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^ Pharynx ^ Esophagus

Lamina Propria Stomach
figure 12.1. Organs comprising the tubular digestive tract.

^ Stomach ^ Small intestine ^ Large intestine

Basic Histological Organization ^ Layers

• Mucosa (mucous membrane). Innermost layer facing the lumen

♦ Epithelium. Either a stratified squamous moist or a simple columnar epithelium

♦ Lamina propria. Loose connective tissue; usually possesses digestive glands

♦ Muscularis mucosae of smooth muscle is usually present.

• Submucosa. Denser connective tissue than the lamina propria. The submucosa possesses Meissner's nerve plexus that supplies innervation to the muscularis mucosae and to digestive glands in the mucosa and submucosa. The submucosa possesses glands in the esophagus and duodenum.

• Muscularis externa of smooth muscle is usually arranged into inner circular and outer longitudinal layers. Auerbach's nerve plexus is located between the two muscle layers and provides innervation to this smooth muscle.

• Serosa (serous membrane) is present if the organ protrudes into the peritoneal cavity, or an adventitia (only the connective tissue portion of the serosa) is present if the organ is retroperitoneal.

Stomach Rugae
figure 12.2. Overview of the layers and components of the tubular digestive tract.

^ Glands

• Exocrine glands, aiding in digestion and/or lubrication, are located in:

♦ Epithelium (e.g., goblet cells throughout the intestines)

♦ Submucosa (e.g., Brunner's glands in the duodenum)

♦ Glands located external to the digestive tract that open into the system (e.g., liver and pancreas)

• Endocrine and paracrine cells, belonging to the diffuse neuroendocrine system (DNES), are located throughout the mucosa of the gastrointestinal tract, influencing the secretion of glands and the motility of the gut.

Variations That Distinguish Each Organ from the Basic Organizational Plan

^ Esophagus

• Epithelium. Stratified squamous nonkeratinized epithelium

• Lamina propria possesses esophageal cardiac glands that are mucus-secreting and are particularly prominent near the junction of the esophagus with the stomach.

• Submucosa has mucus-secreting, esophageal glands.

• Muscularis externa is composed of striated muscle in the upper portion of the esophagus, skeletal, and smooth muscle in the middle portion, and smooth muscle in the lower portion.

• Adventitia. Composed of loose connective tissue.

^ Stomach

• Structures present throughout the stomach

Esophageal glands

Skeletal muscle, smooth muscle or both types

Adventitia

Stratified squamous moist epithelium

Esophageal glands

Skeletal muscle, smooth muscle or both types

Adventitia

Stratified squamous moist epithelium

Epithelium

Cross-section of the esophagus.

figure 12.3.

Cross-section of the esophagus.

♦ Surface epithelium

■ Simple columnar epithelium facing the lumen is modified so that all cells secrete mucus, forming a sheet gland that protects the stomach from its acidic environment.

■ Gastric pit. A channel formed by the invagination of the surface epithelium into the underlying lamina propria; connects the sheet gland with the gastric glands. The length of the gastric pit varies with each stomach region.

♦ Gastric glands

■ Simple, branched tubular glands begin at a gastric pit and extend through the lamina propria to the muscularis mucosae.

■ The region of the gland that attaches to the gastric pit is called the neck region; the base region of the gland is located adjacent to the muscularis mucosae.

■ Secretory cells in these glands vary in each region of the stomach.

♦ Muscularis externa. Subdivisions of this layer frequently interdigitate, making it difficult to distinguish one layer from another.

■ Internal oblique layer

■ Middle circular layer that is modified in the pyloric region to form the pyloric sphincter

■ Outer longitudinal layer is separated from the inner circular layer by Auerbach's plexus, nerve fibers from the autonomic nervous system that supply muscularis externa.

Stomach Rugae
figure 12.4. Cross-section of the stomach.

♦ Rugae. Longitudinal folds of the mucosa and submucosa in the undistended stomach allow for expansion.

• Variations specific to the cardiac region (narrow region adjacent to the esophagus)

♦ Abrupt transition of epithelium from stratified squamous moist of the esophagus to a sheet gland lining the cardiac stomach

♦ Length of gastric pits is about equal to the length of cardiac glands.

♦ Cardiac glands primarily secrete mucus, although other products are also produced. Glands are frequently coiled.

♦ Cardiac glands of the stomach extend into the lower esophagus, forming the esophageal cardiac glands.

• Variations specific to the fundic and body regions (Glands in both regions are called fundic glands.)

♦ Fundic glands are about twice as long as their gastric pits.

■ Stem cells replenish both the surface epithelial cells and cells of the glands. Stem cells are located in the neck region.

■ Mucous neck cells are irregular in shape and stain basophili-cally. They secrete mucus and are located in the neck region.

