Histology of the Lower GI Tract Lecture (Test 1)

Question 1

Small Intestine: Structure and Function

Answer 1

1) Complete Chemical Digestion of food with the combination of Bile, Pancreatic Enzymes, and Intestinal BRUSH BORDER Enzymes
2) Neutralize ACIDIC Chyme from the Stomach with Bicarbonate Ions secreted by the Liver and Biliary Tree, the Pancreatic Duct, and the Intestinal Glands
3) Use Segmentation Contractions to MIX Chyme to facilitate final chemical digestion and Nutrient Absorption
4) Absorb the majority of Nutrients, Electrolytes, Vitamins and Minerals from the Chyme as well as the Water ingested with the foods and majority of the Water secreted into the GI Tract during digestive Gland Secretions
5) Secrete Mucous to LUBRICATE and PROTECT the Stomach and Intestinal Linings

Question 2

Small Intestine: Structure and Function Cont

Answer 2

6) Secrete CCK, VIP, and Secretin (Enteroendocrine Substances) to help REGULATE the Digestive Activities of the Stomach, Liver, Gall Bladder, Pancreas and Intestine
- Longest Part (~22 Feet) of the Gastrointestinal Tract
- Extends from the Pyloric Sphincter to the ILEOCECAL Valve and consists of the Duodenum, the Jejunum, and the Ileum. Wall consists of a Mucosa, Submucosa, a Muscular, and an Outer Serosa

Question 3

Chyme

Answer 3

Mechanical and Chemical Digestion

• Peristalsis mixes food with Gastric Secretions including HCL, Pepsinogen, Gastric Lipase, Intrinsic Factor to create CHYME
• Chyme is the Semifluid mass of partly digested food expelled by the Stomach into the Duodenum

Question 4

Layers of the Small Intestine

Answer 4

1) Serosa
2) Muscular
3) Submucosa
4) Muscular Mucosae

• An INCREASE in the Total Surface Area of the Mucosa reflects the Absorptive Function of the Small Intestine
• Abundance of VILLI in the Small Intestine to help INCREASE the Total Surface Area

Question 5

Four Degrees of Folding

Answer 5

1) PLICAE CIRCULARES (PC)
2) INTESTINAL VILLI (V)
3) INTESTINAL GLANDS (Crypts of Lieberkühn)
4) MICROVILLI on the Apical Surface of the Enterocytes

ENTEROCYTES:
- Epithelial cells lining the Lumen

Question 6

Place Circulares

Answer 6

• Permanent folds of Mucosa and Submucosa

- Begin in the Duodenum, DISTINCT in the JEJUNUM, disappear in the Mid-Ileum

Question 7

Intestinal Villi (Singularis Villi)

Answer 7

• Finger like projections of the Mucosa covering the entire surface of the Small Intestine
• INCREASE SURFACE AREA
• Villi extend DEEP into the Mucosa to form Crypts (of Liberkühn) ending at the MUSCULAR Mucosae
• The length of the Villi depends on the degree of DISTENTION of the Intestinal Wall and the CONTRACTION of Smooth Muscle Fibers in the Villus Core

Question 8

Intestinal Glands

Answer 8

• CRYPTS of Lieberkühn, or Intestinal Glands, are simple Tubular Glands that INCREASE the Intestinal Surface Area
• The CRYPTS are formed by INVAGINATIONS of the Mucosa between adjacent Intestinal Villi
• Cells of the Crypts include absorptive, Goblet, Paneth, and Enteroendocrine Cells

Question 9

Microvilli

Answer 9

• EVAGINATIONS of the Apical part of the ENTEROCYTES to INCREASE SURFACE AREA
• Produce a BRUSH BORDER on the Apical Border of ENTEROCYTES

Question 10

Submucosa and Muscular Externa

Answer 10

• MUSCULARIS MUCOSA is a boundary between Mucosa and Submucosa
• MUSCULARIS EXTERNA is responsible for SEGMENTATION ad PERISTALSIS
• SEROSA is a THIN Layer of Loose Connective Tissue by VISCERAL PERITONEUM (Mesothelium)

Question 11

Mesothelium

Answer 11

• A Membrane composed one SIMPLE SQUAMOUS cells the forms the LINING of Several Body Cavities

MESOTHELIOMA:
- An AGGRESSIVE form of Cancer affecting the Membrane lining the Pericardial, Pleural or Peritoneal Cavities

Ex: A little piece of Peritoneum in the Testes that can allow for Testicular Cancer to occur

Question 12

Segmentation and Peristalsis

Answer 12

SEGMENTATION:
- A pattern of ANNULALR Contraction of the Smooth Muscle Layers in the walls of the Small Intestine which temporarily seems to CUT the Regiona affected into INDIVIDUAL COMPARTMENTS

• This activity serves to MIX the CHYME within the Small Intestine
• It is controlled by the ANS!!!

