Human Embryology and Morphology 13

From Embryology

Keith, A. Human Embryology And Morphology (1921) Longmans, Green & Co.:New York.

Human Embryology and Morphology: 1 Early Ovum and Embryo | 2 Connection between Foetus and Uterus | 3 Primitive Streak Notochord and Somites | 4 Age Changes | 5 Spinal Column and Back | 6 Body Segmentation | 7 Spinal Cord | 8 Mid- and Hind-Brains | 9 Fore-Brain | 10 Fore-Brain Cerebral Vesicles | 11 Cranium | 12 Face | 13 Teeth and Mastication | 14 Nasal and Olfactory | 15 Sense OF Sight | 16 Hearing | 17 Pharynx and Neck | 18 Tongue, Thyroid and Pharynx | 19 Organs of Digestion | 20 Circulatory System | 21 Circulatory System (continued) | 22 Respiratory System | 23 Urogenital System | 24 Urogenital System (Continued) | 25 Body Wall and Pelvic Floor | 26 Limb Buds | 27 Limbs | 28 Skin and Appendages | Figures


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Chapter XIII. The Teeth and Apparatus of Mastication

In previous chapters dealing with the Cranium and Face, many of the changes in the apparatus of mastication have already been mentioned. At the end of the second year the alveolar parts of the palate and mandible are only sufficiently large to carry the milk dentition — which comprises 20 teeth altogether, 8 of these being incisors, 8 milk molars, and 4 canines. During the eruption of the permanent teeth, from the 5th to the 22nd year, space has to be found for the 12 permanent molar teeth, the place of the milk teeth being occupied by the permanent incisors, canine and premolar teeth. Hence the rapid growth of jaws, the enlargement and strengthening of the face, the development of supra-orbital ridges and the upgrowth of the temporal line, which are seen to take place as the permanent teeth come into position. At the same time growth changes affect the muscles of mastication.

Evolution of Teeth

The teeth are products of the skin. Both the cutis or dermis and the epithelium or epidermis enter into their formation. A tooth is a papilla of the dermis which has undergone a peculiar form of ossification (dentine) ; it is coated by an extremely hard substance — enamel — which is formed by the epidermis. Between the placoid scales which cover the skin of the shark and the complicated molar tooth of an elephant, there is a connecting series of intermediate forms. The primitive teeth have a conical or peg-like form, but with the evolution of mastication in the primitive mammalian stock the conical teeth became difierentiated into various and complicated forms — the molar teeth departing very markedly from the primitive simple type. The recognition of the true nature of teeth was delayed by the fact that, during the development, the dental papilla and its epidermal covering are submerged beneath the lining membrane of the mouth.

The Structure of a Tooth

A tooth may be considered as made up of five parts (see Fig. 184) : (1) The pulp, situated within (2) a capsule of dentine ; the exposed part or crown of the dentine is coated by — (3) the enamel ; the embedded part or root by a layer of bone — (4) the crusta petrosa. The root is secured within its socket by (5) the peridental membrane, which acts as a periosteum to both the crusta petrosa and bony wall of the tooth socket. An account of the development of a tooth has to deal with the origin of each of these five parts.


(1) Origin of the Enamel. — The enamel buds are formed by the ectoderm of the stomodaeum. In the 7th week the ectoderm within the labial margin grows within the iinderlying mesodermal tissues so that a narrow semicircular invagination of epithelium is formed within the mandibular arch below, and within the premaxillary and maxillary parts of the primitive palate above. To the plate of ectoderm thus infolded the name of dental lamina or shelf is given. As may be seen in a section of the foetal lower jaw (Fig. 185) the dental lamina is continuous at its origin with the epithelial down growth which separates the lip from the alveolus. From the ingrowing or deep margin of the dental lamina ten epithelial buds arise during the 3rd month, both in the upper and lower jaw. Each of these twenty enamel buds or organs produces the enamel to cover the crown of a milk tooth. Each bud as it deepens and expands comes against a condensed formation in the mesoderm of the jaw — the dental papilla.[1] On the papilla the enamel bud becomes partly invaginated, the mesodermal or odontoblastic bud coming to lie within the invagination (Fig. 185). During the 4th month the deeper stratum of ectodermal cells which cover the papilla change into columnar enamel-producing cells or ameloblasts. The basal part of the ameloblasts is converted gradually into enamel, or to put it somewhat differently, the ameloblasts form and deposit enamel in their bases and thus produce a coating for the dental papilla. Each am^eloblast is gradually converted into an enamel fibre, their more superficial parts are never so converted, but persist as the cuticular membrane (Nasmyth's membrane) which covers the enamel at birth and is soon afterwards worn ofi. The enamel of the milk teeth is completely formed before birth ; and that of the first permanent molar is already partly deposited. From the 5th month onwards the dental lamina — between the tooth germs and the surface of the alveolar margin — -undergoes a gradual disruption and absorption. Isolated masses of the lamina may persist within the gums and in certain cases give rise to masses of dental tissue — odontomes.

