BMS 150 Visceral Anatomy and Histology of the Gastrointestinal Tract PDF

Visceral Anatomy and Histology The Gastrointestinal Tract References: Moore’s Clinically- Oriented Anatomy, Chapter 2 BMS 150 Junqueira’s Basic Histology Text and Week 10 Atlas, Chapter 15 General Anatomy of the Abdomen Part of the trunk bounded...

Visceral Anatomy and Histology The Gastrointestinal Tract References: Moore’s Clinically- Oriented Anatomy, Chapter 2 BMS 150 Junqueira’s Basic Histology Text and Week 10 Atlas, Chapter 15 General Anatomy of the Abdomen Part of the trunk bounded by: ▪ diaphragm superiorly ▪ Musculo-aponeurotic walls anterolaterally ▪ A virtual boundary inferiorly – the pelvic inlet The pelvic and abdominal cavities are continuous – any “boundary” is purely conceptual ▪ Vertebrae posteriorly  General contents of the abdominal cavity: Alimentary canal: Lower esophageal sphincter, stomach Duodenum, jejunum, ileum ▪ Parts of the small intestine will “dip into” the pelvic cavity Cecum, ascending colon, transverse colon, descending colon ▪ Note – the sigmoid, rectum, and anus extend into the pelvic cavity, but will be discussed in BMS 150 Accessory organs, other structures Liver, gall bladder, pancreas, and their ducts Spleen, kidneys Peritoneal folds and many vessels and nerves General contents of the abdominal cavity: Peritoneal reflections have been removed for clarity Peritoneum & Peritoneal Cavity Characteristics of the peritoneum: Transparent, thin membrane that lines the abdominopelvic cavity and is continuous with the serosa of the abdominopelvic organs Parietal peritoneum – lines the interior of the body wall ▪ Pain here is well-localized to the overlying dermatome Exception – peritoneum overlying the diaphragm is referred to C3 – C5 Can sense pressure, cutting, heat, cold, laceration, inflammatory irritation Visceral peritoneum – lines the visceral organs and is continuous with the serosa ▪ Also forms major folds known as mesenteries , omenta, or ligaments ▪ Pain is poorly-localized but can be understood somewhat by knowledge of the embryologic derivatives of some abdominal contents (GI embryology later this semester) Can sense ischemia, inflammation, stretch, chemical irritation Peritoneal Cavity Frontal view – greater omentum sectioned to show underlying structures ▪ Omentum – double-layered peritoneal membrane continuous with serosal surfaces, connected to the stomach Greater and lesser omenta shown here ▪ Mesentery/mesocolon – double-layered peritoneal membrane that surrounds the small intestine (mesentery) and large intestine (mesocolon) at particular sites A clearer view – peritoneal folds Make reference to this slide for the peritoneal content See notes for details  Basics of the major peritoneal folds Greater omentum ▪ One of the largest folds ▪ Extends from the greater (inferior) curvature of the stomach → over the anterior aspect of the abdominal cavity (right next to the abdominal wall) → folds “back up” to join with the transverse colon ▪ Stores a lot of visceral fat (fatty apron) ▪ Contains many lymph nodes and is somewhat mobile – can “wrap around” inflamed or perforated viscus  Basics of the major peritoneal folds Lesser omentum ▪ Extends from the lesser (superior) curvature of the stomach and proximal duodenum → to the inferior aspect of the liver ▪ Clinically-important structures lie within the lesser omentum: Hepatic artery, common bile duct, hepatic portal vein See bottom picture  Basics of the major peritoneal folds The mesenteries Mesentery = double-fold of small intestinal peritoneal lining continuous with the serosa ▪ Lines the jejunum and ileum, binds them to the posterior abdominal wall ▪ Houses many important vessels and nerves ▪ Helps keep the small intestine from being “tangled” Mesocolon – 2 separate double-folds that connect the transverse and sigmoid colon to the posterior abdominal wall ▪ Also lots of blood and lymphatic vessels as well as nerves Basics of the major peritoneal folds The falciform ligament Divides the liver into left and right lobes Attaches to the anterior abdominal