Human structure 2 PDF

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notes on structure and development...

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ABDOMINAL VISCERA 1 • •

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The GI Tract (Hollow) Organs Common structural plan Epithelium: the key to function o Supported by loose CT called lamina propria o Thin muscle layer = muscularis mucosae o Together: Functional unit called mucosa (= In GIT term mucosa indicates a functional unit formed by epithelium, lamina propria and muscularis mucosae = muscle of the mucosa, don’t forget last e) Submucosa CT layer keeps mucosa and muscularis externa connected. It also, contains a network of blood vessels and nerves Muscularis externa: Main muscle layer for moving content of food o Outer longitudinal, Inner circular Serosa: Visceral peritoneum o Visceral peritoneum firmly attached to wall of peritoneum and because the visceral peritoneum is a part of serous membrane, the outermost epithelial layer is called the serosa o Retroperitoneal parts: remaining part not covered by peritoneum, instead fixed to part of abdominal wall ▪ Have some loose CT instead of visceral peritoneum, called adventitia = a fibrous loose CT layer oesophagus doesn’t digest food, simply carries food down to stomach o Doesn’t need specialised epithelium that produces digestive juices o Internal surface is subject to friction from food going through which can damage the epithelium. Thus, need system of replacing lost epithelium o = has stratified squamous epithelium

Glands: Epithelial specialisations • Produce fluid which is then delivered to another surface • Almost all glands are specialisations of some epithelial tissue • Basic concept: o A group of epithelial cells grow to form a column. The column then becomes a hollow tube as cells in the centre die. Cells towards the end of the tube form the secretory part and those towards the epithelial surface form a duct. The duct carries and may alter the secretions o Soon, in hollow tube there are secretory cells towards the end of the tube and cells towards epithelial surface which form a duct that carries these secretions. Tubes can also alter the secretions • Secretory units: Different forms – tubular, round etc. • Secretory and conducting (tube) portion all grow in CT that supports epithelium • Single ducts: Simple glands, most are microscopic • Branched ducts: Compound glands • Glands can be: • In its simplest form, microscopic in the wall of a hollow organ (which part of wall? = in CT that supports epithelium, i.e. lamina propria.

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In complex forms, organs in their own right yet connected to the original epithelium (liver, pancreas, salivary glands) The Stomach Two ends: o Cardia (close to the heart) = junction between oesophagus and stomach o Pylorus (‘Gate’) = junction between stomach and duodenum of SI o why called gate = opens only at certain times. Food stays in stomach for hours and then is gradually released into duodenum o gate opens intermittently to let food flow into duodenum Borders: Greater and lesser curvatures Anatomical parts o Fundus: Broad upper end o Body o Pyloric antrum, last bit is pyloric canal o No clear anatomical boundary Histological (functional) parts (2) o Fundic part (fundus + body) = acid secreting (epithelium capable of producing digestive juices and acid. This is done by glands in lamina propria of stomach) o Non-acid secreting = pyloric part (only produces mucus)

Sphincters Sphincter: generally, a mechanism to ensure movement of material in one direction, and prevent it from flowing backwards • • •

Generally, a thickening of circular muscle In some cases: no anatomical thickening A sphincter ‘guards’ an opening: Controls flow Cardiac sphincter: Prevents Backflow (reflux) Controls flow from oesophagus into stomach AND Prevents backflow of acidic stomach contents into the oesophagus = gastroesophageal reflux • Functional sphincter: No thickening of muscle • Functional factors: ▪ Muscle (smooth) tone = resting contraction ▪ Angle between fundus and oesophagus: When stomach is full, wall of stomach is pressed against oesophagus which prevents backflow of content ▪ Possible action of diaphragm: Oesophagus passes through a muscular part of diaphragm. As the diaphragm contracts, this can pinch the oesophagus off Pyloric sphincter: Controls release into duodenum o Thickening of circular muscle • •

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Peritoneum Covered by visceral peritoneum on both surfaces

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Lesser curvature: two layers of peritoneum (one form anterior and one from posterior side) go as a sheet to connect to the liver = hepatogastric ‘ligament’ Greater curvature – two layers connect to: o Diaphragm = gastrophrenic ligament o Spleen: Called gastrosplenic ligament → Peritoneal fold that connects GC to spleen o From horizontal part: apron hanging over lower abdomen o = Greater omentum. Connects stomach to colon (gastrocolic ligament) before dropping down

The greater curvature has two peritoneal layers connecting it to the diaphragm (gastrophrenic ligament) and to the spleen (gastrosplenic ligament). The horizontal part has the greater omentum – an apron-like sheet that connects the stomach to the transverse colon (gastrocolic ligament) before hanging over the lower abdomen. Greater omentum = an apron-like double peritoneal layer that connects from the horizontal part of the greater curvature to the colon (gastrocolic ligament), before hanging down over the lower abdomen

