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.
- • 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.
- – Cephalad endo-/ectoderm: stomatodeum.
- • 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).
- • 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).
- • 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.
- – Week 9: physiologic umbilical herniation → 90-degree counter-clockwise rotation.
- • 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.
- • 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.
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.
- • 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.
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.
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.
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.
- – Middle rectal arteries (synonym: middle hemorrhoidal artery): abundant interconnecting network of dual blood supply → distinct reticular vascular pattern on endoscopy.
- • 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!
- – Single superior hemorrhoidal vein → splenic vein → portal vein.
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.
- • 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).
Anatomy of the pelvis and anorectum is complex. Thorough knowledge and use of precise terminology are a key distinction of the colorectal specialty.
- • Anus: anal verge, intersphincteric groove, dentate line, anorectal ring.
- • Rectum: valves of Houston, confluens of teniae.
- • 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.
- • 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
- – Confluence of teniae.
- • 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.
- • 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).
- – 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.
- • 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.
- – Separation between innervation, arterial/venous blood supply.
- • 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.
- – Anal transition zone (ATZ, cloacogenic zone) above dentate line: combination of columnar, transitional-cuboidal, and squamous epithelium.
- • 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.
