Histology of the Small & Large Intestines Flashcards
Organ that is the principal site for digestion of food and absorption of digestion products:
Small Intestine
Plicae Circulares
- Transverse folds with a submucosal core
- Cannot lay flat if distended
- Visible in jejunum, less so in ileum
Intestinal Villi
- Mucosal folds that project into the lumen
- Cover the surface of the small intestine
- Covered with a simple columnar epithelium of enterocytes and goblet cells
- Core of loose connective tissue with microvasculature and lacteal
Villi are covered with ___ which further increases surface area
Microvilli
Goblet Cells
- Located in Intestinal Villi
- Provide mucus coating to shield from abrasion and bacterial invasion
Small Intestine: Mucosa
- Simple columnar epithelium with tubular intestinal glands
- Contain Lamina Propria layer and Muscularis Mucosae layer
Small Intestine: Submucosa
- Dense irregular connective tissue
- Neurovasculature, lymphatics and glands may be present
Small Intestine: Muscularis
Consists of inner circular and outer longitudinal layers
Small Intestine: Serosa
Loose connective tissue covered by visceral peritoneum
Small Intestine: Adventitia
Covers retroperitoneal segments
Motility of small and large intestines is controlled by:
ANS
Components of Enteric Nervous System:
- Sympathetic visceral sensory fibers
- Parasympathetic (terminal) ganglia
- Preganglionic and postganglionic parasympathetics
Enteric Nervous System is regulated by:
Extrinsics –> Preganglionic parasympathetics (vagus and pelvic nerves) and postganglionic sympathetics
Intestinal Glands
- Simple tubular glands lined by simple columnar epithelium
- Open onto the luminal surface of the intestine at the base of the villi
4 Types of Cells found in Small Intestinal Glands:
(1) Paneth Cells
(2) M (microfold) Cells
(3) Intestinal Cells
(4) Enteroendocrine Cells
Intestinal Glands are continuous with:
Simple columnar that covers intestinal villi
Paneth Cells: Location
Base of intestinal glands
Paneth Cells
- Large, pyramidal-shaped cells at the base of intestinal glands
- Eosinophilic secretory granules in apical cytoplasm
Paneth Cells: Function
- Functions in innate immunity by secreting antimicrobial substances (lysozyme, alpha defensins)
- Regulate normal bacterial flora via antibacterial action and phagocytosis
Intestinal Stem Cells
- Located in a niche at crypt base, near Paneth Cells
- Function in repopulating epithelial lining
- Daughter cells move out of the crypts then they are able to differentiate into different cell types
Intestinal Stem Cells can Differentiate into:
- Goblet Cells
- Enterocytes
- Enteroendocrine Cells
M (Microfold) Cells
- Epithelial cells specialized for transepithelial transport of particles and microorganisms
- Pocket formed by deeply invaginated membrane; contain lymphocytes and macrophages
M (Microfold) Cells: Location
Ileal mucosa, overlying Peyer Patches (lymphatic nodules in specific segments of the SI)
M (Microfold) Cells: Apical Surface
- Expresses receptors that bind specific macromolecules and Gram-negative bacteria (ex. E Coli)
- Internalized and transferred to immune cells
M (Microfold) Cells function as highly specialized:
Antigen-Transporting Cells
Gastroduodenal Junction: Pyloris of Stomach
- Thick wall due to the presence of the pyloric sphincter muscle
- Pyloric glands at the base of each gastric pit
Gastroduodenal Junction: Duodenum
- Abundant duodenal glands in submucosa, DEEP to the muscularis mucosae
- Secretory mucosa with villi
Duodenum: Mucosal Layer
Simple columnar epithelia with scattered goblet cells
Duodenum: Submucosal Layer
- Duodenal Glands: tubuloacinar mucous glands that produce an alkaline secretion
- Neutralizes acidic chyme
Jejunum: Mucosal Layer
- Simple columnar epithelia with scattered goblet cells
- Long, finger-like villi, villus core contains a well-developed lacteal
- Tubular intestinal glands
- Lymphoid nodules/follicles in the lamina propria
Jejunum: Submucosal Layer
No submucosal glands
Ileum: Mucosal Layer
- Simple columnar epithelia with scattered goblet cells
- Villi typically shorter than jejunum
- Peyer’s Patches: lymphoid nodules (MALT)
- Paneth cells found at base of intestinal glands
Ileum: Submucosal Layers
No submucosal glands
Functions of Large Intestine
- Absorbs water, sodium, vitamins and minerals
- Uptake of sedatives, anesthetics and steroids occurs here
Hallmarks of Large Intestine
- Intestinal glands
- Goblet cells
- NO intestinal villi!!
