Presentation on theme: "DIGESTIVE SYSTEM I. I. Digestive system - General considerations A. Digestive tract - general structure. 1. A hollow tube with lumen of variable diameter."— Presentation transcript:
DIGESTIVE SYSTEM I
I. Digestive system - General considerations A. Digestive tract - general structure. 1. A hollow tube with lumen of variable diameter. 2. Extends from mouth to anus 3. The wall, starting with the esophagus, shows different specializations along its length that relate to function of various components of digestive tract. 4. Various glands (e.g. salivary glands )and organs (liver, gall bladder, pancreas) are also components of this system. 5. The wall of most of the digestive tract consists of 4 major layers, the a) mucosa, b) submucosa, c) muscularis externa and d) adventitia or serosa. In the oral region, only the mucosa is easily defined.
II. ORAL CAVITY A. The oral cavity is lined with a keratinized or nonkeratinized stratified squamous epithelium depending on what region you’re in. 1. This epithelium is often called the mucous epithelium. 2. The transition between the stratified keratinized squamous epithelium of the skin and the stratified non-keratinized squamous epithelium of much of the oral cavity occurs at the lips. 3. The superficial (surface) cells of the non- keratinized epithelium are nucleated (as opposed to those of keratinized epithelium of skin which are not) and have only a few granules of keratin in their cytoplasm. organology/digestive/o_d_3.html
4. Below the stratified squamous epithelium is a layer of loose connective tissue, the lamina propria. a. This lamina propria shows some interdigitation with the stratified squamous epithelium of the oral region as the dermis of the skin does with the epidermis. b. It contains blood and lymph vessels, small glands, nerves and aggregations of lymphocytes.
1frenulum of tongue 2ridge formed by deep lingual vein 3sublingual fold 4sublingual caruncle 5 opening of submandibular duc 5. Together, the stratified squamous epithelium and the lamina propria form the oral mucosa - No muscularis mucosae 6. Sublingual and submandibular salivary glands lie in tissues below the mucosa (this would be the equivalent of the submucosa or sometimes below it). 7. There is no distinct boundry between the lamina propria and the submucosa and no true muscularis externa in oral region - what muscle there is will be striated. No serosa/adventitia. B. The roof of the mouth consists of soft and hard palates. 1. The hard palate has an intramembranous bone backing which is covered by a keratinized mucous epithelium. 2. The soft palate has a core of skeletal muscle and is covered by a non-keratinized mucous epithelium. 1 vestibule 2 hard palate 3 soft palate 4 uvula 5 palatoglossal arch 6 palatine tonsil 7 palatopharyngeal arch 8 posterior wall of oropharynx 9 pterygoid hamulus
C. Tongue 1. The tongue consists of a mass of striated muscle covered by a mucosa consisting of a non-keratinized, stratified, squamous epithelium and lamina propria in most places. a. The mucous epithelium is strongly adherent to the muscle below b. This is because the C.T.of the lamina propria penetrates into spaces between the muscle bundles. 2. Tongue muscle is striated and composed of bundles that are oriented in 3 planes. a. This sort of structure increases both the potential stiffness and the mobility of the tongue.
3. The dorsal surface of the tongue can be divided into two areas by a V-shaped boundary found in the posterior dorsal tongue surface. a. The dorsal surface of the anterior 2/3 of the tongue is covered by various types of papillae. Beneath the surface are serous and mucous glands. b. The posterior 1/3, behind the V, is composed of small bulges that contain lymphatic nodules. * These include the lingual tonsils that consist of lymph nodules surrounding a single crypt. 1anterior 2/3rd of tongue 2posterior 1/3rd of tongue 3palatogossal fold 4palatine tonsil 5fungiform papillae 6circumvallate papillae 7sulcus terminalis 8foramen cecum 9foliate papillae
4. The type of papillae found (see below) depends on what part of tongue you look at.
5. Types of papillae found on the dorsal surface of the tongue. a. Filiform papillae - have an elongated conical shape. * These papillae have a keratinized surface, are the most numerous and are found over the entire dorsal surface. * There are no taste buds in the epithelium covering filiform papillae.
b. Fungiform papillae - these are mushroom shaped. * There are far fewer fungiform than filiform papillae. * They are found interspersed between the filiform papillae over the entire anterior dorsal surface of the tongue. * A few taste buds may be found in the epithelium covering these papillae.
c. Foliate papillae - these are arranged as closely packed folds along the posterior lateral margins of the tongue and are only common in young children. * There are numerous taste buds in the epithelium covering foliate papillae. * Serous glands drain through openings at their bases.
d. Circumvallate papillae - these are extremely large circular papillae which have a flattened surface that extends above the other tongue papillae. About 12 in number. * Circumvallate papillae are distributed in the V region of the posterior dorsal surface of the tongue. * Many taste buds can be found in the epithelium covering their lateral surfaces.
