Transcript The large intestine

Histology of digestive system
Small, large Intestine & Organs Associated
With the Digestive Tract
21th lecture
May 19, 2016
Small Intestine
• The small intestine is the site of terminal food
digestion, nutrient absorption, and endocrine
secretion.
• The processes of digestion are completed in the small
intestine, where the nutrients (products of digestion)
are absorbed by cells of the epithelial lining.
• The small intestine is relatively long—approximately 5
m—and consists of three segments: duodenum,
jejunum, and ileum. These segments have many
characteristics in common and will be discussed
together.
Mucous Membrane
Viewed with the naked eye, the lining of the small intestine shows
a series of permanent circular or semilunar folds (plicae
circulares), consisting of mucosa and submucosa, which are best
developed in the jejunum.
Intestinal villi are 0.5- to 1.5-mm-long mucosal outgrowths
(epithelium plus lamina propria) and project into the lumen.
In the duodenum they are leaf-shaped, but gradually assume
fingerlike shapes moving toward the ileum. Villi are covered by a
simple columnar epithelium of absorptive cells and goblet cells.
(a): The mucosa and submucosa are the inner two of the gut's four concentric layers. (b): They
form circular folds or plicae circulares, which increase the absorptive area.
(d): They are lined by a dense covering of finger-like projections called villi. Internally each villus contains
lamina propria connective tissue with microvasculature and lymphatics . Villi are covered with a simple
columnar epithelium composed of absorptive enterocytes and goblet cells.
(e): At the apical cell membrane of each enterocyte are located dense microvilli, which serve to increase
greatly the absorptive surface of the cell.
Between the villi the covering epithelium invaginates to form short tubular intestinal glands or crypts,
which include stem cells for the epithelium and Paneth cells which prevent intestinal flora from becoming
concentrated in these glands where damage to the stem cells could occur.
Duodenal (Brunner) glands.
Concentrated mainly in the upper
duodenum are large masses of compound
branched mucous glands, the duodenal
glands (DG), with many lobules that occupy
much of the submucosa and may extend
above the muscularis mucosae (MM) into
the mucosa. Many small excretory ducts (D)
extend from these lobules through the
lamina propria and empty into the lumen
among the small intestinal crypts (IC).
Alkaline mucus from duodenal glands
neutralizes the pH of material entering the
duodenum and supplements the mucus
from goblet cells in lubricating and
protecting the lining of the small intestine.
X100. H&E.
Small intestine muscularis and myenteric plexus.
(b): Pale-staining neurons and other cells in one myenteric plexus (MP) are seen between
the two muscle layers. X100. H&E. Along the entire digestive tract autonomic neurons from
the numerous myenteric ganglia and smaller submucosal ganglia innervate the wall and
comprise the enteric nervous system, which is of key importance for gut function and
involved in many digestive problems. Local activity of these neurons is controlled by sensory
neurons and effector neurons supplying the musculature, both of which are intrinsic to the
enteric nervous system. Extrinsic innervation in this system includes parasympathetic
cholinergic nerve fibers that stimulate the activity of smooth muscle and sympathetic
adrenergic nerve fibers that depress muscle activity.
The large intestine
• consists of a mucosal membrane with no folds except in its
distal (rectal) portion and no villi.
• The mucosa is penetrated throughout its area by tubular
intestinal glands lined by goblet and absorptive cells, with a
small number of enteroendocrine cells.
• The absorptive cells or colonocytes are columnar and have
short, irregular microvilli.
• Stem cells for the epithelium of the large intestine are located
in the bottom third of each gland. The large intestine is well
suited to its main functions: absorption of water, formation of
the fecal mass from undigestible material, and production of
mucus that lubricates the intestinal surface.
The large intestine consists of the cecum; the ascending, transverse, descending, and
sigmoid regions of the colon; and the rectum. (a): Anterior view of the large intestine with
the proximal end exposed shows the ileocecal valve at its attachment to the ileum, along
with the blind sac called the cecum and its extension, the appendix. The mucosa has
shallow plicae but no villi. The muscularis has two layers, but the outer longitudinal layer
consists only of three distinct bundles of muscle fibers called taeniae coli (ribbons of the
colon). These bands cause the colon wall to form a series of sacs called haustra. The
serosa of the colon is continuous with that of the supporting mesenteries and displays a
series of suspended masses of adipose tissue called omental appendages.
a): Diagram showing the wall of the large intestine composed of the four typical layers. (b): The
mucosa is occupied mostly by tubular intestinal glands extending as deep as the muscularis
mucosae and by lamina propria rich in MALT. The submucosa is well vascularized. The
muscularis has a typical inner circular layer, but the outer longitudinal muscle is only present in
three equally spaced bands, the taeniae coli (a). b: X80, H&E
Mucosa of the large intestine (colon).