Question 13

Microcirculation of the Small Intestine

Answer 13

• The Intestinal Submucosa is the main distribution site of Blood and Lymphatic Flow
• ATERIOLES derived from the SUBMUCOSAL PLEXUS enter the Mucosa of the Small Intestine and give rise to two Capillary Networks:
  1) The VILLUS CAPILLARY PLEXUS supplies the Intestinal Villus and UUPPER PORTION of the Crypts of Lieberkühn

2) The PERICRYPTAL CAPILLARY PLEXUS supplies the LOWER HALF of the Crypts of Lieberkühn

Question 14

Lacteals

Answer 14

• Lacteal within a VILLUS!!!!!!
• Lacteals convert CHYLE (Lympha contains Lipids absorbed from the meal and packaged in CHYLOMICRONS) from the Intestine to the Lymphatic Circulation and thereby to the Thoracic Duct and then to the Systemic Blood Circulation
• CHYLOMICRONS are Droplets of FAT preserved in the Blood or Lymph after being absorbed from the Small Intestine

Question 15

Innervation and Motility of the Small Intestine

Answer 15

• MOTILITY controlled by the ANS through Submucosal and MYENTERIC PLEXI
• INTRINSIC INPUT received from the Mucosa and Muscle Wall of the Small Intestine
• EXTRINSIC INPUT from the Central Nervous System through the PARASYMPATHETIC (Vagun Nerve) and SYMPATHETIC NERVE TRUNKS
  a) Foregut: Greater Splanchnic Nerve

b) Midgut: Lesser Splanchnic Nerve
c) Hindgut: Least Splanchnic Nerve
- CONTRACTION results in BOTH Segmentation and Peristalsis

Question 16

Duodenum

Answer 16

• BRUNNER’s GLANDS (Mucous) in Submucosa!!!!!!!
• Relatively FEW GOBLET Cells
• Leaf-like Villi

Question 17

Jejunum

Answer 17

• Well developed PLICAE CIRCULARES (Characteristic Feature)!!!!!!!!
• Irregular Villi
• More Goblet Cells
• NO Brunner’s Glands and NO Peyer’s Patches

Question 18

Ileum

Answer 18

• Many Lymphoid Nodules (PEYER’s PATCHES) in LP and Submucosa
• Finger like Villi
• Most Goblet Cells

Question 19

Villi and Crypts of Lieberkühn

Answer 19

• Lined by SIMPLE COLUMNAR EPITHELIUM with 4 Cell Types:
  1) ENTOEROCYTES, Absorptive cells

2) GOBLET CELLS
3) PANETH CELLS
4) ENTEROENDOCRINE CELLS
- Stem, Cells, Paneth Cells, and Enteroendocrine Cells are found IN THE CRYPTS OF LIEBERKÜHN!!!!!!

Question 20

Absorptive Cells/ Enterocytes

Answer 20

• Cells have Apical Domain with a PROMINENT BRUSH BORDER containing about 3000 CLOSELY PACKED MICROVILLI, which INCREASE the Surface Luminal Area 30 fold
• The Microvilli contain Intramembranous Enzymes, including LACTASE, MALTASE, and SUCRASE, for TERMINAL DIGESTION of CARBOHYDRATES!!!!!!
• These Oligosaccharides REDUCE Carbohydrates to Hexoses, which can be Transported into eh Enterocyte by CARRIER PROTEINS!!!!!!

Question 21

Goblet Cells

Answer 21

• COLUMNAR Mucus Secreting cells scattered among Enterocytes of the Intestinal Epithelium
• The Secretory Product of Goblet Cells otnain GLYCOPROTEINS (80% Carbohydrate and 20% Protein) release by EXOCYTOSIS!!!

*** MUCUS HYDRATES to form a PROTECTIVE GEL COAT to SHIELD the Epithelium from MECHANICAL ABRASION and BACTERIAL INVASION!!!