Hepatogastric seems like lesser omentum?? not actually dense CT. thus not true ligaments from last lec. Paste here • Posterior to stomach (part of peritoneal cavity is posterior to stomach o = Lesser sac of peritoneum o Separates it from stomach bed (= organs posterior to stomach but separated by lesser sac) o Stomach bed: With posterior abdominal structures Interior Highly distensible organ • Mucosa (3 layers) thrown into folds • Rugae or rugal folds (= folds of the mucosa, which is the epithelium, lamina propria and muscularis mucosae) • Roughly parallel in pyloric part • In empty stomach, they may enclose a canal along a lesser curvature (gastric canal) o Allows fluids in small quantities to pass along the lesser curvature • Rugae flatten out when stomach is full Small intestine • •

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Longest part of digestive tract Duodenum (12 fingerbreadths) o C-shaped, retroperitoneal o Begins on right of midline and Ends just to the left of the midline Jejunum and ileum o No clear anatomical boundary between them o General direction: Upper left to lower right

o Covered by visceral peritoneum o Mesentery attached to posterior abdominal wall o Blood vessels between the two layers of mesentery Does every organ have its own mesentery? Or is the mesentery just one thing Except retroperitoneal organs which lost their mesentery And every organ in GIT has visceral and P peritoneum Mesentery only in GIT organs right. Is there a peritoneal cavity in GIT organs b/w V and P P Interior • Incomplete circular folds of mucosa (except 2 cm at the beginning) o Called Plicae circulares • Increase surface area and do not flatten when SI is full unlike rugae of stomach (why?) • SA also increased by villi: fine epithelial folds • Gradual change from duodenum to ileum: Wall becomes progressively thin, plicae cicrulares become thinner and microscopic villi also become smaller o Duodenum: thick wall, terminal ileum has thin wall o Duodenum: Large villi (leaf like). Ileum has small, finger like villi o Ileum has lymphoid tissue aggregations, but not seen in old age

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Colon Beginning: Caecum + appendix Ascending and descending colon are retroperitoneal Transverse colon o Has mesentery = transverse mesocolon o Horizontal attachment to posterior abdominal structures o Recall: Like a shelf → Divides abdomen into supracolic and infracolic compartments Sigmoid (s-shaped) colon. V-shaped mesentery (Rectum and anal canal are with the pelvis) Longitudinal muscle in three bands: Taeniae coli (say Tee Ni ae) Taeniae Coli gathers colon into pouches called haustra

^just the sigmoid colon has those? NOPE, entire colon Caecum and Appendix • Caecum = “blind” (cul‐de‐sac, “no through road”). o Almost fully covered with peritoneum. o Three taeniae as in the rest of the colon. o Ileocaecal junction: slit like, with a sphincter. ▪ Sphincter muscle valve that separates the small and large intestine o Opening of the appendix below i‐c valve. • Vermiform (worm-like) appendix

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o Variable size (5-10cm) and position: commonest is retrocaecal o Lymphoid tissue abundant Pain from the appendix: Umbilical Pain from local parietal peritoneum: Right iliac region

ABDOMINAL VISCERA 2 Posterior Wall • Psoas major → lower limb • Quadratus lumborum: L5→12th rib. (T12??) • Diaphragm: Crura (More on diaphragm later.) • Part of transversus abdominis. • All hypaxial muscles. • Fascia: ‘Thoracolumbar’. o Posterior layer: covers epaxial muscles. o Middle and anterior: cover QL. o Psoas major: psoas fascia. Posterior Wall: Arcuate ligaments • Arch like thickenings of fascia. o Between crura: median. o Psoas fascia: medial o Fascia over QL: Lateral. • Note: Median ligament reaches T12 Blood vessels, Kidneys: Overview • Aorta: Entry at T12. o Between Median arcuate ligament and vertebra. o Midline course. • IVC o Passes through diaphragm: T 8. ▪ Right of midline • Kidneys o Both kidneys have a ‘hilum’. o Transpyloric plane (Tp): passes through both hila. o Right slightly lower. o Both reach higher than 12th rib: related to diaphragm, pleura! Kidneys • Fibrous capsule (not shown). • Fat: ‘perirenal’ (*) • Renal fascia. • Muscles posterior to kidneys: Psoas, QL, transversus in that order, medial to lateral. • Left kidney: ribs 11 and 12. • Right: rib 12 Kidneys: Anterior Relations

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Right kidney: o Duodenum is the key! o Transpyloric plane. Left kidney: o Pancreas: fits in the curve of duodenum, rises slightly towards the tail.

