Three Flashcards
Describe the 4 layers of the tubular GI tract.
1) The mucosa is comprised of an epithelial lining which will be either stratified squamous (keratinized or not) or simple columnar depending on its location. Below the epithelial lining will be found an area of connective tissue called the lamina propria (loose areolar connective tissue). The lamina propria is comprised of a number of inflammatory cells, lymphatics, blood vessels, and strands of smooth muscle and collagen. Just below the lamina propria is a discrete band of smooth muscle called the muscularis mucosa.
2) The submucosa is bordered by the muscularis mucosa of the mucosa and on the other side by the muscularis externa. This is an area of dense irregular connective
tissue, which contains large blood vessels, nerves, lymphatics and sometimes, glands. Parasympathetic ganglia can be found here (Meissner’s plexus) as well.
3) The muscularis externa (or propria) is comprised of smooth muscle and usually consists of an inner circular and outer longitudinal layers (except the stomach).
Contraction of the inner layer decreases the diameter of the tract, while contraction of the outer layer shortens its length. Located between these layers are parasympathetic ganglia referred to as “Auerbach’s plexus” that innervate these muscle layers to facilitate peristalsis.
4) The external covering of the tubular portion of the gastrointestinal tract may be comprised of a thin layer of connective tissue called “adventitia.” This is composed of loosely arranged fibroblasts, collagen, and fat. It may contain large blood vessel, nerves, and lymphatics In those areas of the tract that lie within the peritoneal cavity, the adventitia is covered by simple squamous epithelium, which
is then referred to as serosa.
What is the esophagus like grossly? Describe the esophageal mucosa. Submucosa. Muscularis externa. Adventitia.
Esophagus: The esophagus is a straight tube (25 cm long) between the pharynx and the stomach.
Esophageal Mucosa: It is lined on its luminal aspect by stratified squamous epithelium that is not keratinized. The underlying lamina propria of the esophagus is fairly unremarkable and is comprised of fibroblasts, collagen, eosinophils, lymphocytes and plasma cells. However, near the gastroesophageal border, mucous secreting glands may be found within the lamina propria. These glands are difficult to identify on your slides but are called “cardiac glands.” Do not confuse these cardiac glands with those found in the stomach. The muscularis mucosa of the esophagus is unusual as it is discontinuous and will be seen as scattered islands of smooth muscle if the specimen is observed in cross section (Ross figure 16.3, page 480) (Ross figure 17.3, page 523).
Esophageal Submucosa: The esophageal submucosa is comprised of dense irregular connective tissue. A hallmark of the esophageal submucosa is the presence of numerous mucous secreting glands that drain their contents into the lumen. These glands are called “esophageal glands.” Mucous secreted by these glands lubricates the lining of the esophagus and lack the capacity to buffer acid. This area is also rich in large blood vessels, nerves, lymphatics, and ganglia (Ross et.al, figure 16.4, page 480) (Ross figure 17.4, page 523).
Esophageal muscularis externa (propria): The muscularis externa of the esophagus contains either skeletal or smooth muscle depending on where the section is taken from. The upper one-third of the esophagus has a muscularis externa comprised solely of skeletal muscle. The middle one-third of the esophagus has a muscularis externa comprised of a mixture of skeletal and smooth muscle. The distal one third of the esophagus has a muscularis externa comprised entirely of smooth muscle
Esophageal adventitia/serosa: The esophagus displays an external lining comprised mainly of adventitia, except that portion which extends through the diaphragm. This distal portion (1 cm or so) of the esophagus, which enters the peritoneal space, is lined by a serosa..
What are the 4 regions of the stomach? Compare and contrast the 4 regions. How can they be differentiated? Which layers of the stomach are remarkable and unremarkable? What makes them remarkable?
Stomach: The entire stomach is a large
hollow organ that consists of four regions
(cardia, fundus, body, and pylorus). The
stomach plays a role in digestion,
motility, and food storage. The entire
stomach is lined at its luminal surface by a
columnar epithelium. Furthermore, this
epithelial surface displays indentations into
the underlying lamina propria called gastric
pits. The lamina propria of the stomach is
unremarkable except for the fact that it
contains three different kinds of glands,
depending on where the sample was taken
from. *Note: your ability to accurately identify these three types of glands will hinge on
your ability to appreciate the depth of the gastric pits, the overall shape of the glands, and
to identify whether or not the type of secretion is mainly serous or mucus. The muscularis
mucosa, submucosa, and serosa are all unremarkable. The muscularis externa, however,
is different in that it is comprised of three layers of smooth muscle: an inner oblique,
middle circular, and outer longitudinal. The circular layers form sphincters proximally
(cardiac sphincter) and distally (pyloric sphincter) to regulate the passage of food or
chyme.
1) Cardiac glands are found in a discrete area where the esophagus meets the stomach. They have gastric pits which extend 1⁄4 to 1⁄2 of the way into the mucosa, they are highly coiled glands, and the main secretory product is mucus. They stain uniformly with H and E stains.
