Sistema Digestório

	Veterinary Histology UFF Department of Morphology - Biomedic Institute LaBEc - Laboratory of Cellular and Extracellular Biomorphology
	Veterinary Histology Atlas
			Versão em Português

Introduction

Digestive System
Funçtion • Obtain from the ingested food the necessary molecules for the maintenance, growth and other energetic needs of the organism Structure • Hollow Tube composed of a lumen, whose diameter is variable, surrounded by a wall formed by four distinct layers: Mucosa • Epithelial lining • Lamina Propria: Loose Connective Tissue, rich in blood and lymphatic vessels, besides smooth muscle cells
	Muscularis Mucosae • Separates the Mucosa from the Submucosa • Normally composed of two thin sub-layers of smooth muscle cells
	Submucosa • Loose Connective Tissue with many blood and lymphatic vessels • Present a Nervous Plexus or Meissner's plexus • Can contain Glands and Lymphoid Tissue
	Muscularis Layers • Smooth muscle cells with a spiral orientation, divided into two sub-layers, Inner Circular and Outer Longitudinal

	• Between these two sub-layers we find the Myenteric Nervous Plexus or Auerbach’s Plexus
	Serosa • Thin Layer of Loose Connective Tissue, rich in blood and lymphatic vessels, and Adipose tissue • Covered by a thin layer of Simple Squamous Epithelium called Mesothelium
	Adventitia • Thin Layer of Loose Connective Tissue, rich in blood vessels, lymphatic vessels and Adipose tissue • Does not possess Mesothelium • Found where the Digestive Organ is joined with other Organs or structures.
Constituition Tongue • Keratinized(or not)Stratified Squamous Epithelium, depends on the animal’s diet -Canines: Epithelium not keratinized -Equines: Epithelium with little keratin -Felines: Epithelium is keratinized -Bovines: Epithelium not keratinized • Striated Skeletal Muscle (Plexiform) • Loose Connective Tissue highly vascularized and innervated • Adipocytes(Filling) • Acinar Glands : mucous, serous and mixed
	• Taste Buds: Specialized structures that contain taste bud cells, substance detectors capable of drawing the flavor.
Papillae

I - Filiform • Have an elongated cone shape • Numerous • Present in the entire dorsal surface of the tongue • Mechanical action, do not possess taste buds		II - Fungiform • Resembles a mushroom, with a strait base and a more dilated and smooth superior surface portion • Irregularly distributed between the filiform papillae • Possess few taste buds

III - Foliate • Poorly developed in humans, characteristic in rabbits • Consists on two or more parallel folds separated by grooves • Located on the dorsolateral surface of the tongue • Contain many taste buds		IV - Circumvallate • Great circular structures, whose flat surface extends itself to the top of other papillae • A Deep Depression surrounds each papilla • Present glands of von Ebner -Numerous Serous glands located in the fossa of each papilla -Continuous flow of liquid over the taste buds, important for the removal of food particles -Secrete lipase that prevents the formation of a hydrophobic layer over the taste buds • Distributed in the V-shaped boundary, in the posterior section of the tongue