Blood Vessels Etc. • Hilum of the kidney o Renal vein o Renal artery o Ureteric pelvis (Renal pelvis) • Ureters – descend along the psoas Renal artery • Vascular ‘segments’ of the kidney. • Each segment supplied by a branch of the renal artery. • Variations o Accessory arteries: Explained with development. Kidney: Interior • Functional unit: nephron. • A long epithelial tube. • Filtration of blood: Renal corpuscle (1). • Modification (2, 3, 4). o Some essentials taken back, some excreted. o 2: Proximal convoluted tubule. o 3: Loop of Henle. o 4: Distal convoluted tubule. • “Collection” (5). •

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Outer part: cortex. o Corpuscles (Epithelial tube end + capillaries). o Convoluted tubules. Inner part: medulla – pyramids. o Loops of Henle and collecting ducts. o Each pyramid has a tip: papilla. o Numerous CD’s open on the papilla. Minor (c) and major (C) calyces... → Pelvis → ureter

renal papilla drain into minor calyx, which enters the major calyx and then drains into the renal pelvis. Ureters take urine from the renal pelvis to the urinary bladder Abdominal Aorta • Entry behind median arcuate ligament. • T 12 to L 4, midline course.

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Plan of branches o Unpaired branches to GIT x 3 (I believe the coeliac artery, the superior mesenteric, and the inferior mesenteric arteries. o 3x Paired branches to paired organs. ▪ Suprarenals, kidneys, gonads. o Branches to body wall ▪ 4 pairs (5th pair from iliac) o Terminal: Common iliac. o Median sacral.

Inferior Vena Cava • Right of midline: 5 → 8 • Veins from GIT do not join! (Wdym?) • Paired veins: o Different left and right! o Left: Join renal vein. o Right: directly into IVC. • T 10: Passes through central tendon of diaphragm. Diaphragm • Seal between thorax and abdomen. • Therefore: o Attachment all around: o Vertebrae: columns of muscle: crura. ▪ Right longer, thicker: L1 to L3 bodies. ▪ Fans out across midline. o Ribs: 7 to 12. ▪ Therefore, from lower end of sternum, along entire costal margin, 11 and 12. o Xiphisternum. • Muscle bundles converge to central tendon. • Developmentally mainly cervical myotomes 3,4,5. • Phrenic nerve. Passages through Diaphragm • IVC o Right of midline o T 8: Central tendon o Stretches when diaphragm contracts → facilitates venous return. • Oesophagus • Left of midline o T 10: Right crus. o Right crus helps in sphincteric mechanism. • Aorta. (Blood under pressure, diaphragm has no effect.) o Midline, T 12. o Posterior to median arcuate ligament.

The Liver – 1 • Location. • Diaphragmatic surface. o Anterior, lateral mainly. • Visceral surface. o Postero‐inferior. • Porta hepatis: Main entry. o Portal vein (blood from GIT). – Venous blood o Hepatic artery. o Bile duct/s. • Hepatic veins: Join IVC. • Ligamentum teres and ligamentum venosum Hepatic portal vein carries blood and nutrients from the stomach, spleen, intestines and gall bladder to the liver. The hepatic vein carries deoxygenated blood from the liver back to the right atrium of the heart via the inferior vena cava The Liver – Visceral relations • Oesophagus and stomach. o Must be to the left of lesser omentum! • Hepatic flexure (H) of colon. o Lower right corner. o Therefore: duodenum to its left. • Recall: duodenum and right kidney. • R suprarenal: just above right kidney. • IVC in a groove posteriorly. • Bare area bounded by coronary ligament. → Area on diaphragmatic surface that is devoid of peritoneal covering The Liver – 2 • Portal circulation • Plexus in GIT wall • Porta‐systemic anastomoses • Hepatic veins • Portal ‘hypertension Duodenum – Pancreas • Retropritoneal structures • The bile‐pancreatic duct system • Embryologically junctional structures

EMBRYOLOGY OF THE DIGESTIVE SYSTEM The Beginning • Trilaminar Embryo

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o At this point of development, Ectoderm continuous with an epithelial layer called the amnion o Endoderm is continuous with the yolk sac Note cardiogenic area: heart-forming mesoderm is at end of embryo, beyond oral membrane where endoderm and ectoderm are in contact Ectoderm-endoderm contact: o At the head end: oral membrane o Oral membrane: during early embryonic life, the mouth cavity is closed off from the environment by this oral membrane. This membrane has to disappear during the course of development o At the tail end: cloacal membrane Next step: Head and tail folds

