Lab Session 22: Digestive Organs Histology & Anatomy of Organs Along the GI-Tract

Digestive System

Form is highly adapted to function

• anatomy has

  • physical forms → mechanically digest and break down food

  • secretory cells with enzymes → chemically digest molecules

  • large surface area → maximize physiology of absorbing nutrients

General Position of Digestive Organs in the Body

• mesenteries: double layer of peritoneum

  • route blood vessels, lymphatics, nerves

  • hold/support organs in their place

  • store fat, cushioning

• intraperitoneal: most organs in digestive system are suspended from the body wall by dorsal mesenteries, but some are held by ventral mesenteries

• retroperitoneal: organs that have lost their mesenteries during development

Digestive tract

• passageway for blood

• direct digestion & absorption

“The Cephalic Phase”

Food processing that occurs around the oral cavity

• ingestion: the act of bringing food into the oral cavity, i.e. mouth

• mechanical digestion: teeth used to break food into smaller pieces, while tongue shapes the food into a softer bolus

• chemical digestion: saliva contains salivary amylase enzyme → breaks bonds in carbohydrates

Oral Cavity, i.e. Mouth

• walls of the mouth are lined with thick stratified squamous epithelia

  • protects against friction & chemicals

  • reinforced with keratin

• lips/cheeks help keep food in the proper location during ingestion and mechanical digestion

• oral vestibules with gingivae (gums) hold teeth

Palate is divided to perform multiple functions…

• palate → roof of the mouth

  • hard palate: provides a rigid surface for the tongue to push against when chewing

    • ridges → create friction

  • soft palate: more mobile fold that rises to block the nasopharynx when swallowing

    • palatine tonsil/uvula at tip

Tongue is very important to multiple functions…

• tongue: made of skeletal muscle of two types to perform:

  • mechanical digestion of the bolus

  • mixing saliva into the bolus

  • propulsion (swallowing / moving food forward

  • formation of consonant sounds

• intrinsic muscles: not attached to bone, allow tongue to shape

  • speech and swallowing

• extrinsic muscles: attached to bone, alter the position of tongue

Digestive Tract has different Organs, but they all share GENERAL anatomical 4-layer structure

• esophagus → anal canal, the organs of the GI-Tract have the same general 4-layer structure

  • mucosa

  • submucosa

  • muscularis externa

  • serosa

• mucosa: innermost to lumen

  • functions:

    1. secretion: mucus (lubrication), enzymes (digestion), hormones

    2. absorption of nutrients and transfer to bloodstream

    3. protection against pathogens

  • lamina propria: has connective tissue → gives mucosa its shape and stem cells → regenerate tissues

  • muscularis mucosae: an additional thin layer muscle layer, presented to propel mucus → help lubrication and protection

    • form boundary of mucosa

    • contract

• submucosa: surrounds mucosa

  • large amount of connective tissue → help organs distend and regain shape (stomach)

  • provides a path blood vessels, lymph vessels, and nerve fibers (enteric nerve) to move through

• muscularis externa: smooth muscle

  • peristalsis → propulsion of bolus

  • segmentation → mechanical digest

  • has 2 layers

    1. circular layer → contracts (squeeze inward) & lumen’s circumference

    2. longitudinal layer → contracts and shorten’s the tract’s length

• serosa: outermost layer

  • i.e., the visceral peritoneum that lines system along body cavity

  • connective tissue → covered by a single layer of squamous epithelia

Esophagus

Single peristalsis wave moves down to stomach

Propulsion

Throughout GI-Tract, depends on smooth muscle

• longitudinal layer: runs parallel to the lumen → contraction will shorten tube

• circular layer: line circumference the lumen, so contraction will constrict cross-section

• alternating of contraction of these layers will squeeze bolus through the pathway

  • longitude + circular = peristalis

  • circular only = segmentation

“Cephalic Phase” ends with propulsive swallowing

• propulsion (i.e. motility): major process of the digestive system, first occurs during act of swallowing