Teeth and Associated Structures Classification
	Brachydonts • Low-crowned and adapted to a softer diet that results in less dental wear • There is no regeneration of the enamel, during the tooth eruption, the coating of ameloblasts is released
	Hypsodonts • Found in Ruminants, Rodents and Equines • Present a constant eruption, don’t have a definitive crown, neck or root • The organ of the enamel is broken before the tooth eruption and leaves the tissue of the alveolus in contact with the enamel • The cementum is deposited over the enamel • Ameloblasts do not disintegrate in the eruption, they go on with their activities for a while • Infundibular Recess and Pulp Cavity are present, visible according to the degree of wear of the teeth(old age)
Teeth Components
Enamel • Hardest component of the body, however little resistant • Composed 97% of Ca2+ under the form of hydroxyapatite crystals • Not innervated • Possess Retzius’ Striae -Incremental lines(growth lines) that mark the deposition of enamel
• The Organic Matrix is synthesized by ameloblasts -Tall Cylindrical Cells -Nucleus presents a basal orientation -Numerous mitochondria located below the nucleus -Secreting Cell Characteristics (well developed R.E.R. and G.A.) • In brachydont animals, after the end of the enamel synthesis, the ameloblasts form a protective epithelium that covers the crown UNTIL the tooth eruption • Tomes’ Process: Apical extension of the ameloblast, with numerous secretion granules contain proteins of the enamel matrix (amelogenin and enamelin)
Dentin • Mainly composed of collagen fibrils type I, glycosaminoglycans, phospholipids and hydroxyapatites • Pre-Dentin: Matrix at first not mineralized • The Organic Matrix is synthesized by the odontoblasts -They are elongated cells that deposit the matrix only over dentine surfaces -Nucleus with a basal orientation -Numerous mitochondria located below the nucleus -Characteristics of a Secreting Cell(well developed R.E.R. and G.A.) • Tomes’ Fibers (Odontoblastic projections): -Branched apical extension that penetrates the dentin perpendicularly. -Become longer as dentin becomes thicker -Dentinal Tubules: Narrow channels occupied by Tomes’ Fibers • Caries do not develop naturally in animals due to their sucrose-free diet • It is sensitive to stimulations even though it has few amyelinic nerve fibers connected to Tomes’ Fibers • Hydrodynamic Theory: Stimulations cause movements in the fluids inside the dentinal tubules, sensitizing nerve fibers
Dental Pulp • Loose Connective Tissue that contains: odontoblasts, fibroblasts, collagen fibrins and amorphous substance(many glycosaminoglycans) • Highly vascularized tissue with many myelinic nerve fibers that penetrate the apical foramen and branch out • Some Nerve Fibers lose their myelin and extend themselves only a short distance inside the dentinal tubules • Pulp Fibers are sensitive only to pain(single sensory modality)
Periodontal Components
Cementum • Covers the Radicular Dentin • Resembles the bone tissue although it does not contain vessels nor Haversian Systems • It’s labile and reacts to forces in which it undergoes with reabsorption or production of a new tissue • The continuous production of the cementum in the apix(bottom) compensates the physiological wear of the teeth(root/alveolus contact) • Thicker in the apix region, where cementocytes are found: -Cimentocytes are enclosed in lacunae, however practically don’t communicate between each other -Nutrition comes from the Periodontal Ligament -Low metabolic activity for it is not irrigated by blood
Periodontal Ligament • Composed by a special type of Connective Tissue whose fibers are arranged in thick packs(Sharpey’s Fibers) • Penetrate into the cementum of the tooth and osseous walls of the alveolus, allowing limited movements for the tooth • Fibers are organized to support the pressures applied during chewing(dissipation of mechanical forces) • The space between fibers are filled in by GAGs, collagen fibers and oxytalan elastic fibers Alveolar Bone • It is in direct contact with the periodontal ligament • A type of immature bone with collagen fibers not grouped in the lamellar pattern • Piercing Vessels: Blood vessels that pierce the alveolar bone and penetrate the periodontal ligament along the root • Some vessels and nerves direct themselves toward the apical foramen of the root to penetrate the Pulp Gingiva • Mucosa Membrane firmly adhered to the periosteum of the mandible and maxilla bone • Composed of Stratified Squamous Epithelium • Junctional Epithelium: Specialized part of the gingival epithelium that is attached to the enamel through a cuticle (thick basal lamina) • Between the Epithelium and the Enamel, above the Junctional epithelium, is the gingival sulcus(site of tartar accumulation)
