The pancreas. The pancreas lies between the stomach and duodenum. It is composed of a head, body, and tail. It is finely covered with thin connective tissue. The pancreas is actually two glands intimately integrated together. The exocrine portion is a compound acinar gland composed of a single row of pyramidal cells on a basement membrane. These cells are organized into lobules with connnective tissue, vessels, nerves, and ducts being present and penetrating inside. The pancreatic acinar cells produce many different kinds of enzymes seen in their inactive form as zymogen granules. Intercalated ducts exit each acine, having their origin as centrocenar cells. Larger ducts with columnar epithelium eventually become the duct of Wirsung or the duct of Santorini which open at the ampulla of Vater. The endocrine portion of the pancreas is made up of pale staining islet cells, separated from the acine by a reticular network. Alpha cells produce the hormone glucagon to raise blood sugar. Beta cells make up most of the gland (nealy 80%) and are usually near the islets center. These cells produce insulin to lower blood sugar. Other cells, such as deltas, which secrete somatostatin, which inhibits the other endocrine secretion in the islet, and PP cells which inhibit acinar secretion. Thus, endocrine and exocrine portions of the gland act together. The oral cavity. The oral cavity exemplifies the general plan of the gut with an epithelium characterized by glands and projections. A lamina propria is beneath with connective tissue. A muscularis mucosae is followed by a tela submucosa of dense connnective tissue. The muscularis is covered with a serous membrane which suspends the gut. The mouth has an epithelium of stratified squamous epithelium. It has a well developed lamina propria with many papillae, lymphocytes, nerves, and blood vessels. The submucosa is thick with many large vessels and abundant fatty tissue. There is no muscularis mucosa. The soft palate has the surfaces different with the oral side being stratified squamous and the nasal side with pseudo-stratified ciliated columnar. The core is of striated muscles and fibrous tissue. The tongue is a core of striated muscle running in all directions. The lower surface is smooth but the dorsal surface is covered with papillae which are of three types. The most numerous are the filiform which occur in rows diverging from the midline in a V-shape. The superficial cells harden into scales which give a furred appearance to the tongue when not shed. Fungiform papillae are scattered amoung the filiform. They are comprised of a stalk and an upper hemispherical part which give a mushroom appearance. Taste buds may be present on their sides. Circumvallate papillae number 10-12 in number and are sunk into a pit. They are arranged in a V shape. Their lateral surfaces contain about 250 taste buds. The vallate papillae contain the albumenous gland of Von Ebner. Taste buds are also found on the soft palate, the epiglottis, and the posterior wall of the pharynx. They contain an outer taste pore to the outside. Two cell types make up the bud, the supporting cells and neuroepithelial cells which have a taste hair on the free surface. Lingual tonsils and follicles are located on the posterior surface of the tongue. They are much like the surface of the palatine tonsil. These are located in the fauces (opening of mouth into pharynx). They are covered with a lamina propria with a capsule beneath. There are 10 to 20 crypts infiltrated with lymphocytes and heterophils. The lamina propria is filled with nodules. Many small glands are associated with the tonsil. The pharyngeal tonsil (adenoid) is located on the posterior wall of the nasopharynx. These are like the palatines but lack crypts and are covered with pseudostratified, ciliated, columnar epithelium. The salivary glands are located in the mucosa or submucosa. They can be classified according to the type of secretory cells; as cuboidal cells secreting mucous, albuminous glands producing water and ptyalin, and mixed glands. All the glands may be surrounded by basked or myoepithelial cells. Another mode of classification is site of opening. Those that open into the vestibule are the albuminous parotid, the mixed labials in the lips, and the mixed buccals in the cheeks. The mandibular (submaxilar), which is mostly abuminous, and the sublingual open between the tongue and the mandibular. The glands of the root of the tongue are the mixed anter___, under the apex and the posterior linguals, like those of Von Ebner connected with the circumvallate papillae. There are also pure mucous glands of the tongue root. All the glands of the palate are mucous. A