Digestive system
The digestive system consists of:the digestive tract—oral cavity, esophagus, stomach, small and large intestines, rectum, and anus—and associated glands—salivary glands, liver, and pancreasFunction
Large molecules such as proteins, fats, complex carbohydrates, and nucleic acids are broken down into small molecules that are easily absorbed through the lining of the digestive tract, mostly in the small intestine. Water, vitamins, and minerals are also absorbed from ingested foodGeneral Structure of the Digestive Tract
It is a hollow tube composed of a lumen whose diameter varies, surrounded by a wall made up of four principal layers: -mucosa submucosa.- muscularis, serosaThe Oral Cavity
Toral cavity is lined with stratified squamous epithelium, keratinized or nonkeratinized, depending on the region. -The keratin layer protects the oral mucosa from damage during masticatory function and is present mostly in the gingiva (gum) and hard palate. The lamina propria in these regions rests directly on bony tissue.Non keratinized squamous epithelium covers the soft palate, lips, cheeks, and the floor of the mouth. The lamina propria is continuous with a submucosa containing diffuse small salivary glands. In the lips, a transition from the oral nonkeratinized epithelium to the keratinized epithelium of the skin can be observed and called transitional zone(red margin
Tongue:
Is a highly muscular organ composed of striated muscle that function is chewing, swallowing, and speech. The mucous membrane is smooth on the lower (ventral) surface of the tongue. The tongue's dorsal surface is irregularThe upper surface of the tongue is divided into anterior two thirds and posterior one third by V- shaped sulcus called (sulcus terminalis)
. The muscle fibers cross one another in three planes; they are grouped in bundles, usually separated by connective tissue. Because the connective tissue of the lamina propria penetrates the spaces between the muscular bundles, the mucous membrane is strongly adherent to the muscle
FILIFORM PAPILLAE
:The most numerous Found all over the upper surface of the anterior two third of the tongue Tall, narrow and pointed Each papilla has connective tissue core covered by keratinized stratified squamous epithelium.The upper surface of anterior two third shows small projections called papillae which are four types: Filiform papillae. Fungiform papillae. Circumvallate papillae. Foliate papillae.
FUNGIFORM PAPILLAE:
Mushroom shape. Scattered among the filiform papillae. Found at the lateral border and at the tip of the tongue Taste bubs are present in the covering epithelium which is non keratinized stratified squamous epithelium.CIRCUMVALLATE (VALLATE) PAPILLAE
Dome- shape.are 7–12 in number extremely large circular papillae lie in a row immediately in front of the sulcus terminalis.Surrounded by circular furrow, their epithelium is non keratinized stratified squamous epithelium.Contains taste buds.The duct of Von Ebners (serous) gland opens at the base of the furrow surrounding each papillae.FOLIATE PAPILAE
Not well developed in the human. They consist of two or more parallel ridges and furrows Contains many taste buds: taste buds which are specialized sensory organs in which there are apical nerve endings which on being stimulated give rise to nerve impulses that result in sensation of taste at the apex of the bud there is small taste pore that connects the innervation of the bud with the oral cavity.
Three types of cells in the taste bud:- Taste cells (sensory cells). Supporting- sustentacular cells Basal cells( primitive stem cells) can replace the two cells, lie at the base of the bud, rounded in shape
Pharynx:
Lies at the back of the mouth and divided into: Upper part (naso pharynx) lined by pseudostratified ciliated columnar epithelium with goblet cells (respiratory epithelium). Middle part (oropharynx). Lower part ( laryngopharynx) both middle and lower part lined by non keratinized stratified squamous epithelium continous with that of the esophagus.OESOPHAGUS:
In humans the esophagus is covered by nonkeratinized stratified squamous epithelium .In general, it has the same layers as the rest of the digestive tract.At the distal end of the esophagus, the muscular layer consists of only smooth muscle cells that, close to the stomach, form the lower esophageal sphincter; in the mid portion, a mixture of striated and smooth muscle cells; and at the proximal end, only striated muscle cells. Only lower esophagus that is in the peritoneal cavity is covered by serosa. The rest is covered by a layer of connective tissue, the adventitia
The stomach
The stomach is divided into 4 regions: Cardia, fundus, body and pylorus. Because the fundus and body are identical in microscopic structure, only three histological regions are recognized. The mucosa and submucosa of the undistended stomach lie in longitudinally directed folds known as rugae. When the stomach is filled with food, these folds flatten out.surface epithelium that invaginates to various extents into the lamina propria, forming gastric pits. Emptying into the gastric pits are branched, tubular glands The epithelium covering the surface and lining the pits is a simple columnar epithelium.