6. A deep groove encircles the body of the circumvallate papilla. a. Serous (von Ebner’s) glands (serous) drain into the base of this groove. b. The flow of fluid from these glands serves to wash surface of the papilla and clean materials from taste buds so they are ready for new gustatory stimuli. c. Other serous and mucous glands in other parts of oral region serve the same sort of purpose for cleansing other types of papillae. ve/images/ff677.jpg
7. Taste buds a. Oval multicellular structures b. Cells surround a cavity that communicates with the oral cavity via a small pore between the apexes of the cells. c. Dissolved substances enter the cavity through this pore and come into contact with the microvilli of gustatory cells (neuroepithelial sensory cells) of the taste bud. d. These chemical stimuli are transduced to an electrical impulse that is transmitted through afferent axons of cranial nerves 7, 9, and 10 that synapse on the basal portions of the neuro- epithelial cells. e. Action potentials travel along these axons to the portions of the brain responsible for our sense of taste.
f. Taste buds are composed of 3 cell types * Gustatory (taste) cells * Sustentacular (support) cells * Basal cells - stem cells for replacement of gustatory cells ** gustatory cells live for about 7-10 days g. Both gustatory and sustentacular cells have similar structure, i.e. long microvilli that extend into the lumen of the taste bud.
DIGESTIVE SYSTEM 1B ADULT TOOTH STRUCTURE TOOTH DEVELOPMENT
1.In adult humans there are 32 permanent teeth. 2.These are preceded during childhood by 20 deciduous teeth. 3.The tooth lies in a bony socket, the alveolus, that is covered my an oral mucosa called the gingiva (gums) that consists of, a. keratinized stratified squamous epithelium b. lamina propria of loose connective tissue that lies directly adjacent to the bone of the alveolus. Teeth
a. the crown - the portion that protrudes above the gum line. and b. the root - the portion that extends into the alveolus. Odontoblasts extend thin processes (Tome’s fibers), along which the organic matrix of the dentin is secreted. The tooth consists of two major parts, Internally, the tooth consists of a layer of dentin that surrounds a pulp consisting of loose connective tissue, nerves and blood vessels. In the dentin, directly adjacent to the pulp is a layer of specialized cells called odontoblasts - secrete organic matrix that calcifies and forms the dentin.
Dentin is covered by a layer of calcified organic matrix - the enamel a. Hardest substance in body b. Formed by ameloblasts before tooth “erupts” from socket Root region Dentin (mineralized organic matrix) surrounds the pulp. In the root region, the dentin is covered by calcified organic matrix - the cementum - similar to bone, but no haversian system Between the cementum and the bone of the socket lies the periodontal ligament - consists of fibroblasts and associated collagen fibers with glycosaminoglycans in between. a.forms cushion between tooth and bone b.Attaches tooth to bone - Sharpey’s fibers (insert into bone and cementum) Crown region Periodontal ligament Dentin Alveolar bone Lamina propria
Where the gingiva meets the tooth - specialized epithelium - junctional epithelium - binds epithelium to enamel. Junctional epithelium is non-keratinized stratified squamous without “dermal” papillae Bound to enamel by cuticle (looks like extra thick basal lamina - called epithelial attachment of Gottlieb) Cells of junctional epithelium tightly attached to cuticle by hemidesmosomes Between this attachment and the gumline is the gingival sulcus. Lined by sulcular epithelium. This is a non-keratinized stratified squamous epithelium. When dentist probes around your teeth - checking depth of sulcus. If too deep, indicates breakdown between enamel and junctional epithelium - periodontal disease
24 There are a number of terminologies that are used to describe the early development of teeth prior to the cap stage. In some cases, there is disagreement about what a given term represents (e.g. dental lamina, tooth bud). The following description of tooth development tries to make sense out of the available reference material I’ve been able to find; however, be aware that you may see other terminologies used in dental school.