Large Intestine: Mucosal Layer
- Glandular, simple columnar epithelium
- Lamina Propria and muscularis mucosae are present
- GALT is extensive and may penetrate submucosa
Cells of the Mucosal Layer of the Large Intestine:
- Colonocytes
- Goblet Cells
- Stem Cells
- Enteroendocrine Cells
Large Intestine: Submucosal Layer
- Dense irregular connective tissue
- Vascularized
Large Intestine: Muscularis Layer
- Inner layer is typical
- Outer longitudinal layer organized into Taenia Coli (will form haustra of LI)
Taenia Coli
- Aggregates of fascicles of the outer longitudinal layer
- Form three spaced bands
Intraperitoneal portions of the colon are covered by:
Serosa
Colon Histology: Glands
Consist of simple columnar epithelium with NUMEROUS Goblet Cells
Colonocytes
- Located in Large Intestine
- Have short microvilli and dilated intercellular spaces with interdigitating leaflets of cell membrane (sign of active water transport)
Congenital Aganglionic Megacolon (Hirschsprung Disease)
- Occurs due to mutations of the RET gene, leading to failure of NCC development in both plexuses of a bowel segment
- Present in 1 in 5,000 live births
RET Gene
Required for migration and differentiation of NCC
Congenital Aganglionic Megacolon: Short-Segment Disease
- 85% of cases
- Confined to rectosigmoid region
Congenital Aganglionic Megacolon: Long-Segment Disease
- 10% of cases
- Extends past rectosigmoid region to the splenic flexure
Congenital Aganglionic Megacolon: Total Colonic Aganglionosis
- 5% of cases
- Affects entire colon
Congenital Aganglionic Megacolon: Presentation
- Constipation
- Poor-feeding
- Progressive abdominal distention
Congenital Aganglionic Megacolon: Diagnosis
- Delayed BM <48hrs after birth
- Rectal biopsy
- Barium enema
Congenital Aganglionic Megacolon: Treatment
Surgical removal of the affected colon segment
Rectum Proper
- Upper part of rectum
- Typical mucosa with tubular intestinal glands and transverse rectal folds
Anal Canal
- Lower part of rectum
- Contains anal columns with mucosal folds, anal sinuses, and anal glands (extend into the submucosa and muscularis externa)
Anal Sinuses
Depressions between anal columns
Circumanal Glands
- Located in the rectum
- Large apocrine glands surround the anal orifice
3 Features of the Anal Canal:
(1) Anal Columns
(2) Anal Sinuses
(3) Anal Glands
3 Regions of Anal Canal:
(1) Colorectal Zone
(2) Anal Transition Zone (ATZ)
(3) Squamous Zone
Anal Canal: Colorectal Zone
- Upper 1/3 of anal canal
- Simple columnar epithelium that is identical to the rectum
Anal Canal: Anal Transitional Zone
Transition from simple columnar epithelium to stratified squamous epithelium of perianal skin
Anal Canal: Squamous Zone
- Lower 1/3 of anal canal
- Stratified squamous epithelium that is continuous with perineal skin
Internal Anal Sphincter
Thickening of the inner circular layer located at the level of the anus
Retroperitoneal Organs
- Partly covered on one side with parietal peritoneum
- Immobile or fixed
- Divided into primary and secondary classifications
Primary Retroperitoneal Organs
Never had mesentery
Secondary Retroperitoneal Organs
Lost their mesentery during development
Retroperitoneal Organs/Structures:
SAD PUCKER
- Suprarenal glands
- Aorta/ivc
- Duodenum (except first part)
- Pancreas (except tail)
- Ureters
- Colon (ascending, descending) and cysterna chyli
- Kidneys
- Esophagus (anterior, left covered)
- Rectum (upper part)
Primary Retroperitoneal Organs/Structures:
- Kidneys
- Adrenal glands
- Ureters
- Aorta
- IVC
- Rectum
Secondary Retroperitoneal Organs/Structures:
- Duodenum (descending, horizontal)
- Pancreas (head, neck, body)
- Ascending colon
- Descending colon
