Histology of the Digestive, Urinary, and Salivary Systems

Posted on Jun 9, 2024 in Biology

Mucosa

  • Epithelium: Different types based on the region of GIT you are in.
  • Lamina propria (loose areolar connective tissue)
  • Muscularis mucosae (inner circular, outer longitudinal layer of smooth muscle).

Submucosa

  • Made of loose areolar connective tissue
  • The goodies – blood and lymph vessels, submucosal (Meisnner’s) nerve plexus – enteric nervous system (ENS).

Muscularis

  • Skeletal muscles in the mouth, pharynx, upper and middle parts of oesophagus, and external anal sphincter.
  • Smooth muscle in other regions
  • Muscularis: internal (circular) and external (longitudinal);
  • Myenteric (Auerbach’s) nerve plexus (ENS).

Serosa

  • Also called visceral peritoneum
  • Serous (wet and glossy) membrane (covered by serous fluid) around GI organs below diaphragm
  • Areolar conn. tissue contains blood and lymph vessels, simple squamous epithelium (mesothelium).

Oesophagus

  • The upper third of the oesophagus has skeletal muscle fibres; the middle third, a mixture of smooth and skeletal muscle; and the lower third, only smooth muscle.
  • The oesophageal mucosa consists of nonkeratinized stratified squamous epithelium (continuous with that of the pharynx), underlying lamina propria, and prominent muscularis mucosae.
  • Cells can become keratinized if exposed to a degree of trauma
  • Oesophagus upper = skeletal m
  • Middle – both skeletal and smooth
  • Lower – smooth

Cardio-oesophageal junction

  • An abrupt transition occurs in the epithelial lining at the esophagogastric junction. This serrated border, called the Z line, is clinically important, as it is the most common site of oesophageal carcinoma (cells are easily confused)
  • At the Z line, nonkeratinized stratified squamous epithelium of the oesophagus changes to simple columnar epithelium of the stomach, and only basal cells of the oesophageal epithelium continue into simple epithelium of the stomach

Stomach

  • Parietal cells are most numerous in the body of the glands but are also mixed with mucous neck cells in neck areas or with chief cells in basal areas of glands.
  • Parietal cells (HCl + Intrinsic factor) are most numerous in the body of the glands but are also mixed with mucous neck cells in neck areas or with chief cells in basal areas of glands. deeply eosinophilic cytoplasm is due to abundant mitochondria and relative rareness of rough endoplasmic reticulum. (intrinsic factor; glycoprotein that facilitates vitamin B12 absorption in the proximal small intestine)
  • Cuboidal to columnar chief cells, mostly in basal parts of glands, have a round basal nucleus. Their basal cytoplasm is basophilic; secretory (zymogenic) granules make their apical cytoplasm look more granular. ( pepsinogen (inactive pepsin)+ HCl = pepsin)
  • Gastric glands also contain less numerous enteroendocrine cells, scattered with the other cells, that produce gut hormones and are hard to see in routine sections. Special immunocytochemical or electron microscopic methods are needed to identify them with certainty. More than 30 gastrointestinal hormones are produced.
  • The muscularis externa of the stomach is made of three layers of smooth muscle: outer longitudinal, middle circular, and inner oblique

Gastro-duodenal junction

  • The gastroduodenal junction. Its gross anatomy is well delineated, but its histology is not.
  • In contrast to the esophagogastric junction—a discrete squamocolumnar junction
  • The gastroduodenal junction shows a gradual transition from gastric mucosa of the pylorus to villous epithelium of the duodenal mucosa.

Mucosa of the intestine

  • Numerous crevasses lined by glandular epithelium – intestinal glands
  • Cells:
    • Absorptive: most common cell type; tall columnar, brush border. Function = absorption
    • Goblet: secrete mucous.
    • Enteroendocrine: secrete hormones, same as stomach.
    • M cells (microfold cells) modified absorptive, located over enlarged lymph nodes, carry antigens to MALT.
    • Paneth: secrete anti-microbial lysozymes. Located in the base of intestinal glands.

Small intestine

  • Simple columnar epithelium, made of enterocytes and goblet cells, covers the villi.
  • Between the villi, the epithelium dips down to form simple, tube-like invaginations called intestinal glands,
  • Because it is near the stomach, the proximal duodenum has distinctive duodenal mucus-secreting glands (to protect from stomach acid).

