Digestive System Histology Lecture I (2024) PDF

Histology of Digestive system Prof. Dr. Soad A. Treesh. Spring 2024 General Structure of the Digestive Tract The entire gastrointestinal tract (GIT) presents certain common structural characteristics. It is a hollow tube composed of a lumen whose diameter varies, surrounded by a wall made up of...

Histology of Digestive system Prof. Dr. Soad A. Treesh. Spring 2024 General Structure of the Digestive Tract The entire gastrointestinal tract (GIT) presents certain common structural characteristics. It is a hollow tube composed of a lumen whose diameter varies, surrounded by a wall made up of four principal layers: Mucosa, Submucosa, Muscularis, Serosa. The mucosa  Comprises an;  Epithelial lining;  Lamina propria of loose connective tissue rich in blood and lymph vessels and smooth muscle cells lymphoid tissue, +/-glands.  Muscularis mucosae, usually consisting of a thin inner circular layer and an outer longitudinal layer of smooth muscle cells separating the mucosa from the submucosa. The mucosa is frequently called a mucous membrane. The submucosa  Is composed of dense connective tissue with many blood and lymph vessels and a submucosal (also called Meissner's) nerve plexus.  It may also contain glands and lymphoid tissue. The muscularis  Contains smooth muscle divided into two sublayers.  The internal sublayer is circular; the external sublayer, is longitudinal.  The muscularis also contains the myenteric (or Auerbach's) nerve plexus, which lies between the two muscle sublayers, and blood and lymph vessels in the connective tissue between the muscle sublayers. The serosa  Thin layer of loose connective tissue rich in blood and lymph vessels and adipose tissue, and a simple squamous covering epithelium (mesothelium).  In the abdominal cavity, the serosa is continuous with the mesenteries. The Oral Cavity  The oral cavity is lined by the oral mucosa, composed of a wet stratified squamous keratinized, nonkeratinized, epithelium and an underlying connective tissue.  Regions of the oral cavity that are exposed to considerable frictional and shearing forces (gingiva and hard palate) are lined or covered by a masticatory mucosa composed of keratinized stratified squamous epithelium with an underlying dense irregular collagenous connective tissue.  The remainder of the oral cavity is lined or covered by a lining mucosa, composed of a nonkeratinized stratified squamous epithelium overlying a looser type of dense irregular collagenous connective tissue.  The Oral Cavity  lined with stratified squamous epithelium, keratinized or nonkeratinized, depending on the region.  The keratin layer present mostly in the gingiva (gum) and hard palate.  The lamina propria has several papillae similar to those in the dermis of the skin, and rests directly on bony tissue.  Nonkeratinized squamous epithelium covers the soft palate, lips, cheeks, and the floor of the mouth.  The soft palate has a core of skeletal muscle, numerous mucous glands, and lymphoid nodules in its submucosa. Lip  The external surface of the lip is covered by skin with a thin, stratified squamous keratinized epithelium and hair follicles.  The skin transitions to the vermillion zone (red margin) which forms the red portion of the lip, followed by the oral mucosa which lines the inner portion of the lip.  Beneath the oral mucosa are numerous minor salivary glands (labial glands).  The orbicularis oris muscle (skeletal , striated) occupies the central core of the lip.  Lip 12 Lip Oral surface, Str. Squ.non kert. Marginal zone (no glands) Outer surface, skin. Oral mucosa Orbecularis muscle Hair follicles. Sweat glands Sebaceous glands Normal skin 13  Tongue  A mass of striated muscle covered by a mucous membrane.  The muscle fibers cross one another in three planes grouped in bundles, usually separated by connective tissue.  The mucous membrane is smooth on the lower (ventral) surface of the tongue.  The tongue's dorsal surface is irregular, covered anteriorly by a great number of small eminences called papillae.  The posterior one-third of the dorsal surface of the tongue is separated from the anterior two-thirds by a V-shaped boundary.  The tongue is covered by a specialized mucosa that contains multiple types of papillae and taste buds.  Stratified Squamous Non-Keratinized Epithelium.  Dermal Papillae - ridges of connective tissue that project into the epithelium that reduce its mobility and brings blood vessels in close contact with the epithelial cells.  Papillae - parallel ridges on the lateral edges of the tongue separated by deep mucosal furrows (or moats).  Furrows (or Moats) - separate each papillae and receive saliva from the minor lingual glands.  Taste Buds - elliptical structures found in the epithelium of the furrows that contain cells with taste receptors. The circular opening at its apex is the taste pore.  Skeletal Muscle - arranged in three bundles at right angles to each other to allow flexibility and precision in movements of the tongue.  