25 1. Prior to the 6th week of gestation in human embryos, the developing jaws are solid masses of tissue with little differentiation. 2. Tooth development begins during the 5th - 6th week of gestation.
26 3. The first indication is the appearance of a thickened plate of epithelium (labial lamina = vestibular lamina) between the tongue and the upper and lower jaw. This, and the following events occur in both the upper and lower jaw. 4. This thickened epithelium spreads over the inner (oral) jaw surface. 5. An invagination (labial groove) forms in this thickened epithelium. This becomes the vestibule that separates the lip or cheek from the gum.
27 6. The labial (vestibular) lamina overlying the forming gums gives rise to the dental lamina (dental ledge). Neural crest cells in the underlying mesenchyme induce the vestibular lamina epithelium to grow into the surrounding gum tissue. This forms a C-shaped band of tissue in the gums of the upper and lower jaw that is called the dental ledge or dental lamina.
6. The labial or vestibular lamina overlying the forming gums gives rise to the dental lamina. Neural crest cells in the underlying mesenchyme induce the dental lamina epithelium to grow into the surrounding gum tissue. This forms a C- shaped band of tissue in the gums of the upper and lower jaw that is called the dental ledge or dental lamina. In regions where a tooth will form, a further ingrowth of the dental lamina forms the tooth bud. A - dental lamina; B - Mesenchymal neural crest
7. In 10 distinct regions of each jaw, the cells of the dental ledge proliferate rapidly by mitosis forming a cup-shaped structure called the enamel organ that is surrounded by jaw mesenchyme. The enamel organ remains connected to the vestibular or labial lamina by the cord-like remains of the dental lamina. Enamel organ 8. Five enamel organs will develop on the right and left sides of both the upper and lower jaw. These will form the child’s “milk” (primary)teeth.
30 9. The mesenchyme that fills the enamel organ cup will become the dental papilla which eventually forms the dentin and the pulp of the tooth. 10. The enamel organ and dental papilla are surrounded by a sheath of connective tissue called the dental sac. 11. The entire structure is called the cap stage of tooth development. A, Enamel organ; B, Dental lamina; C, Vestibular lamina; D, Dental Papilla; E, Dental sac
The cap stage of tooth development continues to differentiate, forming the bell stage. Concurrent with this, the successional lamina, that will form the secondary tooth later in life, forms as a outgrowth of the remaining dental lamina. 13. This differentiation includes the enamel organ. As is the case for the optic cup, the cup of the enamel organ consists of two adjacent layers of cells that result from the formation of the cup. These are an inner layer of cells (adjacent to the dental papilla) that is called the inner enamel epithelium and an outer layer of cells (adjacent to the dental sac)called the outer enamel epithelium. A - Inner enamel epithelium; B - Outer enamel epithelium; C - Stellate reticulum; D - Successional lamina; E - Dental lamina; F - Dental papilla; G - Dental sac.
The ectodermally derived tissue between these two layers forms a matrix of cells called the stellate reticulum. This matrix is essentially a connective tissue with lots of extracellular material (mainly mucopolysaccharides) between the cells. 15. The inner enamel organ epithelium will eventually differentiate into cells called ameloblasts that will form the enamel of the tooth. 16. Neural crest cells in the dental papilla will form an epthelial layer directly adjacent to the ameloblasts that will differentiate into cells called odontoblasts which will form the tooth dentin. 17. The remainder of the dental papilla will form the dental pulp of the tooth. A - Inner enamel epithelium; B - Outer enamel epithelium; C - Stellate reticulum; D - Successional lamina; E - Dental lamina; F - Dental papilla; G - Dental sac.
33 A - Inner enamel epithelium; B - Outer enamel epithelium; C - Stellate reticulum; D - Successional lamina; E - Dental lamina; F - Dental papilla; G - Dental sac. A, Cervical loop; B, Inner enamel epithelium; C, Outer enamel epithelium; D, Stratum intermedium; E, Stellate reticulum 18. The lips of the cup that forms the enamel organ are called the cervical loop. This structure consists of a portion of the inner and outer enamel epithelium at the region where they join. 19. Research indicates that the inner enamel epithelium portion of the loop is a source of stem cells for the developing ameloblasts (the cells that produce the tooth enamel). The cervical loop will partially degenerate as the root of the tooth develops and will become the Epithelial Root Sheath of Hertwig. In species with continuously growing teeth (e.g. rodents), the cervical loop is retained through adulthood, thus emphasizing its importance in providing stem cells to produce ameloblasts for enamel formation.Epithelial Root Sheath of Hertwig
34 A - Inner enamel epithelium; B - Outer enamel epithelium; C - Stellate reticulum; D - Successional lamina; E - Dental lamina; F - Dental papilla; G - Dental sac. A - Preameloblasts; B - Preodontoblasts; C - Stellate reticullum; D - Dental papilla 20. As differentiation of the inner enamel epithelium proceeds, cells called preameloblasts differentiate from the epithelium, adjacent to the dental papilla. These cells induce neural crest cells in the adjacent dental papilla to differentiate into preodontoblasts.
