Biology 3315 – Comparative Morphology Skeletal Tissues, Teeth, and Dentitions

1. Notochord Cross section of notochord of Lamprey. Note nerve cord above notochord. The notochord has a gelatinous texture but is enclosed in a strong fibrous sheath. This structure is firm yet flexible and helps prevent the body from telescoping when the surrounding muscles contract

2. Cartilages Hyaline cartilage Notice how the cartilage cells (chondrocytes) form small cell groups surrounded by the matrix. The spaces in the matrix that house the cartilage cells are called lacunae.

Fibrous cartilage X-section through a developing knee joint. Notice how the red staining collagen fibers in the fibrocartilage give this cartilage a much rougher appearance than the smooth matrix of hyaline cartilage.

Elastic cartilage Elastic fibers are dark staining, web-like structures. Chondrocytes are reddish-brown.

Calcified cartilage This hardest of cartilages is present in a variety of adult skeletal structures. Examples are the jaws and vertebrae and the ends of the long bones of the frog.

3. Compact (Haversian) bone In this cross section of human bone, identify the following structures: Haversian systems (osteons), Haversian canals, lamellae, lacunae, and canaliculi. Notice the canals joining two Haversian canals together (Volkman’s canal).

4. Bone development Intramembranous or dermal bone development Unlike endochondral bone, which is formed by cartilage replacement, dermal bone is formed by a more direct process. Embryonic mesenchyme, which is a loose, relatively undifferentiated tissue, gives rise to fibroblasts, which in turn connect to one another to begin to lay down the matrix where the bone will develop. The fibroblasts are now termed osteoblasts, which begin to form bone directly (i.e. no cartilage involved). The bone slivers or spicules increase in size via this process until they join one another to form the basic bone pattern. In this slide, a bit of cartilage (purple), which has nothing to do with dermal bone formation, is visible. Orange structures are bone.

Endochondral bone development This slide shows the replacement of cartilage by bone in a developing long bone. Compare the slide to the drawing. Note the longitudinal columns of chondrocytes elarge towards the developing bone.

5. Growth of mammalian long bones The long bones of immature mammals are distinct in having true epiphyses at their ends. These structures are present due to secondary centers of ossification. The gap between the epiphysis and diaphysis is filled (in life) by the epiphyseal or growth plate. Of what material is the plate composed? What is its function? True epiphyseal growth plates do not occur in other groups of . Compare the femora of the young and adult artiodactyls. How many epiphyses are there in the growing animal?

6. Homodonty and polyphyodonty The shark provides a good example of vertebrate dentition in which the teeth are similar in form (=homodont) and function and are continually replaced throughout life (= polyphyodont). Observe the developing replacement teeth inside the margins of the jaw.

7. Teeth of teleost Primitive teleosts, like the salmon, retain numerous teeth on both major bones of the upper jaw (premaxillary, maxillary). In advanced teleosts, such as the marine rockfish shown here, teeth have been lost from the maxillary bone. The mandible of the ling cod demonstrates the homodont, polyphyodont and acrodont dentition typical of most bony fishes. Labels on bones: “d” = dentary, “m” = maxillary, and “pm” = premaxillary 8. Pharyngeal teeth The fifth gill arch of many bony fishes is modified as a tooth-bearing structure and is sometimes termed the "pharyngeal bone". This structure carries true teeth with enamel, dentine, pulp cavities and cement. Pharyngeal teeth guard the entrance to the esophagus and are used to grind and pierce food prior to swallowing. In the case of some teleosts (e.g. carp and goldfish) pharyngeal teeth are the only teeth present. The pharyngeal teeth shown here are from fresh water teleosts (suckers). Does the form of these teeth suggest different functions and diet in these two ?

9. Teeth in Lissamphibia Lissamphibians typically have small homodont, polyphyodont, and pleurodont (only the outer surface of the root touches the bone) teeth. In anurans, the teeth are usually confined to the upper jaw (frogs) or are totally absent (most toads). Both jaws and some of the palatal bones bear teeth in most urodeles. Examine the of the bullfrog under a scope to see the pedicellate of the teeth. Pedicellate teeth are a diagnostic feature of lissamphibians.

10. Teeth in Lepidosauria The skull of the iguana shows teeth that are slightly heterodont, polyphyodont, and pleurodont. Notice the numerous replacement teeth. These do not become attached to the jaw until they are functional. Are there palatal teeth? The homodont, polyphyodont teeth of the alligator are thecodont (tooth held in socket of jaw bone).

