Dr. Randall E. Merchant [email protected]

DIGESTIVE SYSTEM & GLANDS

Objectives

1. Describe the four basic layers that form the walls of the GI tract and know how they vary from one organ to another along the tract.

2. Identify the structures on the CD listed in the laboratory guide.

3. Describe the appearance and be able to recognize the following junctions: esophagus- stomach, stomach-small intestine, small intestine-large intestine, rectum-anal canal, and anal canal-anus.

4. Differentiate the three different types of mucosal glands in the stomach and describe the morphology and function of their component cells.

5. Characterize the differences between the duodenum and the remainder of small intestine as well as the differences between the small and large intestine and appendix.

6. Understand why the pancreas is both an exocrine and endocrine gland. Trace the pathway of exocrine secretions from the acinar cells to the small intestine.

7. Describe the architecture of the hepatic sinusoid and explain the route of blood and bile flow through the liver.

8. Trace the usual route by which proteins, carbohydrates, and fats taken up by the small intestine reach the liver.

9. Understand the anatomic and functional criteria that define the three forms of hepatic lobulation. Be able to identify structures located in a portal canal.

I. General Description and Considerations Regarding the Tubular Digestive Tract

A. Paradox of similarity and Change:

1. All organs of the GI tract are modifications of a simple tube

2. Epithelium of the entire tract is derived from a single embryonic germ cell layer

3. All organs have some function related to digestion

4. All areas of the tract have basically the same histological layers

However,

5. Abrupt changes occur in the epithelium from one organ to the next

6. Drastic modifications of the mucosa occur from one organ to the next

7. Each organ along the tract performs a separate and unique function related to digestion

B. General description of layers common to all organs of the GI tract

1. Mucosa - three components a. epithelium - shape and number of layers dependent upon function of organ - functions include conduction, absorption and secretion - invaginations of epithelium form glands - avascular and sits on a basement membrane b. lamina propria - loose CT - composed of collagen and elastic fibers along with various CT cell types - occasional areas of diffuse and nodular lymphoid tissue - some areas contain varying numbers of exocrine glands - contains small blood vessels, nerves, and lymphatics c. muscularis mucosa - smooth muscle mostly running longitudinally - relative thickness greatest in esophagus - contraction helps empty glands of mucosa & helps transport material along the tube - innervated by post-ganglionic parasympathetic fibers originating from Meissner's plexus

General Plan of the Tubular Digestive Tract

2. Submucosa - classified as dense irregular CT a. coarser fiber bundles than in lamina propria b. large plexi of blood vessels and lymphatics, nerves, and Meissner's plexi (pre-ganglionic fibers from Vagus Nerve) c. contain glands in esophagus and duodenum 3. Muscularis externa - formed of smooth muscle except in upper esophagus a. varies in thickness and number of layers b. usually inner circular and outer longitudinal layers c. innervated by post-ganglionic parasympathetic fibers originating from Auerbach's (myenteric) plexus located between the two muscle layers (pre-ganglionic fibers from Vagus Nerve) d. functions - mainly in peristalsis - maintain tonus and propels food onward - circular layer controls caliber of lumen, also churns and propels food along - longitudinal muscle contraction shortens tube locally

4. Adventitia - dense irregular CT a. contains blood vessels, lymphatics, and nerve fibers b. when covered with visceral peritoneum called a serosa

II. Esophagus

A. General description and functions

1. Length, location, & relationships a. conveys food from pharynx to stomach b. about 10" long, all but final 1" intrathoracic

2. Innervation a. smooth muscle layers innervated by postganglionic parasympathetics from Meissner's and Auerbach's plexi b. vascular smooth muscle innervated by postganglionic sympathetics from cervical and thoracic sympathetic trunks c. skeletal muscle supplied by efferent fibers of Vagus Nerve

3. Functions a. completes warming or cooling of food b. conveys food by gravity and muscular contraction c. glands secrete mucus for lubrication

B. Mucosa

1. Non-keratinized stratified squamous epithelium

2. Lamina propria - contains esophageal cardiac glands (mucus secreting)

3. Muscularis mucosa - mostly longitudinally oriented a. occurs as discreet bundles in upper portion b. continuous and thickest in lower portion c. innervated by post-ganglionic fibers from Meisner’s plexus