complete duct system for the salivary glands includes the intralobular, the striated, the lobular, the sublobular, the interlobular, the primary and the excretory. Epithelium generally gets higher and more stratified as the excretory portion is approached. The esophagus. The structure of the esophagus typifies that of the remainder of the digestive tract. The lumen is bordered by stratified squamous epithelium with many conspicuous papillae. Underlying the epithelium is a submucosa of dense connective tissue. Esophageal glands of the compound tubule alveolar type, which secrete mucous, are abundant in the submucosa. The muscularis mucosae is two layered and made of smooth muscle. The lamina propria is made of thick connective tissue. It contains branched tubular esophageal cardiac glands, one group near the pharynx and another near the entrance of the cardiac stomach. The muscularis externa is two layered and unique. The upper 1/4 is skeletal muscle, the middle part is a mixture of smooth and skeletal, and the lower 1/4 is smooth. The outer serosa is connected to surrounding organs by loose connective tissue called the Tunica adventitia. The empty esophagus is highly folded to provide for easy distension during the passage of a bolus of food. The stomach. The stomach is divided into cardiac, fundic, and pyloric portions, all are characterized by folds called rugae. It is lined with simple columnar epithelium which is pierced by numerous gastric glandular openings. The glands, known as gastric foveolae, differ among the three areas of the stomach. The ring-shaped area around the cardiac region contains compound tubular glands that secrete only mucous. The lamina propria, usually scantry in amount, fills the spaces between glands and muscularis mucosae. The muscularis mucosae is composed of an inner circular and an outer longitudinal layer. It extends between the glands, and its contraction aids in emptying them. The submucosa is of dense connective tissue with many lymphocytes and lymphatic vessels. The muscularis externa is three layered consisting of an outer longitudinal, a middle circular, and an inner oblique layer. The muscularis externa is responsible for strong peristallic contractions that aid in digestion. In the pyloric area, the middle circular layer forms the pyloric sphincter. A serosa covers the stomach and continues as the lesser and greater omenta. The fundic glands are of the simple branched tubular type with four types of cells. The chief or zymogenic cells are found in the lower 1/2 of the tubule. They contain pepsinogen granules, an inactive protease that converts to pepsin in the presence of HCL. Parietal cells occur between zymogenic cells and the basement membrane. These secrete HCl and intrinsic factor, a substance necessary for red blood cell formation. Mucous neck cells occur in the neck of the gland. Enteroendocrine or argentaffin cells are found between the zymogenic cells and the basement membrane but are difficult to distinguish without special staining techniques. These likely secrete hormones such as somatastatin, glucagon, and serotonin directly into the blood. The pyloric glands, which secrete mucous and hormones, are simple branched tubular glands. The intestines. All areas of the small intestine exhibit the same basic "floorplan". The mucosa has large foldings called plicae circularis or valves of Kerbring extending from the pyloris to the middle of the ileum. The epithelium is columnar with a striated border of microvilli capped by a glycocalyx which contains some digestive enzymes. These surface cells are called absorptine cells. Villi are .5 mm to 1.5 mm long. Between the bases of the villi are intestinal glands called crypts of Leiberkuhn. These contain goblet cells, enteroendocrine cells, and at the bottom of the crypts are located Puneth cells that secrete lysozomes which are antibacterial enzymes. Stem cells that replenish all other cell types occupy the basal 1/2 of a gland. The lamina propria is thin and extends into the core of projections of the epithelium called villi. Portal blood vessels which carry amino acids and carbohydrates, and lymphatics which move lipids, are found in the lamina propria. The two layered mucularis mucosae underlies tha lamina propria and makes up part of the core of a villus. Contractions aid in moving digested products into the blood and lymphatic vessels. The submucosa is of dense connective tissue. In the proximal 1/3 of the duodenum are branched tubules called the glands of Brunner. The secretion from these glands is an alkaline mucous which helps neutralize acid stomach chyme. The submucosa also contains a parasympathetic nerve plexus called