Digestive System: Smal1
and Large Intestines
Small Intestine
The small intestine is a long, convoluted tube about 5 to 7 m long; it is the longest section of the
digestive tract. The small intestine extends from the junction with the stomach to join with the
large intestine or colon. For descriptive purposes, the small intestine is divided into three parts:
the duodenum, jejunum, and ileum. Although the microscopic differences among these three
segments are minor, they allow for identification of the segments.
The main function of the small intestine is the digestion of gastric contents and absorption
of nutrients into blood capillaries and lymphatic lacteals.
Surface Modifications of Small Intestine for Absorption
The mucosa of the small intestine exhibits specialized structural modifications that increase the
cellular surface areas for absorption of nutrients and fluids. These modifications include the pli­
cae circulares, villi, and microvilli.
In contrast to the rugae of stomach, the plicae circulares are permanent spiral folds or ele­
vations of the mucosa ( with a submucosal core) that extend into the intestinal lumen. The plicae
circulares are most prominent in the proximal portion of the small intestine, where most absorp­
tion takes place; they decrease in prominence toward the ileum.
Villi are permanent fingerlike projections oflamina propria ofthe mucosa that extend into the
intestinal lumen. They are covered by simple columnar epithelium and are also more prominent in
the proximal portion ofthe small intestine. The height of the villi decreases toward the ileum ofthe
small intestine. The connective tissue core of each villus contains a lymphatic capillary called a
lacteal, blood capillaries, and individual strands ofsmooth muscles (see Overview Figure 13).
Each villus has a core oflamina propria that is normally filled with blood vessels, lymphatic
capillaries, nerves, smooth muscle, and loose irregular connective tissue. In addition, the lamina
propria is a storehouse for immune cells such as lymphocytes, plasma cells, tissue eosinophils,
macrophages, and mast cells.
Smooth muscle fibers from the muscularis mucosae extend into the core of individual villi
and are responsible for their movements. This action increases the contacts of the villi with the
digested food products in the intestine.
Microvilli are cytoplasmic extensions that cover the apices of the intestinal absorptive cells.
They are visible under a light microscope as a striated (brush) border. The microvilli are coated
by a glycoprotein coat glycocalyx, which contains such brush border enzymes as lactase, pepti­
dases, sucrase, lipase, and others that are important for digestion.
Cells, Glands, and Lymphatic Nodules in the Small Intestine
Intestinal glands ( crypts of Lieberkiihn) are located between the villi throughout the small intes­
tine. These glands open into the intestinal lumen at the base of the villi. The simple columnar
epithelium that lines the villi is continuous with that of the intestinal glands. In the glands are
found stem cells, absorptive cells, goblet cells, Paneth cells, and some enteroendocrine cells.
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PART II - ORGANS
Absorptive cells are the most common cell types in the intestinal epithelium. These cells are
tall columnar with a prominent striated (brush) border of microvilli. A thick glycocalyx coat
covers and protects the microvilli from the corrosive chemicals.
Goblet cells are interspersed among the columnar absorptive cells of the intestinal epithe­
lium. They increase in number toward the distal region of the small intestine (ileum).
Enteroendocrine or APUD (amine precursor uptake and decarboxylation) cells are scat­
tered throughout the epithelium of the villi and intestinal glands.
Duodenal (Brunner's) glands are primarily found in the submucosa of the initial portion of
the duodenum and are highly characteristic of this region of the small intestine. These are branched,
tubuloacinar glands with light-staining mucous cells. The ducts of duodenal glands penetrate the
muscularis mucosae to discharge their secretory product at the base of intestinal glands.
Undifferentiated cells exhibit mitotic activity and are located in the base of intestinal
glands. They function as stem cells and replace worn-out columnar absorptive cells, goblet cells,
and intestinal gland cells.
Paneth cells are located at the base of intestinal glands. They are characterized by the pres­
ence of deep-staining eosinophilic granules in their cytoplasm.
m
Peyer's patches are numerous aggregations of closely packed, permanent lyphatic
nod­
ules. They are found primarily in the wall of the terminal portion of small intestine, the ileum.
These nodules occupy a large portion of the lamina propria and submucosa of the ileum.
M cells are highly specialized epithelial cells that cover the Peyer's patches and large lym­
phatic nodules; they are not found anywhere else in the intestine. M cells phagocytose luminal
antigens and present them to the lymphocytes and macrophages in the lamina propria, which are
then stimulated to produce specific antibodies against the antigens.
