Digestive System Notes PDF

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Summary

####### FUNCTIONS Digestion of food Absorption of digested substances Excretion of undigested materials Components of the Digestive system Digestive tract Digestive glands DIGESTIVE TRACT Long tube consists of several segments o Oral cavity o Pharynx o Esophagus o Stomach o Small intestine o Large i...

Description

DIGESTIVE SYSTEM FUNCTIONS -

Digestion of food Absorption of digested substances Excretion of undigested materials

Components of the Digestive system -

Digestive tract Digestive glands

DIGESTIVE TRACT -

Long tube consists of several segments o Oral cavity o Pharynx o Esophagus o Stomach o Small intestine o Large intestine

DIGESTIVE GLANDS -

consists of glands that are embedded in the walls of the digestive tract and the accessory glands (major salivary glands, exocrine portion of pancreas, liver and gallbladder)

GENERAL HISTOLOGIC LAYERS OF THE DIGESTIVE TRACT

MUCOSA

(Tunica mucosa, mucous membrane) Consists of three layers: -

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The walls of the digestive tract has four histologic layers: o Mucosa (mucous membrane) o Submucosa o Muscularis externa o Adventitia/Serosa

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Epithelium which lines the luminal surface of the entire digestive tract. Lamina propria which refers to the loose connective tissue layer under the epithelium and contains fine blood and lymphatic vessels and MALT. Muscularis mucosa which is the thin sheet of smooth muscle fibers that forms the outermost layer of the mucosa.

MUSCULARIS EXTERNA -

SUBMOCUSA -

Made up of loose connective tissue that is usually denser, more abundant and more vascular than lamina propria of the mucosa.

MUSCULARIS EXTERNA -

Consist of two relatively thick smooth muscle coats or layers: Inner layers (muscle fibers are arranged circularly) Outer layers (muscle fibers are arranged longitudinally) Contractions of the muscles in this layer mixes, squeezes and propels food through the digestive tract.

Contains population of cells that acts as the pacemakers of the gastrointestinal motility known as Interstitial cells of Cajal (ICC) which are located between nerve and smooth muscle cells. ICC are indistinguishable from smooth muscle cells using routine histologic preparations but can be distinguished by methelyne blue

staining and zinc-iodide-osmium impregnantion and are best appreciated through electron microscopy and immunocytochemical techniques.

ADVENTITIA/ SEROSA -

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Outermost layer of the digestive tract. In those that are covered by peritoneum the outer covering is lined by mesothelium known as serosa. In areas where there is no peritoneal covering the outermost covering is composed of connective tissue known as adventitia.

DIGESTIVE TRACT

ORAL CAVITY (MOUTH) -

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Initial segment of the digestive tract and divided into two regions: o Vestibule is the region of the mouth that is anterior to the teeth and gums. o Oral Cavity proper Area of the digestive tract in which the food is reduced into smaller bits by the mechanical actions of the teeth, tongue and muscles of mastications and chemical action of the enzymes in the saliva. Roof of the oral cavity proper is formed by the hard and soft palates, the floor mainly by the tongue and the lateral walls by the cheeks.

HISTOLOGIC LAYERS OF THE ORAL CAVITY MUCOSA

SUBMUCOSA

MUSCULARIS EXTERNA

ADVENTITIA

continuous with the skin at the margins of the lips with the mucosa of the pharynx

contains small mucussecreting and tubuloalveolar glands (eg. labial, buccal)

present only in the lips and cheeks; muscles are responsible for mastication and facial expressions

externally located to the muscularis that binds it to the overlying skin

TONGUE -

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It is a muscular organ that plays an important role in mastication, deglutition and speech. Contains most of the sense organs for taste. SULCUS TERMINALIS is the inverted “V” shaped shallow furrow that separate the anterior to the posterior part of the tongue. The apex of the “V” is called the FORAMEN CECUM. posterior attached portion of the tongue is called its “root”

Filiform Papillae -

Fungiform Papillae -

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They are the projections of the mucosa and are confined to the dorsal surface of the anterior tongue. Consists of core of connective tissue that are lined externally with Stratified Squamous Epithelium . In humans, there are composed of three types: Filiform, Fungiform and Circumvallate

Contains connective tissue which are rich in capillaries which appear as pinhead-size red dots to the naked eye. taste buds are occasionally present on their surface

Cirvumvallate Papillae -

LINGUAL PAPILLAE

The most numerous papillae. Slender and tapering and are found all over the dorsal surface of the anterior tongue.

