Digestive System and Derivatives

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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

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Figure 13.1
Fig 13.1 Digestive tract components
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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 extraembryonic
  membranes.
• The endoderm folds upon itself to form a tube
  continuous ventrally with the yolk sac ? forms
  the gut.

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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

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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.

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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
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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)
• Rathkes 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

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Figure 13.4
Fig 13.4 boundaries of the mouth cavity
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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

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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

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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
• 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

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Homodontous Teeth from salamander
Heterodontous Teeth from fox
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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

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Fig 13.37 Salivary glands in a dog
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Fig 13.35 Oral glands of reptiles. Venom glands
derived from Duvernoys gland.
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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

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Fig 7.57 Primary and Secondary palates in
vertebrates
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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 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

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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

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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
• Ultimobranchial Bodies 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
• 1st Pharyngeal Pouch forms spiracle in
  Elasmobranchs
• Forms the tympanic cavity and Eustachian tubes in
  Tetrapods

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Comparative Pharyngeal Pouch Derivatives in
Vertebrates
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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

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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

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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

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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

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Fig 13.42 Ruminant digestion in the bovine
stomach
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Foregut fermentation in Hoatzin digestive system
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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

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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

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Fig 13.27 Stomach and Intestines in
non-mammalian vertebrates
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Figure 13.28
Fig 13.28 Stomach and Intestines in various
mammals
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Fig 13.29 Digestive tracts of various fishes.
Note spiral valves in several species and
elongation of intestine in perch