Question 22

Enteroendocrine Cells

Answer 22

• Inanition to its Digestive Function, the GI Tract is the LARGEST DIFFUSE Endocrine Gland in the Body
• As in the Stomach, these Cells SECRETE PEPTIDE Hormones (Gastrin, Secretin, and CCK) controlling several functions of the Gastrointestinal System
  1) GASTRIN: Stimulates Gastric MOTILITY, HCL (Parietal Cells) and INSULIN
  2) SECRETIN: Stimulates Pancreatic BICARBONATE Secretion and Enhances INSULIN Secretion
  3) CCK: Acts on PYLORIC Sphincter to SLOW Emptying, Stimulates release of Bile and Pancreatic Enzymes

Question 23

Panted Cells

Answer 23

• PANETH Cells secrete ANTIMICROBIAL proteins to Limit Bacteria- Enterocyte contact
• Most of these proteins kill Bacteria DIRECTLY by Enzymatic Degradation of the Bacterial Wall or by Disrupting the Bacterial Inner Membrane
• Antimicrobial Proteins are retained in the Intestinal Mucus Blanket

Question 24

Protection of Small Intestine

Answer 24

The Small Intestine is PROTECTED from pathogens by:
1) A VISCOUS Gel-like Blanket produced by Goblet Cells

2) An intestinal Tight Junction barrier linking Adjacent ENTEROCYTES
3) PEYER’s PATCHES of the Intestinal Epithelium, participating in the CELLULAR Surveillance of Antigens
4) IMMUNOGLOBULIN A (IgA), a product of Plasma Cells secreted by the Intestinal Epithelium and in the Bile, Neurtalizes Antigens
5) The Inactivation of Microbial Pathogens by Antimicrobial Proteins, products of PANETH CELLS
6) The Acidity of the Gastric Juice INACTIVATES Ingested Microorganism
7) The PROPULSIVE Intestinal Motility (Peristalsis) prevents Bacterial Colonization

Question 25

Clinical Significane: Inflammatory Bowel Disease

Answer 25

- A defect in the Protective System accounts for Inflammatory Bowel Diseases, CROHN's DISEASE (involving the Terminal Ileum but also observed in the Large Intestine) - The initial alteration of the Intestinal Mucosa consists in the INFILTRATION of NEUTROPHILS into the Crypts of Lieberkühn - This process results in the destruction of the Intestinal Glands by the formation of CRYPT ABSCESSES and the Progressive ATROPHY and ULCERATION of the Mucosa - Inflammatory process all INFILTRATES the Submucosa and Muscular - Accumulation of Lymphocytes forms AGGREGATES of Cells, or GRANULOMAS - Major complications of the Disease are OCCLUSION of the Intestinal Lumen by FIBROSIS and the formation of FISTULAS in other Segments of the Small Intestine - Segments affected by CROHN's DISEASE are Separated by NORMAL Stretches of Intestinal Segments

Question 26

Fecal Microbiota Transplant

Answer 26

- Procedure in which Fecal Matter is collected from a Tested Donor, mixed with a SALINE or other Solution, Strained, and placed in a patient, by Colonoscopy, Endoscopy, Sigmoidoscopy, or Enema - The purpose of Fecal Transplant is to replace good bacteria that has been Killed or Suppressed, usually by the use of Antibiotics, causing bad bacteria, especially CLOSTRIDIUM DIFFICILE to overpopulate the Colon - This infection causes a condition called C. diff COLITIS, resulting in often debilitating, sometimes FATAL DIARRHEA

Question 27

Large Intestine

Answer 27

- A Major Function of Enterocytes in the Large Intestine is the TRANSPORT of IONS and WATER

Question 28

Large Intestine (Colon) Cont 1

Answer 28

- The Large Intestine is formed by several successive segments: 1) Cecum and Appendix 2) Ascending, Transverse, and Descending Colon 3) Sigmoid Colon 4) Rectum 5) Anus - The mucosa of the Large Intestine is lined by a SIMPLE COLUMNAR Epithelium formed by Enterocytes and Abundant GOBLET CELLS - Enterocytes have SHORT APICAL MICROVILLI - A Major Function of Enterocytes in the Large Intestine is the transport of Ions and Water

Question 29

Large Intestine Cont 2

Answer 29

- Plicae Circulares and Intestinal Villi ARE NOT FOUND beyond the Ileocecal Valve - Numerous opening so the Straight Tubular Glands or Crypts of Lieberkühn are characteristic of the mucosa of the Colon - Secretory products of Goblet Cells LUBRICATE the Mucosal Surface ***MAIN FUNCTION: To absorb Water, Sodium, Vitamins, and Minerals ***ACTIVE TRANSPORT of Sodium removes Water and Concentrates Chyme into SEMISOLID FECES

