"Intestine" - what is it, definition of the term
The intestine, also referred to as the gut or digestive tube, is a continuous muscular conduit that connects the stomach to the anus, consisting of the small and large segments; it facilitates the absorption of nutrients, reclamation of water and electrolytes, and the formation and expulsion of feces. In rodent models such as rats and mice, the gut displays a proportionally elongated small segment with densely packed villi and crypts, adaptations that enhance digestive efficiency and nutrient uptake relative to the animal’s body size.
Detailed information
The small portion of the gastrointestinal system in rodents extends from the pyloric sphincter to the ileocecal valve and measures approximately 30 cm in rats and 20 cm in mice. It is divided into three functional regions: duodenum, jejunum, and ileum. The duodenum receives pancreatic secretions and bile, the jejunum performs the majority of nutrient absorption, and the ileum absorbs bile acids and vitamin B12. The large segment follows the ileocecal valve, comprises the cecum, colon, and rectum, and measures roughly 10 cm in rats and 8 cm in mice. The cecum occupies a substantial proportion of the large portion, serving as a fermentation chamber for microbial breakdown of fiber.
Microscopic architecture is consistent across both species. Each segment displays four layers: mucosa with villi and crypts, submucosa containing blood vessels and nerves, muscularis externa with inner circular and outer longitudinal smooth‑muscle layers, and serosa. Goblet cells increase in density toward the distal colon, providing mucus for lubrication. Enteric nervous system ganglia reside within the myenteric and submucosal plexi, regulating peristalsis and secretion.
Physiological differences include:
- Transit time: Rats exhibit a total gut passage of 4–5 hours; mice complete transit in 2–3 hours.
- Microbial composition: The rat cecum hosts a more diverse anaerobic community, while the mouse cecum contains a higher proportion of Firmicutes.
- Enzyme activity: Lactase activity declines sharply after weaning in mice, whereas rats retain measurable activity into adulthood.
Research applications rely on these characteristics. The rodent gut is a standard model for studying nutrient absorption, intestinal permeability, and microbiome–host interactions. Surgical interventions such as intestinal resection or anastomosis are performed routinely to assess wound healing and postoperative function. Pharmacological testing frequently uses oral gavage to deliver compounds directly to the small segment, allowing measurement of bioavailability and first‑pass metabolism.
Common pathological conditions observed in laboratory colonies include:
- Spontaneous adenomas in the distal colon of aged mice.
- Inflammatory lesions induced by dextran sulfate sodium, which produce ulcerative colitis‑like changes.
- Obstruction caused by cecal impaction, more prevalent in rats due to larger cecal volume.
Maintenance of gut health in these animals requires balanced diets low in indigestible fiber for mice and moderate fiber for rats, regular monitoring of fecal consistency, and prophylactic deworming to prevent parasitic infestation.