Histology and Organ Structure Flashcards
Other than O2 entering through the lungs, where do all the other atoms entering the blood stream come from?
- The digestive system
- Cells use as building blocks
- Nitrogen, carbon, hydrogen, oxygen, sulfur, iodine, calcium, etc
What are the building blocks of proteins, carbohydratees and fats that the digestive system needs to pull out selectively?
- Proteins - amino acids
- Carbohydrates - monosaccharides
- Fats - partially digested lipids like fatty acids and monoglycerides
What are the sphincters in place to prevent backward movement and to facilitate peristalsis?
- Upper esophageal
- Lower esophageal pyloric
- Ileocecal
- Internal and external anal sphincters
The esophagus and anus are unique in the digestive tube for what reason?
- They are not really covered by mucus.
- The mucosae is a tissue type that has a layer of mucus at the surface for protective purposes
- Also helps the digesting normal flora live as well
What are the four definable layers of the digestive tube?
- Mucosa
- Submucosa
- Muscularis externa
- Serosa/adventitia
What is the muscularis externa?
- Comprised of an inner circularly and outer longitudinally arranged layers of smooth muscle and nerve plexes
- Its main function is peristalsis and churning of the lumenal contents
What is the submucosa?
- Contains connective tissue that is usually more dense than in the mucosa, larger blood vessels, nerve plexes, glands and lymphatic nodules
- Lymphoid cells of various types are also scattered throughout
What is the mucosa?
- Comprised of an epithelial layer plus the underlying loose, well-vascularized connective tissue called the lamina propria
- Thin layer of smooth muscle underlying this is referred to as the muscularis mucosae
- Lymphocytes, plasma cells and macrophages will be scattered throughout the lamina propria
• Mucosa. The mucosa is made up of three components: the epithelium , a supporting lamina propria and a thin smooth muscle layer, the muscularis mucosae , which produces local movement and folding of the mucosa. At four points along the tract, the mucosa undergoes abrupt transition from one form to another: the gastro-oesophageal junction, the gastroduodenal junction, the ileocaecal junction and the rectoanal junction.
What is the serosa/adventitia?
- Serosa is an outer covering of squamous epithelial cells separated from the underlying muscular layers by a relatively thin layer of connective tissue
- It is called the adventitia in the case of the esophagus above the diaphragm where the outer squamous layer is absent
• Adventitia. This outer layer of loose supporting tissue conducts the major vessels, nerves and contains variable adipose tissue. Where the gut lies within the abdominal cavity ( peritoneal cavity ), the adventitia is referred to as the serosa ( visceral peritoneum ) and is lined by a simple squamous epithelium ( mesothelium ). Elsewhere, the adventitial layer merges with retroperitoneal tissues.
Describe the organization of the esophagus
- Muscular tube that is 2/3 1/3 split in the type of muscles that control the movements
- Top 2/3 is under skelatal muscle control mostly with a mix of smooth muscle as well
- Bottom 1/3 is all smooth muscle
- No digestion happens here, it’s a muscular tube designed to transport food from the mouth to the stomach, thus it is missing the cornified epithelial digestive layer
- The valve or sphincter to the stomach is not a true or complete one, but made up of tonic muscular contractions
- Heartburn is gastric reflux through this partial sphincter which can be due to alterations in smooth muscle tone.
How is the stomach histologically divided?
• Three histologically distinguishable zones
• Cardia, fundus and pyloris
• Cardia
○ Small area of mainly mucus-secreting glands around the entry of the esophagus
• Fundus
○ Main body of the stomach that secretes acid, peptic digestive products and mucus
• Pyloris
○ Secretes mainly mucus and has a relative preponderance of endocrine cells that secrete the stomach hormone gastrin
What is special about the wall of the stomach?
- Contains substantial longitudinal folds called rugae or plicae mucosae that disappear upon distension
- Outer muscularis externa differs from the basic pattern in that a third oblique layer of smooth muscle is present just lumenally to the circular muscular layer
The stomach differs from the rest of the GI in the architecture of it’s mucosa. How?