2) Gastric glands are found in the fundus and the body of the stomach. They have gastric pits that extend only one-fourth of the way into the mucosa, they are straight glands, and the major secretion is serous. They also stain
heterogeneously with H and E stains (Ross figure 16.8, page 483) (Ross figure 17.8, page 523).
3) Pyloric glands are found in the pylorus (distal stomach). They have the longest gastric pits, extending one-half the width or more of the mucosa. These glands are coiled
and the major secretory product is mucus.These glands stain uniformly with H and E stains. (Ross figure 16.15, page 490) (Ross figure 17.5, page 480).
Describe 6 different cell types in the gastric mucosa.
1) Surface Mucous Cells: The entire stomach lining including the gastric pits is comprised of surface mucous cells. These columnar cells are tall with a single basal nucleus and an abundance of mucin granules in the apical cytoplasm. They release mucus and bicarbonate into the gastric lumen. They play a critical role in modulating gastric mucosal defense (particularly against the back diffusion of acid into the mucosa).
2) Mucous Neck Cells: These simple columnar cells lie below the gastric pits. They, too, secrete mucus. This mucus is more acidic, however, than that secreted
by surface mucous cells.
3) Undifferentiated Cells: These stem cells also lie just below the gastric pits and are thought to represent progenitor cells that migrate up and down to replace
senescent or injured cells (precursor to all other gastric cells). These cannot easily be identified with standard H and E stains.
4) Parietal Cells: These large spherical to ovoid cells are found located high up in the gastric gland below the base gastric pit. They are uni or binucleate and stain intensely eosinophilic with H and E stains owing to an abundance of mitochondria found throughout their cytosol These cells undergo impressive morphologic changes from the resting state to the secretory state. In particular, cytoplasmic tubulovesicles fuse to form a secretory canaliculus. Upon stimulation (by acetylcholine, gastrin, or histamine), up to one million ion pumps are translocated
from cytosolic positions to occupy respective sites on the cell surface. These cells secrete hydrochloric acid and intrinsic factor (required for the absorption of vitamin B12 from the ileum).
5) Chief Cells: Are polygonal to round cells that are found mainly in the base of the gastric glands. These cells stain basophilic with standard H and E stains, owing to an abundance of rough endoplasmic reticulum found in the basal component of these cells. Chief cells secrete the pre enzyme pepsinogen that are housed in secretory granules. In an acidic environment, pepsinogen is cleaved to become pepsin, the active form of this proteolytic enzyme.
6) APUD Cells: See the endocrine lectures for more details on this cell type, but recall that they do not stain with H and E stains.
What is the SI like grossly?
Small Intestine: The human small intestine is approximately five meters long and joins the distal portion of the stomach with the cecum. On its most luminal aspect it contains macroscopic folds referred to as the plicae circulares. It can be divided into three areas:
duodenum, jejunum, and ileum.
Describe the mucosa, submucosa, muscularis externa and adventitia of the SI.
Mucosa: The mucosa of the entire small intestine is thrown into finger like extensions called villi that are bordered laterally by invaginations into the lamina propria, called the crypts of Lieberkuhn (Ross figure 16.19, page 494) (Ross figure 17.19, page 537). The underlying lamina propria is comprised of loose areolar connective tissue which is highly cellular. Smooth muscle, nerves, lymphatics (lacteals), blood vessels, and numerous inflammatory cells (eosinophils, neutrophils, lymphocytes, macrophages, and plasma cells) are normally found in this connective tissue core. The muscularis mucosa lies just below the lamina propria and is comprised of inner circular and outer longitudinal layers. These layers cannot be resolved easily with light microscopy.
Submucosa: The submucosa of the small intestine remains somewhat unremarkable compared to other portions of the gastrointestinal tract. The exception lies in the proximal one-third of the duodenum where Brunner’s glands can be seen. These highly coiled glands secrete alkaline mucus through a system of ducts to help neutralize the acidic chyme entering the proximal intestine from the stomach.
Muscularis Externa (propria): The muscularis externa of the small intestine is comprised of inner circular and outer longitudinal layers of smooth muscle. Located between these layers are well developed parasympathetic ganglia referred to as “Auerbach’s plexus.” Contraction of these layers regulates peristalsis.
Serosa: The entire small intestine is lined by serosa on its antimesenteric border
Describe five different cell types found in the SI?
1) Enterocyte: This columnar cell is found throughout the
mucosa of the small intestine. These cells display a single
nucleus in the base of the cell and prominent microvilli on
their apical aspect. The microvilli are covered by a
glycocalyx and function by increasing cell surface area by
25%. A supranuclear golgi apparatus is apparent and
abundant rough and smooth endoplasmic reticulum is a
common finding. The cells can display interdigitations on
their lateral borders and the cells are joined by tight
junctions and desmosomes at their apical surfaces. The
major function of this cell type is terminal digestion of
carbohydrates and protein.
2) Goblet Cells: A second cell type located throughout the small intestine is the goblet cell. As a rule of thumb, these cells become more dense in the distal small intestine and the entire large intestine. These cells display an “hourglass” shape, a basal nucleus and a large pale staining supranuclear region. These cells release mucin by exocytosis to help lubricate the gastrointestinal
tract (Ross figure 16.23, page 497) (Ross figure 17.23, page 541).