■ Parietal cells are large, spherical, eosinophilic cells that secrete hydrogen and chloride ions and gastric intrinsic factor. They possess numerous mitochondria. An umbrella-shaped canaliculus indents the luminal surface, increasing surface area. Although present throughout the gland, parietal cells are more numerous in the upper regions.

■ Chief or zymogen cells, typical protein-producing cells, predominate in the bases; stain blue with hematoxylin and secrete pepsinogen.

■ Enteroendocrine cells (part of the diffuse neuroendocrine system, DNES) are located on the basement membrane and do not usually reach the lumen of the gland. This population of cells secretes a variety of hormones with endocrine and paracrine influences on digestive activity. Secretory granules cluster toward the basement membrane for their subsequent release into the lamina propria. Most common at the bases of the glands.

• Variations specific to the pyloric region

♦ Pits are longer in pylorus than in the cardiac region.

♦ Pyloric glands, not as coiled as in the cardiac region; primarily secrete mucus.

♦ Enteroendocrine cells are also present here.

♦ Circular layer of muscularis externa is greatly thickened to form the pyloric sphincter.

^ Small intestine

• Subdivided into duodenum, jejunum, and ileum

• Common features of the small intestine

♦ Structures that increase the surface area of the small intestine

■ Microvilli. Increase surface area of absorptive cells and, collectively, form a brush or striated border

■ Villi. Finger-like protrusions of the lamina propria and overlying epithelium into the lumen

- Villi assume different shapes in each of the three intestinal subdivisions.

- A lacteal (blind-ending lymphatic capillary) is located in the center of each villus to absorb digested fat.

- Individual smooth muscle cells lie parallel to the long axis of each villus, "milking" the lacteal contents to the periphery.

■ Plicae circulares. Permanent circular folds formed by an up-welling of the submucosa and its overlying mucosa into the lumen. Villi protrude from the plicae.

Plicae Circulares Villus Crypt
figure 12.5. Longitudinal section through the duodenum (left) and the jejunum/ ileum (right). Note the orientation of the layers of muscularis externa when sectioned longitudinally.

♦ Mucosal epithelium is composed of:

■ Absorptive cells, forming a simple columnar epithelium with microvilli, absorb digested food

■ Goblet cells (unicellular glands) are interspersed among absorptive cells and secrete mucus. These cells increase in number from duodenum to rectum.

♦ Intestinal glands (crypts of Lieberkuhn) are simple tubular glands that begin at the bases of the villi in the mucosa and extend through the lamina propria to the muscularis mucosae. Possess:

■ Absorptive cells

■ Paneth cells possess large, eosinophilic granules whose contents digest bacterial-cell walls.

■ Enteroendocrine cells

♦ Muscularis externa of inner circular and outer longitudinal layers with an intervening Auerbach's nerve plexus

♦ Serosa covers all of small intestine except for the beginning of the duodenum, which is retroperitoneal and possesses an adventitia.

• Variations specific to the intestinal subdivisions

♦ Brunner's glands in the submucosa are present only in the duodenum. These compound tubular glands open into the bases of the intestinal glands and secrete an alkaline mucus to neutralize the acidity of the stomach contents.

♦ Peyer's patches are clusters of 10-200 lymphoid nodules located primarily in the lamina propria of the ileum. Each cluster is positioned on the side of the intestine away from the mesentery and forms a bulge that may protrude into the lumen as well as into the submucosa.

^ Large intestine (colon)

• Mucosal epithelium:

♦ Absorptive cells form a simple columnar epithelium with microvilli.

♦ Goblet cells increase in number toward the rectum and provide lubrication.

♦ A reduced number of enteroendocrine cells is present.

• Intestinal glands (crypts of Lieberkuhn) are very straight in the large intestine.

• No villi or plicae circulares are present in the large intestine.

Crypts Lieberkuhn

• Muscularis externa

♦ Inner circular layer is intact.

♦ Outer longitudinal layer is segregated into three longitudinal bands, the taeniae coli, that are placed equidistantly around the tube. The contraction of the taenia produces permanent saccu-lations in the large intestine, termed haustrae.

• Either an adventitia or a serosa is present, depending on the particular portion of the large intestine.

• The appendix resembles the large intestine except that the outer longitudinal smooth muscle layer is intact. Additionally, abundant lymphoid tissue is present in the lamina propria to protect against invading microorganisms.

• Rectum is a 12-cm-long tube continuing from the sigmoid colon. The mucosa of the rectum is similar to that of the majority of the large intestine. The rectum narrows abruptly to become the anal canal.

• Anal canal. The terminal portion of the intestinal tract is about 4 cm long.

♦ The intestinal glands disappear and the epithelium undergoes an abrupt transition from simple columnar to stratified squa-mous with sebaceous and apocrine sweat glands.

♦ The inner circular portion of the muscularis externa expands to form the internal anal sphincter. The external anal sphincter is composed of skeletal muscle.