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Fig. 184. Showing the parts of an Incisor Tooth.

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Fig. 185. Section through the Lip and Mandible of a Foetus in the third month of development, showing the down-growth of the Dental Shelf.


(2) Origin of the Dentine. — The dental papilla or odontoblastic germ, formed from the mesoderm, corresponds to a depressed skin (dermal) papilla, the enamel cells representing its covering of epithelium. The dental papilla determines the shape of the tooth. In its superficial layers it contains numerous cells, odontoblasts, with branched processes radiating towards the enamel epithelium. By the agency of the odontoblasts a substance is deposited which becomes calcified into dentine or ivory. It is deposited in the matrix round the processes of the odontoblasts. The cavities in which the processes are enclosed form the tubules of the dentine. In rodents especially, but also in all mammals, although only to a slight extent in civilized races of mankind, the odontoblasts react to wear, add new layers of dentine to the wall of the pulp cavity, and thus prevent the pulp from being exposed. The dentine is deposited first in the crown of the tooth beneath the enamel ; the neck is laid down next, and then the root, the last point of all to be formed being the narrow canal at the apex of the root by which the dental vessels and nerves reach the pulp cavity. The dental crowns reach their full size at the time of their formation. Teeth thus differ from all other structures of the body in undergoing no growth subsequent to the period of their development.


(3) The Pulp. — The pulp represents the remnant of the odontoblastic germ enclosed by the dentine. It is made up of a matrix of branching cells and contains the ramifications of the artery, vein and nerve of the tooth. Fine processes of the nerves pass into the dental tubules, while in its peripheral zone are situated cells possessing the characteristics of nerve cells (Mummery).

(4) The Dental Sac.^The foetal tooth, as may be seen from Fig. 186, lies embedded in the alveolus surrounded by a fibrous capsule known as the dental sac. The sac and its contents form a dental follicle. When the enamel bud is invaginated by the dental papilla, the invaginated wall forms the enamel-producing layer, while the invaginating or parietal wall becomes surrounded by a dense layer of mesodermal tissue. The parietal wall is converted into the dental sac. At first (Fig. 185) the dental sac is continuous with the odontoblastic germ ; it becomes separated from the pulp when the root or roots of the teeth are completed. Between the enamel (invaginated) and parietal (invaginating) layers, filling the cavity of the sac, lies a mass of jelly-like epithelium corresponding to the corneous epithelium of the skin. As the crown of the tooth grows it rises within the sac of the enamel germ, and causes the absorption of the gelatinous material (Fig. 187).

(5) The Peridontal Membrane. — The peridontal membrane (Fig. 184) is formed by that part of the dental sac which surrounds the fang of the tooth. The part of the dental sac which surrounds the crown is absorbed during the eruption of the tooth.


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Fig. 186. Showing the stage of development in an Incisor Tooth of a Foetus of six months.

(6) The Crusta Petrosa. — The peridontal membrane is of the nature of periosteum, and contains osteoblasts which deposit the crusta petrosa (bone) on that part of the dentine which forms the fang and also on the inner wall of the alveolus. The centres of ossification in the upper and lower jaw spread round the labial and lingual aspects of the dental sacs, thus enclosing them in a bony gutter or trough. Subsequently septa are developed between the dental sacs, and thus the developing teeth come to be situated in bony crypts. The roof of a crypt is never completed ; a hole or window persists through which the neck of the dental sac emerges to become continuous with the mucous membrane covering the alveolus. The crowns of the teeth erupt at the point of union between the dental sac and alveolar membrane.


Epithelial Remnants of Enamel Organ. — Epithelial remnants of the dental lamina are to be found in the substance of the alveolus up to the end of foetal life or later, and may give rise to cysts of various kinds.


Besides these there are also others which occur within the sac surrounding an uncut tooth, representing remains of the enamel organ. In Fig. 187 is depicted a section of an unerupted first permanent molar tooth, lying within its dental sac, remnants of the enamel organ being shown distributed within the sac from the crown to the growing ends of the roots. We have seen that the enamel organ represents an epithelial sac, only the inner or invaginated wall being concerned in the formation of the enamel, the outer or enveloping layer becoming broken up as shown in Mr. Mummery's figure,[2] to form an interrupted epithelial layer sometimes named Hertwig's sheath.