Ameloblasts; 2 - Enamel; 3 - Dentin; 4 - Odontoblasts; 5 - Pulp 21. The preodontoblasts become odontoblasts as they begin to secrete predentin (which will become dentin). The predentin blocks nutrients from moving from the pulp to the preameloblasts. This causes the preameloblasts to become ameloblasts and begin their secretion of enamel. The odontoblasts and ameloblasts move away from each other as the dentin and enamel layers increase in thickness. 22. As this begins to occur, the developing tooth enters the crown stage. A - Odontoblasts; B - Predentin; C - Ameloblasts; D - Dentin; E - Enamel
36 A - Reduced enamel epithelium; B - Maturative/protective ameloblasts; C - Capillary 23. Once enamel depostion is completed and the crown is fully formed, the enamel organ collapses and the cells form a sheath called the reduced enamel epithelium that covers the crown of the tooth until eruption. This epithelium may be considered stratified and consists of the ameloblast layer, called mature or protective ameloblasts at this point, and the cellular remnants of the rest of the enamel organ.
Once the formation of the crown is completed, the root forms. The inner and outer enamel epithelial layers of the cervical loop region continue to grow toward the future base of the tooth. These tissues form the epithelial root sheath of Hertwig. A - Epithelial diaphragm; B - Radicular pulp cavity; C - Dentin; D - Enamel space; E - Alveolar Bone; F, Root A B C E D F Epithelial root sheath of Hertig 25. The root sheath induces the formation of additional odontoblasts that form the dentin of the root. 26. The central region of the root is called the radicular pulp cavity.
38 A - Epithelial diaphragm; B - Radicular pulp cavity; C - Dentin; D - Enamel space; E - Alveolar Bone; F, Root A B C E D F A - Radicular pulp cavity; B - Dentin; C - Dental sac; D - Point at which epithelial root sheath begins to disintegrate; E - Epithelial diaphram 26. The leading edge of the root sheath turns inward toward the root of the tooth and forms the epithelial diaphram.
Once the root odontoblasts have formed and are secreting dentin, the epithelial root sheath begins to break down. At this time, cells from the dental sac that surrounds the developing tooth migrate to the surface of the newly formed dentin and become cementoblasts. These cells secrete the cementum layer that acts as an attachment region between the root of the tooth and the peridontal ligament. The periodontal ligament binds the root of the tooth to the bone of the alveolar socket. A - Cementoblasts; B - Odontoblasts; C - Predentin A, Enamel organ; B, Dental lamina; C, Vestibular lamina; D, Dental Papilla; E, Dental sac
As the epithelial root sheath degenerates it leaves small groups of cells that are in the peridontal ligament around the root and that are called epithelial rests. A - Epithelial rests; B - Mantle dentin; C - Globular dentin; D - Circumpulpal dentin
Once the tooth is fully formed it is ready to undergo eruption. This process occurs after birth and involves active movement of the tooth such that it penetrates the gum tissues and extends above them. 31. It is likely that there are a number of factors involved in eruption. While there is no consensus on the cause of tooth eruption, there seems to be agreement that root growth, alveolar bone remodeling, and possibly the peridontal ligaments are involved in this process In humans, eruption of the milk (primary) teeth generally begins in the second month and continues until the end of the second year. 33. Thirty-two permanent (secondary) teeth develop, 20 from the successional laminae of the milk teeth and 12 from additional tooth buds along the dental lamina in same manner as primary teeth. The primary teeth will be replaced and 12 additional teeth will be added to the dentition. 34. Twenty-eight of the secondary teeth erupt between the ages of 6 and 13 years. The four wisdom teeth may erupt between 17 and 21 years; however, they often remain impacted.
DIGESTIVE SYSTEM IC Esophagus and Stomach
The wall of the digestive tract starting with the esophagus.
