DIGESTIVE SYSTEM

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Digestive System
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I. Features of the Generalized Vertebrate Gut

• A. Mouth - orobranchial cavity1. Amphioxus -
  filter feeding and associated with gill slits
• 2. Lamprey - oral disc with teeth for
  rasping

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• 3. Gnathostomes- mouth with true jaws
• Stomodeum invaginates from ectoderm of body
  covering (breaks through)
• Rathke's pouch gives rise to anterior pituitary

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B. Teeth

• 1. Origin - evolved from dermal denticles
  associated with the placoid scale

Shark placoid scales and teeth
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• 2. Structure of the toothEnamel - from
  Ectoderm
• Remainder of the tooth arises from mesoderm
• Dentin - interior to the enamel
• Cementum - below the gum line
• Pulp - living region with blood supply and nerves

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3. Trends

• a. Location - some fish - branchial barsothers -
  premaxillary, maxillary, dentary, and palatine
  bones
• b. Number - Polyodont - numerous teeth, variable
  in number -trend is toward reduction in number
• c. Replacement
• polyphydont polymodal - several sets and
  teeth are replaced when lost
• diphyodont - 2 sets as in mammals - deciduous or
  milk and permanent teeth

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• d. Formhomodonty - all teeth have nearly same
  morphology - most vertebrates
• heterodonty - teeth with differing morphology
  and function - mammals some reptilesincisorsc
  aninespremolarsmolars

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More on heterodonty
Permanent teeth
Deciduous teeth
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Tooth Formula

• I - C - P - M
• I - C - P - M
• Primitive mammals 3 - 1 - 4 - 3
• 3 - 1
  - 4 - 3
• Human 2 - 1 - 2 - 3
• 2 - 1
  - 2 - 3
• Lab Mouse 1 - 0 - 0 - 3
• 1 - 0
  - 0 - 3
• Cattle 0 - 0 - 3 -
  3
• 3 - 1
  - 3 - 3

Total number of teeth
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e. Anchorage of tooth to bone

• acrodont - in fish and attached to surface of
  bone
• pleurodont - in amphibians and most reptiles and
  attached to a bony shelf
• thecodont - fossil birds, crocs and gators, and
  mammals and imbedded in socket (deepest in
  mammals)

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C. Glands of the mouth

• Salivary Glands - submaxillary, sublingual,
  parotoid, suborbital, mandibular, zygomatic -
  presence is variable among vertebrates.
• Saliva serves for lubrication and digestion
• Venom Glands - poisonous snakes and evolved from
  salivary glands several times independently

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D. Tongue

• 1. Fish - pad on basihyal bone
• 2. Amphibians - first true tongue - hyoid arch
  here also
• 3. Reptiles, birds, and mammals - muscular tongue
  generally only in mammals and reptiles , others
  flood with lymph or blood
• 4. Associated muscles arise from myotomes of
  arches I, II, III)

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E. Esophagus

• 1. Length - short in fish and amphibians,
• longer in amniotes - to bypass lungs and
  heart

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• 2. Muscles -
• upper esophagus- smooth plus striated
• lower - smooth
• 3. Modifications -
• crop in birds - sac-like expansion of esophagus
  for storage

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• F. Stomach - originally for storage
• 1. Occurrence - absent in lampreys, lungfish,
  some bony fishes
• General trend is for increase in complexity
  elsewhere

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• 2. Regions of the stomach -
• cardiac
• fundus
• pyloric
• Regions designated based on histology (functional
  cell types)

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3. Specialized stomachs

• Stomach (foregut) fermenters - ruminants
• cattle, deer

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G. Intestine

• Small and large intestines in most vertebrates
• Mucosa (lining) adapted for absorbtion with folds
  and cellular structure
• Intestines are short in carnivores, long in
  herbivores

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H. Caecum

• Hindgut fermenters - Caecum houses bacteria to
  digest cellulose

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Relict caecum in humans - appendix
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I. Cloaca

• Common exit for digestive and urogenital tracts
  among vertebrates
• Present embryologically in mammals
• Present in adults of nearly all other vertebrates
  except fish

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Fate of the cloaca in mammalian development
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II. The Liver

• A. Development - outpocketing from gut whose
  constriction remains as bile duct and gall
  bladder is an expanded portion of ductHepatic
  caecum of gut in cephalochordates such as
  amphioxusB. Digestive function - secretion of
  bileC. Other functions detoxification destruc
  tion of RBCs stores glycogen fatty materials
  - cholesterol

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III. Pancreas

• A. Origin - dorsal and ventral outpocket of gut
  maintaining union with prancreatic duct
• B. Functions1. Exocrine function produces
  digestive enzymes 2. Endocrine function
  produces two hormones -
• insulin and glucagon