Types of cells in the gastric mucosa:
Oxyntic (parietal )cells:- Present in the upper part of the gastric gland characterized by:- Pyramidal cells with central spherical nuclei and eosinophilic cytoplasm. The surface area of the cell increase as a result of deep circular invaginations of plasma membrane forming intercellular canaliculus. These cells produce intrinsic factors which binds with vitamin B12 to make it absorbable by the digestive tract, also they secrete a diluted solution of hydrochloric acid (HCl) Chief ( peptic) cells or enzyme- producing cells Enteroendocrine cellsChief ( peptic) cells or enzyme- producing cells:
Present in the lower part of the gastric gland. They have large basal nuclei and RER, the cytoplasm is filled with zymogene granules which contains the inactive enzyme pepsinogen which is discharged into the gastric lumen where it is converted by the gastric acid into the active enzyme pepsin.
Salivary Glands:
Are exocrine gland, they empty their secretion into the oral cavity by their ducts. Classified into minor and major. Minor salivary glands: found mainly in the sub mucosa of the mouth Major salivary glands: 3 large pairs of gland: Parotid submandibular sublingual.Fundus and Body: Surface epithelium is thrown into short gastric pits; the lamina propria is filled with long straight tubular gastric glands some of them open at the bottom of the gastric pits.
Cardia: . Its mucosa contains simple or branched tubular cardiac glands... the surface epithelium composed of mucous-secreting cells with few parietal cells.
Gastric esophageal junction
Fundus and BodyPylorus This region shows deep gastric pits into which open tubular mucous glands extending down to the muscularis mucosa similar to the cardiac glands.These glands are lined by mucous cells and scattered parietal cells particularly close to the pyloric sphincter, the glands secrete mucin and enzyme lysozyme.
Small intestine
Mucosa:- the small intestine shows structural modification to increase it’s surface area for absorption, so the lining mucosa and submucosa are thrown into numbers of folds or plicae which are most prominent in the jejunum .the surface of plicae arranged into intestinal villi which are long outgrowths of the mucosa consists of epithelium and lamina propriaThe epithelium of the villi is continuous with that of intestinal glands and consists of 5 types of cells:-
Enterocytes ( absorptive cells) :-Tall columnar cells with rounded nuclei, the apex of the cells shows brush border consists of about 3000 microvilli, the cytoplasm contains mitochondria, Golgi apparatus, Lysosomes and ribosoms. Goblet cells:-Lie between the absorptive cells and increase in number in the jejunum and ileum and most numerous in the terminal ileum. The cytoplasm is fully expanded with mucin
Paneth cells :-have basal nuclei and prominent large eosinophilic granules in their apical cytoplasm, these granules secrete lysozyme which protects against infection. Enteroendocrine cells:-mainly lie in the lower third of the gland, triangular in shape. Stem cells
Jejenum: is the main absorptive site, finger like villi, plicae (most prominent).
Duodenum: entirely retroperitoneal, leaf shaped villi, Brunner’s gland in the submucosa,.Ileum: aggregation of lymphoid tissue to form large nodules called (Peyer’s patches)
Large intestine Mucosa:The epithelium of large intestine: contains no folds and no villi but there is a mixture of absorptive cells and mucous cells (goblet cells) arranged as simple, straight, non branching tubular glands (intestinal glands).goblet cells are filled with large mucin granules
The absorptive cells are columnar and have short irregular microvilli, they are less numerous because they are compressed between the large mucous (goblet cellsThe appendix is a blind- ending tubular diverticulum arising from the cecum, it has a narrow lumen caused by the presence of lymphoid follicles in its wall
Anal canal lined by columnar epithelium at it’s upper end and this changes to a non keratinizing stratified squamous epithelium at the level of pectinate or dentate line in this line , the mucous membrane forms a series of longitudinal folds, the rectal columns of Morgagni. Small branched tubular (anal glands) open into the anal canal just above the pectinate line
the lamina propria contains a plexus of large veins that,when excessively dilated, produces hemorrhoids. The inner circular smooth muscle layer of rectum form the Internal sphinecter of anal canal. While The external longitudinal muscle of the rectum loses it’s fibers at the level of puborectalis muscle of pelvic floor and continuous as a fibroelastic septum between the internal and external sphincter.