Book - Vertebrate Zoology (1928) 18

From Embryology

Vertebrate Zoology G. R. De Beer (1928)

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Chapter XVIII The Teeth

Teeth and the denticles (or placoid scales) of the dogfish are identical in that they consist essentially of a hollow cone of dentine, inside which is a pulp-cavity, and outside which is a layer of enamel. The dentine is formed from the mesoderm of the skin, and the enamel is produced from the overlying ectoderm. The denticle or tooth is formed below the surface of the skin, and is subsequently erupted through it. In the dogfish the denticles are not restricted to the borders of the mouth, but occur all over the surface of the body. In a few bony fish such as Polypterus, Lepidosteus, and catfish, denticles also occur over the surface of the body ; but in the remainder, and all higher vertebrates, teeth are restricted to the mouth. In addition to those on the premaxilla, maxilla, and dentary, teeth may be carried by the prevomer, parasphenoid, palatine, and pterygoid bones in lower vertebrates ; in the bony fish teeth may even be carried on the branchial arches. In Selachians, the teeth are loosely attached to the underlying skeleton by connective tissue. In bony fish, they are firmly fixed on to the underlying bone by " cement, a modified form of bone, which is absorbed when the tooth is shed. In some cases the teeth may be hinged. In higher forms the bone grows round the base of the teeth, which thus come to lie in grooves (pleurodont) or sockets (thecodont). In Sphenodon and Chamaeleo the teeth are fused on to the edge of the bone (acrodont condition) and are not replaced.


The teeth of Osteolepidoti and of the earliest amphibia are peculiar in that their walls are thrown into folds, giving a characteristic appearance when seen in section, and which is responsible for the term Labyrinthodontia which is applied to the earliest amphibia. In snakes the teeth may be grooved or even hollow and converted into poison-fangs. The poisonous secretion passes in the groove or tube and is inserted as with a hypodermic needle into the tissues of the prey.


Living Chelonia have no teeth, but they were present in the primitive fossil Triassochelys. The same applies to birds, which are toothless to-day, but which originally possessed teeth, as is shown by the fossil Archaeopteryx and others.


DeBeer1928 fig121.jpg

Fig. 121. Transverse sections through the lower jaw of mammalian embryos showing the development of the teeth. A, early stage, the dental lamina (dl) has grown in from the ectoderm ; B, a tooth-germ has been formed on the dental lamina, the cells of which (ectodermal) produce the enamel (e) ; beneath the enamel the mesodermal cells produce dentine (dn). C, the development of the first or milk-tooth imt) is nearing completion : beneath it the dental lamina has formed another tooth-germ which will produce a permanent tooth (pt) ; the dentary bone (d) encloses the teeth in a socket, ftn, floor of the mouth ; /, lip ; Mc y Meckel's cartilage ; mp, mesodermal papilla ; pc, pulp-cavity ; t, tongue. (C/. Method of development of the denticle, Fig. 120.)


Fig. 122. The origin of teeth in the dogfish. A, inner side of one half of the upper jaw, showing the rows of reserve teeth ; B, section through the lower jaw ; the smallest teeth are the most recently formed. Mc, Meckel's cartilage.


The teeth of mammals and of those extinct reptiles which were on the mammalian line of descent differ from those of other vertebrates in that they are not all similar, but differ in shape in the various regions of the mouth. This condition is called heterodont, as opposed to the homodont condition when the teeth are all similar.


The most anterior teeth are the incisors, and (except in some Marsupials) they are never more than three in number on each side in each jaw. In the upper jaw they are carried on the premaxilla. Next come the canines, the premolars, and the molars. The molars differ from the premolars in that there is only one set of them, whereas the premolars are represented by a lacteal or " milk " dentition followed by a permanent set which replaces them. In a few mammals, such as the toothed whales, the teeth are all similar, but this is a secondary and degenerate condition.


Another difference between the teeth of mammals and those of other vertebrates lies in the fact that they arise in two sets, or, in other words, they are replaced once only (except for the molars which are not replaced at all). Other vertebrates have perpetual replacement of teeth as and when the exist- ing ones wear out. The mammalian condition is called diphyodont, that of other vertebrates polyphyodont. However, it is probable that the two sets of teeth of the mammal are not to be regarded as simply an abridgement and reduction of the many sets of teeth of, say, the crocodile, for the following reason. The ectoderm, which sinks down beneath the surface of the skin of the mouth to produce the enamel, forms a long band extending parallel to the edge of the jaw, known as the dental lamina. The rudiments of the teeth appear on the outer side of the dental lamina in two families ; one from the middle of the side of the lamina, and the other from its base. The teeth formed by one family of rudiments grow up and are intercalated between the teeth formed by the other family. When, in the crocodile, for example, a tooth has been formed, another tooth arises beneath it from the same rudiment, and this second tooth will eventually push out and replace the first. But any given tooth is only replaced by a tooth belong- ing to its own family, and which has arisen from the same part of the dental lamina. In the mammal there are the same two families of tooth- rudiments, but each rudiment gives rise to one tooth only. Further, owing to the reduction in size of the jaw, there is not room for both families of teeth at the same time. One family appears first, and gives rise to the lacteal or " milk " dentition. Later on, the other family appears and forms the permanent dentition which pushes out and replaces the lacteal teeth. In the mammal, therefore, replace- ment is effected by a tooth of one family displacing a tooth of the other family ; in the other vertebrates replacement is brought about by the displacement of a tooth by another tooth belonging to the same family. It is probable that the molars belong to the permanent family, the corresponding lacteal teeth having been suppressed.


