Digestive System: Oral Cavity & Alimentary Canal

BIO 408

Histology

Dr. D. L. Daley

Oral Mucosa

n Composed of a wet stratified squamous keratinized, nonkeratinized or parakeratinized epithelium & underlying connective tissue

n The gingiva, dorsal surface of the tongue and hard palate are lined with masticatory mucosa (parakeratinized to completely keratinized stratified squamous epithelium - dense irregular connective tissue is found below)

n The remainder of the oral cavity is lined or covered with lining mucosa (nonkeratinized stratified squamous epithelium)

Lips

n Three zones or regions

n External aspect - covered with thin skin, associated with sweat glands, hair follicles & sebaceous glands

n Vermillion zone - continuous with the above - the pink region - thin skin - no sweat glands & hair follicles - capillary loops of dermal papillae are close to the surface & impart the pink color

n Mucous (internal) aspect - always wet - lined with stratified squamous nonkeratinized epithelium - dense irregular connective tissue below - mostly mucous glands and minor salivary glands

Teeth

n 20 deciduous or milk teeth - replaced by 32 permanent teeth - 20 succedaneous teeth and 12 molars

n Each tooth - suspended in a bony socket - the alveolus via peridontal ligament (dense, irregular collagenous connective tissue)

n The gingiva (gums) also supports the tooth & its epithelium seals the oral cavity from the subepithelial connective tissue spaces

Teeth

n Crown - portion of a tooth that is visible in the oral cavity

n The region within the alveolus - root

n The portion of the tooth the crown & the root is the cervix

n Three calcified substances enclose a soft, gelatinous connective tissue called the pulp

Mineralized Components

n Enamel - hardest substance in the body - translucent - color due to underlying dentin

n Consists of 96% calcium hydroxyapatite & 4% organic material and water

n Calcified portion of enamel is large crystals coated with thin layer of organic matrix - enamelins (keratin-like high-molecular weight tyrosine rich glycoproteins

n Enamel is elaborated daily by ameloblasts

Mineralized Components

n Dentin - second hardest tissue of the body - it is yellowish

n High degree of elasticity - protgects overlying brittle enamel

n Composed of 65-75% calcium hydroxyapatite, 20% to 25% organic material; and about 10% bound water

n Most of the organic material associated with is type I collagen associated with proteoglycans and glycoproteins

n Odontoblasts produce dentin - cells in the pulp and have thin processes extending into the tunnel-like spaces in the dentin - dentinal tubules

Pulp

n Loose gelatinous connective tissue - rich in proteoglycans and glycoaminoglycans

n Extensive vascular and nerve supply & some lymph circulatory elements

n Outer - odontoblast zone - next a cell-free zone - then cell-rich zone is deepest zone - surrounds the pulp core

n May have some calcified elements called pulp stone (denticles)

Palate

n Composed of the hard palate, soft palate and uvula

n Separates the oral and nasal cavities from each other

Tongue

n Largest structure of the oral cavity

n Extremely mobile - large intertwined mass of skeletal muscle fibers

n Extrinsic muscles - originate outside the tongue

n Responsible for moving the in & out & side to side

n Intrinsic muscles - originate in and insert into the tongue

n Responsible for altering the shape of the tongue

Tongue

n The dorsal surface - two unequal regions

n Larger anterior two thirds & smaller posterior one third

n The two regions are separated by a shallow, V-shaped groove, the sulcus terminalis - apex points backwards and contains a deep concavity, the foramen cecum

Lingual Papillae

n Four types of papillae

n Numerous filiform papillae - slender & give the tongue a velvety appearance to the dorsal surface

n Covered by stratified squamous keratinized epithelium - help scrape food off a surface

n No taste buds

n Fungiform papillae - like a mushroom - covered by stratified squamous nonkeratinized epithelium

n Taste buds on the dorsal aspect of the papillae

Lingual Papillae

n Foliate papillae - located along the posterior aspect of the tongue - appear as vertical furrows

n Have functional taste buds in the neonate but these degenerate by the second or third year of life

n Ducts of minor salivary glands of van Ebner empty into the furrows at the base of these papillae

n Circumvallate papillae - V shaped arrangement just anterior to the sulcus terminalis

n Taste buds are present

Taste Buds

n About 3000 taste buds

n Each is composed of 60 to 80 spindle shaped cells in a oval structure

n Narrow end of the taste bud - located in the free surface of the epithelium - projects into an opening, the taste pore, formed by squamous epithelial cells that overlie the taste bud

n Four types of cells (I, II, III & IV (basal stem cell)

n Nerve fibers synapse on the taste cells

Alimentary Canal

n Tubular portion of the digestive tract - where food is churned, liquefied, digested, water and nutritional elements are absorbed and undigested material stored and eliminated

n Four layers

n Mucosa, submucosa, muscularis external and the outer serosa (adventitia)

