Transcript Digestive Tract and Derivatives

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 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
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
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
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