21. Anatomy, Histology, & Embryology of the Large Intestine

Anatomy, Histology, & Embryology of the Large Intestine

*Considered partially intraperitoneal, and partially retroperitoneal, partially midgut & hindgut

Anatomy of the Large Intestine

• Location: occupies a large area in the abdominal cavity, from ileocecal jxn to anus, approx 1. m long

• Distinguishing Characteristics: Has the teniae coli, the three enlargements of outer longitudinal layer of musc externa, haustra – the bumps formed between each teniae (like little houses), and omental (epiploic) appendices – the peritoneum-covered sacs of fat attached all along the colon

  • 3 tenia are:
    • Omental – near attachment of greater omentum
      
    • Mesocolic – @ attachment of mesocolon
      
    • Tenia Libre (free)
      

• Surface Projection: Don’t need to know all of it, just how to find cecum, and appendix

  • location of cecum = intersection between transtubercular and midinguinal line

  • appendix – clinically important to treat appendicitis. Can point in any direction. Located in two ways:

    • Make a line starting at umbilicus → ASIS, and point at jxn between 1^st and 2^nd third = McBurney’s point

    • Make a line between umbilicus and mid point of inguinal ligament, and find jxn b/w mid and lat third

  • 

• Topography: will be discussed with each part of colon

• Function: convert liquid contents of ileum into semisolid feces by absorbing water, salts, and electrolytes. Also secretes mucus onto the feces, to lubricate them for easier defecation/

• Parts:

1.  Cecum – blind sac at proximal end of cecum, in right iliac fossa, covered by peritoneum, but has no mesentery attachment, can be palpated if filled with gas or feces about 2.5 cm about midpoint of inguinal ligament, location of ileocecal opening (leading from SI

2. Vermiform appendix – a narrow worm like sac that projects off the cecum (location listed above), possible lymphatic function b/c has lymph tissue in wall, suspended from ab wall via mesoappendix, normally 6-10 cm long, and 2-3 cm in diameter.

• Clinical Note – If inflammed, will spasm, and cause PAIN. (NOTE that most pain felt in the abdominal organs is sensed by peritoneal coverage), but pain funnily enough, usually felt in epigastrium, not at location of appendix

3. Asc Colon – from cecum → hepatic (R colic) flexure, is secondary retroperitoneal. And is covered anteriorly by peritoneum, and not posteriorly. Histologically speaking, this means the anterior side has serosa and the posterior side has adventia. Passes over R kidney. SP = 3 fingers wide lat to R mid inguinal line

4. Transverse Colon – from R colic flexure → L colic flexure. SP of R flexure = Just below R lobe of liver @ transpyloric line. SP of L flexure = 3 fingers above transpyloric line, and 2 fingers lateral to L midinguinal line

• NOTE – the prox 2/3 of transverse colon is MIDGUT, and distal 1/3 of transverse colon is HINDGUT.
• Held in place by the transverse mesocolon, and sticks to posterior wall of lesser sac, lies along inferior border of pancreas, and continous posteriorly with parietal peritoneum, and is intraperitoneal
• L colic flexure is attached to diaphragm via phrenicocolic ligament
• Crosses Liver, Stomach, pancreas, and is imbedded into greater omentum

5. Descending Colon – is also secondary retroperitoneal, and runs from L colic flexure → sigmoid colon, Peritoneum covers it ant only, passes anterior to left kidney. SP = 2 fingers lat to L midinguinal line

6. Sigmoid Colon – S shaped, links desc colon to rectum @ S3, intraperitoneal, has a peritoneal attachment called sigmoid mesocolon, with left ureter and left common iliac bifurcation behind it

Note: Rectum and Anal Canal discussed below.

• Blood Supply: Blood up until distal 1/3 of transverse colon is supplied by Sup. Mesenteric a, as it is MIDGUT, after it is HINDGUT, so blood supplied by Inf Mesenteric a. Veins follow the arteries

  • Sup mesenteric a:

    • cecum = iliocecal a (same as iliocolic a)

    • appendix = appendicular br off iliocecal a

    • asc colon and prox 2/3 transverse colon = iliocecal and right colic a.

  • Inf mesenteric a:

    • dist 1/3 transverse colon = middle colic a, left colic a

    • desc colon = left colic a, sigmoid a

    • sigmoid colon = sigmoid a

• Lymph Drainage

  • Lymph vessels also tend to follow arteries. Lymph drains into paracolic nodes → then flow into lymph nodes named for the arteries that supply the area. Meaning, if area is supplied by middle colic a, lymph flows to middle colic lymph nodes.

• Innervation

  • PNS – (+) digestion, secretion, vasodilation.