Origin of the Permanent Teeth

From the dental shelf, besides the buds for the milk teeth, there grow inwards, during the latter part of the 3rd month of development, so as to lie on the lingual aspect of the milk buds, processes of ectoderm which form the enamel of the ten teeth which replace the milk teeth (Figs. 185, 156 and 158). The three permanent molars of each side arise from a process which prolongs the dental lamina backwards behind the part from which the enamel buds of the milk teeth arise (Fig. 188). The first molar is the earliest of all the permanent teeth to undergo development. The permanent teeth are formed in exactly the same manner as the milk set. They develop on the lingual aspect of the roots of the milk teeth (Fig. 186), and if the milk teeth be roughly extracted the permanent bud may also be torn out. Being developed deeper in the alveolus than the milk teeth, the neck of the dental sac is more elongated, and has been named the gubernaculum dentis under the belief that it serves to guide the teeth during eruption. The opening by which the gubernaculum emerges from the crypts of the permanent incisors and canines is seen on the lingual side of the alveolus near the sockets of the corresponding milk teeth. In the case of the premolars, the openings lie within the crypts of the milk molars (Carter).


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Fig. 187. Epithelial remnants in the Dental Sac of a first Permanent Human Molar. (Howard Mummery.)


Dentigerous and other Cysts of the Jaw

Cysts with epithelial walls, containing fluid, teeth or other dermal contents, occasionally develop in the jaw.[3] They are formed from epithelial remnants of the dental lamina which normally breaks up and disappears completely, or from detached parts of the enamel buds.


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Fig. 188. Mucous Membrane covering the posterior part of the Alveolus of a newly born Child with the Dental Shelf still attached to it. Proceeding backwards from the end of the dental shelf is seen the " molar process," which gives rise to the three permanent molar teeth. The crown of the second milk molar and the germ of the second premolar are also shown. (After Rose.)


Number of Dentitions

In many lower vertebrates, such as sharks, the dental lamina gives off constantly a series of buds, so that as soon as one tooth is lost another springs up in its place from behind (Fig. 189). In mammals generally, as in man, the dental lamina gives off only two series of buds — one for the milk set and another for the permanent set. In marsupials it gives off only one series, so that the first set of teeth is never replaced by a second. Thus in the most primitive vertebrates there is a succession of teeth, owing to the fecundity of the dental shelf. In man there are only the primary and secondary broods, but it is possible that occasionally representatives of a 3rd brood may be produced, for there are cases on record where a permanent tooth has been replaced by another late in life.

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Fig. 189. Diagrammatic Section across Dental Shelf of a Shark showing a Succession of Dentitions. (After Vialleton.)

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Fig. 190. Premolar Tooth of a Carnivorous Mammal to show the Primitive Cone, Cmgulum and Secondary Cusps springing from the Cingulum. (Marett Tims.)


Morphology of Human Teeth

The crowns of all the human teeth[4] seem to be modifications of the same type, all being evolved from the simple conical tooth found in fishes and reptiles (Figs. 190, 191). The conical peg-like tooth is to be regarded as the most primitive type, and in man vestigial teeth of this type occasionally occur. A modified example of the type is seen in the premolars of carnivorous mammals (Fig. 190). Here the base of the peg-shaped crown is surrounded by a ring of enamel — the cingulum. From the conical tooth was evolved the tritubercular type, one in which the crown carries three tubercles or cusps, two on the labial side of the crown and one on the lingual margin (Fig. 191, A). Secondary cusps arise from the cingulum (Marett Tims), and by the fusion of these with the original cone the two outer cusps are produced, while the inner cusp arises within the cingulum. The canine retains the conical form of crown ; the prominence or heel on the lingual aspect of the crown represents the inner cusp ; occasionally this cusp is well developed on the human canine (Farmer). The cutting edge of the incisors represents the two outer cusps ; when newly cut, the incisor crowns show five serrations ,or cuspules. In the premolars or bicuspids the outer cusp, as may be seen in many of the lower primates, is really double.

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Fig. 191. A. The Tritubercular Type of Tooth. The corresponding cusps are shown in the crowns of an Incisor (B), Canine (C), Bicuspid (D), Upper Molar (E), and a Lower Molar (F).