Regional Differences in the Small Intestine
The duodenum is the shortest segment of the small intestine. The villi in this region are broad,
tall, and numerous, with fewer goblet cells in the epithelium. Branched duodenal (Brunner's)
glands with mucus-secreting cells in the submucosa characterize this region.
The jejunum exhibits shorter, narrower, and fewer villi than the duodenum. There are also
more goblet cells in the epithelium.
The ileum contains few villi that are narrow and short. In addition, the epithelium contains
more goblet cells than in the duodenum or jejunum. The lymphatic nodules are particularly large
and numerous in the ileum, where they aggregate in the lamina propria and submucosa to form
the prominent Peyer's patches.
Large Intestine (Colon)
The large intestine is situated between the anus and the terminal end of the ileum. It is shorter and
less convoluted than the small intestine. It consists of an initial segment called the cecum, and the
ascending, transverse, descending, and sigmoid colon, as well as the rectum and anus.
Chyme enters the large intestine from the ileum through the ileocecal valve. Unabsorbed
and undigested food residues from the small intestine are forced into the large intestine by strong
peristaltic actions of smooth muscles in the muscularis externa. The residues that enter the large
intestine are in a semifluid state; however, by the time they reach the terminal portion of the large
intestine, these residues become semisolid feces.
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PART II - ORGANS
FUNCTIONAL CORRELATIONS: Duodenum
A characteristic feature of the duodenum are the branched tubuloacinar duodenal
(Brunner's) glands in the submucosa. Their excretory ducts penetrate the muscularis
mucosae to deliver their secretions at the base of intestinal glands. Duodenal glands secrete or
release their product into the lumen in response to the entrance of acidic chyme from the
stomach and parasympathetic stimulation by the vagus nerve.
The main function of the duodenal glands is to protect the duodenal mucosa from the
highly corrosive action of the gastric contents. Also, alkaline mucus and bicarbonate secre­
tions from the duodenal gland secretions that enter the duodenum buffer or neutralize the
acidic chyme to provide a more favorable environment for digestive enzymes that enter the
duodenum from the pancreas.
Duodenal glands are also believed to produce a polypeptide hormone called urogas­
trone. This hormone inhibits hydrochloric acid secretion by the parietal cells in the stomach
and increases epithelial proliferation in the small intestine.
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PART II - ORGANS
Small Intestine
Paneth cells, located in the bases of intestinal glands, are exocrine cells and produce lysozyme,
an antibacterial enzyme that digests bacterial cell walls and destroys them. Paneth cells may
also have some phagocytic functions. Thus, these cells have an important function in control­
ling the microbial flora in the small intestine.
Enteroendocrine cells in the small intestine secrete numerous regulatory hormones,
including gastric inhibitory peptide, secretin, and cholecystokinin (pancreozymin). To
release these hormones directly into the capillaries, the secretory granules in these cells are
located in the base of the cells, which are adjacent to the lamina propria and the capillaries.
Once these regulatory hormones enter the bloodstream, they control the release of gastric and
pancreatic secretions, induce intestinal motility, and stimulate contraction of the gallbladder
to release bile, among other functions.
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PART II - ORGANS
FUNCTIONAL CORRELATIONS: Peyer's Patches in the Ileum
The lamina propria and submucosa contain numerous and large aggregates of large lymphatic
nodules, called Peyer's patches. Overlying these lymphatic patches are specialized epithelial
cells, called the M cells. The cell membranes of M cells show deep invaginations that contain
both macrophages and lymphocytes. The lymphatic nodules of Peyer's patches contain
numerous B lymphocytes, some T lymphocytes, macrophages, and plasma cells. M cells
continually sample the antigens of the intestinal lumen, ingest the antigens, and present them
to the underlying lymphocytes and macrophages in the lamina propria. The antigens that
reach the underlying lymphocytes and macrophages then initiate the proper immunologic
responses to these foreign molecules.
Small Intestine
The small intestine performs numerous digestive functions, including (1) continuation and
completion of digestion (initiated in the oral cavity and stomach) of food products (chyme)
by chemicals and enzymes produced in the liver and pancreas, and by cells in its own mucosa;
(2) selective absorption of nutrients into the blood and lymph capillaries; (3) transportation
of chyme and digestive waste material to the large intestine; and (4) release of different hor­
mones into the bloodstream to regulate the secretory functions and motility of digestive
organs.