Minor Salivary Glands - found throughout the tongue.  Serous Glands - secrete a fluid that contains digestive enzymes.  Mucous Glands - secrete a fluid that contains mucus (glycoproteins known as mucins). Orientation of skeletal , striated muscle (Tongue ) 2 1 3  squamous epithelium glands.  At low power, the normal tongue has an overlying squamous epithelium beneath which is a lamina propria. The bulk of the tongue is composed of skeletal muscle. Scattered throughout the tongue, but more prominent toward the back of the tongue, are minor salivary glands. Papillae  Papillae are elevations of the oral epithelium and lamina properia that assume various forms and functions.  There are four types ;  Filiform Papillae.  Fungiform Papillae.  Foliate Papillae.  Circumvallate Papillae. Filiform Papillae  Filiform papillae have an elongated conical shape; they are quite numerous and are present over the entire surface of the tongue. Their epithelium, which does not contain taste buds, is keratinized. Fungiform Papillae  Fungiform papillae resemble mushrooms in that they have a narrow stalk and a smooth- surfaced, dilated upper part.  These papillae, which contain scattered taste buds on their upper surfaces, are irregularly interspersed among the filiform papillae. Foliate Papillae  Foliate papillae are poorly developed in humans.  They consist of two or more parallel ridges and furrows on the dorsolateral surface of the tongue and contain many taste buds. Circumvallate Papillae  Circumvallate papillae are 7– 12 extremely large circular papillae whose flattened surfaces extend above the other papillae. They are distributed in the V region in the posterior portion of the tongue. Numerous serous (von Ebner's) glands drain their contents into the deep groove that encircles the periphery of each papilla.  Taste buds  Onion-shaped structures each one containing 50–100 cells.  The bud rests in the basal lamina, and in its apical portion the taste cells project microvilli that poke through an opening called the taste pore.  Most of the cells are actually the taste cells, while others have a supportive function, secreting an amorphous material that surrounds the microvilli in the taste pore.  Undifferentiated basal cells are responsible for the replacement of all the cell types.  Tastants dissolved in saliva contact the taste cells through the pore, interacting with taste receptors (sweet and bitter tastes) or ion channels (salty and sour tastes) on the surface of the cells.  The result is a depolarization of the taste cells, leading to the release of neurotransmitters that will, in turn, stimulate afferent nerve fibers connected to the taste cells.  This information will be processed by central gustatory neurons.  It is believed that each tasting stimulus generates a unique pattern of activity across a large set of neurons, which explains taste discrimination.  The receptors for bitter tastants, recently identified, belong to a family estimated to have 40–80 members. In the near future other families of taste receptors will certainly be identified as well. Pharynx  Lined by stratified nonkeratinized squamous epithelium in the region continuous with the esophagus and by ciliated pseudostratified columnar epithelium containing goblet cells in the regions close to the nasal cavity.  The mucosa of the pharynx also has many small mucous salivary glands in its lamina propria, composed of dense connective tissue.  The constrictor and longitudinal muscles of the pharynx are located outside this layer. Nasopharynx and Oropharynx Epithelium Oropharynx Nasopharynx Esophagus  Is a muscular tube transport foodstuffs from the mouth to the stomach and to prevent the retrograde flow of gastric contents.  Transport is achieved by peristaltic contractions and relaxation of the esophageal sphincters (upper and lower).  Covered by nonkeratinized stratified squamous epithelium.  In general, it has the same layers as the rest of the digestive tract.  In the submucosa are groups of small mucus- secreting glands, the esophageal glands, whose secretion facilitates the transport of foodstuffs and protects the mucosa.  In the lamina propria of the region near the stomach are groups of glands, the esophageal cardiac glands, that also secrete mucus.  At the distal end of the esophagus, the muscular layer consists of only smooth muscle cells that, close to the stomach, form the lower esophageal sphincter; in the mid portion, a mixture of striated and smooth muscle cells; and at the proximal end, only striated muscle cells. Only that portion of the esophagus that is in the peritoneal cavity is covered by serosa. The rest is covered by a layer of connective tissue, the adventitia, that blends into the surrounding tissue. Gastro-esophageal junction 37 Praxis Lip Tongue Filiform papillae Foliate Papillae Fungiform Papillae Circumvallate Papillae Taste buds Oesphagous Gastro-esophageal junction 51 End of Lecture 1 Good Luck