Oral Cavity and Digestion in Vertebrates

BIO308/508Comparative Anatomy

The Oral Cavity (Buccal Cavity)• initial mouth in an embryo is an

invagination called the stomodeumstomodeum.• stomadeum is separated from

embryonic gut by a buccopharyngeal buccopharyngeal membranemembrane.

• first part of embryonic gut just posterior to mouth is pharynxpharynx.

• remainder of gut is alimentary alimentary canalcanal.

• oral cavity is lined with ectoderm.

• pharynx and alimentary canal lined with endoderm.

Divisions of Digestive System

The Oral CavityProblemProblem - 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.

Landmarks for Boundary of Oral Cavity

Nasal placodeNasal placode - a thickening of the ectoderm that gives rise to nasal sac.

Hypophyseal pouchHypophyseal pouch - ectoderm that gives rise to pituitary gland.

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.• basihyalbasihyal and ceratohyalceratohyal cartilages push forward and upward to create a fold in floor of oral cavity - becomes primary tongueprimary tongue.• some fish have a “tongue” with teeth for holding prey against roof of mouth.

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 hyoid apparatusapparatus.

• hyoid apparatus made of elements from hyoid arch and first (and sometimes second) branchial arches.

The Tongue of an Amniote

Consists of:– primary componentprimary component that is derived from mesenchyme of hyoid arch (homologous to primary tongue

– glandular componentglandular component that contributes to some sensory structures

– pair of lateral lateral lingual swellingslingual swellings from mesenchyme of mandibular arch and contribute to musculature of tongue.

The Digestive System• basic function of digestive system is

to extract nutrients from foods.Extraction occurs through the following

processes:1. mechanical breakdown of food (chewing

in mammals or grinding of food in gizzard of birds)

2. chemical breakdown of food3. temporary storage of food4. absorption of nutritional components

through gut wall5. transportation of food along digestive

tract to sites for breakdown and absorption

6. elimination of undigested wastes

The Alimentary CanalWith exception of lampreys and hagfish (i.e. living

agnathans), there are at least 3 divisions of alimentary canal:1. esophagus2. stomach (absent in lampreys and hagfish)3. intestine

• esophagus and stomach typically known as foregutforegut.• intestine with its specializations is the hindguthindgut.

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 cropcrop of birds; storage site for food along ventral side of esophagus.

Alimentary Canal

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:1. glandular - cardiacardia (only in mammals), fundusfundus

and pyloruspylorus2. non-glandular - may form from esophagus

(e.g. ruminant mammals)1. cardia secretes mucous; between fundus and

esophagus.2. fundus is digestive region; secretes enzyme

pepsinogen and hydrochloric acid (converts pepsinogen into active form pepsin).

3. pylorus secretes mucous that helps to neutralize acid in stomach.

Specializations of the Stomach

• stomach of birds and crocodiles divided into:– proventriculusproventriculus (corresponds to fundus)– gizzardgizzard (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.

Ruminant Stomach4 compartments to

ruminant stomach:1. rumen2. reticulum3. omassum4. abomassum

• rumen , reticulum and omassum derived from esophagus.

• abomassum is derived from stomach (fundus).

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 valvesspiral valves in sharks.– pockets of intestine called pyloric caecapyloric 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 villivilli (singular is villus).

– cells of villi may have extra projections called microvillimicrovilli.

– intestine divided into small intestine and large intestine.

Villi and Microvilli

The Intestine• anterior region of intestine is the duodenumduodenum.

• connected by ducts from pancreaspancreas and gall bladder of liverliver.

• pancreas secretes enzymes for digestion of fats, carbohydrates and proteins.

• gall bladder temporarily stores and releases bile that emulsifies fats.

• small intestinesmall intestine principal area for digestion and absorption.

• large intestinelarge intestine will absorb water, but mostly transport undigested materials to rectum where wastes are stored temporarily.

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 proctodeumproctodeum.

• at first separated from gut by a cloacal membranecloacal 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.