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| The large intestine consists of the colon, cecum, appendix, rectum and
anal canal. We will look at slides of the colon, appendix and anorectal junction. The
principal functions of the colon are the reabsorption of electrolytes and water and the
elimination of undigested food and waste. The cecum is a blind pouch just distal to the
ileocecal valve whose histology resembles that of the colon. The appendix is a thin,
finger-like extension of the cecum. The rectum is the dilated distal portion of the
alimentary canal. Its histology resembles that of the colon, but it is distinguished by
transverse rectal folds in its submucosa and the absence of tenia coli in its muscularis
externa (see below). The anal canal is the most distal part of the alimentary canal,
extending from the anorectal junction to the anus. |
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The mucosa appears smooth at the gross level because it has no villi. Numerous
straight, tubular glands are present. They extend all the way to the muscularis mucosae.
The glands and the surface are lined with simple columnar epithelium whose cell types are
as described for the small intestine. However Paneth cells are usually absent in the adult
human and enteroendocrine cells are rare. Columnar absorptive cells and goblet cells are
abundant. Goblet cells are more prevalent in the crypts than along the surface, and their
number increases distally toward the rectum. The mucus facilitates the passage of the
increasingly solid colonic contents, and covers bacteria and particulate matter. The
absorptive cells have short, irregular microvilli, and although they secrete a glycocalyx,
it has not been shown to contain digestive enzymes. The absorptive cells actively
transport electrolytes. Water is also absorbed as it passively follows the electrolytes.
As in the small intestine, undifferentiated cells are found at the base of the crypts.
The lamina propria is highly cellular. It is particularly rich in lymphoid cells and
and lymph nodules may interrupt the regular spacing of the crypts and extend into the
submucosa (this is particularly evident in the appendix). The extensive development of
GALT reflects the abundance and variety of microorganisms and noxious end products of
metabolism. As in the small intestine, lymphatic vessels form a network around the
muscularis mucosae. However, no lymph vessels extend into the lamina propria between
colonic crypts. The muscularis mucosae has a circular and longitudinal layer. |
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The submucosa is quite dense, similar to that of the small intestine.
The muscularis externa consists of an inner circular and outer longitudinal layer. The
inner circular layer is typical, but the outer longitudinal layer of the colon is very
thin, except for three extremely thick longitudinal bands, called teniae coli. Bundles of
muscle from the teniae coli penetrate the circular layer at irregular intervals. These
discontinuities in the muscularis externa allow segments of the colon to contract
independently. A contraction in one segment (2-5 cm) will peak over about 30 seconds, and
disappear during the next 60. The lumen may almost be occluded during a contraction,
allowing all the fecal matter to be in touch with the colon wall. The fecal matter is dug
into and rolled - much like spading earth. The longitudinal layer contracts at the same
time as the circular layer. Unstimulated portions between the contracting segments bulge
outward, forming saccules or haustra. The next contraction would be in another area. All
but 80 ml of the daily load of 450 ml of chyme are aborbed. Peristaltic movements in the
colon (longitudinal layer) result in mass movements distally of colonic contents. They are
not frequent (about once a day in a typical person). |
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The entire transverse colon is covered with a serosa, whereas parts of the ascending
and descending colon have an adventitia. |
 Fig. 34 Low power view of the colon  
Figure 34 shows a low power view of the mucosa, submucosa and a bit of
the muscularis externa of the colon. The luminal surface of the mucosa appears slightly
damaged. The glands extend downward from the surface to the muscularis mucosae. They
appear paler than the surrounding lamina propria. In some glands, the lumen can be seen,
especially in the one toward the left near the "mu". (If the field of view were
extended a bit higher, we would see this gland opening into the lumen of the colon.) The
lamina propria is very cellular. The submucosa shows areas of varying densities, the
densest band in this particular section lies just above the muscularis externa. Some blood
vessels and a large lymph nodule can be seen. The lymph nodule (which originated in the
GALT of the mucosa) has disrupted the muscularis mucosa. Part of the inner circular layer
of the muscularis externa is seen at the bottom of the figure. |
 Fig. 35 Low power view of the colon with glands cut obliquely
Figure 35 is very similar to Figure 34, but the glands have been cut
at an oblique angle. Therefore they appear as circular profiles and the fact that they are
straight is not obvious. This type of sectioning is quite frequent. Numerous blood vessels
are seen in the submucosa. |
Fig. 36 Higher power view of mucosa of colon 
Figure 36 shows a higher power view of the mucosa of the colon. The
cellularity of the lamina propria separating the glands is more evident at this
magnification. In one area, a lymph nodule has formed near the base of some glands. (As
you know, these can extend into the submucosa). In one gland, the section cuts through the
apical ends of the cells lining it. Most of this gland looks like it is filled with
bubbles, but the uppermost part has been sectioned straight. |
Fig. 37 High power view of colonic glands 
Figure 37 shows a high power view of the cells lining a colonic gland
(crypt). The epithelium is simple columnar, with basal nuclei. The cells seen are all
absorptive cells or goblet cells. The absorptive cells have a thin striated border which
appears as a reddish line at the apical end of the cell. The goblet cells are pale and
wider apically. Along the surface epithelium (not shown), absorptive cells predominate, in
the crypts, goblet cells become more abundant, and their abundance increases distally
toward the rectum.