Meisner's plexus. The muscularis externa is of two well developed layers. The parasympathetic ganglia of Auerbach occur between the two layers of the muscularis externa. Its action regulates secretion and mobility. Externally the serosa is typical of the rest of the gut and is continuous with the mesenteries that surround it. The small intestine consists of a short proximal duodenum, a middle long jejunem, and a distal long ileum. All possess a similar compostion except for the presence of the glands of Brunner in the duodenum. Plasma cells and lymphatic nodules which are involved in the immune response are numerous in the small intestine. Aggregations of nodules are known as Peyer's patches. Products made by the immune cells of the lamina propria act as an immunological barrier on the surface of the epithelial cells. Peyer's patches are covered with M-cells which move macromolecules from the lumen to the lymphocytes in the nodules. The ileum joins the colon at a valve called the ileocoecal valve which is a double fold of the mucosa and submucosa. The large intestine or colon consists of the small proximal cecum which gives off the appendix. The remainder of the colon is the ascending, transverse, descending, and segmoid parts. All parts are divided by plicae into bulging areas called havstra. The mucosa has a simple columnar lining until the most distal part which is stratified squamous. There are no villi but glands of Leiberkuhn are present. Many goblet cells occur throughout. The only secretory product is mucous. The lamina propria, muscularis mucosa, and submucosa are much like those of the small intestine. The muscular externa is layered with the inner circular layer typical and the outer longitudinal layer is organized into three bands called taenia coli. The serosa forms protuberances of fat on the colon called appendices epiploica. The appendix is structured identically to the colon with very large numbers of nodules being present. In the anal region the anucosa becomes folds as the rectal colums of Morgagni. Crypts of Lieberkuhn are absent. Stratified squamous epithelial takes over from columnar at 2 cm above the anal orifice, at the pectinate line. The inner circular layer of the muscularis externa thickens and becomes the internal anal sphincter. Skeletal muscle surrounds the distal end of the anal canal, and makes up the voluntarily controlled external anal sphincter. The liver. The liver possesses a unique blood supply in the form of a hepatic portal system. Blood from the digestive tract passes through liver sinusoids from which it is ultimately collected into the hepatic vein that empties into the inferior vena cava. The liver is said to have almost 100 functions including metabolism of all types of foods, detoxification, production of bile, synthesis of blood proteins, manufacture of cholesterol, immunity, storage of glycogen and fat, and destruction of red blood cells among others. A tough connective tissue capsule, Glissons's capsule, covers the liver and is overlain by another capsule. Glisson's capsule penetrates inside to divide the liver up into its basic unit, the lobule. A lobule is a polygon of 0.7 to 2 mm in diameter. Liver sinusoids are supported by a reticular network and are lined with macrophages called Kupffer cells. Hepatocytes are arranged as zones of activity with the center being a zone of permanent repose, the next outer one being the zone of permanent function and the outer one being the zone of variable activity. the liver qualifies as both an endocrine and an exocrine gland, exocrine because it secretes bile into a duct system. The structure of the liver is intimately related to blood vessels with an hepatic vein in the center and a portal triad at the corners. A portal triad contains an hepatic portal vein, an hepatic artery, a bile duct and lymphatics. Blood from the hepatic portal vein percolates through endothelial lined sinusoids that exist between the parenchymal hepatic cords and ends up in a central vein in the middle of the lobule. Food products from the gut are processed as they pass over the hepatocytes. The hepatic artery supplies blood laden with oxygen. Hepatic portal blood and hepatic arterial blood move into the liver. Bile, made by hepatocytes, is collected into bile canaliculi and flows in the opposite direction, toward the outside. Blood collected in the central vein also passes in an outward direction. Bile canalicule are the result of adjacent hepatocytes possessing plasmalemmas with tight junctions. The juncture with the bile duct is difficult to demonstrate but canalicule open collect into interlobular ducts and form an anastomosing network surrounding the branches