Fig. 123. Diagrams showing the^relation of the mammalian to other modes of tooth-succession. (After Bolk.) A, diagrammatic view of the outer side of the dental lamina (dl) of the lower jaw, showing the alternation between tooth-germs at the side (s) and at the base (b) of the dental lamina. B, diagrammatic representation of tooth-replacement in reptiles ; the teeth formed from the tooth-germs at the side of the dental lamina are shaded : those formed from the tooth- germs at the base of the dental lamina are white ; the tooth-germs produce several teeth, which replace other teeth formed originally from the same tooth-germ as themselves. C, diagrammatic representation of tooth- replacement in mammals ; each tooth-germ produces one tooth only, and the teeth formed from the tooth-germs at the base of the dental lamina replace those formed from the tooth germs at the side of the dental lamina. e, ectoderm.


The Marsupials have a peculiar mode of reproduction in that the young are born very early and in a very undeveloped condition. They are attached to the nipples of the mother and continue their development in her pouch, or marsupium. During this period no teeth are required, and it is found that in Marsupials the lacteal dentition is reduced ; in fact only one tooth (the third premolar) is replaced. That this is a secondary reduction is proved by the fact that in extinct forms replace- ment took place in more of the teeth. There is another point of interest in the teeth of the Marsupials, which refers to the fact that they are the only mammals in which more than three incisors are found on each side. The probable explanation is that in this region of the mouth, the teeth of one family are not replaced by the teeth of the other, but that both families of teeth are erupted together, the members of the two families intercalated as in the crocodile. Behind the canine, however, the families of teeth replace one another as in other mammals. The marsupials, then, are intermediate between the reptiles (simultaneous presence of teeth of both families all over the jaw with complete intercalation) and the higher mammals (no intercalation of teeth of the two families).


Fig. 124. Types of teeth (the different teeth are not drawn to the same scale).A, longitudinal section through a human molar, showing : c, cement (here restricted to the base of the tooth) ; d, dentine ; e, enamel ; pc, pulp- cavity ; r, roots or fangs ; such a tooth is short and low in the crown, and conforms to the type called brachyodont. B, longitudinal section through a premolar of the horse ; cement, dentine and enamel all enter into the com- position of the crown of the tooth, and as the hardness of these substances differs, they are worn away to different extents ; such a tooth is long and high in the crown, and conforms to the type called hypsodont. C, longi- tudinal section through the incisor of a rabbit, showing the open (" root- less ") pulp-cavity or persistent pulp (pp). D, view of the crown of an upper molar of a pig, showing the separate cusps characteristic of bunodont teeth. E, crown of an unworn, F, crown of a worn lower molar of a camel, showing the crescent-shaped ridges joining the cusps, characteristic of selenodont teeth. G, crown of an unworn, H, crown of a worn lower molar of a tapir, showing the transverse ridges joining the cusps, character- istic of lophodont teeth. I, inner side view of a tritubercular (upper) molar, in relation to J, a tuberculo-sectorial (lower) molar ; t, talonid. K, diagram of the relative positions of the cusps of tritubercular molars (dotted lines) of the upper jaw, and tuberculo-sectorial molars (full lines) of the lower jaw ; an, anterior side ; ou, outer side. L, simple peg-like tooth of a reptile.


The primitive shape of the molar teeth in the mammal is three-cusped or tritubercular in the upper jaw, while those of the lower jaw have three cusps and a posterior " heel " or talonid, and are called tuberculo-sectorial. The three cusps of the upper teeth form a triangle or " trigon," with the apex pointing inwards ; the three cusps of the lower teeth form a " trigonid," with the apex pointing outwards. They are so arranged by this means that the teeth of the upper and those of the lower jaw fit into and work against one another. This type of molar was evolved from the primitive reptilian type in which each tooth had but one cusp. The original cusp is represented by the outer cusp of the trigonid in the lower molars, while in the upper molars the original cusp has been split into two and is represented by the two lateral cusps of the trigon. The remaining cusps and the talonid were subsequently developed in relation to the " fit " of the teeth on one another. The number and arrangement of the cusps may be much modified in the different groups, but the primitive forms of most groups of mammals have molars of this tri- tubercular and tuberculo-sectorial type.


When the cusps remain separate as in the pig, the tooth is called bunodont. In other forms, the cusps may be joined to one another by ridges running at right angles to the length of the jaw, as in the tapir (lophodont condition). In others, again, the cusps are splayed out to form crescents running in the line of the length of the jaw, as in the camels (selenodont condition).


