscularis externa, and serosa layers. At the rectoanal junction differences are observed. The anal canal is lined with stratified squamous epithelium and possesses no muscularis mucosa (Anal Canal 1). At the anal opening, the epithelium becomes continuous with the skin, a stratified squamous keratinized epithelium (Anal Canal 2).
The accessory glands of the digestive system include the salivary glands in the oral cavity and the digestive glands in the abdominal cavity, including the pancreas, liver, and gall bladder.
The structures of salivary glands were studied in the "epithelium" laboratory. Review them and be able to identify serous (parotid) (Serous Gland) , mucus (sublingual) (Mucus Gland), and mixed (submandibular) (Mixed Gland) glandular units.
The pancreas is both an exocrine and endocrine organ. The organ is covered by a thin connective tissue capsule, from which septa penetrate the gland, dividing it into lobules (Pancreas 1). The exocrine secretory units, the acini, are packed within the lobules. The acini are serous secretory units, composed of cuboidal cells with round nuclei at the base, surrounded by intense basophila (representing large amounts of RER) and acidophilic cytoplasm (zymogen granules). Occasionally, a centroacinar cell, located in the center of the acini units, can be found. These cells lack zymogen granules and have a central nucleus. They are part of the intercalated duct system of the gland (Pancreas 2). The endocrine portion of this gland is composed of clumps of pale staining cells housed in structures known as the Islets of Langerhans (Islet 1). The details of their organization will be studied later in the endocrine laboratory.
The details of the duct system of the pancreas should be studied. The main pancreatic duct enters the organ and branches into numerous interlobule ducts. These ducts are found within the connective tissue of the septa and are composed of simple columnar epithelium (Pancreas 3). The interlobule ducts give rise to the intralobular ducts which enter the lobules. These ducts are ensheathed with dense connective tissue and are composed of a simple cuboidal epithelium (Pancreas 4). The intralobular ducts give rise to the intercalated ducts which enter the individual acini. These ducts are composed of a flattened cuboidal or squamous-like epithelium (Pancreas 5).
The liver is the largest gland in the body, providing both exocrine and endocrine products. There is no division of labor in the liver, each hepatocyte produces both exocrine and endocrine secretions. Therefore, each hepatocyte must (1) abut a passageway that connects with a duct system for exocrine secretions and (2) abut a blood vessel to receive the endocrine secretions.
The liver is divided into lobules, but contains very little connective tissue in humans. In the pig, lobules are outlined by connective tissue septa, and in all cases, by the arrangement of liver cells and the associated blood supply (Liver 1). The hepatocytes are arranged in anastomosing cords radiating out from a central vein. Between cords are blood sinusoids, which receive the endocrine secretions (Liver 2). The sinusoids are lined by endothelial cells and Kupffer cells which are immobilized macrophages (Liver 3). The surface of the hepatocytes, opposite that which abuts the blood sinusoids, abuts other hepatocytes. Between the hepatocytes, on this surface, small intercellular clefts called the bile canaliculus are found. These canaliculi transport the exocrine secretions to the bile ducts and out of the liver (Bile Canaliculi). The periphery of the lobules are defined by portal tracts, which consist of: (1) a portal vein, (2) a hepatic artery, (3) a lymphatic vessel, and (4) a bile duct (Liver 4).
The gallbladder is a pear-shaped sac off the hepatic duct. It is lined with a folded simple columnar epithelium which sits on a lamina propria, arranged in small villus projections. There are no muscularis mucosae or submucosal layers. The muscularis externa contains smooth muscle bundles running in all directions. A thick layer of fat and loose connective tissue is observed between the externa and serosa (Gall Bladder).
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