of the portal vein. The bile ducts parallel the portal system, first displaying cuboidal epithelium and later columnar epithelium. The main ducts from different lobes fuse to form a common hepatic duct. After joining the ceptic duct from the gall bladder the duct is called the ductus choledochus. The extra hepatic ducts have tall columnar epithelium, smooth muscle, and connective tissue. The structure of the gall bladder is a blind-ended fundus, a body, and a neck wich is continuous with the cystic duct. It is a hollow, pear shaped organ attached to the posterior surface of the liver, measuring about 10 cm by 4 cm. The wall is complex with the mucosa thrown into folds called rugae. The epithelium is columnar but lacks the goblet cells characteristic of the rest of the digestive tract. A lamina propria and smooth muscle layer appear bound together. Perinusclar connective tissue is surrounded by a serosa. The cystic duct as it exits the neck is thrown into folds called the spiral valve of Heister. The choledcohal duodenal junction is complex. It comprises the duodenal wall traversed by bile and pancreatic ducts. Most of the length is an oblique passageway through the submucosa of the duodenum. The associated bile and pancreatic ducts are covered with the muscular sphincter of Oddi which is made of the sphincter pancreatises, the Fasciculi longitudinales, the sphincter ampullae, and the sphincte choledochres. The opening to the duodenum is guarded proximally by a contractile window in the duodenum and distally by the valvules of the ampulla of Vater. This complex arrangement retains bile during fasting but relaxes under hormonal stimulation when fats reach the digestive tract. Concentration of bile salts for emulsification of fats takes place in the gall bladder. Hormonal stimulation causes the musculare's to contract. The Kidney. The kidney is a compound tubular gland with a filtration apparatus called a glomerulus. The gross structure of the kidney is bean-shaped with the ureter emerging from the hilus. A tough collagenous capsule surrounds the organ. A cavity in the hilus, the sinus, houses the pelvis or the expanded end of the ureter in the form of cup like calyces. The kidney is made of tubules which are grouped together into about 10 pyramids arranged around the renal sinus. The apex or papilla of each pyramid inserts into a cup-like minor calyx of the ureter. The cortex of the kidney occurs between bases of the pyramids beneath the capsule. The cortical areas project inward and are referred to as renal columns. Taken together a pyramid and its adjacent cortex constitute a lobe. The structural and functional unit of the secretory part of the kidney is the nephron which produces urine. Drainage tubules which connect to nephrons are called collecting ducts. The nephron begins in the pars convoluta region of the cortex as Bowman's capsule which is an expanded pushed in part of the beginning of the tubule. A knot of capillaries, the glomerulus, fits into the capsule forming a renal corpuscle. Simple squamous epithelium of Bowman's capsule is visceral where it touches glomerulur endothelium and parietal on the other side of the urinary space of Bowman's capsule. Podocytes are specialized visceral epithelial cells with filtration slits between them. Urine filters between the podocytes into the urinary space and then into the nephron. The golmerulus enters the corpuscle beginning as an afferent arteriole. In this region smooth muscles from a juxtaglomerular apparatus, which secretes a blood pressure regulating hormone renin. The glomerulus is a capillary tuft with two arteriole sides. The exit vessel is the efferent arteriole. The urinary pole of the capsule is where the nephron becomes the proximal convoluted tubule. Both afferent and efferent arterioles are located at the pole of the corpuscles. This arteriole arrangement facilitates filtration of blood plasma. The blood supply to the kidney is complex. The renal artery divides into dorsal and ventral branches which in turn become interlobular arteries supplying the renal columns between pyramids. The interlobulars become arciform arteries between cortex and medulla. Afferent arterioles are branches from interlobulars. Efferents collecting from the glomerulus form another capillary net around the convoluted tubules. Corpuscles deep within the cortex show their afferent capillary bed entering the medulla along side the renal tubule, an arrangement called the vasa rectae. More superficial capillary beds are known as peritubular. The corpuscle becomes the proximal convoluted tubule. These tubules make up most of the cortical region called the pars convoluta. The proximal tubule is