Anatomy and Physiology




Embryologic Colorectal Development



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Overview



Knowledge of embryologic development facilitates the understanding of congenital malformations and many other disease processes.



Embryonic period: first 8 weeks; fetal period: from 9th week to birth.




Landmarks




  • • Ectoderm → epidermis, nervous system.
  • • Mesoderm → mesenchymal tissues: muscular and connective tissue component of intestinal tract.
  • • Endoderm → GI tube to form epithelia and parenchymatous tissues of visceral/thoracic organs.
  • • Fusion zones:

    • – Cephalad endo-/ectoderm: stomatodeum.
    • – Caudad endo-/ectoderm: proctodeum.




Developmental Details



Weeks 2–4 (Embryonic Disc → Early Organ Layout)




  • • Three germ layers: ectoderm, mesoderm, endoderm.
  • • Notochord: primordial axis of the embryo → axial skeleton and inductor of neural plate (→ neuroectoderm).
  • • Day 21: heart begins to beat → circulation.
  • • Craniocaudal and lateral folding of 3 layers → formation of cranial and caudal ends → head fold (stomatodeum), tail fold (proctodeum).



Weeks 4–8 (Morphogenesis/Organogenesis)




  • • Week 4: formation of primordial gut: foregut, midgut, hindgut → cloacal membrane.
  • • Formation of cloaca → urinary, genital, and rectal tracts empty through the same opening.
  • • Week 5: migration of neural crest cells along spinal cord → sympathetic ganglia; proximal to distal migration of neural crest cells to internal organs.
  • • Week 6: formation of levator ani.
  • • Week 7: fusion of urorectal septum with cloacal membrane (perineal body) → partitioning of cloaca into dorsal and ventral part; division of cloacal sphincter into posterior (external anal sphincter) and anterior (bulbocavernosus, transverse perinei muscles) parts.
  • • Week 8: rupture of anal membrane → communication of hindgut with amniotic cavity; formation of internal anal sphincter (hindgut).



Weeks 9–12 (Tissue and Organ Differentiation)




  • • Midgut → small intestine including most of duodenum, colon from cecum to splenic flexure: arterial supply by SMA.

    • – Week 9: physiologic umbilical herniation → 90-degree counter-clockwise rotation.
    • – Week 10: return of intestines to the abdomen → 180-degree counter-clockwise rotation, 90-degree horizontal rotation of duodenum/pancreas to the right.

  • • Hindgut → colorectum from splenic flexure to anus, bladder, most of the urethra: arterial supply by IMA.

    • – Formation of anal canal: proctodeum = fusion zone between hindgut (proximal) and ectoderm (distal) → dentate line, separate blood supply above/below.




Clinical Focus and Pathology




  • • Remnant of primitive streak → sacrococcygeal teratoma.
  • • Remnants of notochord → chordoma.
  • • Intestinal malrotations → incomplete rotation, incomplete fixation (eg, mobile cecum), volvulus.
  • • Incomplete return of intestines → omphalocele, umbilical hernia.
  • • Persistent yolk stalk → Meckel diverticulum, omphaloenteric fistula.
  • • Failure of recanalization → intestinal duplication.
  • • Failure of neural crest cells to migrate to distal bowel → Hirschsprung disease.
  • • Abnormal partitioning of urorectal septum → anorectal malformations.





Vascular Anatomy



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Overview



Knowledge of the vascular anatomy is prerequisite for the performance of safe and oncologically correct surgery. Vascular anatomy to the colorectal organs is characterized by 3 circulatory systems: systemic arterial network, systemic venous network, and portal venous network.




Landmarks




  • • SMA/SMV: at duodenum part III.
  • • IMA: at the aorta.
  • • IMV: at the inferior edge of the pancreas.
  • • Splenic flexure: transition from superior to inferior mesenteric vessels.
  • • Dentate line: border between visceral and systemic circulation.




Anatomic Details



Small Intestine



Arterial:




  • • Supply mostly from SMA, partly from celiac trunk, some proximal collateralization, but end arteries at the level of the bowel.
  • • SMA: 1stbranch: pancreaticoduodenal artery, 2nd: mid-colic artery, 3rd:vascular arcades to small bowel.



Venous:




  • • Paralleling the arterial supply → SMV → portal vein.



Colon (Figure 3–1)




Figure 3–1.



Vascular anatomy of the colon.




Arterial:




  • • Supply from SMA and IMA → 3–4 major (named) vessels with significant anatomic variation.
  • • Ileocolic artery/right colic artery: last branch of SMA → terminal ileum, right colon, hepatic flexure.
  • • Mid-colic artery: 2nd branch of SMA → transverse colon (1st branch = pancreaticoduodenal artery). Anatomically special situation: transverse colon more distal to small bowel, but its arterial run-off more proximal than blood supply to small bowel.
  • • Left colic artery: 1st branch of IMA → splenic flexure to descending colon.
  • • Superior hemorrhoidal (superior rectal) artery: 2nd branch of IMA → sigmoid colon/upper rectum.
  • • Critical watershed areas: marginal artery of Drummond (variability on both the left and right colon). Griffith point (at splenic flexure): diminutive or absent (5%) marginal artery junction between SMA and IMA; extra connection between SMA and IMA: arch of Riolan (60%). Sudeck point: watershed between sigmoid colon and rectum.



Venous:




  • • Drainage through SMV or IMV to portal vein system. Limited collaterals to systemic circulation.
  • • Right colon to transverse colon: drainage collecting into SMV → parallel course to SMA → fusing with splenic vein at portal vein confluens.
  • • Splenic flexure to rectosigmoid colon: drainage collecting into IMV → course separating from IMA but targeting to pancreas tail → fusion with splenic vein.