Oropharynx • Extension of the Oral Cavity that connects it to the Esophagus • Lined by Keratinized (or not)Stratified Squamous Epithelium, depends of the species • Typical Lamina Propria, containing the tonsils, individual lymphoid nodules and disperse leucocytes. • Present numerous papillae, similar to the dermal papillae of the skin • There is no Muscularis Mucosae, a thin layer of elastic fibers separates this space from the submucosa • Typical Submucosa containing mucous glands • Muscularis Layer formed by striated skeletal muscle fibers without any particular orientation • Present Adventitia for there is a union of the oropharynx to the esophagus
Esophagus
	• Muscular tube whose function is to transport the food from the mouth to the stomach • Lined by Keratinized(or not)Stratified Squamous Epithelium, depends on the species • Typical Lamina Propria • Muscularis Mucosae formed by well fragmented smooth muscle fibers
	• Typical Submucosa containing mucous secreting esophageal glands, that facilitate the transport of food and protect the mucosa • Muscularis Layer: Fibers are in different directions to facilitate the peristaltic movements -Upper Third: Only Striated Skeletal Muscle Fibers -Middle Third: Mix of Smooth muscle with Striated Skeletal muscle -Inferior Third: Only Smooth Muscle Fibers • Present Adventitia for there is a union of the esophagus with the trachea • Present Serosa in the Esophagus Region that is in the Peritoneal Cavity
Curiosities • The facility of ruminants and dogs to vomit and/or regurgitate is linked to the distribution of Striated Skeletal Muscle • Fecal Emesis and Regurgitation are not physiological events in dogs • Regurgitation is a physiological event in Ruminants • Even though equines posses Striated Skeletal Muscle in 2/3 of their esophagus, their vomit is very rare, because the caudal esophageal sphincter has enough tonus to remain closed during gastric dilation to a point where a stomach rupture occurs without the occurrence of vomit.
Stomach • It is an organ that accomplishes exocrine and endocrine functions, digesting the food and secreting hormones • Continues the digestion of carbohydrates begun in the mouth • Adds an acid fluid to the ingested food • Transforms the food into a viscous mass(chime) through muscular activity • Promotes the initial digestion of proteins through the pepsin enzyme Division Monogastric Animals Structure
	• Lined by Mucous Simple Columnar Epithelium(all of the cells secrete an alkaline mucus) • The mucous forms a thick layer of gel that protects the cells from the stomach’s acidity • Occlusive Junctions between superficial cells and gastric pits form a protective barrier against the acid • HCl, Pepsins and Lipases are considered endogenous factors that damages the mucosa
	• Gastric Pits: -Epithelium that suffers invagination towards the lamina propria -In these pits characteristic glands of each region debouch secretion
	• Typical Lamina Propria containing smooth muscle cells and lymphoid cells • Submucosa with dense connective tissue containing blood and lymphatic vessels
	• Muscularis Layer contains Smooth Muscle Fibers oriented in three main directions: -Inner Oblique -Middle Circular -Outer Longitudinal • Present a Thin Serosa
Cell Types Stem-cells • Low cylindrical cells with an oval nucleus with a more basal location • Possess a high mitotic rate and originate other cell types • Apical Regeneration: Superficial Mucous Cells and Pit Cells • Basal Regeneration(responses are slower than the superficial mucous cells): Mucous Neck or Parietal and Cells Zymogenic or Enteroendocrine Cells • Abundant in the neck, and a few in the isthmus of the fundus Mucous Neck Cells • Irregular shape with basal nucleus and secreting granules in the apical region • Produce a mucous secretion that is less viscous than the mucous of the Surface Cells • Can differentiate into surface lining cells and/or Gland Lining Cells • Found in groups or isolated between the parietal cells in the neck region of the gastric cells