Innervation of the Digestive Tract

n Autonomic nervous system

n Sympathetic and parasympathetic components

n Interacts with the Enteric NS

n Enteric Nervous system - runs the length of the alimentary canal from esophagus to anus

n Self-contained nervous system composed of repeating ganglia known as Meissner’s submucosal plexus and Auerbach’s mysteric plexus

n Controls the secretory and motile functions of the system

Esophagus

n Muscular tube about 25 cm in length - from oral pharynx to stomach - usually collapsed - opens during swallowing

n Mucosa - three layers

n Stratified squamous nonkeratinized epithelium

n Langerhans cells - antigen presenting cells - phagocytose & degrade antigens into small polypeptides called epitopes

n Also produce major histocompatability complex (MHC) II molecules - these attach to the epitopes & display the MHC II - epitope complex on the cell’s surface - then migrate to lymph nodes and present the complex to lymphocytes

Esophagus

n Mucosa

n Lamina propria - houses esophageal cardiac glands near the pharynx and juncture with the stomach

n Muscularis muscosae - only a single layer of longitudinally oriented smooth muscle

n Thicker near the stomach

Esophagus

n Submucosa - dense, fibroelastic connective tissue

n Houses esophageal glands - tubuloacinar glands - mucus and serous cells

n Serous cells - release proenzyme pepsinogen and antibacterial agent lysozyme

Esophagus

n Muscularis externa - two layers of both skeletal and smooth muscle

n Inner circular muscle

n Outer longitudinal muscle

n Upper third - skeletal muscle

n Middle third - skeletal & smooth muscle

n Lower third - only smooth muscle

n Adventitia - fibrous outside coating until the esophagus pierces the diaphragm - then it is called a serosa - squamous epithelium & connective tissue below

Stomach

n Most dilated portion of the alimentary canal

n Sac-like - accommodate 1500ml of food & gastric juices at max

n Bolus of entering food is mixed with gastric juice to form chyme

n Four regions - Cardia, fundus, body (corpus) and pylorus

Stomach

n Fundic muscosa

n Simple columnar epithelium

n Surface lining cells - thick mucous producers - mucous traps bicarbonate ions - neutral pH at the cells surface

n The cells continue into gastric pits - form their epithelial lining

n Regenerative cells - found near the opening of the gastric pits - stem cells

n Lamina propria - lightly vascularized loose connective tissue - mostly 15 million gastric (fundic) glands

Fundic Glands

n Three regions - isthmus, neck and base

n Simple columnar epithelium - 6 cell types

n Surface lining cells - as describe

n Mucous neck cells - produce soluble mucous - mixed with & lubricates the chyme

n Regenerative cells - stem cells - among the mucous neck cells

Fundic Glands

n Parietal (oxyntic) cells - pyramid-shaped cells

n Upper half of fundic glands

n Produce HCH & gastric intrinsic factor (necessary for B₁₂ absorption)

n Chief (Zygomatic) cells - columnar cells

n Base of fundic glands

n Produce proenzyme pepsinogen, rennin & gastric lipase

Fundic Glands

n DNES cells (APUD or enteroendocrine cells) - grouped or individually dispersed

n Argentaffin cells, APUD cells, DNES cells (13 types) and enteroendocrine cells

n These cells produce endocrine, paracrine and neurocrine hormones

Stomach

n Muscularis mucosae - three layers of smooth muscle

n Inner circular

n Outer longitudinal

n Occasional third layer - circular & not always evident

Stomach Mucosa Differences

n Cardiac region

n Shallow gastric pits - base of the glands highly coiled

n Cells of the glands are mostly mucous neck cells, a few DNES cells and parietal cells - no chief cells

n Pyloric region - same cells types as cardiac region

n Mucous neck cell predominates - in addition to mucous these secrete lysozyme (antibacterial)

n Glands are highly convoluted and branched

Stomach

n Submucosa - dense irregular connective tissue - rich vascular & lymphatic supply

n Muscularis externa - three layers of smooth muscle

n Innermost oblique, middle circular (form pyloric sphincter in pyloric region) & outer longitudinal

n Serosa - wet simple squamous epithelium & loose subserous connective tissue

Small Intestine

n Small diameter - 3 meters long

n Large intestine - 1.5 meters long

n First 20-30 cm - duodenum

n Next 2/5 - jejunum

n Last 3/5 - ileum

n Empties through ileocecal valve in large intestine

Small Intestine

n Modifications of the luminal surface

n Plicae circulares (valves of Kerckring) - transverse folds of the submucosa and mucosa - form semicircular to helical elevations - increase surface area by 2-3 times

n Villi - finger-like projections of the lamina propria

n Core is capillary loops & a lacteal and a few smooth muscle fiber in connective tissue

n Also increase surface area - 10 times

n Microvillae - modifications of the apical plasma membrane of epithelial cells - 20 fold incr in surface area

n Invaginations of the epithelium into the lamina propria - form crypts of Liberkühn (glands)