• swallowing (deglutition) involves tongue, pharynx, esophagus

• tongue/mouth = voluntary skeletal muscle to initiate the process

• pharynx/esophagus = involuntary smooth muscle to move bolus into the stomach

Stomach

Storage and digestion

• storage location for food to be digested

• protein digestion: primary chemical digestion role

  • also produces lipases → fat breakdown

• performs large amount of mechanical digestion → turns the food into chyme

Modification of 4-layers

• muscularis: circular and longitudinal layers of smooth muscle, also has third innermost layer of smooth muscle

• oblique layer: adds “pummel” to motion that smooth muscle can do → stomach performs large mechanical digestion

• mucosa = modified for the digestive roles of the stomach with gastric glands

  • gastric glands entry = gastric pit

Myenteric Plexus:

Neuron Ganglia for the Enteric Nervous System

• myenteric plexus: hard to see layer between the circular layer and longitudinal layers present in the muscularis externa

• location of neuronal ganglia from neurons of the enteric nervous system → regulates digestion and sends feedback signals

Pacemaker cells are located in the Myenteric Plexus

• interstitial cells of Cajal (ICC): neurons from enteric nervous sytem (ENS) that act as pacemakers

  • unstable membrane potential

  • auto-depolarize to generate rhythm

  • in myenteric plexus between muscle

• connected to smooth muscle cells by gap junctions → electrical impulse is relayed directly through cells

• even though they are “auto-rhythmic,” still innervated by ENS / ANS at varicosities

Stomach Physiology

Storage and DIgestion

• gastric juice contains

  • hydrochloric acid → breakup the ECM, holds cells together, denatures proteins

  • pepsin → once the larger food/cellular components are exposed, the pepsin enzymes digests proteins into amino acids

• gastric gland in the stomach has different cell types:

  • parietal cells → secrete H+ and Cl- separately & intrinsic factor

    • more light color

    • less parietal cells the deeper into the glands

  • chief cells → secrete pepsinogen → pepsin when mixed with HCl (low-pH) & lipase

    • take up stain because they have more secretory granules

    • more chief cells the deeper into the glands

  • mucous cells → secrete mucus to protect lining from gastric juice

  • enteroendocrine cells → secrete a variety of hormones & gastrin

Small Intestine

Primary Site for Digestion and Absorption

• 3 structural divisions

  1. duodenum (first part / entry)

  2. jejunum (highest absorption)

  3. ileum (vitamins, salts / exit)

• tremendous surface area → absorption of nutrients

• major organ of digestion and absorption

Digestion throughout the body

• stomach does a difficult job of digesting proteins → small intestine digests everything

Duodenum is an entry point for digestive enzymes

• most digestion occurs in duodenum, where numerous enzymes enter via ducts from many accessory organs:

  • pancreas: sends lipases, proteases, and nucleases → breakdown fats, proteins, and nucleic acids

  • liver: makes bile salts, which are first stored in the gallbladder (breakdown fats)

• also makes its own enzymes

Jejunum and ileum perform most of the absorption

• maximizes surface area for absorption → forms a brush border layer

• vili

  • projection into the lumen → increase surface area

• goblet cells

  • more light color, or cut out

  • produce mucus

• intestinal crypt

  • deep groove

  • increase surface area

• enterocytes

  • absorb material

  • have microscopic extensions called microvili

• larger extensions, called circular folds (plicae) in the small intestine

Lamina Propia (not submucosa) extends into vili

• lamina propia (i.e. mucosa) tissue connective/structural tissue of the vili, and not submucosa

• crypts generally extend down to muscularis mucosa, at the boundary with the submucosa

Blood and Lymph Vessels transport absorbed nutrients

• blood vessels: transport hydrophilic amino acids and sugars via capillaries

• lymph vessels: transport hydrophobic fats and glycerides via lacteals

Small intestine: more mucus secreted by glands

• intestinal glands: aka mucosal glands, secrete mucus from mucosa

• brunner’s glands: aka duodenal glands, secrete mucus from submucosa

Small intestine: secretes more than mucus / enzymes

• peyer’s patches: lymphatic tissue/vessels present in the submucosa of ileum, near the entry to large intestine → secretes antimicrobial agents to stop infection

• mucus associated lymphoid tissue

  • malt: immunity role