C. Submucosa

1. Contributes to large longitudinal folds of esophagus - these smooth out during swallowing

2. Contains esophageal glands - compound tubuloalveolar, mucus secreting

D. Muscularis externa - determines level of histological section

1. Upper 1/3 - composed of almost all skeletal muscle - because initiation of swallowing is voluntary

2. Middle 1/3 - a mixture of smooth and skeletal muscle

3. Lower 1/3 - composed only of smooth muscle a. at junction with stomach this layer forms a physiologic sphincter that prevents regurgitation of the stomach contents b. peppermint, alcohol, and nicotine will relax the sphincter, help you feel less "full" c. reflux of stomach contents into this area causes feeling of heartburn

4. Smooth muscle innervated by post-ganglionic parasympathetics from Auerbach's plexus, skeletal muscle by spinal nerves

E. Adventitia

1. Intrathoracic portion blends with surrounding dense irregular CT

2. Below diaphragm covered with visceral peritoneum

III. Stomach

A. General description and functions

1. Role in digestion and storage a. acidifies and converts swallowed food into a viscous fluid called chyme b. mucosa produces digestive enzymes

2. Four gross anatomic vs. three histological divisions a. gross anatomic divisions - cardiac, fundic, body, and pyloric b. histological divisions - cardiac, fundic, and pyloric - based on gland structure - no sharp transitions between divisions

3. Rugae - large longitudinal folds formed by the mucosa and submucosa, most prominent when stomach empty

B. Mucosa

1. Surface a. esophageal-gastric junction - abrupt change from stratified squamous to simple columnar b. gastric pits - fine furrows, lead into glands c. simple columnar epithelium - forms sheet gland, all mucus secreting, mucus has a basic pH - continuous with epithelium of gastric pits - cells anchored to each other by cell junctions - secretions form protective barrier from strong acidity of stomach content - cells continuously sloughed off - survive only about 3 days

2. Gastric gland structure & nomenclature a. simple, branched tubular glands - occupy entire thickness of mucosa b. each gland has a neck, body, and base

3. Lamina propria a. occupies space between glands and pits b. richly vascularized and slips of smooth muscle from muscularis mucosa

4. Cardiac glands a. location - in cardiac region of stomach, near esophageal orifice b. length of gland approximately equal to depth of pit c. component cells - mostly mucus secreting cells with a few parietal and enteroendocrine cells d. primary function is to produce a protective mucus secretion

5. Fundic glands a. located in fundic and body regions of the stomach b. one or more open into each gastric pit, glands are much longer than their pits are deep c. are classified as simple branched tubular d. are closely packed and orientated perpendicular to surface e. are the major source of gastric juice

6. Cells of the fundic glands a. mucous neck cells - appear basophilic with H&E stain • relatively few, located in necks of glands • produce an acidic mucous secretion

b. parietal cells - appear acidophilic with H&E stain • the greater proportion of cells in upper part of glands • have a secretory canaliculus - increases surface area • contain an abundance of mitochondria (why cytoplasm eosinophilic) • secrete H+ and Cl- • secrete gastric intrinsic factor - binds to vitamin B12 and this allows it to be absorbed in small intestine, the vitamin is needed for proper RBC maturation - a lack of this factor (or vitamin) causes pernicious anemia

c. chief cells - appear basophilic with H&E stain • dominant cell type in the lower half of glands • have zymogen granules in their apical cytoplasm • secrete pepsinogen - converted to pepsin by low pH - important enzyme in the digestion of proteins

d. enteroendocrine cells - variable staining with H&E • a collection of at least 12 different cell types each of which secrete a specific endocrine hormones • found mostly in bases of glands • granules mostly on side of basal lamina • some produce gastrin - stimulates secretions of chief and parietal cells and increases gastric motility • some synthesize serotonin - triggers contraction of smooth muscle of blood vessels - leads to a slowing of gastric activity • secretions enter blood (endocrine) or affect adjacent cells (paracrine)

6. Pyloric glands a. located in distal 4-5 cm of stomach b. empty into wide, deep pits c. are simple, branched tubular which d. are very coiled e. composed of almost completely of mucus secreting cells with a few parietal cells and enteroendocrine cells

C. Accessory coats

1. Submucosa - dense irregular CT

2. Muscularis externa - inner oblique, middle circular, and outer longitudinal layers a. pyloric sphincter - formed by middle circular layer b. Auerbach's plexus between circular and longitudinal layers c. functions in mixing and emptying contents of stomach d. emptying of stomach due to peristalsis and intermittent relaxation of the pyloric sphincter

3. Serosa - loose CT covered with mesothelium (visceral peritoneum)