Gastrointestinal System I

• The gastrointestinal system consists of the alimentary canal and its associated organs:

1. Tongue;
2. Teeth;
3. Salivary glands;
4. Pancreas;
5. Liver and
6. Gallbladder

• The lumen of the alimentary canal is physically and functional external to the body (the body is topologically a 1-hole donut).
• Food is broken down both physically and chemically as it passes through the GIT.
• Absorption occurs chiefly through the walls of the small intestine.

• Undigested food and other matter are excreted as faeces.

• The alimentary mucosa is the surface across which most substances enter the body.
• The functions of this mucosa include:

1. Barrier function: barrier to the entry of noxious substances, antigens and pathogenic organisms.
2. Immunologic functions: lymphatic tissue serves as the first line of defence.
3. Secretory function: digestive enzymes, hydrochloric acid, mucin and antibodies.
4. Absorptive function: absorption of metabolic substrates.

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• The oral cavity consists of the mouth and its contents.
• It is divided into the vestibule and the mouth cavity proper.

• The vestibule lies between the lips, cheeks and teeth.
• The oral cavity proper lies behind the teeth and is bounded by the hard and soft palate superiorly, the tongue and floor of the mouth inferiorly and the entry of the oropharynx posteriorly.

• The ducts of the salivary glands empty into the oral cavity.
• The major salivary glands include the: parotid gland, submandibular gland and the sublingual gland.

• The major salivary glands have relatively long ducts from the secretory portion of the gland.
• The minor glands from the submucosa of the tongue and lining of the oral cavity have relatively short ducts.

• Lymphatic tissue is organised into a ring of immunologic protection.
• This consists of the: palatine tonsil (the tonsils), the pharyngeal tonsil (the adenoids) and lingual tonsils.

• The oral cavity is lined by masticatory mucosa, lining mucosa and specialised mucosa.

• It is on the gingivae and hard palate.
• It is keratinised or parakeratinised stratified squamous epithelium.

• Parakeratinised epithelium is similar to keratinised epithelium but the cells of the stratum corneum retain their nucleus, and their cytoplasm does not stain as intensely with eosin.

• As in the skin, the depth and number of papillae is dependent on the relative immobility of the masticatory mucosa.
• This protects it from frictional and shearing forces.

• This is on lips, cheeks, alveolar mucosal surface, floor of mouth, inferior surfaces of the tongue and soft palate.
• In these sites, it covers striated muscle (lips, cheeks and tongue), bone (alveolar mucosa) and glands (soft palate, cheeks, and inferior surface of the tongue).

• The epithelium is generally non-keratinised.
• In some places it may, however, be parakeratinised (e.g., the epithelium of the vermilion border of the lip).

• The non-keratinised lining epithelium is thicker than the keratinised epithelium.
• It consists of 3 layers:

1. Stratum basale;
2. Stratum spinosum;
3. Stratum superficiale.

• The cells of the mucosal epithelium are similar to those of the epidermis of the skin.
• They include: keratinocytes, Langerhan's cells, melanocytes and Merkel's cells.
• The sharp contrast between the alveolar mucosa and the rest of the lining epithelium is in the numerous deep papillae of the alveolar mucosa compared to the shallow papillae of the rest.

• This is restricted to the dorsal surface of the tongue, where it contains papillae and taste buds.

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• The striated muscles of the tongue are arranged in bundles that generally run at right angles to each other (in 3 planes).
• The dorsal surface of the tongue is divided into anterior 2/3 and a posterior 1/3 by a V-shaped depression; the sulcus terminalis.

• Papillae cover the dorsal surface of the anterior portion of the tongue.
• Many papillae cover the dorsal surface anterior to the sulcus terminalis.
• These papillae include: filiform, fungiform, circumvallate and foliate papillae and some contain taste buds.

• The papillae and their associated taste buds constitute the specialised mucosa of the oral cavity.
• There are 4 types of papillae described.

• These are most numerous in humans and are the smallest.
• They are conical, elongated projections of epithelium and connective tissue.
• It has keratinised (parakeratinised) stratified squamous epithelium.

• This epithelium does not contain taste buds.

• They are distributed over the entire dorsal surface, with their tips pointing back.
• Filiform papillae appear to form rows that diverge from the midline and parallel to the arms of the sulcus terminalis.

• These are mushroom-shaped projections.
• They tend to be more numerous at the tip of the tongue.
• Taste buds are present in their stratified squamous epithelium.

• These are large, dome-shaped structures that reside in the mucosa just anterior to the sulcus terminalis.

• Each papilla is surrounded by a moat-like invagination of stratified squamous epithelium that contains numerous taste buds.

• Ducts to the lingual salivary glands (of von Ebner) empty their serous secretions into the moats.

• These are parallel low ridges separated by deep mucosal clefts.
• They are aligned at right angles to the long axis of the tongue.

• They occur at the lateral edge of the tongue.

• The dorsal surface of the posterior portion of the tongue exhibits smoother bulges that reflect the presence of lingual tonsils.

• They appear as oval, pale staining bodies that extend through the thickness of the epithelium.

• There is a small opening on the epithelial surface at the apex of the bud.
• This is known as the pore of the taste bud.

