Digestive System and Derivatives

1. Digestive System and Derivatives • Derivatives include respiratory system, liver, pancreas, gall bladder and endocrine structures • All are endodermal in origin • Digestive System includes digestive tract • Mouth & Pharynx — Small Intestine • Esophagus — Large Intestine • Stomach — Cloaca (or derivative) • Also includes associated digestive glands: liver, pancreas and gall bladder

2. Figure 13.1 Fig 13.1 – Digestive tract components

3. Embryonic Origin of Digestive Tube • Embryonic Origin of Digestive Tube by 2 Basic Methods • Cyclostomes, Actinopterygians, and Amphibians = gastrulation provides a “tube-within-a-tube” arrangement. Inner tube is endodermally derived and becomes gut. • All other vertebrates have: • The epiblast oriented on top of the hypoblast in flat sheets. The hypoblast is continuous peripherally with the endoderm of the prospective yolk sac. • Development of head, lateral body, and tail folds separate the embryo from extraembryonicmembranes. • The endoderm folds upon itself to form a tube continuous ventrally with the yolk sac  forms the gut.

4. Development of Openings to Gut Tube • Protostomes = blastopore forms the mouth; the anus is derived secondarily • Includes Annelids, Molluscs, and Arthropods • Deuterostomes = blastopore becomes the anus; the mouth forms later as an independent perforation of the body wall • Includes Echinoderms and Chordates • In vertebrate development the head turns downward over the surface of the yolk, forming an ectodermal pocket (stomodeum) which represents the primitive mouth cavity

5. Development of Openings to Gut Tube • Stomodeum is separated from the pharyngeal region of the gut by a membrane (pharyngeal membrane) that eventually breaks down so that the oral cavity and pharynx become continuous. • Proctodeumis similar invagination at the posterior end of the gut, separated from the gut by the cloacal membrane that eventually disappears, leaving a tube open at both ends. • The mouth and teeth are derived from ectoderm.

6. Figure 13.2 Fig 13.2 – Embryonic formation of the digestive system Early amniote embryo Generalized amniote embryo Ventral view of isolated gut Lateral view of differentiating gut

7. Development of Openings to Gut Tube • The boundary of the mouth ideally is the junction of the stomodeum (ectodermal) with the pharynx (endodermal). • In practice, definite anterior and posterior limits to the mouth are difficult to establish, and differ among vertebrate groups. • Landmarks used in distinction as markers of the mouth (ectodermally derived) include: • Nasal Pits (= nasal placodes) • Rathke’s Pouch (= hypophyseal pouch) • Evolutionary trend: toward inclusion of more ectoderm inside the mouth in advanced forms • Primitively, stomodeal structures are forced outside the mouth through differential growth

8. Figure 13.4 Fig 13.4 – boundaries of the mouth cavity

9. Mouth Cavity • Lined by skin, includes teeth and salivary glands as components • Teeth are homologous with the integument of some fishes and placoid scales (denticles) of shark skin • Location of teeth • Fish = found on palate (roof of mouth), margins of jaw, gill arches • Amphibians/Reptiles = found on some bones of the palate and margins of maxillary, premaxillary and dentarybones • Mammals = found only on margins of maxillary, premaxillary and dentarybones

10. Mouth Cavity • Evolutionary trend in mammals = reduction in numbers of teeth from primitive to advanced mammals • Primitive mammal number is 44 (humans with 32) • Whales have an increased number as a specialization to their very large mouth • Birds have no teeth, except for primitive Mesozoic forms (associated with reduced weight for flight) • Turtles also lack teeth; instead have a hard, keratinized beak • Number of generations of teeth is reduced from primitive (continuous replacement) to advanced (1 or 2 sets) vertebrates

11. Degree of Tooth Differentiation • HomodontousCondition = all teeth are similar, generally conical in shape • Most vertebrates • HeterodontousCondition = specialization of teeth • Typical state for a few reptiles, Therapsids, and Mammals • Teeth include: • Incisors(front) - used for cropping • Canines- behind the incisors, used for tearing • Molars(cheek teeth) - furthest back in mouth, used for chewing • Teeth in heterodontous vertebrates are used for capture or cropping of food and chewing • Chewing aids in digestion by increasing surface area of food available for digestion • This increases digestive efficiency and provides energy necessary to support high rates of metabolism of mammals

12. Homodontous Teeth from salamander Heterodontous Teeth from fox

13. Salivary Glands • Formed from invaginations of the mouth lining • Mucous Glands =produce mucous; lubrication of food • Serous Glands =watery secretion containing enzymes; initiates digestion of carbohydrates (salivary amylase) • Mixed Glands = mucous and serous secretions • Snake venom glands are modified serous salivary glands