- Facing the open cavity of the stomach are surface mucous-secreting cells
- These are arranged in a myriad of folds along with an underlying lamina propria
- Between the epithelial folds are spaces or gastric pits
- Beginning at the bottom of the mucous secreting pits the epithelium changes and continues deep into the mucosa as one or more tubular gastric glands
- The gastric glands contain a number of differentiated epithelial cells that are crucial for the unique funciton of the stomach: to begin the digestion of food at an acidic pH
- The lumen of the stomach essentially extends to the very bottom of the lumen of the gastric glands, although these glands secrete components outwards
- Humans have 3-4 million of these
How often are surface cells replaced in the gastric epithelium?
- Every 3-5 days.
- Main point here is that they are super rapidly dividing and constantly replenishing themselves
- Such a high stem cell reservoir as well as a high mitotic rate will make these cells super susceptible to certain mitotically inhibitory drugs
The stomach’s stem cells have two different routes they can take upon differentiation. What are these?
- They can divide and rise upward to the mucous secreting cells
- OR they can migrate downward to the various specialized cells in the gastric glands
What do the surface mucous cells do?
- They contain large vesicles full of stomach mucins and bicarbonate
- Discharge these vesicles locally onto the surface to provide a viscous protective layer that shelters the epithelial cells against stomach acid and abrasion from churning chyme
- Directly covering the short microvilli of the these surface cells is a more structured layer of cell surface glycoproteins, the glycocalyx
What are the chief cells and what do they do?
- These cells are involved in the beginning of digestion and play less of a role in GI protection
- Chief cells are typical protein secretors with apical granules and an elaborate basal RER
- The primary secretory product is pepsinogen which gets converted to an active protease, pepsin, in the presence of acid
- Pepsin is a protease that functions optimally at low pH
- Chief (or peptic) cells can divide or be derived directly from stem cells
What are the different layers of a gastric pit?
• Pit
○ Contains the surface mucous cells
• Isthmus
○ Cell architecture is mixed
• Neck
○ Mucous neck cells, which make a different mix of protective proteins?
○ Parietal cells start popping up here
• Fundus
○ Not to be confused with the gross fundus of the stomach as an entire organ
○ Parietal cells and chief cells, with chief cells more represented the deeper you go
What do the parietal cells do?
• Unique acid-producing cells specialized to pumping H+ ions into the lumen of the gastric glands against a pretty steep gradient
• Does this using a H+/K+ - ATPase
• The pH of the gastric juice thus secreted is 1-1.5, so production of acid is a vigorous, energy demanding activity
• These cells have an extensive set of microvilli bordering canaliculi that extend well into the cell so they have an enormous surface area for pumping protons into the lumen
• They also contain a large number of mitochondria, characteristic of cells partaking in major transport functions
• Parietal cells are stimulated to produce acid by secretion of the local hormone gastrin and histamine
• Parietal cells also secrete an important glycoprotein required for uptake of vitamin B12 - intrinsic factor
○ Super important for RBC production in bone marrow
What is up with Zollinger-Ellison syndrome?
- Excessive secretion of gastrin results in overproduction of HCl by parietal cells
- It cannot be adequately neutralized in the small intestine and leads to duodenal ulcers and complications
What are the enteroendocrine cells and what do they do?
• There are many flavors of these cells through the GI tract
• Members of a widely distributed population collectively referred to as APUD cells
○ Amine precursor uptake decarboxylation
• Many of these cells are in the GI tract, but they do not have to be
• STOMACH -
○ G cells that secrete gastrin (pyloris mostly)
○ A-cells that secrete glucagon
○ EC-cells that secrete serotonin
○ D-cells that secrete somatostatin (everywhere but middle portion of stomach)
• Cells are oriented towards vascular side, not lumenal side
What do the A cells secrete and where are they?
• Cell ○ A cell • Location ○ Stomach and small intestine • Hormone produced ○ glucagon • Hormone action ○ Stimulates glycogenolysis by hepatocytes, thus elevating blood glucose levels
What do the D cells secrete and where are they?
• Cell ○ D cell • Location ○ Stomach, small and large intestine • Hormone produced ○ somatostatin • Hormone action ○ Inhibits release of hormones by enteroendocrine cells in the vicinity
What do the EC cells secrete and where are they?
• Cell ○ EC cell • Location ○ Stomach, small and large intestines • Hormone produced ○ Serotonin ○ Substance P • Hormone action ○ Increases peristaltic movement