	Parietal Cells • Round or Pyramid-shaped Cells with a central spherical nucleus, cytoplasm intensely acidophilic • Produce H+ and Cl- • Gastrin and Histamine are potent stimulators of HCl production • Gastrina possesses a trophic effect in the gastric mucosa, stimulating its growth • Are mainly present in the upper half of the Gastric Glands, scarce in its base
Zymogenic Cells • Secreting Characteristics, very basophilic and with cytoplasmic granules containing Pepsinogen • Excrete pepsinogen that in contact with the stomach’s HCl transforms into pepsine. • Present in the inferior region of the gastric glands Enteroendocrine Cells • Produce hormones: -Serotonin: At the bottom of the Stomach, responsible for the increase of the intestinal mobility -Somatostatin:Inhibits the release of some hormones such as gastrin • Located in the base of the Gastric Glands
Histological Regions
	Cardia • Transition between esophagus and stomach • Transition from Stratified Squamous Epithelium of the Esophagus to Mucous Simple Columnar of the Stomach • Submucosa contains cardiac glands that produce mucous and lysozyme(antibacterial) • Present few Parietal Cells
	Fundus (Corpus e Fundus are Histologically identical) • Lined by a highly ridged Mucous Simple Columnar Epithelium • Fairly thick mucosa • Submucosa contains fundic glands that produce mucus • 3 to 7 glands open at the bottom of each gastric pit • Regions of Gastric Glands: I-Isthmus • Mucous cells in differentiation that will substitute the pit and surface cells • Undifferentiated Stem-cells • Parietal Cells II-Neck • Undifferentiated Stem-cells • Parietal Cells • Mucous Neck Cells(different from the mucous cells of the isthmus) • Enteroendocrine cells III-Base • Parietal Cells • Zymogenic Cells • Enteroendocrine Cells
	Pylorus • Lined by Mucous Simple Columnar Epithelium • Possess Deep Gastric Pits where Pyloric glands open(mucus and lysozyme) • Present Enteroendocrine cells that secrete gastrin intercalated by Mucous Cells • The Middle Muscle Layer is thicker so to form a pyloric sphincter
Polygastric Animals Forestomach • Lined by Keratinized Stratified Squamous Epithelium • Are non-glandular
	Rumen (Paunçh) • Small Rumen Papillae • Do not possess Muscularis Mucosae (lamina propria and connective tissue of the submucosa are fused) • Have a mechanical function • Curiosities: -Produce Volatile Fatty Acids -Depend on protozoa and bacteria to digest -Timpanism -Gases
	Reticulum (Bonnet) • Reticulum' folds with a honeycomb aspect and spear-shaped • Muscularis Mucosae projects itself a little, present in the apex of the papilla
	Omasum (Manyplies) •Omasum' folds • Omasum' Folds with smaller papillae • Muscularis Mucosae projects itself a lot, forming an axis of smooth muscle in all of its extension. There is also a layer of smooth muscle of inner circular. • Retains food, it’s the coarsest papillae
True Stomach-Obomasum(rennet-bag) • Same conformation as the common stomach • Regions: Nonglandular, Cardia, Fundus and Pylorus
Small Intestine • Final part of the food digestion, nutrients absorption and endocrine secretion • Possess many modifications that enhance the contact surface and consequently the absorption: -Long length(5 meters) -Folds in Mucosa and Submucosa -Villi and Microvilli -Absorptive Cells(Enterocytes) Type of epithelial cell of the superficial layer of the small and large intestines These cells break-down molecules and transport them inside the tissue
Structure
	• Lined by Simple Columnar Epithelium with a Striated Border and Goblet cells(vary according to the portion of the intestine)
	• Present intestinal villi: Elongated Finger-like structures • Crypts of Lieberkühn: Opening of Simple Tubular Glands • Great amount of blood and lymphatic capillaries, besides veins • Myenteric Plexus of Auerbach • Submucosa Nerve Plexus of Meissner
