Digestive System and Derivatives - PowerPoint Presentation

Slide1

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

Slide2

Figure 13.1

Fig 13.1

– Digestive tract components

Slide3

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

.

Slide4

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

Slide5

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.

Slide6

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

Slide7

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 formsPrimitively, stomodeal structures are forced outside the mouth through differential

growth

Slide8

Figure 13.4

Fig 13.4

– boundaries of the mouth cavity

Slide9

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

Slide10

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

Slide11

Degree of Tooth Differentiation

Homodontous

Condition = all teeth are similar,

generally conical in shape Most vertebratesHeterodontous

Condition = specialization of

teeth

Typical state for

a few reptiles, Therapsids, and

Mammals

Teeth

include:

Incisors

(front) - used for

croppingCanines - 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

Slide12

Homodontous

Teeth from salamander

Heterodontous

Teeth from fox

Slide13

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 secretionsSnake venom glands are modified serous salivary glands

Slide14

Fig 13.37

– Salivary glands in a dog

Slide15

Fig 13.35

– Oral glands of reptiles. Venom glands derived from

Duvernoy’s

gland.

Slide16

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

Slide17

Fig 7.57

– Primary and Secondary palates in vertebrates

Slide18

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

Slide19

Other Pharyngeal Derivatives

Parathyroids

- not present in fishes; present in all

tetrapodsAmphibians 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

Slide20

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 5

th

pharyngeal pouch in all vertebrates

except mammalsSecrete Calcitonin, so they are presumably homologous with C-cells of mammalian

thyroid gland1st

Pharyngeal Pouch forms

spiracle

in

Elasmobranchs

Forms

the tympanic cavity and Eustachian tubes in

Tetrapods

Slide21

Comparative

Pharyngeal Pouch

Derivatives in

Vertebrates

Slide22

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

Slide23

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

Slide24

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

Slide25

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

occursThe 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

Slide26

Fig 13.42

– Ruminant digestion in the bovine stomach

Slide27

Foregut fermentation in Hoatzin digestive system

Slide28

Intestine

Majority

of digestion and absorption occurs here

 Sharks and some other fishes have a spiral intestine = cigar-shaped body with spiral valve internallyGreatly

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

Slide29

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

Slide30

Fig 13.27

– Stomach and Intestines in non-mammalian vertebrates

Slide31

Figure 13.28

Fig 13.28

– Stomach and Intestines in various mammals

Slide32

Fig 13.29

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