14. Fig 13.37 – Salivary glands in a dog

15. Fig 13.35 – Oral glands of reptiles. Venom glands derived from Duvernoy’s gland.

16. Palate • Forms roof of mouth • Composed of bone, lined by epithelium and connective tissue • Fish, Amphibians and Birds have only a primary palate present • Crocodilians and mammals also have a secondary palate, which allows simultaneous chewing and breathing in mammals, and breathing while mouth is submerged in crocodiles • Secondary palate separates nasal passages from mouth

17. Fig 7.57 – Primary and Secondary palates in vertebrates

18. Pharynx • Shared region between digestive and respiratory systems – Respiratory system represents a derivative of the digestive tract. • Other pharyngeal derivatives • Thyroid- present in all vertebrates, always derived as outpocketing from floor of 1st pharyngeal pouch • Fish = thyroid tissue becomes dispersed along the ventral aorta in adults • Tetrapods= remains as a single or bilobedgland • Function = produces Thyroid Hormones that increase metabolic rate and regulate early development and growth • C-cells are also present (only in mammals); produce Calcitonin which decreases blood calcium levels by reducing bone resorption

19. Other Pharyngeal Derivatives • Parathyroids- not present in fishes; present in all tetrapods • Amphibians and Reptiles = derived from ventral regions of pouches 2-4 • Birds = from ventral regions of pouches 3-4 • Mammals = from dorsal regions of pouches 3-. • Secrete parathyroid hormone which increases blood calcium levels by promoting bone resorption

20. Other Pharyngeal Derivatives • Thymus - found in all vertebrates except Cyclostomes • Derived from various pouches in the different vertebrate groups • Function: immunological role, production of T-lymphocytes  cell-mediated immunity • UltimobranchialBodies = derivatives of ventral part of 5th pharyngeal pouch in all vertebrates except mammals • Secrete Calcitonin, so they are presumably homologous with C-cells of mammalian thyroid gland • 1stPharyngeal Pouch forms spiracle in Elasmobranchs • Forms the tympanic cavity and Eustachian tubes in Tetrapods

21. Comparative Pharyngeal Pouch Derivatives in Vertebrates

22. Digestive Tube Proper • General Sequence: anterior to posterior is Esophagus  Stomach  Intestine  Cloaca (or anus) • Esophagus: • Function = food transport; secretes mucus to aid passage • Birds show specialized Crop = sac-like structure adapted for food storage

23. Stomach • None present in Cyclostomes, chimeras, lungfish, and some teleosts (primitive condition) • When present, functions in food storage, physical treatment of food, initiates digestion • Food storage is the primary function (and probably the original evolutionary function) • Physical treatment evolved somewhat later as food is taken in large chunks • Digestion probably is latest function to evolve

24. Stomach • Birds and Crocodiles • Muscular tissue of stomach is concentrated posteriorly as a gizzard • Anterior stomach is glandular (Proventriculus) • Because birds lack teeth, many will swallow small pebbles (grit) that lodge in the gizzard and aid in grinding food • Functional analog to teeth in mammals

25. Stomach • Ruminant Mammals (Cud-chewing Ungulates) • Possess ruminant stomach with 4 chambers • When food is eaten it enters rumen and reticulum which reduce the food to pulp • Microorganisms are present that aid in the breakdown of complex carbohydrates in plant material • The cud is then regurgitated for more chewing • After chewing the cud, the remasticated material passes to omasum and abomasum where physical and chemical processing similar to normal mammalian stomach occurs • The rumen, reticulum, and omasum are derived as modifications of esophagus; abomasum is the true stomach • Ruminant-like digestion occurs in one bird, the Hoatzin • Folivorous(eats leaves) bird with foregut fermentation similar to ruminant digestion • Enlarged crop & lower esophagus house symbiotic bacteria

26. Fig 13.42 – Ruminant digestion in the bovine stomach

27. Foregut fermentation in Hoatzin digestive system

28. Intestine • Majority of digestion and absorption occurs here • Sharks and some other fishes have a spiral intestine = cigar-shaped body with spiral valve internally • Greatly increases surface area for absorption • Increased surface area in Tetrapods is by elongation and coiling of intestines along with folding of internal surfaces • Intestine is longer in herbivores than in carnivores because plant matter is more difficult to digest

29. Intestine • Evolutionary Trend in intestine structure = increased intestinal surface area (primitive  advanced) associated with higher metabolic rates in advanced vertebrates • Hagfish lack spiral valve; poorly developed in lampreys • Spiral valve is present in sharks and some other fishes • Elongation and coiling with internal folding in Tetrapods

30. Fig 13.27 – Stomach and Intestines in non-mammalian vertebrates

31. Figure 13.28 Fig 13.28 – Stomach and Intestines in various mammals

32. Fig 13.29 – Digestive tracts of various fishes. Note spiral valves in several species and elongation of intestine in perch