wall ▪ Embryologically interesting – the distal edge that connects to the anterior abdominal wall (round ligament) There are many more anatomically- is the remnant of the interesting but “too in-depth” anatomical umbilical vein… so it terms for these peritoneal structures connects to the All are important surgical landmarks umbilicus  Navigating the peritoneal cavity Many structures are not fully “surrounded” by peritoneum ▪ Either the entire surface does not really contact the peritoneum or the surface of the structure is not completely surrounded by the peritoneal lining ▪ Lots of terms for this, but we’ll just call them all “retroperitoneal” (this isn’t strictly anatomically correct): Most of the duodenum, parts of the ascending and descending colon, anal canal Pancreas Kidneys, adrenal glands, ureters Aorta and inferior vena cavae  Navigating the peritoneal cavity There are certain “sacs” or bursa that are present within the peritoneal cavity that are clinically-relevant (and very surgically-relevant) The omental bursa is behind the stomach and lesser omentum ▪ Can enter it via the omental foramen – just to the right and posterior to the border of the lesser omentum The greater sac includes all compartments (see next slide)  Navigating the peritoneal cavity FYI compartments: ▪ The supracolic (above the transverse colon and mesocolon) ▪ The infracolic (below the transverse colon and mesocolon) Abdominal Arterial Vasculature - Overview Also known as the splanchnic circulation (see next slide) Arteries branch off the abdominal aorta at 3 major sites: ▪ Celiac trunk – gives rise to: Left gastric artery, common hepatic artery, splenic artery Pancreas, liver, gallbladder, stomach, duodenum, spleen Supplies the structures of the embryologic foregut (more later) ▪ Superior mesenteric artery – gives rise to arteries that supply: pancreas, stomach, small intestine, as well as the large intestine up to the point of the transverse colon Supplies the structures of the embryologic foregut and midgut ▪ Inferior mesenteric artery – gives rise to arteries that supply the rest of the large intestine and superior anus (embryologic hindgut) Circulation of the GI tract Hepatic artery Abdominal Venous Vasculature - Overview The venous circulation is a portal circulation ▪ Portal circulation = capillary networks that are in series with each other ▪ i.e.: artery → capillary → portal vein → capillary → vein → right atrium Celiac trunk Inf. mes. Inf. vena Hepatic artery cava portal vein Sup. mes. artery Sup. mes. Splenic Hepatic vein vein vein Abdominal Venous Vasculature - Overview Inferior mesenteric vein joins with the splenic vein The splenic vein and the superior mesenteric vein come together to form the hepatic portal vein The hepatic portal vein carries poorly-oxygenated but nutrient- rich blood to the liver from most of the organs within the abdominal cavity artery → capillary → portal vein → capillary → vein → right atrium Celiac trunk Inf. mes. Inf. vena Hepatic artery cava portal vein Sup. mes. artery Sup. mes. Splenic Hepatic vein vein vein Circulation of the GI tract  General GI tract histology Although each organ is different, the tract follows a common theme (the accessory organs are unique) ▪ From lumen → outer wall, the layers are: Mucosa – absorption, secretion, chemical digestion, many endocrine functions, some immune functions Submucosa – secretion, lots of blood vessels, contains a large plexus of neurons (submucosal or Meissner’s plexus), some immune functions Muscularis – two to three layers of smooth muscle, main function is propulsion, another large neuronal plexus exists here (muscular or Auerbach’s plexus) Serosa/adventitia – connective tissue that anchors the GI tract and at the same time allows mobility – forms the peritoneum General GI tract histology Mucosa: epithelial lining, lamina propria, & muscularis mucosa Epithelial lining consists of: ▪ Epithelium: simple columnar with apical microvilli in high-absorption areas (small intestine) stratified squamous or cuboidal in other areas Apical microvilli