Arranged along the sulcus terminalis. They are the largest (1 to 3 mm in diameter) but the least (only 6 to 14) of the lingual papillae. They are cylindrical structures whose base is surrounded by canal or moat. about 200-300 taste buds are embedded on its lateral surface

Taste Buds -

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Organs that are responsible for the sense of taste. They are ovoid structures that are embedded in the epithelium of circumvallate and fungiform papillae. They are consist of densely-packed fusiform cells. Cells of the Taste Buds

PHARYNX -

Funnel-shaped fibromuscular tube that are common to both digestive and respiratory system.

ESOPHAGUS -

Long muscular tube (20 to 25 cm) that serves as a passageway for food from the pharynx to the stomach

In light microscope: 3 types are distinguishable namely: -

Sustentacular or Supporting cells Neuroepthelial or Gustatory cells Basal Cells

In electron microscope, 5 types are distinguishable” -

Type I (Sustentacular Cells) Type II Type III Type IV (Basal Cells)

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Type V (outer boundary of the taste buds)

Cells of the Taste Buds

(LIGHT MICROSCOPE) -

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Sustentacular or Supporting Cells Neuroepithelial or Gustatory Cells o SIMILARITIES: Both slender, spindle shape cells whose free surface is covered by microvilli. o DIFFERENCE: Sustentacular cells are darker staining cells than neuroepihelial cells. Basal Cells: round cells located at the base of the taste buds and serve as stem cells for other cell type.

HISTOLOGIC LAYERS OF THE ESOPHAGUS MUCOSA

SUBMUCOSA

MUSCULARIS EXTERNA

ADVENTITIA

forms a longitudinal folds which flatten out when swallowed materials passes through the organ

thicker thatn the lamina propria; consists of connective tissue, elastic fibers, blood vessels and esophageal glands

outer layer consists of longitudinal mucle fibers, while inner layer consists of circular muscle fibers

found in the outermost layer

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Pyloric region tapering distal portion of the stomach that is 6ontinuous to the duodenum.

STOMACH -

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J- shaped hollow organ located in the left upper quadrant of the abdomen. presents a left or lateral border (great curvature), a medial border (lesser curvature), an anterior surface and a posterior surface In this organ the food is thoroughly softened, mixed and converted into CHYME, an acidic semi fluid mixture of food and gastric juice.

Mucosa of the stomach -

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Grossly the stomach is subdivided into: -

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Cardia portion of the stomach immediately surrounding the esophageal opening. Fundus is the dome-shaped portion of the stomach. Body comprises the bulk of the organ

The mucosa and part of the submucosa forms longitudinal anastomosing folds or wrinkles (rugae) At intervals, the surface epithelium invaginates into the lamina propria to form shallow grooves (gastric foveolae or pits)

GLANDS OF THE STOMACH -

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They are named according to their location. They are all confined in the mucosa. They are simple tubular or simple branched tubular glands that open in the bottom of the gastric pits known as isthmus. Consists of three types namely: o Fundic (Gastric glands) o Cardiac glands o Pyloric glands

Fundic Glands -

Also known as gastric glands.

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Located in the fundus and body of the stomach. The most numerous and longest of the glands of the stomach. Consists of five types of cells: o Oxyntic (parietal) cells o Zymogenic (chief) cells o Mucous neck cells o Stem cells o Enteroendocrine cells.

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They have secretory granules that contains pepsinogen the precursor of the enzyme pepsin.

Cells of the Fundic Glands Parietal or Oxyntic cells -

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Pyramidal in shaped with oval or round nucleus The most easily identified cells of the fundic glands in the H &E preparation due to its intense cytoplasmic eosinophilia. Produce hydrochloric acid that is responsible for the acidity of the gastric juice and intrinsic factor, a glycoprotein needed for the absorption of Vitamin B 12 in the terminal region of the ileum.

Chief or Zymogenic cells -

Most numerous cells of the fundic glands especially in he basal region of the glands.

Mucous neck cells -

Generally confined in the neck of the fundic glands. Contains numerous secretory granules that contain mucin. cells comprising the surface epithelium of the stomach

Stem cells -

Play in important role in renewing the cell population of the stomach.