The gland shown is bounded immediately on the right by another gland (sectioned more
obliquely). Some lamina propria is seen on the left. In addition to lymphocytes, the
lamina propria has many plasma cells and fibroblasts, as well as macrophages and other
cells. (We don't identify cell types.) The GALT is more highly developed than in the small
intestine. The lamina propria also contains fine collagen and reticular fibres. At the
extreme left of Figure 37, the nuclei of the epithelial cells of another crypt can be
seen. |
Fig. 38 Low power view of muscularis externa with tenia coli 
Figure 38 shows a low power view of the muscularis externa of the
colon. The outer longitudinal layer can be seen to be expanding into a tenia coli. The
full extent of the tenia coli is not shown. |
Fig. 39 Low power view of the appendix 
Figure 39 shows a low power view of the appendix. The structure of the
appendix is very similar to that of the colon. However, the crypts are farther apart and
not as well-developed. The most notable feature of the appendix is that lymph nodules
arising from GALT completely "take over". They can be found in the mucosa or
submucosa, and very frequently the muscularis mucosae is completely disrupted. In the area
shown, such a disruption occurs at the bottom of the field of view, where a huge lymph
nodule occupies most of the mucosa and submucosa. Toward the top of the section, several
large lymph nodules lie in the submucosa, but it can still be distinguished from the
mucosa with its glands and lamina propria. When scanning the appendix at a higher power,
you would periodically come across sections of the muscularis mucosae. There are no teniae
coli in the appendix, the muscularis externa is typical. At this magnification, it is hard
to distinguish its 2 layers. (They have separated slightly toward the bottom of the
figure.) The serosa is not distinguishable at this magnification. Much debris is seen in
the lumen, as the luminal surface of the tissue has been damaged. |
Fig. 40 Higher power view of mucosa and submucosa of appendix 
Figure 40 shows a higher power view of the luminal part of the
appendix wall. In the mucosa, the more irregular structure of the glands can be seen. The
muscularis mucosae is obscured by GALT. It is found, when visible, running along the base
of the glands, as in the colon. Two large lymph nodules are seen in the submucosa, whose
connective tissue is very dense. |
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The rectum is usually divided into two parts. The upper part (about 12-17 cm) largely
resembles the colon, but the crypts are longer, more widely spaced and lined almost
entirely with goblet cells. The rectum also has more abundant diffuse lymphatic tissue and
lymph nodules, and lacks teniae coli in the muscularis externa.
The lower part of the rectum (about 2.5-3.5 cm) is the upper part of the anal canal.
Here the mucosa is thrown into about 8 permanent, longitudinal folds (columns of
Morgagni), which terminate distally about 1.5 cm from the anal orifice. The bases (distal
ends) of the columns are connected by transverse folds of the mucosa, called the anal
valves.
Above the anal valves, the mucosa is lined by simple columnar epithelilum with many
goblet cells. Crypts are present. At the level of the anal valves (transition to lower
part of anal canal), the epithelium becomes non-keratinized stratified squamous. There are
no crypts. The non-keratinized epithelium extends to the anal orifice, where it becomes
epidermis. (Aside from the stratified squamous, keratinized epithelium at the anal
orifice, other typical skin structures, such as hairs, sebaceous glands and sweat glands,
appear.) Along the way to the epidermis of the anal orifice, different types of epithelium
may be seen in the anal canal, such as stratified cuboidal or stratified columnar.
At about the level of the anal valves, the muscularis mucosae subdivides into divergent
strands which soon disappear. The submucosa of the anal canal has a rich plexus of blood
vessels (hemorrhoidal plexus). The veins are often tortuous and lack valves. This,
together with their size, is condusive to the formation of hemorrhoids.
The circular layer of smooth muscle of the anal canal is thick and forms the internal
anal sphincter. The external anal sphincter is formed by skeletal muscle. It lies just
inside the levator ani muscles which also act as a sphincter. |
Fig. 41 Low power view of the anal canal, showing transition from upper to
lower 
Figure 41 shows a low power view of the anal canal. The upper
(colon-like) part is at the right and the lower part is at the left. The simple columnar
epithelium of the mucosa, which contains crypts, gives way to stratified squamous
epithelium. Farther distally (to the left), different types of epithelium appear. In the
region of the arrow, the epithelium is stratified columnar (cannot be identified as such
at this magnification). The muscularis mucosae is at the point of dissipating into a few
strands and disappearing. (To the right of the area in the field of view it appears as a
much more definitive structure.) A lymph nodule is seen directly below the epithelium at
the left. Numerous blood vessels are seen in the submucosa. |
Fig. 42 Low power view of lower anal canal with lymph nodule 
Figure 42 shows the lymph nodule seen in figure 41, as well as the
underlying submucosa, in which numerous large blood vessels can be seen. This figure is at
the same magnification as the previous one. The epithelium just above the lymph nodule is
stratified squamous, below the lymph nodule it is stratified columnar. |
 Fig.
43 Upper part of anal canal with crypts
Figure 43 shows a higher power view of the upper part of the anal
canal. Crypts are present, and the muscularis mucosae is distinct. The area shown here is
proximal to the upper anal canal shown in Figure 41. |
Fig. 44 Island of stratified squamous epithelium among crypts in upper
anal canal 
Sometimes islands of stratified epithelium appear in the upper anal canal as it
approaches the lower canal. An island of stratified squamous epithelium among the crypts
is shown in Figure 44. The muscularis mucosae is not within the field of
view. Note the cellularity of the lamina propria, with many lymphocytes seen between the
crypts. |
 Fig. 45 Stratified squamous keratinized epithelium at the anal orifice
Figure 45 shows the beginning of stratified squamous keratinized
epithelium toward the end of the anal canal. The cells at the surface have lost their
nuclei. |
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