Tipos Celulares Stem-Cells • Low cylindrical cells with an oval and basal located nucleus • Possess a high mitotic rate and originate other types of cells Absorptive Cells (Enterocytes) • Tall Cylindrical Cells with an oval and basal located nucleus, granulated acidophilic cytoplasm, presence of striated border • Absorb nutrients produced during digestion • These cells are in constant renovation since there is a desquamation of apical cells • Located in three portions of the Small Intestine, and situated in all of the villi portion Goblet Cells • Produce mucous that protects and lubricates the intestine’s lining • Located in great amounts close to ileum, and situates itself between the absorptive cells Paneth Cells • Pyramid-shaped cells with acidophilic granules and basal nucleus • Have an antibacterial activity because they produce lysozyme that destroys the bacterial cell wall • Located in the basal portion of the glands M Cells • Specialized Epithelial Cells that line the lymphoid follicles of Peyer’s Patches • The Basal Lamina below them is discontinuous, facilitating the intestinal traffic • Possess many basal invaginations containing lymphocytes and antigen-presenting cells, like macrophages • Capture antigens through endocytosis and transport them to the underlying lymphoid cells and macrophages • Participate in the immunological defense of the intestines • GALT(Gut-Associated Lymphoid Tissue) -Plasmacytes that synthesize IgA -Occlusive Intercellular junctions -Present macrophages and lymphocytes located in the mucosa and submucosa
Division
	Duodenum • Tube of approximately 2 to 3 centimeters in diameter that connects the stomach to the small intestine • Villi with a leaf-like aspect, close to the ileum they acquire a finger-like shape • Lined by Simple Columnar Epithelium with Striated Border and some RARE Goblet Cells • Present many Absorptive Cells and Paneth Cells • M cells and Goblet cells are characteristic of this region • Typical Lamina Propria • Muscularis Mucosa without any peculiarities
	• Submucosa containing duodenal glands or Brünner Glands that produce mucus -This mucus protects the mucosa against the acidity of the gastric acid and neutralizes the pH of the chime -These glands open at the bottom of the Crypts of Lieberkühn • Muscularis Layer with an Inner circular and Outer Longitudinal layer and present Myenteric Plexus of Auerbach • Typical Serosa
Jejunum • Portion of the small intestine that is comprised between the duodenum and ileum • Villi are narrower and less numerous than in the duodenum, finger-like shape • Lined by Simple Columnar Epithelium with Striated Border and SOME Goblet Cells • Present many Absorptive Cells and Paneth Cells, some Goblet Cells • M cells are not characteristic of this region • Typical Lamina Propria • Muscularis Mucosae without any peculiarities • Submucosa does not contain any glands, present Nerve Plexus of Meissner • Muscularis Layer with inner circular and outer longitudinal layer and presents Myenteric Plexus of Auerbach • Characteristic Serosa
	Ileum • Part of the small intestine in which the chemical digestion continues, after passing through the duodenum and jejunum • Villi with a bat shape, folds are not present • Lined by Simple Columnar Epithelium with Striated Border and MANY Goblet Cells • Present many Absorptive Cells, Paneth Cells, Goblet Cells and M Cells • Characteristic Lamina Propria with lymphoid nodules(Peyer’s Patches) • Muscularis Mucosae without any peculiarities • Submucosa does not contain any types of glands, has Peyer’s Patches and Nerve Plexus of Meissner • Muscularis Layer with an inner circular and outer longitudinal layer and present myenteric plexus of Auerbach • Characteristic Serosa
Large Intestine
	• Consists of a Mucosa Membrane filled with Goblet Cells, no Folds or Villi • Functions are water absorption, formation of feces and production of mucus • Lined by Simple Columnar Epithelium with a Striated Border(irregular and spaced) and MANY Goblet Cells • Present many Absorptive Cells and Goblet Cells, and some Enteroendocrine Cells • M Cells and Paneth Cells are not typical of this region • Lamina Propria with many lymphoid cells and nodules that frequently reach the submucosa • Muscularis Mucosae without any peculiarities • Typical Submucosa with lymphoid nodules • Muscularis Externa formed by three thick laminae(taenia coli) • Typical Serosa only until the neck after that it continues as an adventitia

Rectum • Terminal part of the digestive system, after the large intestine and before the annus • Very thick Muscularis Externa • Adventitia
Annus • The annus is the orifice at the end of the large intestine where the feces and intestinal gases are eliminated. • Transition from the Simple Columnar Epithelium to the Stratified Squamous, typical of the tegument • Lamina Propria containing a plexus of large veins • Adventitia

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