(a): Transverse section of the colon shows the muscularis externa (ME), including a taenia
coli cut transversely in the lower part of the figure, the submucosa (S), the mucosa (M) filled
with tubular intestinal glands. Some of these glands are cut longitudinally, but most seen
here are cut transversely. X14. H&E. (b): Transversely cut glands are seen to consist of simple
columnar epithelium surrounded a tubular lumen (asterisk) and embedded in lamina propria
(LP) with many free lymphocytes. Lymphocytes can also be seen penetrating the epithelium
(arrow). X200. H&E.
Vermiform Appendix
A blind evagination off the
cecum,
the appendix, has a very
small lumen,
fewer glands in its mucosa,
and no taeniae coli.
The laminar propria and
submucosa are generally
filled with lymphocytes and
lymphoid follicles, making
the appendix a significant
part of the MALT. X40. H&E.
Rectum
• Intestinal glands are
straight, like test tubes.
• A continuous coat of
longitudinal muscle is
present.
• Absence of taenia.
Mucosa of the recto-anal junction.
The simple columnar epithelium with tubular glands that lines the rectum (left side) changes
abruptly to stratified squamous epithelium in the anal canal (right side), as seen in this
longitudinal section. The connective tissue of the lamina propria is seen to contain many free
lymphocytes. X40. H&E.
Organs Associated With the Digestive
Tract
• The organs associated with the digestive tract
include the salivary glands, the pancreas, the
liver, and the gallbladder.
• Products of these organs facilitate transport and
digestion of food within the gastrointestinal tract.
• The main functions of the salivary glands are to
wet and lubricate ingested food and the oral
mucosa, to initiate the digestion of carbohydrates
and lipids with amylase and lipase, and to secrete
protective bacteriostatic substances such as the
immunoglobulin IgA, lysozyme, and lactoferrin.
Salivary Glands
Submandibular gland is a mixed
serous and mucous gland (serous
cells predominate) and shows
well-stained cells in serous acini
(A) and (S) and pale-staining
mucous cells (M) grouped as
tubules in this tubuloacinar gland.
Small intralobular ducts (ID) drain
each lobule, but these are not
composed of columnar cells with
well-developed striations. X340.
H&E.
Features and Functions of the Salivary
Glands
• Main functions:
– Produces and secretes saliva
– Cleanses the mouth
– Dissolves food chemicals so they can be tasted
– Moistens food, squeezing it into a bolus
– Begins the chemical breakdown of food
• Salivary amylase: starch
Pancreas
• The pancreas is a mixed exocrine-endocrine
gland that produces both digestive enzymes
and hormones.
• A thin capsule of connective tissue covers the
pancreas and sends septa into it, separating
the pancreatic lobules.
• The secretory acini are surrounded by a basal
lamina that is supported by a delicate sheath
of reticular fibers and a rich capillary network.
Microscopic Anatomy of the Liver
• Liver Lobules: structural unit of liver
• Hepatocytes: liver cells contained within the lobules
• Hepatic portal vein & Hepatic Artery: the circulation of the
liver, they bring blood into the liver where it is filtered through
the liver sinusoidal capillaries
• Kupffer cells: remove debris
• Filtered blood drains into the central vein, then to the hepatic
vein, and eventually to the inferior vena cava
• Bile (produced by hepatocytes) drains into the bile duct after
passing through portal triad
• Bile then shipped to gallbladder for storage
Functions of the Liver
• Largest internal organ
• Functions:
– Filters and processes nutrient-rich blood of
carbohydrates, proteins, and lipids from intestine
– Production and regulation of cholesterol
– Production of bile which emulsifies fats
– Removes drugs and hormones from circulation
– Storage of vitamins and minerals
Cut transversely hepatic lobules are
polygonal units showing plates of
epithelial cells called hepatocytes
radiating from a central venule (C). In all
cases peripheral connective tissue of
portal areas with microvasculature and
small bile duct (D) branches can be seen
and in humans as in other mammals these
are present at the boundaries between
two or more hepatic lobules. The vessels
near the bile ducts branches are a venule
(V) off the portal vein and an arteriole (A)
off the hepatic artery. Both X150. H&E.