Head and Tail Folds 6:49 • At the head end o Heart tube position changes o Fold deepens and heart tube will be closer to its final location o Cranial end of gut tube apparent • At tail end: Caudal end of gut tube • Allantois – diverticulum from the tube (gut?); o Between endoderm and yolk sac, o Explained with urogenital system • Yolk sac relatively smaller and maintains communication with the cavity of the gut tube • Communication between yolk sac and gut tube results in three parts of gut tube • Foregut at head end, • Part opposite communication with yolk sac is midgut • Part at tail end is hindgut –beyond communication area Lateral folds • Complete the ‘tube’ formation. • Folds from lateral sides - R and Left • Lateral plate mesoderm splits and there is a cavity between the two layers called coelomic cavity • In the abdomen, the coelomic cavity is represented by the peritoneal cavity (part of original coelomic cavity) When folding is complete, the right and left ends meet each other. • Visceral layer of lateral plate mesoderm (that goes along with the endoderm) forms a complete tube around the endoderm • •

The coelomic cavity now complete around the gut tube and body wall The mesentery connecting parietal and visceral peritoneum o Since the coelomic cavity surrounds the gut on all sides except at the dorsal border, there are two epithelial layers with some CT in between. It is through this CT that blood vessels reach the gut wall

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Endoderm only gives rise to lining epithelium of GI tract, and glands Most Muscle + all Connective tissue: mesodermal o The surrounding area – lamina propria, muscle layer, submucosa all develop from mesoderm

Further… • Yolk sac relatively smaller: Communication between yolk sac and midgut narrows • Between the liver in the abdomen and the bending heart tube in the thorax, there is the diaphragm • Part of foregut cranial to the abdomen • At this point we consider only the abdominal part Mesenteries Mesentery connecting gut tube with posterior abdominal wall = dorsal mesentery • Dorsal mesentery is common for foregut, midgut and hindgut. Thus, from the diaphragm reaching down to the pelvis, there is a continuous line of mesentery connecting the gut tube to the posterior abdominal wall • •

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However, under the diaphragm the foregut is connected to the anterior abdominal wall by another layer of mesentery = ventral mesentery Major organ that arises from the abdominal foregut is the stomach, which is called gaster. The mesentery of the foregut is therefore called ventral mesogastrium and dorsal mesogastrium Thus, the VM connects abdominal foregut to anterior abdominal wall under the diaphragm. The DM connects it to the posterior abdominal wall Dorsal mesogastrium is continuous with the dorsal mesentery of the midgut and hindgut Small yolk sac o Communication between yolk-sac and midgut is seen as a tube called the vitello-intestinal duct (Vitellos = latin term for yolk sac). o Define VI duct: a duct connecting the yolk sac to the midgut to allow communication between the yolk sac and gut tube o VI duct is at the midpoint of the midgut Well defined umbilicus and cord o Cord contains yolk sac and allantois o Two arteries (umbilical) from aorta going to the placenta o Blood from placenta comes back to embryo through umbilical vein, which joins the ventral mesogastirum

Diverticulum allantois is now long and extends into the proximal portion of the umbilical cord So the proximal end of umbilicus has two arteries, one vein, allantois and part of VI? tract Foregut derivatives • Abdominal oesophagus

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Cranial half of duodenum (first half of the C shape) Entire stomach o Initially in sagittal plane o Ventral curvature: lesser o Dorsal curvature: becomes greater curvature o As it develops, the stomach rotates o Instead of being in median plane, after rotation it is now more in coronal plane o Lesser curvature on right side o Greater curvature on the left side

Foregut mesenteries Mesogastrium: Ventral and dorsal Liver develops in ventral mesogastrium – two parts • In ventral mesogastrium, between the two layers of mesentery – a group of cells from the endodermal lining of GI tract spread into ventral mesogastrium to form the liver • Liver forming cells migrate from the endoderm outside the wall of the digestive tube into VM, where they start proliferating • As liver grows up to the diaphragm, it divides the ventral mesogastirum into two parts o On the anterior side, between liver and abdominal wall, there is the part of V mesogastrium called the falciform ligament o Falciform means sickle shaped (the instrument for cutting grass) o Function of falciform ligament: Connects liver to abdominal wall o Between liver and foregut, there is another fold which is a part of the VM called the lesser omentum o Thus, the lesser omentum: Is a peritoneal flap that connects the liver to the lesser curvature of the stomach LO: A double peritoneal layer that connects the liver to the lesser curvature of the stomach. Summary: • Thus, falciform L and lesser omentum are two subdivisions of the ventral mesogastrium (VM). These subdivisions are brought about by the liver, which develops between the foregut’s ventral mesogastrium and* abdominal wall Spleen develops in dorsal mesogastrium → splits it into three parts • Dorsal to foregut, there is an organ not connected to the digestive system but develops along with it = spleen •

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Largest part: Greater omentum o part of stomach caudal to spleen: from horizontal lower part of greater curvature = greater omentum Cranial to spleen, the dorsal mesogastrium connects the stomach to the diaphragm

o Gas...