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Digestive System and Derivatives

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Title: 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 extraembryonic
    membranes.
  • 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.
  • 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.

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)
  • 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

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

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
  • 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

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 Duvernoys 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 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

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
  • 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

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
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