in many parts of the tract greatly increase SA, and often the mucosa and sometimes the submucosa are also folded to further increase SA ▪ Goblet cells are often present (secrete mucous) Mucous protects/lubricates the GI tract, forms a water layer for diffusion of nutrients, and helps “store” IgA ▪ Enteroendocrine cells are often present General GI tract histology Mucosa cont... ▪ Lamina propria has MALT, blood + lymphatic vessels, some glands – loose connective tissue A lot of mast cells present, not necessarily associated with lymphatic nodules ▪ Muscularis mucosa forms the border between the mucosa and submucosa Increases folding of the mucosa layer As it moves, ensures all absorptive cells have access to the contents of the lumen Enteroendocrine cells (DNES) Part of the epithelial lining – lots in the stomach and small intestine Can be open or closed ▪ Open - contact the lumen and can sense luminal contents ▪ As they are activated they secrete their granules along the basal surface, towards the blood and other cells Thus can have paracrine (most common) and endocrine function ▪ Closed - do not contact the lumen, thus they are dependent on other sources of input to regulate secretion i.e. hormones or nervous system input  Important Enteroendocrine Cells Cell Location Hormone (Stimulus) Main Hormonal Functions Somatostatin Generally “turns down” the Stomach, duodenum, D pancreas (many different stimuli cause release) release of hormones from nearby cells ECL – stomach ECL – histamine (stimulated by vagus) ECL – stimulates acid EC, ECL EC – stomach, small and EC – serotonin, substance P production large intestines (mechanical, neural, endocrine) EC – increased motility Gastrin (amino acids in the stomach, Increases secretion of G* Stomach vagal stimulation, gastrin-releasing stomach acid peptide) CCK (fats and proteins in the Pancreatic enzyme secretion, duodenum) gallbladder contraction, I* Small Intestine satiety Inhibits gastric acid secretion Glucagon-like peptide (amino acids & Insulin secretion, satiety L Small intestine carbs) Inhibits gastric acid secretion Motilin (fasting) Migrating motor complex Mo* Small intestine Secretin (acid in small intestine, Bicarbonate and water especially duodenum) secretion from pancreas S* Small intestine Inhibits gastric acid secretion and gastric emptying Enteroendocrine cells (DNES) What should we know from that chart at this point? Know the “bold & underlined” information ▪ The cells highlighted are key regulators of GI physiology that impact: Motility in the GI tract Acid secretion Secretion of pancreatic enzymes and bicarbonate Secretion of bile ▪ We’ll go through the rest as we learn more small intestinal physiology General GI tract histology Submucosa: ▪ Large blood vessels and lymphatics ▪ Submucosal plexus Meissner’s plexus is thought to regulate the secretory activity of the tract, and also convey sensory information from the lumen to other parts of the gut or the CNS ▪ The lymphatic nodules may also be found here ~ 80% of the antibodies made in the body are in the GI tract – epithelial cells can take up antibodies produced in the nodules and translocate them into the lumen High concentration of lymphocytes and macrophages in these nodules Histology of the GI Immune System GALT: ▪ MALT – smaller nodules rich in macrophages and lymphocytes, found in the mucosa (lamina propria) ▪ Peyer’s patches – very large (extends right through to the submucosa) nodules that may be cm in length found mostly throughout the distal small intestine (jejunum, ileum) In the epithelium overlying Peyer’s patches are M (microfold) cells ▪ M-cells selectively endocytose antigens and present them to dendritic cells and lymphocytes – important in regulating the immune response to intraluminal antigens Peyer’s Patches General GI tract histology Muscularis: ▪ Most parts have an inner circular and outer longitudinal layer of smooth muscle Stomach has an additional oblique layer ▪ Muscular nervous plexus (Auerbach’s plexus) is found between the two layers, and is thought to mainly regulate the