In the upper molar teeth, to the three primary cusps which form a cup, a fourth has been added (see Fig. 191, E). The two outer or buccal cusps are distinguished as the A.E. cusp (antero-external), the P.E. cusp (posteroexternal) ; the two inner as the A.I. (antero-internal) and P.I. (posterointernal). In the upper molars the cusps are situated alternately and the P.E. and A.I. cusps are united by an oblique enamel ridge, which represents the posterior margin of the crown of the primitive tritubercular tooth (Fig. 191, E). In the upper molar teeth of civilized races, especially in their wisdom teeth, the 4th or posterior internal cusp is often absent, the primitive tritubercular tooth thus reappearing. In the lower molars two cusps have been added to the three primary ones, making five in all. The fifth cusp is situated at the posterior border of the crown ; the others are arranged in opposite pairs. The fifth cusp has become lost in the 2nd and 3rd lower molars of civilized races. Harrison found in Sphenodon, a primitive type of lizard, that concrescence or fusion of the simple peg-like teeth takes place in the posterior part of the jaw ; it is possible that the molar teeth of mammals may have originated thus (Marett Tims). Gemmination may occur in human incisors ; the incisor bud divides so that two crowns are produced on one root.[5]


The Roots

The upper molar teeth have three roots, two outer and one inner, but in the wisdom teeth, especially of civilized races, the roots are usually fused. The lower molars have two roots, but each root appears to be essentially double in nature. In lower primates the upper bicusps have three roots, but in man these are usually fused so as to form one or sometimes two roots. The lower bicuspids have usually one root, but as in lower apes, they may have two. The roots are the last parts to be formed. When the roots of the molar teeth come to be developed, the base of the dental papilla is differentiated into three parts — round each of which a root is formed (Fig. 187). In that peculiar ancient and extinct race of men — known as the Neanderthal race — the dental papilla and pulp cavity were very large and the roots were short and wide. Thus in Neanderthal teeth — the condition is occasionally seen in a modern tooth — the pulp cavity almost descended to the tips of the roots.[6]

Eruption of the Teeth

The eruption[7] of the milk teeth commonly covers a period of eighteen months, beginning in the 6th with the lower incisors and ending in the 24th or 30th with the 2nd milk molars. The eruption of the permanent teeth occupies a period of about eighteen years, beginning with the 1st permanent molar in the 6th year and ending about the 24th with the 3rd molar. The milk molars are replaced by the permanent premolars. In civilized races the third molars or wisdom teeth frequently remain embedded in the alveolus or may be quite absent. The upper wisdom tooth is developed in the posterior border of the superior maxilla, which bounds the spheno-maxillary fissure in front. In growth backwards of the maxillary antrum the posterior border of the superior maxilla becomes rotated into the alveolar border, thus bringing the wisdom teeth into position (see Fig. 176). The inferior wisdom teeth are developed in the alveolus on the inner aspect of the ascending ramus.


A fourth molar sometimes appears behind the third. The original primate stock is supposed to have had three incisors and four premolars on each side, yet a supernumerary incisor or premolar is a rare abnormality. The upper lateral incisor may be very small or even absent, there being a distinct tendency towards the disappearance of this tooth in civilized races. If the teeth are too large for the jaw, a not uncommon condition in civilized races owing to a diminished growth of the bony palate, they appear in irregular positions.

Mechanism of Eruption

As regards the mechanism which causes teeth to erupt there is still some degree of uncertainty. One naturally infers that the growth of the root will tend to force the crown upwards and the tissues over the crown to atrophy. The process of eruption is a much more complex one than the mere formation of a root. It is well known that a rootless tooth may cut the gum, while in another case the root may form and yet the tooth remain embedded in the jaw. Eruption is a definite growth movement — allied in nature to the mechanism which leads to the extrusion of a foreign body by the tissues. During the eruption of a tooth there is not only an absorption of the overlying tissues of the gum — probably due to pressure — but there is also the positive growth of the peridontal tissues at the base of the tooth-sac which, as it presses the tooth towards the surface, moulds the surrounding wall of the dental crypt into a suitable alveolar socket. Thus the formation of the socket or alveolus appears to be part of the mechanism of eruption. Mr. J. T. Carter regards the gubernaculum dentis as playing an effective part in tooth eruption.[8]


Effect of Civilization

Mention has been made of the fact that the eruption of the last molars in highly civilized peoples may be long delayed or arrested ; in a small proportion of individuals these teeth may be quite absent. When the teeth and jaws of ancient European races are compared with those of their successors, certain changes are very evident. These are (1) the crowns of the teeth in the ancient races are much worn ; (2) the palate is well formed, and large enough to carry the teeth without crowding or irregularity ; (3) the wisdom teeth are in position, but usually show a reduction in size and development ; (4) diseased and carious teeth are uncommon ; (5) the edges of the incisor teeth come into apposition in biting. In modern Europeans the degree of wear or erosion is slight ; the palate is often vaulted, contracted and the teeth crowded and misplaced ; the wisdom teeth are often unerupted or absent ; diseased teeth are extremely common ; the edges of the lower incisors ascend behind the crowns of the upper (scissors bite). The cause or causes of these remarkable changes are ill-understood, but it is probable that some or all will be traced to the nature of our modern dietary.


