Digestive System I Oral cavity & teeth. FIRST CATCH YOUR FOOD. Oral (buccal) cavity adapted to the way food is gathered and type of food ingested. Tunicates, cephalochordates, lamprey larvae are jawless and filter feeders.
Digestive System I Oral cavity & teeth
FIRST CATCH YOUR FOOD Oral (buccal) cavity adapted to the way food is gathered and type of food ingested Tunicates, cephalochordates, lamprey larvae are jawless and filter feeders. Water drawn in by cilia, expansion of buccal cavity etc. Food particles trapped in mucous, mucous moves down gut by ciliary action.
JAWS – larger prey Fish are suction feeders (mostly) or ram feeders (overtake prey with mouth open). sharks – combination of suction and ram-feeding • Open = • Epaxial muscles lift head • hypaxial muscles • hyoid muscles • mandibular muscles • Lower jaw to produce enormous gape. • Some enlargement of pharynx to produce suction
Suction feeders e.g. bony fish Increase in number and motility of bony elements Open – virtually all the elements move (kinetic skull) to rapidly expand the pharynx and generate great suction force Similar suction feeding in aquatic salamanders
TERRESTRIAL VERTS One of the major changes in evolution onto land was feeding mechanism. No water to suck in. Co-ordinated movements of mandible and tongue replace it. 4 phases of feeding cycle in all terrestrial verts: 1. Slow opening Kinetic skull - transverse hinge across skull. Pterygoid muscles move p[terygopid bones forward & snout lifts up. Slight lowering of mandible
2. Fast opening Sudden opening to max gape – depressor mandibulae, mandibulohyoideus 3. Fast closing Mandible brought back into position –adductor mandibulae
4. Slow closing/power stroke Crushing cutting slicing of prey The champions of cranial kinesis
Cranial kinesis in birds, allows a good repertoire of abilities for different foods, shock absorption for woodpeckers.
MAMMALS – same 4 phases, NON-KINETIC SKULLS. New muscles e.g digastric (opening) and temporalis (closing), new movements – grinding, cutting, depending on carnivore or herbivore
TEETH Present in some part of oral cavity of all gnathostomes unless secondarily lost. ‘Outside in’ theory – similarity to scales of sharks and placoderms had dentine-like scales led to suggestion of teeth from scales. But presence of pharyngeal teeth in teleosts and discovery of pharyngeal teeth in jawless vertebrate fossils suggests ‘Inside out’ theory. Teeth are excellent for fossilisation and are as diagnostic of a species as DNA
Tooth development – neural crest cells, remarkably similar to bony scales and even hairs & feathers. Cap stage Bell stage Blue = neural crest Odontoblasts from neural crest, ameloblasts from epithelium
Adult tooth Dentine with enamel on exposed surface Kept in place by cement and/or periodontal ligament
Tooth attachment to jaw Therapsid ancestors of mammals and mammals – more food, can withstand stronger forces Most vertebrates – POLYPHYODONT – new tooth develops under old one, complex cycles of replacement from front to back ensuring that >50% of teeth fully functional
Most mammals – DIPHYODONT – 2 sets, deciduous (milk) and permanent All these teeth are there as tooth germs, no re-development of teeth. Toothed whales are MONOPHYODONT – 1 set. Most vertebrates have teeth similar shape = HOMODONT
HETERODONT teeth = mammals = teeth specialized for different functions INCISORS CANINE PREMOLARS MOLARS
Dental formula - humans = I 2/2, C 1/1, P2/2, M3/3 INCISORS – cutting, cropping and picking up. Specialised in rodents, enamel on front surface, continually grows. Tusks are highly modified incisors. Mouse = I 1/1, M 3/3
CANINES – seizing piercing and killing, especially in carnivores Pig-deer from Malaysia - defense
PREMOLARS and MOLARS – combined cutting and crushing, most complex and variable. These teeth have 2 or more roots and crown has cusps and crests First mammals had 3 cusps in linear sequence Then they became triangular and the number of cutting surfaces increased Then they became square = BUNODONT Much better crushing and grinding action for omnivores and herbivores, no sharp cones, now they are low hillocks
In specialized herbivores – elephants, rodents, horses, rhinos instead of 4 cusps many more have formed and they all fuse together to form ridges or LOPHS = LOPHODONT molars. Also here the premolars have become molars
ORAL GLANDS Fish don’t have them except for occasional mucus secreting cell Lampreys have pair of glands that secrete anti-coagulant. Terrestrial verts have well developed salivary glands. SALIVARY GLANDS Modified in some snakes to produce neurotoxins & vampire bats to produce anticoagulants Produce saliva = lubricates, moistens, begins digestion (a-amylase), antiseptic (IgA & lysozyme)
mandibular • sublingual • parotid • Some species have • zygomatic, buccal, • molar
Salivary glands are exocrine (ducted), merocrine (type of secretion). • 2 cell types in a salivary gland • serous cell – dark staining, produces a watery, thin secretion • mucus cell – pale staining, produces a thick secretion