Digestive System

Updated 3 November 2006

Functions

A. Transportation and storage - ingestion (opportunistic feeders), defecation (bolus, control: peristalsis/sphincters)

B. Physical treatment - reduce size add mucous (increase surface area)

C. Chemical treatment - break down into simple substances (enzymatic, bacterial fermentation); chyme

D. Absorption - large surface area, high vascularization

Structure

A. Embryonic development (slide 1)

B. Components (slide 2)

1. Oral (buccal) cavity

a. boundaries (stomodeum-ecto, pharynx-endo; palatoglossal arches)

b. phylogenetic differences (slide 3)

2. Lips - soft epidermis or keratinized (birds, turtles)

a. may be manipulative (mammals - facial muscles)

b. cheeks (mammals; DRAW)

3. Internal nares (choanae); posterior movement (slide 4)

4. Tongue

a. primary - fishes, aquatic amphibians (hypobranchial musculature)

b. definitive - most amphibians, amniotes

1) sensory (many squamates)

2) manipulate food (mammals)

3) capture food (hyoid - many amphibians, chameleon, anteater, woodpecker); Video

4) articulation (man)

5. Oral glands (especially terrestrial vertebrates: moisten food, poison prey); e.g., lamprey

Teeth

A. Why study?

1. Well preserved in fossil record (only record available for some taxa)

2. Reflect diet/foraging strategy (adaptive)

3. Taxon-specific stability in lower groups (use as taxonomic character)

B. Functions

1. Prey capture

2. Mastication (primarily mammals)

3. Defense

4. Sexual/social interactions (courtship, territorial behavior, dominance)

C. Structure (slide 5)

1. Crown, root, pulp, dentine, cement, enamel, cusp

D. Attachment

1. Fibrous (many fish) or bony connection (most tetrapods)

2. Types slide 6

a. acrodont (on top) - teleosts, amphibians, some lizards

b. pleurodont (lingual shelf) - most squamates

c. thecodont (sockets) - thecodonts (order Thecodontia), crocodilians, mammals

E. Replacement (wear and tear, loss)

1. Polyphyodont (most), diphyodont (mammals)

2. Patterns and timing (no impairment of function)

a. e.g., reptile (2 sets relative to age, replaced in waves from anterior®posterior)

b. replaced from below (most) or rear (elephants; slide 7)

F. Vertebrate survey

1. Edentate (no teeth) - mysticete whales, recent birds & turtles, advanced anteaters, some frogs, recent monotremes, agnathans

2. Phylogenetic trends - reduction in number and location, more specialization in structure and function

3. Fish - most homodont, conical, numerous throughout oral cavity

a. major exceptions - hinged teeth, broad crushing plates

b. pharyngeal teeth (not in oral cavity)

4. Amphibians - most homodont, conical, numerous; slide 8

a. labyrinthodonts - infolding of enamel (also some crossopterygians)

b. lissamphibians - pedicelate

5. Reptiles - many heterodont, greatly reduced number

a. synapsids (slide 9)

b. some lizards (cuspidate)

c. snakes (aglyphous, opistoglyphous, proteroglyphous, solenoglyphous; DRAW)

6. Mammals (chewers - important)

a. most heterodont (exceptions: ondontocete whales, primitive anteaters)

b. few teeth, restricted to jawbones

c. highly specialized dentition (slide 10)

1) incisors (rooted in premaxilla) – nipping

2) canines (1st rooted in maxilla) – stabbing

3) premolars (no deciduous precurser) - crushing, cheek tooth

4) molars (deciduous precurser) - crushing, cheek tooth

d. dental formula

1) primitive placental formula: 3/3, 1/1, 4/4, 3/3 = 44

2) cat: 3/3, 1/1, 3/2, 1/1 = 30

3) rat: 1/1, 0/0, 0/0, 3/3 = 16 (diastema)

4) man: 2/2, 1/1, 2/2, 3/3 = 32

5) deer: 0/3, 0/1, 3/3, 3/3 = 32

e. specializations

1) incisors: elephant tusks, Monodon, tarsiers (slide 11), vampire bats

1) canines: walrus slide 11, saber tooth cats (Box 13.3, slide 12; slide 23)) cheek teeth: carnivory: carnassial (p⁴/m₁) – shearing

2) herbivory: brachyodont (vs. hypsodont) - increase cusp height; slide 13

3) frequently rootless (beaver skull - DEMO)

Gut fine structure (slide 14)

A. Mucosa - mucous epithilium, secretory, absorptive (active layer)

1. Structure of mucosa varies along length and defines gut type (slide 15)

B. Submucosa - connective tissue

C. Muscular - circular and longitudinal (peristalsis)

D. Serous - secretory epithilium, conective tissue

Gut regions (endoderm derivatives)

A. General

1. 2 regions always differentiated (foregut, hindgut; pyloris in between)

2. Phylogenetic trend toward more foregut differentiation

3. Specific regions below may not be differentiated in all vertebrates

4. Highly adaptable (even in closely-related species) - reflects diet

B. Pharynx - simple passageway in adults; important derivatives in embryo (e.g., thymus, tonsil, parathyroid, thyroid, middle ear, auditory tube)

C. Esophagus (transportation)

1. Mucus, ciliated (keratinized in birds, turtles); distensible (temporary storage)

2. Crop (birds, storage, pigeon milk secretion); slide 16

D. Stomach

1. Mucosal structure (glandular and non-glandular regions)

2. Esophageal (non-glandular except mucus; sometimes keratinized)

3. Cardiac (transitional [mammals only], mucus)

4. Fundic (highly glandular: digestive, enzymes, HCL [absent in ant eaters])

5. Pyloric (transitional, mucus)

6. External morphology roughly reflects lining types

a. recognize cardiac, fundic, pyloric

b. major exceptions (not differentiated in some, e.g., lamprey)

c. may concentrate muscular portion posteriorly (gizzard (=ventriculus): crocodilians, birds (gastroliths; grit); substitute for teeth? slide 17

7. Complex stomachs (ruminant ungulates; slide 18, slide 19)

a. rumen (temporary storage from opportunistic ingestion)

b. reticulum (cud forming); regurgitate and ruminate

c. omasum (physical treatment)

d. abomasum (=cardiac, fundic, pyloric [“true” stomach])

E. Intestine (chemical digestion and absorption)

1. How increase process time and/or surface area?

a. intestinal folding

b. villi and microvilli slide 20

c. spiral valve slide 21

d. increase length (mammalian carnivore 2-6X body length; herbivore 20-25X)

e. caeca - variable in number and size; teleosts (few-many (slide 22); absorption), rabbit (fermentation; coprophagy; man-appendix)

f. regions

2. Small intestine - relatively long, digestion and absorption

a. duodenum (most anterior) receives secretory digestive products from liver, pancreas

3. Large intestine (=colon) - relatively short, water absorption and feces formation; terminates in cloaca or rectum (some fishes, therian mammals)

4. “Large” and “small” intestine not differentiated in gross morphology (e.g., diameter) in many vertebrates

READ

A. Associated glands of digestion

B. Function and evolution of the digestive system