1. Mucosa - Mucous layer a. epithelial lining b. lamina propria of loose connective tissue - includes blood, lymph vessels, and sometimes glands. c. Smooth muscle region called muscularis mucosae lies below the lamina propria. d. The mucosa forms selectively permeable barrier to the contents of digestive tract. In the stomach and small intestine, it takes part in both the digestion of food and in the small intestine, the transport of nutrients into the body. e. The movements caused by the muscularis mucosae act to increase contact of the mucosa epithelium with food in the digestive tract. f. glands may be present in lamina propria. A. The wall of the digestive tract (starting with the esophagus) can be divided into 4 layers, pathphys/digestion/basics/gi_microanatomy.html MedEd/Histo/HistoImages/hl8-04.jpg
1. Mucosa f. The lamina propria is often rich in macrophages. Aggregations of “free” lymphoid cells and lymph nodules may also be present * This area can be said to act as a barrier to bacterial invasion. * These cells in lamina propria (and also the submucosa) act to help prevent spread of infectious organisms into the body via the digestive tract.
2. Submucosa a. Another layer of loose C.T. with many blood and lymph vessels that lies just below the muscularis mucosae. b. This layer also contains nerve plexuses each called a Meissner's plexus (parasympathetic). c. Glands and Lymph nodules may also be present in this layer.
3. Muscularis externa - smooth muscle layer a. In most regions, circular and longitudinal bands of smooth muscle. b. closest to lumen - circular smooth muscle. c. furthest from lumen = longitudinal smooth muscle.
3. Muscularis cont. d. C.T. lies between these two layers e. A myenteric (Auerbach's) nerve plexus and blood and lymph vessels are present in this layer of C.T. between the two muscle layers (sympathetic and parasympathetic components). f. Note - while the circular and longitudinal muscle layers are responsible for gross movements of the digestive tract, we shouldn't forget that the muscularis mucosae causes movements of the mucous layer that are independent of the rest of the digestive tract. MedEd/Histo/HistoImages/hl8-30.jpg
g. The longitudinal, circular and oblique smooth muscle layers of the stomach undergo relatively slow rhythmic contractions that act to mix ingested food with various stomach secretions and also move the food through the digestive tract. h. These muscles can also respond to adverse conditions with very strong and rapid contractions. (Often seen at frat parties.)
4. Serosa or adventitia a.Thin layer of loose C.T. b.Around most of the esophagus it is an adventitia - blends with surrounding tissue c.In peritoneal cavity, it is a serosa covered externally by a simple squamous epithelium. d.The simple squamous epithelium is a continuation of the lining of the peritoneal cavity - mesothelium e. This layer is rich in blood and lymph vessels as well as adipose cells
III. Esophagus A. Basically a muscular tube that transports food from mouth to stomach. B. Lining same as in much of the oral cavity - nonkeratinized stratified squamous epithelium. C. Layers same as general digestive tract as outlined above. 1. mucosa a. epithelium b. lamina propria c. muscularis mucosae 2. submucosa 3. muscularis externa a. inner circular muscle b. outer longitudinal muscle 4. adventitia or serosa
D. Specializations of esophageal tissues 1.Tubuloacinar mucus secreting glands called esophageal glands proper are present in the submucosa. Their ducts extend to the esophageal lumen. 2.Small branched mucus secreting glands are also sometimes found in the lamina propria near the stomach. These are called esophageal cardiac glands The muscular layer of the esophagus changes from striated muscle near mouth, to smooth muscle near stomach. Why would this be the case? 4. A serosal layer with a simple squamous epithelium as its outermost component is only found in the peritoneal cavity, below the diaphragm and near the stomach. The rest of the esophagus has an adventitia - outer layer of loose C.T. that blends with surrounding tissues.
