1. Oral Cavity and Digestion in Vertebrates
BIO308/508
Comparative Anatomy

2. The Oral Cavity (Buccal Cavity)
initial mouth in an embryo is an invagination called the stomodeum.
stomadeum is separated from embryonic gut by a buccopharyngeal membrane.
first part of embryonic gut just posterior to mouth is pharynx.
remainder of gut is alimentary canal.
oral cavity is lined with ectoderm.
pharynx and alimentary canal lined with endoderm.

3. Divisions of Digestive System

4. The Oral Cavity
Problem - exact boundary between oral cavity and pharynx not the same in all vertebrates.
i.e. features that are inside the oral cavity in some vertebrates are on the outer surface of the head in other vertebrates.
How can we tell this?
two landmarks identify the location of the mouth on a vertebrate.

5. Landmarks for Boundary of Oral Cavity
Nasal placode - a thickening of the ectoderm that gives rise to nasal sac.
Hypophyseal pouch - ectoderm that gives rise to pituitary gland.

6. The Oral Cavity - The Tongue
a true tongue only in tetrapods.
sharks and fish lack a true tongue - their tongue is a fold in floor of pharynx without musculature.
basihyal and ceratohyal cartilages push forward and upward to create a fold in floor of oral cavity - becomes primary tongue.
some fish have a “tongue” with teeth for holding prey against roof of mouth.

7. The Tongue of a Tetrapod
tongue of tetrapods is mobile (i.e. muscular).
only posterior part is homologous to primary tongue.
muscles of tongue attach hyoid apparatus.
hyoid apparatus made of elements from hyoid arch and first (and sometimes second) branchial arches.

8. The Tongue of an Amniote
Consists of:
primary component that is derived from mesenchyme of hyoid arch (homologous to primary tongue
glandular component that contributes to some sensory structures
pair of lateral lingual swellings from mesenchyme of mandibular arch and contribute to musculature of tongue.

9. The Digestive System
basic function of digestive system is to extract nutrients from foods.
Extraction occurs through the following processes:
mechanical breakdown of food (chewing in mammals or grinding of food in gizzard of birds)
chemical breakdown of food
temporary storage of food
absorption of nutritional components through gut wall
transportation of food along digestive tract to sites for breakdown and absorption
elimination of undigested wastes

10. The Alimentary Canal
With exception of lampreys and hagfish (i.e. living agnathans), there are at least 3 divisions of alimentary canal:
esophagus
stomach (absent in lampreys and hagfish)
intestine
esophagus and stomach typically known as foregut.
intestine with its specializations is the hindgut.

11. The Esophagus and Stomach
primitively, there is little differentiation between the esophagus and stomach.
esophagus is short in aquatic vertebrates (fish and many amphibians) and simply merges with stomach.
tetrapods typically have a neck and a much longer esophagus.
mucous usually secreted by cells in wall of esophagus to lubricate food; cilia may be present to help move food.
one specialization of the esophagus is the crop of birds; storage site for food along ventral side of esophagus.

12. Alimentary Canal

13. The Stomach
stomach is primitively little more than a temporary storage site for food before it enters the intestine.
stomachs of advanced vertebrates tend to be divided into the following regions:
glandular - cardia (only in mammals), fundus and pylorus
non-glandular - may form from esophagus (e.g. ruminant mammals)
cardia secretes mucous; between fundus and esophagus.
fundus is digestive region; secretes enzyme pepsinogen and hydrochloric acid (converts pepsinogen into active form pepsin).
pylorus secretes mucous that helps to neutralize acid in stomach.

14. Specializations of the Stomach
stomach of birds and crocodiles divided into:
proventriculus (corresponds to fundus)
gizzard (corresponds to pylorus)
proventriculus is site for enzymatic breakdown of food.
gizzard is site for mechanical breakdown of food by hard objects (e.g. pebbles) that are swallowed.
stomach of specialized herbivorous mammals such as cows and deer (ruminants) has four chambers for digestions of plants.
plant cell walls contain cellulose which must be broken down to extract nutrients from interior of cell.

15. Ruminant Stomach 4 compartments to ruminant stomach: rumen reticulum
omassum
abomassum
rumen , reticulum and omassum derived from esophagus.
abomassum is derived from stomach (fundus).

16. The Intestine
site for completion of digestion and absorption of nutrients.
ability to absorb dependent upon available surface area so often specializations to increase surface area and/or slow passage of food.
spiral valves in sharks.
pockets of intestine called pyloric caeca in most bony fish (they will lack a spiral valve); increase time for passage of food.
increase length of intestine (especially for herbivores).
development of thousands of tiny projections of wall of intestine called villi (singular is villus).
cells of villi may have extra projections called microvilli.
intestine divided into small intestine and large intestine.

17. Villi and Microvilli

18. The Intestine anterior region of intestine is the duodenum.
connected by ducts from pancreas and gall bladder of liver.
pancreas secretes enzymes for digestion of fats, carbohydrates and proteins.
gall bladder temporarily stores and releases bile that emulsifies fats.
small intestine principal area for digestion and absorption.
large intestine will absorb water, but mostly transport undigested materials to rectum where wastes are stored temporarily.

19. The Cloaca
the end of the intestine in many vertebrates opens into the cloaca (Latin meaning ‘sewer’).
forms during development as a second invagination called the proctodeum.
at first separated from gut by a cloacal membrane that will break down.
cloaca is small chamber where digestive, urinary and reproductive systems empty.
cloaca is absent in some fishes and most mammals.