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Can be differentiated into surface mucous cells or gastric cells.

Enteroendocrine cells/ Enterochromaffin cells/ Argentaffin cells -

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Hormone producing cells of glands of the stomach, large and small intestines. They are columnar, ovoid or pyramidial cells . cells that are also found in the surface epithelium of the stomach In the stomach the type of enteroendocrine cells are: o G cells which secrets Gastrin o EC cells which secrets Serotonin o D cells that secrets Somatostatin o ECL or Enterochromnaffin like cells which secrets Histamine

Cardiac Glands -

The least numerous of the gastric glands. Also contains all the cell types of the gastric glands in which the most numerous is mucous neck cells.

Pyloric Glands -

Shorter but more coiled than fundic and cardiac glands.

GASTRIC MUCOSAL BARRIER -

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SMALL INTESTINE

prevents HCl and the gastric enzymes from damaging the mucosa of the stomach It has three components: o compact epithelium o layer of mucus o bicarbonate ions

HISTOLOGIC LAYERS OF THE STOMACH MUCOSA

SUBMUCOSA

MUSCULARIS EXTERNA

ADVENTITIA

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thick, made up of connective tissue; well supplied by blood vessels, nerves and GALT

thick; composed of 3 layers: inner obliquely arranged smooth muscle fibers, circularly arranged muscle fibers and outermost longitudinally muscle fibers

enveloped by peritoneum, hence, the outermost layer is serosa

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Segment located between the stomach and the large intestine. The organ which are responsible in the absorption of nutrients from the digested food. anchored to the posterior wall by means of the mesentery Composed of three segments:

DUODENUM, JEJUNUM, ILEUM EPITHELIUM OF THE SMALL INTESTINE -

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Its surface is simple columnar It has four types of cells: enterocytes, goblet cells, enteroendocrine cells and M-cells The microvilli in the small intestine forms a striated area called Brush border or striated border

Lamina Propria and Muscularis mucosae of the small intestine -

consists of loose connective tissue and smooth muscle fibers lymphocytes, mast cells, plasma cells and macrophaes are also present embedded within is the intestinal glands or crypts of Lieberkuhn whicha has 5 cell types: o Stem cells (most numerous) o Enterocytes o Goblet cells o Paneth cells o Enteroendocrine cells

HISTOLOGIC LAYERS OF THE SMALL INTESTINES MUCOSA exhibit modifications that are designed to increase the surface area for digestion and absorption, these 3 modifications are: horizontal mucosal folds, intestinal villi and microvilli

SUBMUCOSA

MUSCULARIS EXTERNA

ADVENTITIA

supplied with blood vessels, lymphatic vessels abd GALT

comprised of inner concentric and outer longutudinal layer

Most external layer is serosa except for the 2nd and 3rd segments of the duodenum not covered by peritoneum

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LARGE INSTESTINES -

It is the most distal segment of the digestive tract. Consists of several segments (arranged from most proximal to most distal): o Cecum o Vermiform appendix o Ascending colon o Transverse colon o Descending colon o Sigmoid colon o Rectum o Anal canal

The segment of the digestive tract which are responsible in the absorption of water and some electrolytes occurred which is a necessary step in the formation of semisolid stool. Contains bigger diameter than small intestines. contains segments: cecum, vermiform appendix, ascending colon, transverse colon, descending colon, sigmoid colon, rectum and anal canal differs from the small intestine because of several features: o its wall forms pouches called sacculations or haustrae o small sacs of fat-filled peritoneum (appendices epiploicae) dangles from the external surface of its wall o most of the muscle fibers collects into three bands called taenia coli

HISTOLOGIC LAYERS OF THE LARGE INTESTINE SUBMUCOSA

MUCOSA devoid of horizontal mucosal folds (vales of Kerckring); no villi but crypts of Lieberkuhn are present instead Cells that populate its epithelium: enterocytes, goblet cells, enteroendocrine cells and M-cells

thick, richly supplied with GALT and has no glands

MUSCULARIS EXTERNA

ADVENTITIA

atypical, because the outer layer of smooth muscle fibers does not surround the organ evenly, but mostly forms equidistant taenia coli

Outermost layer is serosa because the greater part is covered by peritoneum, the serosa contains large amount of adipose tissue; adventitia part if found on the ascending and descending colons

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Consist of poorly developed muscularis mucosa and muscularis externa. Its outermost covering is serosa.

