Large Intestine (Colon) - Axon

n Gastrointestinal Tract
The proximal colon to the splenic flexure derives its
blood supply from the superior mesenteric artery through
the ileocolic, right colic, and middle colic branches. The
remainder of the colon is supplied by the left colic and
sigmoid branches of the inferior mesenteric artery. The inferior
mesenteric artery and iliac vessels provide blood to the
rectum. Veins accompany the arteries and share their names.
The large bowel venous drainage enters the portal circulation
except for the distal rectum, which drains into the
systemic circulation through the middle and inferior rectal
veins. In portal hypertension, this area can serve as a portalsystemic
shunt and can be a site of varices. The lymph node
drainage is divided into those lymph nodes close to the
bowel wall (e.g., pericolic, perirectal) and those that follow
the blood vessels (e.g., mesenteric).
The vagus nerves supply stimulatory nervous activity to
the ascending colon and proximal transverse colon. The
remainder of the large bowel is supplied by pelvic postganglionic
parasympathetic nerves. The inhibitory nervous
activity is derived from the superior and inferior mesenteric
plexuses.
The large bowel mucosa is composed of a single-cell
layer of colorectal epithelium covering the lumen and lining
the crypts, the supporting lamina propria and a small
smooth muscle band referred to as the muscularis mucosae.
The normal colorectal luminal surface of the mucosa is
straight; the glands are made up of tubules (crypts) that are
tightly packed, parallel, nonbranching, and closely approximating
the muscularis mucosae (Fig. 23-1). The appearance
of the colonic tubules is similar to rows of test tubes
placed in a rack. Goblet cells interspersed between colorectal
absorptive cells line the colonic tubules. Scattered neuroendocrine
cells can be observed usually near the base of
the crypt and contain basally oriented eosinophilic cytoplasmic
granules. The lamina propria contains a modest
amount of mixed inflammatory cells including plasma cells,
lymphocytes, eosinophils, mast cells, and macrophages.
Intraepithelial lymphocytes are present normally but usually
are fewer than 6 per 100 colorectal epithelial cells. The
muscularis mucosae is arranged into two layers (an inner
circular and outer longitudinally running layer) and is
usually thin and regular. The submucosa is typically devoid
of inflammatory cells. Scattered mucosal and submucosal
lymphoid follicles are normally encountered, especially in
younger individuals. In areas of mucosal lymphoid follicles,
the mucosal architecture may be distorted and the muscularis
mucosae can be incomplete. Flattened epithelial cells
known as M cells overlie the mucosal lymphoid aggregates.
The epithelium of the M-cell zone typically contains numerous
intraepithelial lymphocytes. 1-3 Paneth cells with their
basal nuclei and luminal cytoplasmic refractile red granules
are seen in the base of colonic crypts but are considered
normal only in the cecum and proximal ascending
colon. 2,3
Mucosal biopsy interpretation can be hampered by
changes associated with bowel preparation and with the
trauma of the biopsy procedure itself. Changes ascribed to
bowel preparation include decreased intraepithelial mucin,
increased numbers of mitotic figures, surface apoptosis with
karyorrhectic debris in the superficial lamina propria, and
small numbers of neutrophils and eosinophils in surface or
crypt epithelium. 2-7 Edema and recent hemorrhage into
tissues not associated with other degenerative or inflammatory
changes most likely represent biopsy-related trauma.
Muciphages (foamy macrophages containing faintly
periodic acid–Schiff [PAS]–positive material) are often
present in the lamina propria of the large bowel especially
the rectum where they most likely represent a nonspecific
response to mucosal injury (i.e., trauma). 8,9 Muciphages
should be distinguished from xanthelasma/xanthomatous
polyp, which can also occur in the large intestine (Fig.
23-2).
LARGE BOWEL TUMORS
Common Adenomas and Malignant Polyps
Figure 23-1 ■ Normal colonic mucosa. The luminal surface is straight, and
the colonic tubules are tightly packed, parallel, nonbranching, and closely
approximating the muscularis mucosae.
An adenoma, defined as a benign intraepithelial neoplasm
composed of epithelial cells exhibiting cytologic dysplasia,
is considered the precursor lesion of most colorectal
carcinomas. 1,10-12 Dysplasia is characterized by decreased
intraepithelial mucin, epithelial nuclear enlargement with
hyperchromasia, nuclear stratification, and increased
numbers of mitotic figures. Large bowel adenomas are
highly prevalent in Western societies. The frequency of
these tumors markedly increases after age 40 years and
reaches a peak at age 70 years. Adenomas are usually asymptomatic
but large ones may bleed.
Adenomas usually produce a raised endoscopically or
grossly detectable abnormality, generally a protrusion
or polyp that can often be further subclassified as sessile or
pedunculated. Some adenomas appear flat; some may cause
mucosal depressions. Adenomas occur singly or can be