Question 30

Mucosa of Large Intstine

Answer 30

Four Cell types present in the Surface Epithelium and Tubular Glands: 1) Simple Columnar Absorptive Cells with Apical Microvilli 2) Predominant Goblet Cells 3) Stem Cells 4) Enteroendocrine Cells

Question 31

Enterocytes

Answer 31

- A Surface Simple Columnar Epithelium formed by ABSORPTIVE Enterocytes and Goblet Cells - Enterocytes have SHORT APICAL Microvilli, and the cells participate in the Transport of IONS and WATER - All regions of the Colon Absorb Na+ and Cl- ions facilitated by Plasma Membrane Channels that are regulated by MINERALOCORTICOIDS - ALDOSTERONE Increases the number of Na+ Channels and Increased the Absorption of Na+. Na+ Ions entering the Absorptive Enterocytes are EXTRUDED by an Na+ PUMP!!!!! - GOBLET CELLS secrete Mucus to LUBRICATE the Mucosal Surface and serve as a PROTECTIVE BARRIER

Question 32

Glands of Lieberkühn

Answer 32

- Contain Goblet, Enteroendocrine Cells and Stem Cells | - PANETH Cells are not Obsered (May be present in Cecum)

Question 33

Tania Coli

Answer 33

- A Characteristic feature of the Large Intestine, formed by FUSED BUNDLES of the Outer Smooth Muscle Layer - CONTRACTION of the TANIA COLI and the INNER CIRCULAR SMOOTH MUSCLE layer produces PERIODIC SACCULAR Structures called HAUSTRA!!!!!

Question 34

Appendix

Answer 34

- A Small blind- ending DIVERTICULUM form the Cecum - The most important features of the Appendix is the THICKENING of ITS WALL, which is mainly due to LARGE ACCUMULATIONS of LYMPHOID TISSUE in the LAMINA PROPRIA and SUBMUCOSA - Intestinal Villi are usually Absent, and Crypts do not occur as frequently as in the Colon - There is often FATTY TISSUE in the Submucosa - The Muscular Externa is thinner than in the remainder of the Large Intestine and the outer, Longitudinal Smooth Muscle Layer of the Muscular Externa does NOT AGGREGATE INTO TAENIA COLI!!!!! ***Researchers now say that the Appendix acts as a Safe House for Good Bacteria. The Body uses this to essentially "Reboot" the Digestive System when one suffers from a bout of DYSENTERY or CHOLERA

Question 35

Rectum

Answer 35

- The Rectum, the Terminal portion of the Large Intestine and a continuation of the Sigmoid Colon, consists of two regions: 1) The Upper Region, or RECTUM PROPER 2) The Lower Region or ANAL CANAL, which extends from the Anorectal Junction to the Anus - This part has KERATINIZED STRATIFIED SQUAMOUS EPITHELIUM

Question 36

Ano-Rectal Junction

Answer 36

- Simple Columnar Epithelium of the Rectal Mucosa is replaced by a STRATIFIED SQUAMOUS EPITHELIUM (Epithelial Transformation Zone or PECTINATE LINE), and the Inner Circular Layer of Smooth Muscle Thickens to form the INTERNAL ANAL SPHINCTER - Beyond this region, the Anal Mucosa is lined by a KERATINIZING STRATIFIED SQUAMOUS EPITHELIUM and the Submucosa contains SEBACEOUS and SWEAT GLANDS. The EXTERNAL ANAL SPHINCTER

Question 37

Clinical Consideration: Colorectal Tumors

Answer 37

- Develop from a POLYP, a Tumoral Mass that protrudes into the Lumen of the Intestine. Some Polyps are Non-neoplastic and are relatively common in persons 60 years and older - POLYPS can be present in Large Number (100 or more) in FAMILIAL POLYPOSIS SYNDROMES such as FAMILIAL ADENOMATOUS POLYPOSIS - FAMILAIL POLYPOSIS and COLORECTAL TUMORIGENESIS is determined by a DEFECT in the protein ADENOMATOUS POLYPOSIS COLI (APC) Gene - Excess of this protein, ACTIVATES genes leading to COLORECTAL TUMORIGENESIS - Spread of Cancer into he Lymphatic Vessels can follow the vessels that supply the parts of the Colon with the Cancer Ex: Right Colic Flexure Cancer ---> Superior Mesenteric Lymph Node Ex: Sigmoid Colon ----> Inferior Mesenteric Lymph Node