  • • Midgut = from CN X, which carries pre ggl fibers to submucosal/myenteric plexus, which sends short post ggl fibers within the wall of the organ

  • • Hindgut = from S2-4, via sacral splanchnic n

  • SNS – (-) digestion, secretion, vasoconstriction

  • • Midgut = fibers from sup mesenteric ggl run with lesser splanchnic n, which run with arterial branches of sup mesenteric a

  • • Hindgut = preggl fibers from L1-3 lumbar n → Inf mesenteric ggl → post ggl n fibers go to hindgut

• Peritoneal ligament mentioned above

Histology Large Intestine #53 H&E

Structures to Identify:

• All Layers
• simple columnar epith
• Crypts of Leiberkuhn
• lymph tissue (GALT)
• goblet cells
• a/v
• tenia (if present)
• Myenteric plexus
• Submucosal plexus

General information

• Wall has same basic layers as SI, but NO kerkring folds or villi or paneth cells

• Layers: Mucosa, Submucosa, Musc. Ext, and either serosa & adventia

Mucosa

• simple columnar epith with brush border and goblet cells (loads of them) – absorbative functio

• Has intestinal glands = crypts of Leiberkuhn

• LP = lymphoreticular connective tissue, location of Crypts of Leiberkuhn (intestinal glands), goblet cells, enteroendocrine cells (GEP cells), absorbative cells

• has collagen table,layer of collagen which assists the resorbed water to enter capillaries via passive diffusion

• well developed GALT (primary lymphatic nodules for production of Iga + secondary ones at times)

• B lymphocytes formed in secondary follicles become APCs called follicular dendritic cells, found at border b/w mental zone and germinal center (larger eosinophillic nucleus)

• pericryptal fibroblast sheath

• NO lymph vessels (slow rate of metastasis in colon cancer)

• stem cells located at bottom of crypt = zone of replication

• Old goblet cells become tall and thin = tuft cells

• Basement membrane very well defined

• Musc. Mucosae present

Submucosa

• lymph nodules might be seen, nothing else remarkable

• fibroblasts, fibers, support cells, etc

• Meissner’s nerve plexus

Muscularis Externa

• very unusual, has same inner circular layer and outer longitudinal, but outer layer is primarily condensed into three muscular bulges called teniae coli, maybe some longit muscle in between them, but very thin, if at all.

• 3 Teniae = Tenia mentali, mesocolica, & liber ( last one free, faces down, no attachment)

• No tenia coli in appendix and in rectum, longit layer is continuous there

• can find a/v here

• location of Myenteric plexus

External Layer

• may be either subserosa/serosa or adventia → if slice taken from anterior side of LI, then is serosa, but if from posterior side, then has adventia. Histologically mostly the same, adventia is thicker a bit.

• If serosa, will have lots of adipose – indicating the epiploic appendices

Large Intestine #54 *AZAN

• nucleus = red/purple

• cytoplasm = pink

• RBC = red

• collagen = blue

• musc = red

Vermiform Appendix # 52 * H&E

Structures to Identify

• irregular lumen

• lymph nodules w/ germinal centers

• all 4 layers : mucosa, submucosa, musc externa, serosa

• a/v

• glands

General Information

• described as worm like structure, arises from cecum and forms sac about 8cm long

• b/c not open, can easily get inflammed and has to be surgically removed

• may have role in B lymphocyte maturation, presence of mature lymphatic nodules, larger appearance in children

• wall of appendix like SI, but no valves of Kerkring, or villi, so in that respect, ressembles colon

Mucosa

• simple columnar epithelium with goblet cells and enterocytes

• LP – sometimes invaginates into submucosa and musc. Mucosae (which is barely visible in some areas, and is discontinuous)

• has MANY lymph nodules

• goblet cells, enteroendocrine cells

• lymphoreticular CT

Submucosa

• normal structures, dense CT and extra a/v

• eosinophillic, lots of collagen

• Meissner’s plexus

Musc Ext

• inner circular and outer longitudinal layer
• NO tenia, layers are continuous the whole way around, they unite at origin of appendix
• Myenteric plexus between 2 layers

NOTE – Appendix is intraperitoneal, so is totally covered with serosa

Comparison of Histology of SI v LI

Embryology of Large Intestine

• Epithelium (includes goblet cells, paneth cells in SI, and enteroendrocrine cells) is derived from endoderm

• all other layers of LI are derived from visceral mesoderm (mean LP, musc mucosa, submucosa, musc ext, serosa or adventia)

• MIDGUT = Cecum, Appendix, Asc colon, prox 2/3 transverse colon

• Midgut forms U shaped loop (midgut loop) that herniates through the primitive umbilical ring into extraembryonic coelom @ wk 6
• Midgut loop has cranial end, and caudal end – LI rotates 180 degrees → sup root goes below and behind inf root
• Caudal end forms cecal diverticulum, (cecum and appendix), and the rest of the caudal limb forms lower ileum, and parts of colon mentioned
• After rotation of 270 degrees counter clockwise around the superior mesenteric a, the cecum bud ascends to visceral surface of liver → R colic flexure
• Cecum falls to R Iliac fossa → asc/transverse/desc colon formed

• HINDGUT = distal 1/3 transverse colon, desc colon, sigmoid colon, rectum, anal canal

• Cranial end of hindgut → becomes the parts of colon mentioned
• terminal end of hindgut is endoderm lined pouch called cloaca, which touches surface ectoderm of proctodeum to form cloacal membrane
• Cloaca split into ant and post areas by urogenital septum, which splits it into an anterior ugogenital area and posterior recto-anal area
• Cloacal membrane becomes anal membrane posteriorly, and urogenital membrane anteriorly

Anim = http://www.rvc.ac.uk/Review/abdomen/HTML/intestinal2.htm

Anim2 = http://www.indiana.edu/~anat550/gianim/gutrot/gutrot.html