It is characteristic of mammalian molars to have divided roots or " fangs." Normally, a tooth grows to a certain size (not very big), and after that the pulp-cavity becomes almost closed at the base. Such a tooth may have one or more " roots " or fangs, and when these have formed, the tooth ceases growing. This is the brachyodont type, the name being derived from the fact that the teeth are comparatively short, and as a rule their possessors do not make use of them for grinding hard materials. Where the diet consists of resistant material which requires grinding, and in other cases where the teeth are subjected to hard wear, the pulp-cavity remains widely open at the base, and the teeth are capable of continuous growth. These teeth are described as being " rootless," or possessing persistent pulps, and from the fact that they are usually long, this condition is known as hypsodont. Examples of hypsodont teeth are to be found in the premolars and molars of the horse, the incisors of the rabbit, the incisors (tusks) of the elephant, and the canines of the boar, to mention only a few.


In the carnivores (cats and dogs) one tooth in each jaw on each side becomes enlarged and modified for tearing flesh, forming the so-called carnassial tooth. It is the last premolar (4th) in the upper jaw and the first molar in the lower jaw. Other carnivores (bears and seals) do not have the carnassial tooth well developed.


In addition to dentine and enamel, it is common for the teeth of mammals to have a complete or partial covering of bone which is called " cement." This may be restricted to the roots of the teeth, as in man, or it may form a complete covering over the crown before the tooth is erupted, as in ungulates. After the tooth has been erupted and projects above the gums, it is subjected to wear, and its different constituents become worn away according to their softness. The hardest substance is the enamel, and next comes the dentine, and lastly the cement which is the softest. The result of the unequal wear in teeth like those of the elephant or of some rodents is that the crown is not smooth but becomes ridged like a file, and such teeth are as efficient as mill-stones grinding against one another. In some of the Edentates (sloths and armadilloes) the teeth have no enamel, while in others (ant-eaters) and in some whales there are no teeth at all.


There is no difficulty in tracing the teeth of vertebrates back to the denticles of the Selachians, and of some of the Ostracoderms. It has been suggested that denticles also gave rise to dermal bone by fusing together. This is very im- probable. Denticles are composed not of bone but of dentine, which differs from bone in that the cells which secrete it do not remain in it but migrate out. Denticles are often found attached to true scales or dermal bones, but these are developed independently from the denticles.


It can be said that the dermal bones and scales develop in relation to the denticles, but not from them.


The so-called teeth of Petromyzon, of Ornithorhynchus, and of the tadpole of the frog are epidermal horny structures, and have nothing whatever in common with the true teeth.

Literature

Bolk, L. Odontological Essays. Journal of Anatomy. Vol. 55, 1921 ; Vol. 56, 1922 ; and Vol. 57, 1922.

Mummery, J. H. The Microscopic Anatomy of the Teeth. Henry Frowde and Hodder & Stoughton, London, 191 9.

Tomes, C. S. A Manual of Dental Anatomy, Human and Comparative, Churchill, London, 1808.


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Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)
Vertebrate Zoology 1928: PART I 1. The Vertebrate Type as contrasted with the Invertebrate | 2. Amphioxus, a primitive Chordate | 3. Petromyzon, a Chordate with a skull, heart, and kidney | 4. Scyllium, a Chordate with jaws, stomach, and fins | 5. Gadus, a Chordate with bone | 6. Ceratodus, a Chordate with a lung | 7. Triton, a Chordate with 5-toed limbs | 8. Lacerta, a Chordate living entirely on land | 9. Columba, a Chordate with wings | 10. Lepus, a warm-blooded, viviparous Chordate PART II 11. The development of Amphioxus | 12. The development of Rana (the Frog) | 13. The development of Gallus (the Chick) | 14. The development of Lepus (the Rabbit) PART III 15. The Blastopore | 16. The Embryonic Membranes | 17. The Skin and its derivatives | 18. The Teeth | 19. The Coelom and Mesoderm | 20. The Skull | 21. The Vertebral Column, Ribs, and Sternum | 22. Fins and Limbs | 23. The Tail | 24. The Vascular System | 25. The Respiratory system | 26. The Alimentary system | 27. The Excretory and Reproductive systems | 28. The Head and Neck | 29. The functional divisions of the Nervous system | 30. The Brain and comparative Behaviour | 31. The Autonomic Nervous system | 32. The Sense-organs | 33. The Ductless glands | 34. Regulatory mechanisms | 35. Blood-relationships among the Chordates PART IV 36. The bearing of Physical and Climatic factors on Chordates | 37. The origin of Chordates, and their radiation as aquatic animals | 38. The evolution of the Amphibia : the first land-Chordates | 39. The evolution of the Reptiles | 40. The evolution of the Birds | 41. The evolution of the Mammalia | 42. The evolution of the Primates and Man | 43. Conclusions | Figures | Historic Embryology



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