ACCESSORY GLANDS AND ORGANS OF THE DIGESTIVE SYSTEM

SALIVARY GLANDS -

It includes all the digestive glands that open into the oral cavity. Classified according to size: Major Salivary glands

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VERMIFORM APPENDIX -

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It is short, narrow and wormlike tubular evagination of the cecum. Its epithelium is made up of TALL COLUMNAR CELLS with microvilli, with few goblet cells and enteroendocrine cells, short crypts of Lieberkuhn. Its lamina propria and submucosa are infiltered with GALT.

Located outside the oral cavity but their secretions empty into the mouth via large duts. Minor Salivary glands

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Small glands that re embedded In the lamina propria of the oral cavity.

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Their collective secretions are called saliva.

Saliva -

It is a clear watery fluid that contains two digestive enzymes (lingual lipase and ptyalin or salivary amylase), glycoproteins, mucins,

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immunoglobulins, lysossomes, proteins and inroganic ions. It serves as solvent for substances that stimulate taste buds. It facilitates speech and protects the teeth, tongue and other oral tissues.

Two types:

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Three types:

MAJOR SALIVARY GLANDS -

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They are all compound tubuloalveolar glands which are made up of functional component (parenchyma) that consist glandular epithelium and (stroma) which is the connective tissue framework. The stroma consists of a capsule, septa, lobes and lobules They are primarily exocrine glands They secrete substances only in response to appropriate stimulus. Also secrets epidermal growth hormone. Consists of three pairs of major salivary glands: parotid glands, submandibular glands and sublingual glands.

o Intercalated o Striated excretory ducts (those that are outside the lobules)

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o interlobular o lobar o main excretory Intercalated duct (directly drains an acinus or a secretory lobule Striated duct (formed by the union of intercalated ducts)

Parotid Glands -

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It is the largest salivary glands located one of each side of the body just below the anterior to the pinna of either ear at the region of the angle of the mandible. They are purely serous glands, thus its secretory units consists of typical serous cells. It has the best developed duct system with its main duct known as Stensen’s duct which opens into the vestibule of the oral cavity opposite the upper second molar tooth.

Submandibular Gland

(Submaxillary Gland) DUCT SYSTEM OF THE MAJOR SALIVARY GLANDS -

Intralobular ducts (ducts within the lobules)

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Located in the submandibular fossae of the inner aspect of the mandible. It is a mixed glands but most of its secretory units are serous. Its main excretory duct is called Wharton’s duct which opens into the oral cavity underneath the tongue.

Sublingual Glands -

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Smallest of the major salivary glands. They are almond shaped located on the floor of the mouth, underneath the tongue. It is also a mixed glands but most of its secretory units are mucuos acini.

PANCREATIC ACINI -

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PANCREAS -

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It is a soft, pinkish organ, 12 to 15 cm in length . It is an organ that function both as exocrine and endocrine glands. It consists of a head, body and tail The stroma of the pancreas consists of a capsule, septa, reticular tissue and lobules The lobules consists of: islets of Langerhans, secretory unirs and intralobular ducts As exocrine glands, its secretions contains digestive enzymes that are vital for protein, carbohydrate and fat digestion and these secretion is collectively known as pancreatic juice. It is also a serous-secreting compound tubuloacinar (tubuloalvoelar) glands that are structurally resembles the parotid glands.

grape-like clusters of epithelial cells In H & E prep, the epithelial cells have round basally-located nucleus that has one or more prominent nucleoli Their apical cytoplasm is eosinophil while their basal cytoplasm is

basophilic

DUCTS OF THE PANCREAS -

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The secretion of the pancreatic acini drain into intercalated ducts, whose walls consist simple squamous or simple low cuboidal epithelium, which has pale-staining cells called centroacinar cells Walls of interlobular ducts consist of simple cuboidal or simple columnar epithelium The interlobular ducts merge to form the main excretory duct (duct of Wirsung) and accessory duct (duct of Santorini)

LIVER -

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It is the largest gland of the body. Located right under the diaphragm and fits into the diaphragm’s concavity. It is divided into several unequal lobes( right, left, quadrate and caudate) it acts as an endocrine (it produces erythropoietin and thrombopoietin) and exocrine glands (secret...