Structures Identified in This Section

Overview of tubular digestive tract Adventitia Epithelium Esophagus Glands

Lamina propria Large intestine Mucosa

Muscularis externa Muscularis mucosae Plicae circulares Serosa

Small intestine

Stomach

Submucosa

Esophagus

Adventitia Capillaries Cardiac glands Cardiac stomach Epithelium, stratified squamous moist Esophageal glands Gastric pits Gastric pits, openings Gastroesophageal junction Lamina propria Mucosa

Muscularis externa Muscularis mucosae Skeletal muscle (ls and xs) Smooth muscle Squamous cells Submucosa

Stomach

Auerbach's plexus Brunner's glands Cardiac glands Chief cells Collagen fibers Enteroendocrine cells Fundic glands

Gastric glands Gastric glands, bodies Gastric glands, branching Gastric glands, necks Gastric pits

Gastroduodenal junction Intestinal glands Lamina propria Lymphoid nodule Mast cells Meissner's plexus Mucosa

Mucous neck cells Mucigen

Muscularis externa Muscularis externa, middle circular layer Muscularis externa, outer longitudinal layer Muscularis mucosae Neuron cell body Parietal cells Peripheral nerves Plasma cells Pyloric glands Pyloric sphincter Secretory granules Serosa Sheet gland

Sheet gland, stem of cell Stomach, cardiac Stomach, fundic and body Stomach, pyloric Submucosa Villi

Small intestine

Absorptive cells

(enterocytes) Brunner's glands Brunner's glands, ducts Duodenum Enteroendocrine cells

Epithelium, simple columnar

Large intestine

with microvilli

Anal canal

Epithelium, simple squamous

Anal sphincter, external

Germinal center

Anal sphincter, internal

Goblet cells

Anus

Intestinal glands

Apocrine sweat glands

Jejunum/ileum

Diffuse lymphoid tissue

Junctional complex

Hair follicle

Lacteal

Intestinal glands

Lamina propria

Lymphatic nodules

Lumen

Mucosa

Lymphoid nodule

Muscularis externa

Mesentery

Muscularis externa, inner

Microvilli

circular layer

Mucosa

Muscularis externa, outer

Muscularis externa

longitudinal layer

Muscularis mucosae

Recto-anal junction

Paneth cells

Rectum

Peyer's patch

Serosa

Plasma cells

Submucosa

Plicae circulares

Taeniae coli

Serosa

Submucosa

Villi

Major Digestive Glands

Pancreas

Overview

^ Located in the abdomen in the curve of the duodenum and divided into a head, body, and tail

^ Is both an exocrine and an endocrine gland

• The exocrine portion produces an alkaline secretion containing digestive enzymes that empties into the duodenum.

• The endocrine portion secretes insulin, glucagon, and somatostatin that regulate blood glucose levels.

Microscopic Anatomy

^ Exocrine pancreas

• Compound acinar gland; the acinar cells secrete numerous digestive enzymes that breakdown proteins, carbohydrates, lipids, and nucleic acids.

• Cells show polarity with basal RER and apical secretory granules.

♦ Ducts begin as centroacinar cells located within the acini.

♦ Intercalated ducts are lined with simple cuboidal epithelium. Centroacinar cells and cells of the intercalated ducts secrete bicarbonates to neutralize the acidity of the stomach contents (chyme) entering the duodenum.

♦ Striated ducts are not present.

♦ Interlobular ducts lead into one or more excretory ducts that empty into the duodenum.

• Resembles the parotid gland except the pancreas has centroacinar cells and fewer ducts.

• Secretion is regulated by cholecystokinin and secretin from enteroendocrine cells in the small intestine.

^ Endocrine pancreas (islets of Langerhans)

• Small clusters of cells, richly supplied by fenestrated capillaries, are scattered throughout the exocrine pancreas; these clusters show no orderly arrangement of secretory cells within the cluster.

• Predominate cell types and secretions

♦ A cell (alpha cell). Secretes glucagon, which elevates glucose levels in the blood

♦ B cell (beta cell). Secretes insulin, which lowers blood glucose levels; predominant cell type

♦ D cell (delta cell). Secretes somatostatin, which modulates release of the other two major hormones

• Individual cell types cannot be distinguished with hematoxylin and eosin.

Liver

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Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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Responses

  • reece
    Where is the mucosae located?
    8 years ago
  • Susan
    Is your large intestine a digestive organ?
    8 years ago
  • Folco Clayhanger
    Where is the lamina located in the digestive tract?
    8 years ago
  • dexter
    Is there a lacteal in the lamina propria of a villus?
    7 years ago
  • Kira
    Where is the lamina propria located?
    7 years ago
  • Einojuhani
    What is specific about the duodenum lamina propria?
    6 months ago

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