Muscles of Mastication

The four muscles of mastication[9] — the temporal, masseter, external and internal pterygoids arise in the mandibular arch. A single muscular mass is apparent at the end of the first month ; during the second month it is differentiated into its several parts — the internal pterygoid being the first to separate from the common mass. The masseter and external pterygoids are derived from the primitive temporal muscle. The external pterygoid is a late addition ; even in man it is often imperfectly separated from the temporal. The muscles of mastication differ from the ordinary striated muscles of the body in being derived from the musculature of a visceral arch. Their motor nerve — the motor root of the Vth — represents the splanchnic nerve of the second segment of the head (see p. 99). The somatic motor nerve of the segment is the 4th or trochlear nerve ; the somatic musculature of this segment is represented by the superior oblique. The sensory nerves of the teeth — the 2nd and 3rd divisions of the Vth nerve — represent the skin or somatic sensory fibres of the second or mandibular segment of the head. It will be thus seen that the apparatus of mastication has been evolved in connection with the second cephalic segment — the neuromere of this segment being the second of the mid-brain. The manner in which the muscles of mastication are attached to the skull, and the extent to which they modify cranial characters have been already mentioned (p. 155). The evolution of the temporo-mandibular joint has also been alluded to (p. 177).



  1. A. Masur, Anat. Hefte, 1907, vol. 35, p. 263 (Dev. of Dental Pulp) ; J. Howard Mummery, The Microscopic Anatomy of the Teeth, 1920.
  2. J. Howard Mummery, Phil. Trans. 1919, vol. 209 (B), p. 305.
  3. P. Adloff, Anat. Anz. 1912, vol. 40, p. 177 (Abortive Dental Buds).
  4. A. C. F. Etemod, Verhand. Anat. Oesellsch. 1911, p. 144 (Bicuspid Theory of Teeth); Sir C. S. Tomes, Manual of Dental Anatomy ; Prof. L. Bolk, Versuch. einer Losung der Oebissprobleme, Jena, 1913 ; Amer. Journ. Anat. 1916, vol. 19, p. 91 ; Journ. Anat. 1921, vol. 55, p. 138 ; T. Wingate Todd, Introduction to Mammalian Dentition, 1918; D. M. Shaw on use of dental cusps, Journ. Anat. 1918, vol. 52, p. 97.
  5. J. T. Wilson and J. P. Hill, Quart. Journ. Mic. Sc. 1907, vol. 51, p. 137 (Tooth Formation in Monotremes) ; W. Ramsay Smith, Journ. Anat. and Physiol. 1907, vol. 42, pp. 126, 226 (Morphology of Teeth of Austrahan Natives).
  6. Keith and Knowles, Journ. Anat. and Physiol. 1911, vol. 46, p. 12.
  7. G. Fischer, Anat. Hefte, 1909, vol. 38, p. 617 (Eruption of Permanent Teeth).
  8. See Brit. Dent. Journ. 1904, Feb.
  9. Professor F. H. Edgeworth, Quart. Journ. Mic. Sc. 1914, vol. 59, p. 573.
Historic Disclaimer - information about historic embryology pages 
Mark Hill.jpg
Pages where the terms "Historic Textbook" and "Historic Embryology" 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 and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Human Embryology and Morphology: 1 Early Ovum and Embryo | 2 Connection between Foetus and Uterus | 3 Primitive Streak Notochord and Somites | 4 Age Changes | 5 Spinal Column and Back | 6 Body Segmentation | 7 Spinal Cord | 8 Mid- and Hind-Brains | 9 Fore-Brain | 10 Fore-Brain Cerebral Vesicles | 11 Cranium | 12 Face | 13 Teeth and Mastication | 14 Nasal and Olfactory | 15 Sense OF Sight | 16 Hearing | 17 Pharynx and Neck | 18 Tongue, Thyroid and Pharynx | 19 Organs of Digestion | 20 Circulatory System | 21 Circulatory System (continued) | 22 Respiratory System | 23 Urogenital System | 24 Urogenital System (Continued) | 25 Body Wall and Pelvic Floor | 26 Limb Buds | 27 Limbs | 28 Skin and Appendages | Figures