Rectum



Arterial:




  • • Blood supply to rectum from two sources: IMA and internal iliac arteries.
  • • IMA: → superior rectal artery (synonym: superior hemorrhoidal artery) → rich reticular anastomotic network in the rectal submucosa with extensive collaterals.
  • • Internal iliac (hypogastric) arteries:

    • – Middle rectal arteries (synonym: middle hemorrhoidal artery): abundant interconnecting network of dual blood supply → distinct reticular vascular pattern on endoscopy.
    • – Inferior hemorrhoidal arteries.

  • • Variably present: median sacral artery (arises from posterior surface of the aorta and descends behind rectum to tip of the coccyx).



Venous:




  • • Blood from anorectum collects in arteriovenous plexuses → drainage through:

    • – Single superior hemorrhoidal vein → splenic vein → portal vein.
    • – Bilateral middle hemorrhoidal veins → internal iliac veins → IVC.
    • – Bilateral inferior veins: external and internal hemorrhoidal plexus communicate → partial blood flow from internal hemorrhoidal plexus → pudendal veins → IVC.
    • – Caveat: close proximity to rectum, but not associated with it: presacral veins!



Anal Canal and Pelvic Floor



Arterial:




  • • Blood supply mostly from internal iliac artery.
  • • Middle hemorrhoidal artery → wide intramural network of collaterals.
  • • Extrapelvic pudendal artery → inferior hemorrhoidal artery.



Venous:




  • • Via wide venous network of middle and inferior hemorrhoidal veins: not exposed to effects of portal hypertension.




Clinical Focus




  • • Oncologic resection: to follow arterial supply and lymphatic and venous drainage.
  • • Hematogenous tumor spread: colon: portal vein system (→ liver); rectum: portal vein system (→ liver) and systemic circulation (→ lungs).
  • • Intestinal ischemia: colon: watershed areas at splenic flexure, rectosigmoid junction, right colon; rectum: because of extensive collaterals virtually no risk for ischemia (unless previous surgical interruption of routes or complete aortic occlusion).




Pathology




  • • Ischemic colitis.
  • • Anatomic variability.
  • • Rectal varices.
  • • Hemorrhoids.





Anorectal Landmarks



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Overview



Anatomy of the pelvis and anorectum is complex. Thorough knowledge and use of precise terminology are a key distinction of the colorectal specialty.




Landmarks




  • • Anus: anal verge, intersphincteric groove, dentate line, anorectal ring.
  • • Rectum: valves of Houston, confluens of teniae.




Anatomic Details



Anorectum (Terminal Portion of GI Tract)




  • • Embedded in osseous pelvis, surrounded by urogenital organs, muscular, ligamentous, and connective tissue structures.
  • • Maintenance of fecal continence: stopper-equipped reservoir, controlled expulsion mechanism for feces.



Rectum (Pelvic Segment of Large Intestine)




  • • Partially extraperitoneal.
  • • Proximal start: rectosigmoid junction, defined as:

    • – Confluence of teniae.
    • – Endoscopic (rigid sigmoidoscope) 12–15 cm proximal to anal verge.
    • – Inadequate definitions: position of peritoneal reflection, level of sacral promontory

  • • Distal end: pelvic floor, upper end of anal canal.
  • • Nonmobilized rectum: 3 distinct endoluminal curves that form folds: valves of Houston.
  • • Lymph drainage: upper two-thirds of the rectum → primarily draining to inferior mesenteric and paraaortic nodes; lower one-third of the rectum → multidirectional drainage: along superior hemorrhoidal artery and IMA, along middle hemorrhoidal vessels to lateral pelvic side wall to internal iliac lymph nodes.



Anal Canal




  • • Definitions:

    • – Surgical: approximately 2–4 cm long: between anal verge and anorectal ring (proximal level of levator-external anal sphincter complex) → correlates with digital or sonographic exam.
    • – Anatomic: based on histologic architecture along the canal.
    • – Functional: high-pressure zone (manometry).

  • • Intersphincteric groove between internal and external anal sphincter around level of anal verge.
  • • Narrowing of rectum into anal canal → change of smooth mucosal lining to plicated appearance: columns of Morgagni, crypts.
  • • Dentate line: ~ 1–2 cm proximal to anal verge = embryologic fusion point between endoderm and ectoderm:

    • – Separation between innervation, arterial/venous blood supply.
    • – Separation of lymphatic drainage: above dentate line → drainage to inferior mesenteric and internal iliac nodes; below dentate line → drainage to superficial inguinal lymph nodes.

  • • Crypts: cryptoglandular complex with 4–8 apocrine anal glands from intersphincteric space that empty via anal ducts through IAS into anal canal.
  • • Epithelia:

    • – Anal transition zone (ATZ, cloacogenic zone) above dentate line: combination of columnar, transitional-cuboidal, and squamous epithelium.
    • – Anal canal between dentate line and anal verge: anoderm, ie, modified squamous epithelium without appendages.
    • – Anal margin (outside anal verge): radial skin folds, thicker skin, pigmentation, skin with adnexal tissues.



Anus




  • • Virtual orifice, ie, anal canal not visible from outside; even with lateral traction, the sphincter reflex results in an immediate contraction that keeps it closed.
  • • Normal position: midline, ~ 60% of distance from coccyx to posterior vulva/scrotal base.


Jan 14, 2019 | Posted by in GASTOINESTINAL SURGERY | Comments Off on Anatomy and Physiology
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