muscular movements of the GI tract Serosa/Adventitia: ▪ Esophagus has an adventitia (no mesothelium, dense connective tissue) ▪ Serosa forms the outer layer of the rest of the tract – loose connective tissue covered by a simple squamous mesothelium Mesothelium is continuous with the fluid-secreting peritoneum in the abdominal cavity Many large blood and lymphatic vessels are found within the mesentery/serosal layer Enteric Nervous System A basic schematic of the enteric nervous system is shown in the next slide Much of the activity of the GI tract is autonomous, though input from the autonomic nervous system is essential for normal function ▪ Autonomic nervous system efferents can impact muscular movements (Auerbach’s plexus interactions) or secretions from glands in the mucosa and submucosa (Meissner’s plexus interactions) Auerbach’s (myenteric) plexus is located between circular and longitudinal muscle layers, Meissner’s (submucosal) plexus is found diffusely within the submucosa ▪ Local reflexes can also regulate coordinated muscular movements and secretions  Enteric Nervous System Note the nerves that travel with the vessels within the mesentery Efferents from the ANS Afferent sensory information as well Anatomy and Physiology, 2nd ed. Fig. 23.3 Peritoneal membrane – a closer look Peritoneal membranes have a similar surface area (~ 1.7 m2 ) to the total skin surface area Although the entire surface is covered in secretory squamous mesothelium, only 50 – 75 mL (4 – 5 tablespoons) of peritoneal fluid is present ▪ Due to the constant circulation and absorption of peritoneal fluid ▪ Small particles are absorbed by venous pores and enter the portal circulation ▪ Larger particles are absorbed by lymphatic capillaries and enter the thoracic duct Lymphatic absorption is responsible for draining extra fluid that is produced during inflammatory processes Excess accumulation of peritoneal fluid = ascites  General Histologic Features - Esophagus Mucosa – non-keratinized stratified squamous epithelium with some glands near the LES Submucosa – some mucous-secreting glands Muscularis – upper part is striated muscle, lower part is smooth muscle, middle part is a transition between the two Outer layer is adventitia, SSE – stratified squamous epithelium; D – not serosa duct from submucosal glands; MM - ▪ Does not secrete fluid muscularis mucosa; GL – glands in submucosa  General Histologic Features - Stomach Mucosa – simple columnar epithelium arranged into pits and glands that run deep into the lamina propria ▪ Glands are deep to the pits ▪ Function of the glands varies depending on region of the stomach (acid secretion in fundus and body) ▪ Mucous neck cells are found throughout – secrete alkaline mucous that protects the stomach from secreted acid Muscularis – three layers instead of two ▪ Inner layer is oblique Serosa is continuous with the greater and lesser omentum  General Histologic Features – Small Intestine Mucosa: ▪ 3 levels of “folding” to optimize surface area: Plicae circulares, villi, and microvilli (on the surface of enterocytes) The submucosa also occupies the plicae circulares ▪ Peyer’s patches found in the ileum ▪ Crypts are depressions in between villi Submucosa: ▪ Large Brunner glands can be found in the duodenum (protective against stomach acid ▪ Peyer’s patches in the ileum extend from lamina propria all the way to submucosa Muscularis and serosa: ▪ Typical muscular layer, serosa  General Histologic Features – Large Intestine Mucosa – arranged into tubular intestinal glands that penetrate deep into the lamina propria ▪ simple columnar epithelium, fewer microvilli than in the small intestine ▪ Many goblet cells that secrete mucous ▪ Lots of MALT nodules in lamina propria Muscular layer is unique ▪ Circular layer is continuous, like other areas of the alimentary canal ▪ Longitudinal layer is arranged into 3 separate bands known as the teniae coli  Gross Anatomy of Large Intestine Note the teniae coli Longitudinal muscle layer is discontinuous and therefore somewhat weak Will review when we discuss diverticulosis