IV. Stomach A. There are 3 major regions of the stomach, each with a different histologic structure. 1. The cardia - cardiac stomach 2. The body (corpus) and fundus 3. The pylorus - pyloric stomach
B. The inner surface of the stomach is thrown into folds called rugae that include both mucosa and submucosa. 1. The mucosa of the rugae is also folded. These folds form invaginatoins, such that the basal areas of the invaginations penetrate into mucosal lamina propria. 2. The upper portion of these invaginations in the mucosa are called the gastric pits - foveolae gastricae. a. The epithelial lining of the the pits and general surface area of the stomach consists of simple columnar epithelium of mucous secreting cells in all parts of the stomach. 3. The gastric glands of the stomach connect to the bottoms of the gastric pits. The cellular structure of these glands is different in the different parts of the stomach. digestion/stomach/anatomy.html
C. Cardiac stomach 1. A narrow circular region where esophagus connects to stomach. 2. Mostly simple, unbranched, slightly coiled, tubular cardiac glands in lamina propria - produce mostly mucous. 3. Cell types present a. mucous secreting cells b. lysozyme secreting cells c. enteroendocrine cells d. a few parietal cells
D. Body (corpus)and fundus 1. Has branched, tubular gastric glands. 2. Each gastric gland is divided into 3 regions a. isthmus b. neck c. base
Gastric glands of fundus and body of stomach - structure
3. Five cell types are present in gastric glands of the body and fundus a. Isthmus mucous cells - present in isthmus. * Similar to mucous secreting epithelial cells of gastric pit region. * Secrete neutral mucus that protects surface from acid. organology/digestive/images/ff746.jpg
b. Neck mucous cells May be present in clusters in neck of gastric gland (also found in other regions of gland), secrete acid glycoprotein mucus. Irregular in shape with basal nucleus. Thought to be stem cells for other cell types in the gastric glands
* Most present in upper half of gland. Fewer in basal portion of gland. * These are rounded or pyramidal cells with spherical nucleus and eosinophilic (acidophilic) cytoplasm. * These are the cells that produce hydrochloric acid. * When examined with EM, deep invaginations of plasmalemma into cytoplasm can be seen that form intracellular canaliculi. Lined with microvilli. * Large number of mitochondria and a descrete golgi apparatus are present near the base of each cell. * There are NO SECRETORY GRANULES. organology/digestive/o_d_36.html digestive/images/ff758.jpg c. Parietal (oxyntic) cells
* Resting cell and actively secreting cell have different structure. ** In the resting cell there are many tubulovesicular structures that can be seen below the plasmalemma in the apical region. ** In the active cell, the tubulovesicular structures fuse with the plasmalemma to form many microvilli in the canaliculi and thereby increase the surface area through which HCl can be actively transported into lumen of canaliculi. *Both neural and hormonal factors cause secretion by these cells.
* Predominant basally in glands. * Typical protein synthesizing and secreting cell structure. * Granules (membrane bound vesicles) of inactive enzyme pepsinogen in cytoplasm. * When inactive pepsinogin is released into acidic environment of stomach it is activated. * Forms proteolytic enzyme pepsin. * Also, some lipase is secreted. stoImages/hl8A-43.jpg tive/o_d_36.html d. Chief (zymogenic) cells (basophilic)
e. Enteroendocrine cells - hormonal Amine Precursor Uptake and Decarboxylation (APUD) cells, Argentaffin cells (old terminology) There are a number of different types of these cells. Granules of secretory material collect at base of cell and are released into underlying tissue. Have hormonal function. At least one kind (Argentaffin cell)releases hormone 5-hydroxytryptamine (5-HT) also called serotonin 5-HT stimulates the activity of smooth muscle and can greatly increase the movements of the muscle layer of the stomach. Also found in intestinal glands. es/hl8A-45.jpg es/hl8A-50.jpg (Actually, this is an intestinal gland) mages/enteroendocrine.gif
Enteroendocrine cells might be considered a sort of diffuse endocrine gland.
E. Pylorus 1. Has deep, branched, gastric pits that connect to pyloric glands that have similarities in structure to glands of both the cardiac and body/fundic portions of stomach. 2. These glands secrete lysozyme (into gland lumen) and gastrin (into surrounding tissues - enteroendocrine cells). a. Gastrin is a hormone that stimulates the secretion of HCl by parietal cells that are mostly in body and fundus of stomach. tive/images/ff765.jpg
F. Cells of epithelial lining of stomach are constantly being replaced. 1. This is accomplished by mitotic activity of cells in neck regions of gastric glands (neck mucus cells). 2. Cells move from mitotic regions in two directions, a. Upward to replace the columnar mucous secreting cells*. * Replacement of these cells occurs every 5 days. b. Downward to replace cells of gastric gland epithelium, e.g. chief cells and parietal cells**. ** This replacement is relatively slow. G. The muscle layer of the stomach has 3 sub layers instead of External muscle sublayer is longitudinal. 2. Middle muscle sublayer is circular. 3. Inner muscle sublayer is oblique. H. Serosa