The Large Intestine
- Low power view of the appendix
- Higher power view of mucosa and submucosa of appendix
- The Anorectal Junction
- Low power view of the anal canal, showing transition from upper to lower
- Low power view of lower anal canal with lymph nodule
- Upper part of anal canal with crypts
- Island of stratified squamous epithelium among crypts in upper anal canal
- Stratified squamous keratinized epithelium at the anal orifice
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.
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.
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).
The entire transverse colon is covered with a serosa, whereas parts of the ascending and descending colon have an adventitia.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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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