Intestinal Mucosa

n Epithelium - simple columnar epithelium

n Surface absorptive cells - apical surface has a brush border (microvillae) covered by a thick glycocalyx

n Goblet cells - unicellular glands - smallest number in duodenum - produce mucinogen - hydrated its mucin - a component of mucus

n M cells - microfold cells - phagocytose & transport antigen from lumen to lamina propria

Intestinal Mucosa:

n Lamina Propria

n Loose connective tissue - of the villi - extends down to the muscularis mucosae

n Crypts of Liberkühn - simple tubular or branched tubular glands

n Open into the intervillar spaces as perforations of the epithelial lining

n Composed of surface absorptive cells, goblet cells, regenerative cells, DNES cells and Paneth cells (produce antibacterial lysozyme)

n Muscularis Mucosae - inner circular smooth muscle layer and outer longitudinal smooth muscle layer

Intestinal Submucosa

n Dense irregular fibroelastic connective tissue

n Intrinsic innervation from parasymapthetic submucosal (Meissner’s) plexus

n Bruners glands (duodenum)

n Tubuloalveolar glands

n Produce mucous, bicarbonate rich fluid & urogastrone (human epidermal growth factor) - inhibits production of HCl & increases the rate of epithelial cell mitosis

Intestinal Muscularis Externa & Serosa

n Muscularis externa - inner circular smooth muscle layer & outer longitudinal smooth muscle layer

n Auerbach’s mysenteric plexus - between the two muscle layers

n Serosa- surrounds whole small intestine except parts of the duodenum

Lymphatic & Vascular Supply of the Small Intestine

n Blind ending lymph capillaries, lacteals, are in the core of the villi

n These empty into the submucosal lymphatic plexus - then to a series of lymph nodes - then to the thoracic duct - then into the junction of the left internal jugular and subclavian veins

n Capillary loops are adjacent to the lacteals & drain into vessels that empty into the submucosal vascular plexus - then to the portal vein on the way to the liver

Large Intestine

n Composed off the cecum, colon (ascending, transverse, descending and sigmoid), rectum and anus

n Approximately 1.5 meters long

n Absorbs water and ions from the chyme and compacts chyme into feces for elimination

n Appendix is a blind outpocketing of the cecum

Colon

n Receives chyme from the small intestine through the ileocecal valve

n No villi but richly endowed with crypts of Liberkühn similar to those of the small intestine

n Except no Paneth cells

n Number of goblet cells increases from the cecum to the sigmoid colon

n The crypts of Liberkühn are fewer but deeper

Colon

n The lamina propria, muscularis mucosae and submucosa of the colon are similar to the small intestine

n However, the muscularis external is unusual, since the outer longitudinal layer is not continuous along the surface - rather is gathered into three narrow ribbons of muscle fascicles - called taeniae coli

n The continuous tonus of the taeniae coli puckers the colon into sacculations called haustra coli

n The serosa - numerous fat filled pouches - appendices epiploicae

Rectum and Anal Canal

n Histologically the rectum resembles the colon

n The crypts of Liberkühn are fewer and deeper

n The anal canal - constricted continuation of the rectum is about 3-4 cm long

n The crypts of Liberkühn are short and few in number

n Mucosa shows longitudinal folds - anal columns - these meet and form anal valves

Anal Canal

n Anal mucosa - simple cuboidal from the rectum to the pectinate line ( at the level of the anal valves) - then stratified squamous to the external anal orifice

n Lamina propria - fibroelastic connective tissue - anal glands (rectoanal junction)

n Cicumanal glands - distal end of the anal canal

Anal Canal

n Submucosa - fibroelastic connective tissue

n Two venous plexuses - internal hemorrhoidal plexus (above the pectinate line) and the external hemorrhoidal plexus - at eh junction of the junction of the anal canal and the external orifice, the anus

n The muscularis externa - inner circular layer thickened at the pectinate line - internal anal sphincter

n Skeletal muscles of the floor of the pelvis form an external anal sphincter muscle - surrounds the fibroelastic sheet and internal anal sphincter - later under voluntary control

Appendix

n Veriform appendix - 5 to 6 cm long diverticulum of the cecum

n Usually occupied by debris

n Resembles the colon histologically -except smaller in diameter

n Richer supply of lymphoid elements and contained more DBNES cells in it crypts of Liberkühn