Vertebrate Digestive System Development

Vertebrate Digestive System

• Digestion involves four main processes: ingestion, digestion (physical and chemical), absorption, and waste elimination.

Development of the Digestive System

• The coelom forms from the splitting of the lateral plate mesoderm (hypomere).
• The pleuroperitoneal cavity in reptiles comes from the coelom.
• The pleural cavity is the chest cavity, and abdominal cavity is the peritoneal cavity.
• The parietal peritoneum is the part closest to the body wall.
• The visceral peritoneum is the part that touches the viscera.
• Dorsal and ventral mesenteries are folds of the peritoneum.
• The gut itself comes from the endoderm.
• The foregut gives rise to cranial digestive organs such as the liver, pancreas, even the beginning of the small intestine.
• The midgut gives rise to the majority of the small intestine and part of the large intestine.
• The hindgut gives rise to the caudal most large intestine and the cloaca.

General Morphology of the Gut Wall

• A typical gut wall has mucosa, submucosa, muscularis, and serosa.
• Mucosa is the innermost layer and may have villi.
• Submucosa is a highly vascular layer.
• The muscularis layer is smooth muscle.
• Serosa is the outermost layer and releases a watery fluid.
• The visceral peritoneum is a synonym for serosa.

Gut Motility: Peristalsis and Segmentation

• Peristalsis moves food along the digestive track in a wave-like contraction.
• Reverse peristalsis occurs when a cow regurgitates its cud or when vomiting.
• Segmentation is a stationary contraction and is more of a mixing type of contraction.
• Peristalsis involves muscular contractions used to transport food through the digestive tract with one way movement in a caudal direction, resulting in high-speed food propagation with minimal mixing.
• Segmentation is a muscle process in the small intestine mixing backwards and forward to mix the chyme allowing better mixing, resulting in slow speed propagation with thorough mixing

Oral Cavity/Oropharyngeal Cavity

• If there is no subdivision of the mouth and pharynx it is called the oropharyngeal cavity.
• The tongue can extend from the mouth in amphibians.
• In the oral cavity the tongue starts in amphibians, with the ability to extend the tongue out of the mouth to obviously capture food.
• In sharks, the tongue cannot move, but helps stabilize the food.
• The palate is the roof of the oral cavity.

Glands of the Oral Cavity

• Glands in the oral cavity release venom (snakes), salivary gland which makes saliva to help lubricate food
• Duvernoy's and venom glands are part of a snake's secretory system, primarily in predatory behavior.
• Anticoagulants are released in lampreys.
• Nutrients may be released in catfish.
• Mucus is released in the mouth.
• There are three major pairs of salivary glands: sublingual, submandibular, and parotid.

Teeth

• Teeth are primarily dentin surrounded by enamel.
• Initially, teeth started as dermal armor becoming dermal plates, and the placoid scales gave rise to teeth as well.
• Dentin forms the majority of the tooth made by odontoblasts.
• Enamel is the hardest substance in the body due to its mineral content.
• The pulp cavity contains the blood vessels and nerves.
• Cementum covers the root of the tooth and is acellular bone.
• Tooth attachment
• Acrodont: peak of jaws; teleosts.
• Pleurodont: inner surface of jaws; amphibians, lizards.
• Thecodont: sockets; crocodiles, extinct birds, mammals.
• Sets of teeth
• Polyphydont: many sets, typical of most vertebrates.
• Diphydont: two sets, most mammals.
• Monophydont: one set, beluga whale.
• Homodont teeth have the same shape, most vertebrates.
• Heterodont teeth have different shapes depending on function.
• Dental Formulas
• Human: 2-1-2-3/2-1-2-3 = 32
• Cat: 3-1-3-1/3-1-2-1 = 30
• Cow: 0-0-3-3/3-1-3-3 = 32
• Cows dental formula indicates that they tear the grass rather than chop it.

Pharynx

• The pharynx in tetrapods allow opening to airways, middle ear, and esophagus.
• The pharynx is part of the digestive tract that exhibits pharyngeal pouches.
• The pharynx is respiratory in fish.
• In tetrapods, it is in the throat, is for swallowing, and is the location of tonsils in mammals.

Esophagus

• The esophagus can close in fish so the stomach doesnt become filled with respitory water
• Birds may have a crop for digestive enzyme and food storage.
• In doves, pigeon milk is an esophageal secretion for the nestlings.

Stomach

• Key features include one or more chambers, a pylorus, a pyloric sphincter, and a greater and lesser curvature.
• The greater omentum is for mammals only.
• The stomach contains gland and secrete enzymes which break down protien
• The stomach consists of pylorus, pyloric sphincter, greater and lesser curvature.
• Greater omentum, mammals only.
• Proventriculus is the digestive enzymes of birds and crocodiles.
• The Gizzard is a grinding mill in bird.
• A typical ruminant stomach has four parts: rumen, reticulum, omasum, and abomasum.

Stomach Physiology

• The stomach receives, stores, liquefies, and mixes food.
• Chyme is the product of these processes.
• Zymogenic/chief cells make pepsinogen, which breaks down protein.
• Parietal cells make HCl, which breaks down protein, activates pepsinogen, and is anti-microbial.
• The gastric glands are made up of a variety of secretory cells such as parietal cells, chief cells, mucous neck cells, and enteroendocrine cells.
• The surface mucous cell secretes mucus
• The mucous neck cell secretes mucus that is thin and acidic
• The parietal cell secretes hydrochloric acid for intrinsic factor
• The zymogen chief cell secrete pepsinger and a gastric lypase
• The Enteroendocrine G cells secrete hormones for gastrin.

Intestine

• In fish, the intestine is straight, has no small or large intestine, and has a spiral valve.
• Coiling and Ceca are present.
• The small intestine has three parts in tetrapods: the duodenum, jejunum, and ileum.
• Villi increases surface area for absorption. Villi also contain blood vessels & lacteals for absorption
• The Duodenum stomach is attached to the stomach, while curving around the head of the pancreas
• The jejunum attaches anteriorly to the duodenum
• The ileum extends from the jejunum to the large intestine
• Most nutrient absorption occurs in the small intestine

Large Intestine in Tetrapods

• Cecum/ceca may be present in amniotes.
• The colon is the majority of large intestine.
• The rectum is the terminal segment of the large intestine.
• Functions to support formation adn storage of feces, with some water reabsorption and fermentation in the herbivores.

Liver & Gall Bladder

• Liver embryological is formed from diverticula of foregut and midgut
• Lesser omentum supports ducts and vessels
• Gall Bladder store bile
• Bile production
• Common Bile Duct is formed by hepatic and cystic ducts & goes to duodenum
• The liver also produces
• Glucose storage
• Bile secretion
• Amino acid deamination
• Clotting factors
• Blood formation in fish
• The bile, created in the liver, flows through the left and right hepatic ducts, and then joins with the cystic duct (connected to the gall bladder) and enter the start of the small intestine.
• Bile acts to breaks down dietary fat in the small intestine into minute droplets.

Pancreas

• Exocrine portion makes digestive juices that travel through pancreatic duct.
• Acinar cells are the exocrine cells.
• Juice contains amylase, lipase, and protease.

Cloaca

• Receives digestive, urinary, excretory, and reproductive systems
• Not present in most placental mammals