Jejunum

  • The jejunum has the largest surface area for luminal secretion and absorption
  • It has a thicker wall and a wider lumen compared to the ileum
  • Plicae circulares are thicker, taller, and more numerous in the jejunum
  • Most of the cells found in the simple columnar epithelium are enterocytes – tall columnar absorptive cells and contain oval shaped nuclei that are located basally
  • Apical border usually striated in appearance
  • The core of each villus = highly vascularized lamina propria, systemic capillaries, and one larger lymphatic lacteal (for the transport of fats to the liver).

Ileum

  • The ratio of goblet cells:enterocytes is the greatest in ileum
  • Large amounts of gut associated lymphoid tissue (GALT) MALT
  • Peyers patches (aggregated lymphoid nodules) in the LP – most common in the distal ileum
  • Function = immune response and serve as a source of plasma cells (but diminish in size and number with age)
  • Secretory Paneth cells occur in all parts of the SI but ^ in ileum at bases of crypts (role in aiding epithelial renewal by secreting antimicrobial agents e.g. lysozyme into the intestinal crypts.

Specialisations

• Duodenum: villi are often leaf or ridge shaped; Brunner’s glands. • Jejunum: the main absorptive site, most complex finger like villi. • Ileum: greatest development of gut-associated lymphoid tissue – Peyer’s patches; may have leaf like villi. Histology of colon (large intestine) • The colon lacks villi • Columnar (absorptive) cells Columnar (absorptive) cells are the most numerous of the epithelial cell types. Although there is some variation in their structure, they all bear apical microplicae (microvilli) which are shorter and less regular than those on enterocytes in the small intestine. • Mucous (goblet) cells Mucous cells have a similar structure to those of the small intestine but they are more numerous. They are outnumbered by absorptive cells for most of the length of the colon but they are equally frequent towards the rectum, where their numbers increase. • Microfold (M) cells Microfold cells are similar to those of the small intestine. Rectum and Anal Canal Histology • At anorectal junction we start to see a change in epithelium from simple columnar to stratified squamous epithelium below the pectineal line • Lower end of the anal canal is continuous with the perianal skin…this should give us clues on what epithelial lining we can expect. What is Saliva? Why do we need it? • pH ~ 6.7-7.4 • Lubricates and protects oral tissues • Immune in function – aid in control of microbial flora in oral cavity • Starts digestion of complex carbohydrates by secreting α- amylase • Produced by salivary glands • Salivary glands = compound tubuloacinar glands Major Salivary Glands • Parotid gland = serous. • Submandibular gland = mostly serous and partially mucous. • Sublingual gland = almost completely mucous. • Mucous has the same refractive index as cytoplasm. • At times, we can find myoepithelial cells on top of the acini which will help with expulsion of saliva along the course of the duct. Ductal system of Salivary Glands • Basal striations are folds within the basal surface to maximize SA for ion channels. • Intercalated ducts – close to acini (simple cuboidal lining). • Striated ducts – become more columnar. • Interlobular duct = from the intralobular duct as they enter the CT between lobules – septas join together until the main duct is formed. • All digestive organs including pancreas and liver develop from gut tube and maintain contact via their ducts. Parotid Gland • Parenchyma enclosed by fibrous capsule. • Septa divides it into lobes and lobules. • Septa act like a conduit for blood vessels and autonomic nerves. • Parotid – branched tubuloacinar glands and is composed of clusters of elongated, branched serous acini. • Pyramidal serous cells that surround a central lumen form each acinus. • Cells = round basal nuclei and granular cytoplasm that is basophilic at the base and more eosinophilic toward the apex. • Nuclei are acidophilic and organelles are basophilic. • Number of granules in the cytoplasm = dependent on the phase of secretion. • A basement membrane surrounds each acinus and encloses a few flat myoepithelial cells. • Duct – starts off as one layer of squamous or cuboidal = intercalated. • Striated ducts are present but not common. • Duct continues as striated duct aka columnar cells with basal striations. • Intralobular connect to interlobular ducts (stratified cuboidal-columnar and then pseudo). Submandibular: • Egg shaped • Floor of oral cavity • Its watery secretion accounts for 60% of the saliva it produces • Most of it is serous but also contains mucous acini • Has many striated ducts and fewer intralobular Sublingual: • Smallest • Minor salivary glands scattered in lips, tongue cheeks and are mixed • Produces mucin • Serousmucous acini mixed = mucous lighter straining towards central lumen • Flattened serous cells around the surface of the acini • striated ducts – short almost absent altogether. Liver histology • Hepatocytes: cuboidal shaped • Hepatic sinusoids: lined with a thin discontinuous epithelium (deliver blood to central vein); • Kupffer cells (stellate sinusoidal macrophage) – detect and engulf bacteria and breakdown aged erythrocytes. • Portal space contains portal triads: branch of hepatic artery, branch of portal vein and bile duct (also contains lymph vessels). • Blood flows from portal triad through sinusoidal channels into a central vein → inferior vena cava > right atrium of the heart. • Bile canaliculus located between hepatocytes • Bile: made in hepatocytes, collects in bile canaliculi, used or stored in gall bladder. Exocrine product of the liver. • Flows in the opposite direction to blood. • Gallbladder stores and concentrates bile • Lining of the GB mucosa = simple columnar cells with short microvilli • Water from the lumen is absorbed by the GB cells ( ^ concentration of bile). Pancreas: (Endocrine). • Hormones produced in the pancreatic islets (islets of Langerhans) • Glucagon stimulates glucose production via gluconeogenesis & glycogen breakdown • Insulin promotes glucose uptake by cells and glycogen production & storage • Cells and what they produce: ü Alpha – glucagon ü Beta – insulin ü Delta – somatostatin ( inhibits the exocrine portion of the pancreas) ü Gamma – Pancreatic polypeptide (inhibits the endocrine portion of the pancreas and pancreatic secretion of fluid, bicarbonate, and enzymes. Exocrine: • Secretion of pancreatic juice into the duodenum • Pancreatic juice is released into duodenum via main and accessory pancreatic ducts • Pancreatic juice contains enzymes for the digestion of proteins and fats, alkaline fluid rich in bicarbonate ions for neutralising stomach acid • Trypsin > secreted by pancreas as trypsinogen (protein) • Amylase- digestion of complex carbohydrate like starch • Lipase – digestion of fats • Centroacinar cells secrete bicarbonate fluid • The digestive enzymes of the pancreas are released in a ‘gift pack’ form known as Zymogen Granules – the enzymes are active once they reach the duodenum – if not, pancreas will digest itself. 