• There are 3 principle cells types found in the taste buds:

1. Neuroepithelial cells;
2. Supporting (sustentacular) cells;
3. Basal cells.

• Neuroepithelial cells and supporting cells are mature elongated cells that extend from the basal lamina to the taste pore.
• Through the taste pore, the tapered apical surface of each cell extends microvilli.

• The basal cells are located near the basal lamina and are believed to be the stem cells of the other two cell types.

• The taste buds react to only 4 stimuli; sweet, salty, bitter and acid.

• The musculature of the tongue is supplied by the hypoglossal nerve (CN XII).
• There are, in addition, sympathetic and parasympathetic innervation of the blood vessels and glands.

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• The adult tooth consists of 4 distinct structure components: the enamel and the cementum on the outside, the dentin beneath and the pulp in the central pulp cavity.

• The enamel covers the crown of the tooth.
• The enamel layer ends at the neck or cervix of the tooth at the cementoenamel junction.

• Enamel is the hardest substance in the body.
• It contains 96-98% hydroxyapatite.
• Each enamel rod spans the full thickness of the enamel layer.

• Striations seen in the enamel (contour lines of Retzius) may represent evidence of rhythmic growth of the enamel in the developing tooth.

• Enamel is produced by ameloblasts.
• These are narrow, highly polarised columnar cells.
• They are directly adjacent to the developing enamel.

• At the apical pole of the cell is a process, Tome's process, which is surrounded by the developing enamel.

• This covers the root of the teeth.
• Cementum is thin layer of bone-like material secreted by cementocytes and their processes.

• This is a calcified substance that forms most of the tooth's substance.
• It contains less hydroxyapatite than enamel, about 70%, but more than that found in bone and cementum.

• Dentin is formed by odontoblasts which forms an epithelial layer over the inner surface of the dentin.
• The layer of odontoblasts retreats as the dentin is laid down.
• This leaves odontoblast processes embedded in the dentin in narrow tubules known as dentinal tubules.

• Predentin is the newly secreted organic matrix and is closest to the cell body of the odontoblasts.
• This substance has yet to be mineralised.

• The dental pulp cavity is a connective compartment bounded by tooth dentin.
• This is occupied by pulp, which is a loose connective tissue that is richly vascularised and innervated.

• The alveolar processes of the mandible and maxilla contain the sockets or alveoli for the roots of the teeth.
• Alveolar bone proper is a thin layer of compact bone.
• It forms the wall of the alveolus.

• This is a fibrous connective tissue joining the tooth to the surrounding bone.
• This ligament is also known as the periodontal membrane.

• This provides:

• Attachment;
• Support;
• Bone remodelling (during movement of tooth);
• Nutrition of adjacent structures;
• Proprioception;
• Tooth eruption.

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• The major salivary glands are paired glands with long ducts.
• These include the: parotid, submandibular and sublingual glands.

• The minor salivary glands are located in the submucosa of different parts of the oral cavity.
• These include the: lingual, labial, buccal, molar and palatine glands.

• The acini of salivary glands contain serous cells (protein secreting), mucous cells (mucin secreting), or both.
• Thus, three types of acini are described:

1. Serous acini;
2. Mucous acini;
3. Mixed acini.

• They are generally spherical.
• Serous cells contain large amounts of rER, free ribosomes, a prominent Golgi complex and numerous secretory granules.
• The secretions are stored in zymogen granules found in the apical cytoplasm.

• These are usually more tubular.
• Most mucous cells contain large numbers of mucinogenic granules in their apical cytoplasm.
• Thus, in routine H&E preparations, these cells have an empty appearance.

• Some mucous acini have a cap of serous cells.
• These caps, or serous demilunes, secrete into the highly convoluted intercellular space, between the mucous cells.

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• The lumen of the salivary acini is continuous with the duct system.
• The duct system can be considered to have 3 sections:

1. Intercalated ducts;
2. Striated ducts;
3. Excretory ducts.

• Serous glands have well-developed intercalated and striated ducts that modify their serous secretions (absorption of specific components and secretion of additional components).

• Mucous glands, in which the secretions are not modified, have very poorly developed intercalated ducts and they may not be recognisable in H&E sections.
• Moreover, they do not display striated ducts.

• There are located between a secretory acinus and a larger duct.
• They are lined by low cuboidal cells.
• These cells possess carbonic anhydrase activity.

• They secrete the bicarbonate ion and absorb the chloride ion.

• These have numerous infoldings of the basal plasma membrane, which are seen as basal striations.
• Their epithelial lining changes from simple cuboidal to simple columnar.
• These reabsorb sodium and add potassium.

• These travel in the interlobar and interlobular connective tissue.
• The excretory ducts connect ultimately with the oral cavity.
• Their epithelium changes from cuboidal to stratified cuboidal to pseudostratified columnar.

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• These glands are totally serous.
• Its duct enters the oral cavity opposite the 2^nd upper molar.

• These glands are mixed glands that are most serous in humans.
• Some mucous acini capped with serous demilunes are found among the predominantly serous acini.

• The small sublingual glands are mixed glands that are mostly mucous secreting in humans.
• Some of the predominant mucous acini have serous demilunes.
• The mucous secreting unit may be more tubular than purely acinar.

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