69: Phylogeny and development of the digestive system Flashcards
What are the primary functions of the digestive system?
o Ingests and digests food.
o Absorbs nutrients from digested food.
o Removes undigested remnants from the body.
What are the major subdivisions of the digestive tract?
o Oral Cavity (Cavitas Oris): Initial site for food intake and digestion.
o Pharynx: Passageway for food moving from oral cavity to esophagus.
o Esophagus: Conducts food to the stomach.
o Stomach (Gaster, Ventriculus): Main site of digestion with acidic environment.
o Intestines (Small and Large): Absorbs nutrients; the large intestine absorbs water and forms feces.
o Anus or Cloaca: Excretes digested waste material.
What are the accessory organs of the digestive system and their functions?
o Tongue (Lingua): Aids in food manipulation and taste sensation.
o Teeth (Dentes): Involved in the mechanical breakdown of food.
o Oral (Salivary) Glands: Secrete enzymes that begin carbohydrate digestion.
o Liver: Produces bile, crucial for lipid digestion.
o Gallbladder: Stores and concentrates bile.
o Pancreas: Produces digestive enzymes and bicarbonate.
What is enzymatic digestion and how are enzymes activated?
o Digestive enzymes are hydrolases, secreted as inactive zymogens to prevent self-damage.
o Pancreatic enzymes are activated by entero kinase.
o Amylase, secreted by the pancreas (and in some mammals by salivary glands), breaks down starch.
o The increase in starch availability due to agriculture has influenced human dietary patterns.
How do protochordates feed and digest food?
o Feeding: Primarily filter feeders using a large pharynx with slits to trap food particles.
o Digestion: Movement of water and food facilitated by cilia.
o Liver Evolution: Originates as an outgrowth of the intestine.
What evolutionary changes occurred in vertebrates related to feeding?
o Pharynx: Became muscular, supporting a shift from filter feeding to predation.
o Feeding Mechanism: Reliance on cilia for moving food became impractical.
What are cyclostomes and gnathostomes, and how did teeth evolve?
o Cyclostomes: Represent the only surviving jawless vertebrates (agnathans).
o Gnathostomes (Jawed Vertebrates): Include all other extant vertebrates, featuring active predation advantages due to jaws.
o Teeth Evolution: Evolved alongside jaws, with cartilaginous fishes displaying true teeth homologous to placoid scales.
How did the stomach evolve in cartilaginous fishes?
o Predatory Adaptation: The evolution of the stomach is attributed to their predatory nature, allowing them to ingest large portions of food at irregular intervals.
o Functional Development: Initially served as a storage area for ingested solids, evolved to secrete hydrochloric acid (HCl) and pepsin for protein digestion.
o Pancreatic Development: Represents a novel feature in cartilaginous fishes, enhancing their digestive capabilities.
What is the derivation and formation of the digestive system in mammals?
o The digestive system originates from the endoderm, with mesodermal contributions for structural support, and terminal compartments covered by ectodermal epithelium.
o During gastrulation, the endoderm specifies and forms the primitive gut or archenteron, a hollow tube under the notochord.
o The blastopore, the initial opening, develops into the anus or cloaca, a primitive trait in vertebrates.
How is the primitive gut connected to the yolk sac in mammals?
o A segment of the yolk sac is incorporated into the embryo to form the primitive gut.
o Temporarily, the middle part of this gut stays connected to the yolk sac through the vitelline duct or yolk stalk.
How do accessory digestive organs form?
Accessory organs like the liver and pancreas develop from outpouchings of the primitive gut tube, influenced by molecular signals.
Describe intestinal formation in mammals.
The intestines experience physiological umbilical herniation, temporarily protruding into the umbilical cord before retracting back into the abdominal cavity.
How does the mouth form in mammals?
o After neurulation, the anterior ectoderm invaginates to form the stomodeum or primary oral cavity.
o The stomodeum contacts the expanding endoderm of the pharynx, initially separated by the buccopharyngeal membrane, which soon ruptures to connect the oral cavity to the pharynx.
What is the role of the cecum in reptiles and other amniotes?
Fully developed, plays a crucial role in absorbing water and salts.
What are the major evolutionary changes in the digestive system of amphibians?
o Salivary Glands: Evolved to moisturize food, adapting to terrestrial feeding.
o Tongue: Became muscular and motile, actively moving food within the mouth.
o Esophagus: Lengthened to facilitate the movement of food from the mouth to the stomach.
o Intestine: Lost its spiral fold, adapting to different dietary needs.
o Cecum: Developed at the junction of the small and large intestines, aiding in fermentation and digestion.