lined with pyramidal cuboidal epithelium with an obvious striated border. After many loops the tubule directs to the periphery, returns to the pars radiata (medullary ray), and enters the upper part of the medulla. The small loop which enters the upper part of the medulla is made of a descending limb lined mostly with squamous epithelium. This bend then becomes an ascending limb, which is cuboidal lined, and enters the pars radiata of the cortex where it returns to its corpuscle and attaches to the afferent arteriole of that corpuscle. The role of the loop of Henle is to recirculate sodium ions and make urine hypertonic to the blood when appropriate. A disc of tall thin cells in this region is called the macula densa. The loop of Henle soon becomes contorted as the distal convoluted tubule. In cross section the distal tubule is smaller than the proximal with lower cuboidal cells. Physiologically the glomerulus filters, the proximal tubules resorb ( push substances from the tubule back into the blood) or secrete (pull substances from efferent arterioles back into the tubule). Sections of the cortex are potentially confusing but can be distinguished on the following criteria. The pars convoluta or labyrinth contains elements that are primarily circular. These include the renal corpuscle, the proximal convoluted tubule, and the distal convoluted tubule. The pars radiata on the other hand, has many longitudinally arrayed elements which include the proximal tubule, the distal tubule, and the non-urine producing collecting tubule. The collecting tubule is of cuboidal epithelium and is difficult to distinguish from the distal tubule. The excretory ducts from the pars radiata continue into the medulla where they fuse with others in a branching fashion to form the central branchings. These tubules are lined with simple cuboidal epithelium which increase in height toward the apex of a papilla becoming more like columnar epithelium. The ducts of Bellini open at the apex of a papilla (pyramid), forming a sieve-like area called the area cribrosa. Urine drains from the area cribrosa and collects in a minor calyx of the renal pelvis. The epithelium of the calyx is transitional. The exretory passages of the urinary system consist of clyces, pelvis, ureter, bladder and urethra. Except for the urethra all the passages are of similar composition. The inner mucosal surface is of transitional epithelium increasing in thickness toward the bladder. The empty passageways are usually thrown into large folds which include lamina propria of dense connective tissue closely attached to a well developed muscularis which undergoes peristalsis. The urinary passages lack a submucosa. The innermost muscular coat is longitudinally arranged and the outer is circular. From the lower portion of the ureter on down an external longitudinal layer is developed. A large connective tissue adventitia covers most of the passageways with a smoother serous membrane adherent to the proximal portion of the bladder. The ureters are separated from the bladder by connective tissue valves. A smooth muscle sphincter is located at the internal opening of the bladder into the urethra. The female urethra is 15-30 mm long with the mucosa thrown into deep folds. The lining is generally stratified squamous epithelium. The lamina propria is like that of the male with lacunae of Morgagni and glands of Littre. A thick smooth muscularis is present with a sphincter well developed at the distal end. The male urethra is 8-20 cm long and is divisible into 3 parts. The proximal most part is the short pars prastatica. The prostate glands surrounds this portion. To the right and left of the midline are the 2 openings of the ejaculatory ducts and several openings of the prostatic glands. The epithelium of the pars prostatica is transitional. The pars prostatica is followed by the short pars membranacea extending from the apex of the prostate to the bulb of the corpus cavernosa penis. Epithelium is either stratified or pseudostratified columnar. The most distal part of the male urethra is the pars cavernosa or penile urethra. It is about 15 cm long and passes through the penis. The epithelial lining is either stratified or pseudostratified columnar epithelium except at the distal margin where it changes to stratified squamous. All portions of the male urethra possess a lamina propria with considerable elastic tissue. Inner longitudinal and outer circular muscle layers lie outside. Branching down from the epithelium are recesses called the lacunae of Morgagni. Deep tubules, the glands of Littre, extend from the lacunae into the lamina propria.
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