The urinary system comprises a set of structures which contribute to the elimination of wastes from the body, and the maintenance of homeostasis. Comprises: • 2 kidneys • 2 ureters • Urinary bladder • Urethra. Function of the Urinary System Remove waste products from the blood Production, storage and voiding of urine Contribute to the regulation of: • Arterial blood pressure • pH of body fluids • Volume and composition of blood • Blood glucose levels (via gluconeogenesis) Production of hormones: – calcitriol (the active form of vitamin D, ↑Ca) – erythropoietin (stimulates the production of erythrocytes) Enzymatic blood pressure regulation (by secreting renin) – cardiovascular + kidneys. KIDNEYS 1- Renal cortex 2- Renal medulla Functional unit: Nephron. The Nephron The functional unit of the kidney (~1 million), consists of: • Renal corpuscle – filters blood plasma • Glomerulus – capillary network • Glomerular (Bowman’s) capsule – double-walled cup surrounding glomerulus that receives the filtrate • Renal tubule – filtered fluid passes into (reabsorption and secretion): • Proximal convoluted tubule (most reabsorption occurs here) • The loop of the nephron (Henle) (ascending and descending limbs) • Distal convoluted tubule • Nephrons drain into collecting ducts (in renal pyramids of medulla) • Two types: cortical and juxtamedullary. Cortical : 80 – 85% of nephrons Renal corpuscle in outer portion of cortex and short loops of Henle extend only into outer region of medulla • Thick ascending limb of the loop of Henle. • Tubules receive secretion from peritubular capillaries which also reabsorb some of the filtrate (water and solutes) into blood. Juxtamedullary: 15-20% of nephrons • Renal corpuscle deep in cortex and long loops of Henle extend deep into medulla. • The ascending limb of the loop of Henle has a thin and thick portions. • Tubules receive secretions from peritubular capillaries and vasa recta which also reabsorb some of the filtrate. • Enable kidney to secrete very concentrated urine. Efferent arteriole • Takes filtered blood back into circulation • Thinner – less resistance. Afferent arteriole • Brings filtrate towards glomerulus • Thicker than efferent arteriole • Has juxtaglomerular cells (baroreceptors). Afferent arteriole: – Lined by endothelium and surrounded by smooth muscle and granular cells – Granular (Juxtaglomerular) cells: specialized cells in the wall of the afferent arteriole that release renin in response to decreased perfusion or increased sympathetic tone. Renal Corpuscle • Visceral layer: modified simple squamous e. – podocytes (pedicels, filtration slits allow water and small molecules to filter into corpuscle space) • Parietal layer: simple squamous e. • Capsular (Renal/Bowman’s) space. Blood not filtered passes into efferent arteries. Proximal Convoluted tubule: • Simple cuboidal ep with microvilli = fuzzy lumen • 100% of glucose, amino acids, vitamins and plasma proteins are reabsorbed • 60-85% of water reabsorbed • PCT cells secrete hydrogen into tubular fluid • PCT = where filtrate, becomes tubular fluid. How to find a PCT: • Simple cuboidal epithelium • Central nuclei • Very acidophilic cytoplasm • Many long microvilli: brush border • Lumens often fuzzy and hazy. • Found in the cortex of the kidney Function of PCT: • Reabsorption of all organic nutrients, all proteins, most water, all glucose, and electrolytes • Secretion of of organic anions and cations, H+, and NH4 + • Hydroxylation of vitamin D and release it to the capillaries. Nephron Loop (Loop of Henle) • Has descending and ascending limbs with a loop in between • Both limbs have thick and thin segments • Thick = cuboidal lining • Thin = squamous lining. Distal Convoluted tubule: • Simple cuboidal ep with short and sparse microvilli = clear lumen • Mainly reabsorption/secretion of NaCl and H2O • DCT is always towards the afferent arteriole…this relationship allows the nephrons to regulate blood pressure • Intercalated cells = control blood pH • Macula densa cells = Chemoreceptors. Structure: – Composed of two limbs (thick and thin) – Thick limb – Simple cuboidal epithelium with no microvilli, but many mitochondria – Thin Limb: Simple squamous epithelium with few mitochondria. Function of Nephron Loop: Passive (thin limb) and active (thick limb) reabsorption of Na+ and Cl–.How to find a DCT: • Simple cuboidal epithelium • Smaller than PCT • Less acidophilic cytoplasm than PCT • Short microvilli • Lumens clear • Found in the cortex of the kidney Function of DCT: • Reabsorption of electrolytes Special features: – MACULA DENSA. Macula densa : • Initial, straight part of the distal convoluted tubule in contact with the arterioles • Regulation of the rate of Na+ absorption in DCT: by aldosterone (adrenal glands) • Cells become more columnar and closely packed • Apical nuclei. COLLECTING DUCTS: Formed by joining several connecting tubules together Histologic features: • Simple cuboidal epithelium • In the medulla of the kidney • Principal and intercalated (microvilli) cells Function: • Water reabsorption • Carrying the filtrate to aminor calyx Papillary duct (or duct of Bellini) • Formed by merging of several medullary collecting ducts approaching the apex of each renal pyramid • Deliver urine directly into the minor calyx. Ureters – Function: tubes that transport urine from renal pelvis to urinary bladder – Follow an arrangement of layers Ø Mucosa (Transitional epithelium & lamina propria); § Cuboidal or low columnar cells: an intermediate region containing from one to several layers of cells § Umbrella cells: a superficial layer of large bulbous or elliptical cell; sometimes binucleated Ø Muscularis Externa (Inner longitudinal, outer circular – opposite of GIT) Ø Adventitia. Urinary Bladder: – Follow an arrangement of layers Ø Mucosa (transitional epithelium & lamina propria) Ø Muscularis Externa (Inner longitudinal, middle circular, and outer longitudinal – opposite of GIT) Ø Serosa (superior part of urinary bladder)/adventitia (inferior part of urinary bladder). Urethra Carries the urine from the bladder to the exterior In females: 3- to 5-cm-long tube In males: 1- Prostatic urethra: 2- Membranous urethra: 3- Spongy urethra:. Male urethra 1- Prostatic urethra: • 3-4 cm long • Extends through the prostate gland • Lined by urothelium 2- Membranous urethra: • A short segment • Passes through an external sphincter of striated muscle • Lined by pseudostratified columnar epithelium 3- Spongy (penile) urethra: • About 15 cm in length • Enclosed within erectile tissue of the penis • Lined by pseudostratified columnar epithelium with stratified squamous epithelium distally. Female urethra • Lined initially with transitional epithelium which then transitions to nonkeratinized stratified squamous epithelium distally.