3) Undifferentiated Cells: Undifferentiated cells (stem cells) of the small intestine are found towards the base of the crypts of Lieberkuhn They are columnar but otherwise unremarkable. They replace senesce and injured cells through migration and observed differentiation.
4) Paneth Cells: These cells are led at the very base of the crypts of Lieberkuhn and are readily identifiable by their intense staining with H and E stains. These very large cells have a single nucleus, well developed rough endoplasm reticulum and numerous large secretary granules in the apical cytoplasm. These granules contain lysozyme, a substance which helps to degrade the cell walls of certain types of bacteria (Ross figure 16.24, page
498) (Ross figure 17.24, page 542).
5) APUD Cells: A variety of APUD cells are found in the mucosa of the gastrointestinal tract. These are unicellular endocrine glands, which regulate a variety of functions of the gastrointestinal tract. As a rule, APUD cells do not stain with H and E stains.
What are Peyer’s patches? How can the 3 sections of the SI be differentiated?
Duodenum: Presence of Brunner’s glands, fewer goblet cells, and no “Peyer’s Patches.”(Ross et.al, figure 16.27, page 500) (Ross figure 17.27, page 544).
Jejunum: No Brunner’s glands, more goblet cells, and no “Peyer’s Patches.”
Ileum: No Brunner’s glands, even more goblet cells, and has “Peter’s Patches.”
Peyer’s Patches are large aggregates of confluent lymphatic nodules found usually in the ileal mucosa, but sometimes found extending into the submucosa. They are comprised of groups of lymphocytes, plasma cells, and macrophages. Overlying the Peyer’s Patch at its luminal surface the epithelial lining of the mucosa is comprised of specialized antigen presenting cells called “M” cells. These special cells transport luminal antigen to the underlying lymphocytes. M cells are difficult to observe with H and E preparations.
What is the large intestine like grossly? What is its function? What are the layers like?
Large Intestine (Colon): Is comprised of the cecum, ascending, descending, and transverse colon, as well as the sigmoid colon and the rectum. The function of the colon is to make the final adjustments in tonicity of fecal material by absorbing salt and water.
Large Intestine Mucosa: In contrast to the small intestine, the large intestine lacks villi. In comparison to the small intestine the large intestine does exhibit well-developed crypts which extend down to the level of the muscularis mucosa. APUD cells may be found in the colon but their density is considerably less than that found in the small intestine. The lamina propria is similar to that seen in the small intestine. The mucosal lining is comprised of simple columnar epithelial cells with an abundance of goblet cells. The columnar cells of the colon serve an absorptive capacity and goblet cells secretions serve to lubricate the colon. The number of goblet cells detected in the large intestine increases towards the rectum. The muscularis mucosa of the colon is unremarkable Paneth cells are
not found in the large intestine (Ross figure 16.33, page 505)(Ross figure 17.33, page 548).
Large Intestine Submucosa: The submucosa of the large intestine is unremarkable, but may contain isolated lymphatic nodules.
Large Intestine Muscularis Externa: The muscularis externa of the colon is unique in that it is limited to a well developed inner circular layer and three distinct bands of
muscle that make up the longitudinal layer. These three longitudinal bands are called taeniae coli in gross anatomy. Contraction of these bands facilitates excretion of feces.
Large Intestine Serosa: The large intestine is lined on its external surface by a serosa
Describe the gross and microscopic histology of the appendix.
Appendix: The appendix is a
small finger-like blind ended
pouch extending from the wall
of the cecum. It resembles the
large intestine, having a colonic
epithelium but lacks taeniae
coli. Fecal material is often
found in the lumen of the
appendix, even after sectioning.
Numerous (isolated and
confluent) lymphatic nodules
are found throughout the
mucosa – and submucosa. The
overall impression of the
appendix is that it is a tissue in
a chronic state of inflammation
(Ross figure 16.35, page 506)
(Ross figure 17.35, page 550).
Describe the layers of the anal canal.
Mucosa: Represents the most distal portion of the gastrointestinal tract. is 3 4 cm long and lined at its luminal aspect by either a columnar epithelium at its origin towards the rectum: or stratified squamous epithelium, which is nonkeratinized moving distally
toward the proximal anus. Finally, at the distal anus the canal is lined by a stratified squamous epithelium which is keratinized. Specialized glands may be found in the
lamina propria. These are referred to as circumanal glands and represent specialized specialized apocrine sweat glands. Hair follicles and sebaceous glands are also associated with the anus. The lamina propria is
comprised of fibroelastic connective tissue. The muscularis mucosa is unremarkable.
Submucosa: The anal submucosa is noteworthy because of the presence of the internal and external hemorrhoidal plexuses.
Muscularis Externa: The mucosa consists of inner circular and outer longitudinal layers of smooth muscle. The inner circular layer will form the internal anal sphincter. Skeletal muscle from the floor of the pelvis forms the external anal sphincter.