11. Tooth development and structure This micrograph shows developing teeth in a mammalian embryo. Locate the tooth bud and identify the ameloblasts, enamel, odontoblasts, and dentine. From what germ layers (embryonic tissue) do these structures develop? On the sectioned mammalian tooth locate the following: crown, root, dentine, pulp cavity, and cement. Cement is a specialized, acellular bone. What are the principal structural differences between enamel and dentine? How does dentine differ from typical bone?

12. Mammalian tooth replacement Unlike typical reptiles, mammals are limited to two sets of teeth (diphyodonty). Compare the deciduous (milk teeth) of the young dog with the permanent teeth of the adult.

13. Carnassial teeth The and jaws of the wolf and domestic cat illustrate two types of carnassial or sectorial dentitions. That of the wolf is more generalized - how? Can you determine the dental formula of each specimen?

Members of the order Carnivora are characterized by a pair of carnassial teeth specialized for shearing. This shearing is always produced by the upper fourth premolar (P4) and the lower first molar (M1). Notice that the cat lacks a heel on the lower carnassial tooth. What are the functions of the several types of teeth composing the dentition?

14. Lophodont teeth Lophodont teeth have nearly straight ridges that lie at right angles to the tooth row. The simple lophodont teeth present in kangaroos have only two lophs and are low crowned (brachydont). More complex grinding surfaces are found on the cheek teeth of many rodents, including the South American capybara shown here. Notice that cement contributes significantly to the occlusal surface in the capybara but not in the kangaroo. Do the different forms of the teeth in these two herbivores suggest different movements of the jaws during feeding?

15. Elephant tooth What type of occlusal (grinding) surface is represented by this tooth? Identify the three tissues forming the surface of the elephant tooth. Compare the teeth of the elephant to those of the horse. What are some of the similarities and differences?

16. Bunodont teeth Bunodont cheek teeth are adapted primarily for crushing and some grinding. The dentitions of the monkey, bear, and pig illustrate this form of tooth in three orders of mammals (Primates, Carnivora, Artiodactyla). Mammals with bunodont teeth are more or less omnivorous or frugivorous (fruit eating).

17. Dentitions for crushing shellfish Compare the teeth of the river otter with those of its close relative, the sea otter. The river otter eats mainly fish, whereas the sea otter specializes on several types of shellfish (e.g. sea urchins, molluscs). How do the two dentitions reflect these dietary preferences?

18. Selenodont teeth The crescent-shaped folds of enamel on the surface of the cheek teeth are characteristic of selenodont dentitions. The folded enamel encloses softer dentine and cement. The differential hardness of enamel, dentine, and cement causes the grinding surfaces of the teeth to become more complex as they wear.

Note the unerupted molars in the juvenile elk. This specimen also has upper canines, as it is male. The lower canine tooth of the cow is incisiform so the dental formula is: IO/3, CO/1, P3/3, M3/3. In advanced artiodactyls, the upper incisors are lost and replaced by a tough pad of keratinized skin. A deer skull is also shown.

19. Dentitions for fish-eating Simple, homodont teeth are characteristic of fish-eating (piscivorous) vertebrates. Why? Examples shown here are the alligator gar, caiman, sea lion, and porpoise (see illustration). Homodonty is secondary in marine mammals, as is the multiplication of teeth in the porpoise.

20. Evergrowing teeth Mammalian teeth subject to excessive wear, such as the gnawing incisors of rodents and lagomorphs, are open rooted to permit continuous growth throughout the animal's life. Growth rates for teeth of this type may exceed 10 inches per year. The sectioned jaw of this muskrat reveals the full extent of the enlarged incisor. Notice that the root has extended posteriorly to a position behind the molar teeth! The three cheek teeth of the muskrat are high crowned and also open rooted to offset wear.

21. Hypsodont dentitions Many grazing mammals extend the wear life of their cheek teeth by developing very high crowns. In young adults, the crowns extend far below the gum line into the areas of the jaws normally occupied by the roots. As the surface of the tooth wears, the crown is pushed upward to compensate.

Compare the sectioned pronghorn antelope tooth (hypsodont) with that of the mule deer (brachydont). Deer are browsers and feed on relatively soft food, while pronghorns are grazers and feed on tough grasses, sagebrush, and other abrasive foods. Pronghorn teeth are thus subject to much higher rates of wear. Note the relative development of the crown and its placement in the jaw of these two artiodactyls. Can you identify the cement?

22. Ant and termite eating In mammals that feed largely or exclusively on ants and termites, the teeth are reduced (armadillo) or entirely lost (anteater). Instead, a long, sticky tongue is used to capture insect food. Why should suppression of the teeth be associated with a diet of this type?