Digestive System and Derivatives Derivatives include respiratory system

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 – Esophagus – Stomach — Small Intestine — Large Intestine — Cloaca (or derivative) • Also includes associated digestive glands: liver, pancreas and gall bladder

Figure 13. 1 Fig 13. 1 – Digestive tract components

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: a. The epiblast oriented on top of the hypoblast in flat sheets. The hypoblast is continuous peripherally with the endoderm of the prospective yolk sac. b. Development of head, lateral body, and tail folds separate the embryo from extraembryonic membranes. c. The endoderm folds upon itself to form a tube continuous ventrally with the yolk sac forms the gut.

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

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. • Proctodeum is 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.

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

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

Figure 13. 4 Fig 13. 4 – boundaries of the mouth cavity

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 dentary bones – Mammals = found only on margins of maxillary, premaxillary and dentary bones

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

Degree of Tooth Differentiation • Homodontous Condition = all teeth are similar, generally conical in shape – Most vertebrates • Heterodontous Condition = specialization of teeth – Typical state for a few reptiles, Therapsids, and Mammals – Teeth include: 1. Incisors (front) - used for cropping 2. Canines - behind the incisors, used for tearing 3. 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

Homodontous Teeth from salamander Heterodontous Teeth from fox

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

Fig 13. 37 – Salivary glands in a dog

Fig 13. 35 – Oral glands of reptiles. Venom glands derived from Duvernoy’s gland.

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

Fig 7. 57 – Primary and Secondary palates in vertebrates

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 1 st pharyngeal pouch – Fish = thyroid tissue becomes dispersed along the ventral aorta in adults – Tetrapods = remains as a single or bilobed gland – 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

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

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

Comparative Pharyngeal Pouch Derivatives in Vertebrates

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

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

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

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

Fig 13. 42 – Ruminant digestion in the bovine stomach

Foregut fermentation in Hoatzin digestive system

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

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

Fig 13. 27 – Stomach and Intestines in non-mammalian vertebrates

Figure 13. 28 Fig 13. 28 – Stomach and Intestines in various mammals

Fig 13. 29 – Digestive tracts of various fishes. Note spiral valves in several species and elongation of intestine in perch