On the surface epithelium, goblet cells secrete mucus that lubricates, coats, and protects
the intestinal surface from the corrosive actions of digestive chemicals and enzymes. The outer
glycocalyx coat on absorptive cells not only protects the intestinal surface from digestion, but
also contains numerous enzymes required for the terminal digestion of food products. These
enzymes are produced by absorptive epithelial cells.
Absorption of nutrients into the cell interior occurs via diffusion, facilitated diffusion,
osmosis, and active transport. Intestinal cells absorb amino acids, glucose, and fatty acids­
the end products of protein, carbohydrate, and fat digestion, respectively. Amino acids, water,
various ions, and glucose are transported through intestinal cells into the blood capillaries
present in the lamina propria of the villi, from which they pass to the liver via the portal vein.
Most of the long-chain fatty acids and monoglycerides, however, do not enter the capillaries,
but instead enter the tiny, blind-ending lymphatic vessels, called lacteals, that are also located
in the lamina propria of each villus. The presence of smooth muscle fibers in the villi causes
contractions of the villi and move the contents of the lacteals from the villi into larger lymph
vessels in the submucosa and into the mesenteries.
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PART II - ORGANS
FUNCTIONAL CORRELATIONS: Large Intestine
The principal functions of the large intestine are to absorb water and minerals ( electrolytes)
from the indigestible material that was transported from the ileum of the small intestine and
to compact them into feces for elimination from the body. Consistent with these functions, the
epithelium of the large intestine contains columnar absorptive cells (similar to those in the
epithelium of the small intestine) and mucus-secreting goblet cells, which produce mucus for
lubricating the lumen of the large intestine to facilitate passage of the feces. No digestive
enzymes are produced by the cells of large intestine.
Histologic Differences Between the Small and Large Intestines (Colon)
The large intestine lacks plicae circulares and villi that characterize the small intestine.
Intestinal glands are present in the large intestine and are similar to those of the small intes­
tine. However, they are deeper (longer) and lack the Paneth cells in their bases. The epithelium
of the large intestine also contains different enteroendocrine cells.
Although present in the small intestine, goblet cells are more numerous in the large intes­
tine epithelium. Also, the number of goblet cells increases from the cecum toward the terminal
portion of the sigmoid colon. The lamina propria of the large intestine contains many solitary
lymphatic nodules, lymphocyte accumulations, plasma cells, and macrophages.
In contrast to the small intestine, the muscularis extema of the large intestine and cecum
shows a unique arrangement. The inner circular smooth muscle layer is present. However, the
outer longitudinal muscle layer is arranged into three longitudinal muscle strips called taenia
coli The contractions or tonus in the taenia coli forms sacculations in the large intestine,
called haustra (see Overview Figure 13).
CHAPTER 13 - Digestive System: Small and Large Intestines
305
9 Temporary fold
(mucosa and submucosa)
1 Intestinal glands
1 D Intestinal glands
(longitudinal and
cross section)
r 2Lining
"'
-1 epithelium
I (with goblet
cells)
I 3 Lamina
I propria
I 4 Muscularis
l mucosae
5 Submucosa
8�
�
6 Muscularis
externa
11 Muscularis
mucosae
12 Submucosa
13 Lymphatic
nodule
14 Myenteric
plexus
15 Taeniae coli
7 Serosa
8 Myenteric
plexus
16 Blood vessels
17 Adipose cells
FIGURE 13.10 • Large intestine: colon wall (transverse section). Stain: hematoxylin and eosin. Medium
magnification.
CHAPTER 13 - Digestive System: Small and Large Intestines
307
1 Lining epithelium
with goblet cells
2 Muscularis
mucosae
3 Lamina propria
-c--�e-':i�- 9 Lymphatic nodule
with germinal
4 Germinal center
(of lymphatic nodule)
5 Intestinal glands
6 Diffuse lymphatic
tissue
FIGURE 13.11 • Appendix (panoramic view, transverse section). Stain: hematoxylin and eosin.
Low magnification.
CHAPTER 13 - Digestive System: Small and Large Intestines
309
9 Advenlilia
1 Surface epithelium
10 Lymphatic nodule
2 Lamina propria
11 Muscularis
mucosae
4 Intestinal glands
in mucosa
5 Lumen
12 Adipose cells
13 Muscularis
externa:
a. Inner circular
layer
b. Outer
longitudinal layer
6 Venule
14 Parasympathetic
ganglia of myenteric
(Auerbach's) plexus
8 Submucosa
15 Arteriole
and venule
FIGURE 13.12 • Rectum (panoramic view, transverse section). Stain: hematoxylin and eosin. Low magnification.
CHAPTER 13
Summary
Digestive System: Small and Large Intestines
Small Intestine
• Long, convoluted tube divided into duodenum, jejunum,
and ileum
• Duodenum is the shortest segment with broad, tall, and
numerous villi
• Digests gastric contents and absorbs nutrients into blood
capillaries and lymphatic lacteals
• Transports chyme and waste products to large intestine
• Releases numerous hormones to regulate secretory ftmc­
tions and motility of digestive organs
• Amino acids, water, ions, glucose and other substances are
absorbed and transported in blood capillaries
• Long-chain fatty acids and monoglycerides are transported
by lymphatic lacteals
• Contains numerous permanent surface modifications that
increase cellular contact for absorption
• Plicae circulares are spiral folds with submucosa core that
extend into intestinal lumen
• Villi are fingerlike projections of lamina propria that extend
into the intestinal lumen
• Microvilli are cytoplasmic extensions of absorptive cells that
extend into intestinal lumen
• Microvilli are coated with brush border enzymes that digest
food products before absorption
• Villi contain a core of connective tissue with capillaries,
lacteal, and smooth muscle strands
• Lamina propria is filled with lymphocytes, plasma cells,
macrophages, eosinophils, and mast cells
• Smooth muscle strands in lamina propria of villi induce
their movement and contractions
Cells of Small Intestine
• Absorptive cells with microvilli covered by glycocalyx are
most common in intestinal epithelium
• Goblet cells, interspersed between absorptive cells, increase
in number toward distal region
• Enteroendocrine cells are scattered throughout the epithe­
lium and intestinal glands
310
• Secretory granules of enteroendocrine cells located at base
of cells and close to capillaries
• Enteroendocrine cells secrete numerous regulatory hor­
mones for the digestive system
• Undifferentiated cells in the base of intestinal glands replace
worn-out luminal cells
• Paneth cells with pink eosinophilic granules in cytoplasm
are located in the intestinal glands
• Paneth cells produce the antibacterial enzyme lysozyme to
control microbial flora in intestine
• M cells are specialized cells that cover the lymphatic Peyer's
patches
Glands of Small Intestine
• Intestinal glands located between villi throughout the small
intestine
• Intestinal glands open into the intestinal lumen at the base
of the villi
• Duodenal glands in the submucosa of duodenum are char­
acteristic of this region
• Duodenal glands penetrate muscularis mucosae to discharge
mucus and bicarbonate secretions
• Bicarbonate secretions enter base of intestinal glands and
protect duodenum from acidic chym e
• Polypeptide urogastrone from duodenal glands inhibits
hydrochloric acid secretions
Lymphatic Accumulations in Small Intestine
• Peyer's patches are numerous aggregations of permanent
lymphatic nodules
• Peyer's patches found primarily in the lamina propria and
submucosa of terminal part of intestine
• Overlying Peyer's patches are specialized M cells, which are
not anywhere else in the intestine
• M cells show deep invaginations that contain macrophages
and lymphocytes
• M cells sample intestinal antigens and present them to
underlying lymphocytes for response
CHAPTER 13 - Digestive System: Small and Large Intestines
Large Intestine
• Situated between anus and the terminal end of ileum
• Shorter and less convoluted than small intestine
• Consists of cecum, ascending, transverse, descending, and
sigmoid sections
• Semifluid chyme enters through ileocecal valve
• At terminal end, semifluid residues become hardened or
semisolid feces
• Main function is the absorption of water and electrolytes
Epithelium consists of simple columnar epithelium with
increased number of goblet cells
311
• Goblet cells produce mucus for lubricating the canal to facil­
itate passage of feces
• No enzymes or chemicals produced, but enteroendocrine
cells are present in the epithelium
• No plicae circulares, villi, or Paneth cells are present; intesti­
nal glands are deeper
• Increased numbers of solitary lymphatic nodules with cells
are present in lamina propria
• Muscularis externa contains inner circular layer with outer
layer arranged in three strips, taenia coli
• Contractions of taenia coli form sacculations or haustra