Book - Human Embryology and Morphology 5
|Embryology - 15 Apr 2021 Expand to Translate|
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Chapter V. Development and Mokphology of the Teeth
- Links: Tooth Development
The Structure of a Tooth
A tooth may be considered as made up of five parts (see Fig. 47) :
(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 imbedded 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.
Origin of the Enamel
The enamel is formed by the epiblast of the stomodaeum. At the sixth week the epiblast within the labial margin grows downwards so that a narrow semicircular invagination of epithelium is formed. To the plate of epiblast thus infolded the name of dental shelf is given ; its position is marked superficially by an epithelial crest — the dental ridge (Fig. 48). From its ingrowing or deep margin ten epithelial buds arise, 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 grows downwards and inwards from the surface and comes against a condensed formation in the mesoblast of the jaw — the dental papilla. On the papilla the enamel bud becomes partly invaginated, the papilla coming to lie within the invagination (Figs. 48 and 49). The epithelium covering the papilla becomes a layer of columnar enamel-producing cells or ameloblasts. The basal part of the ameloblasts are converted gradually into enamel, or to put it somewhat differently, they form and deposit enamel in their bases and thus produce a coating for the dental papilla. Each ameloblast is gradually converted into an enamel fibre, but their more superficial parts are never so converted but persists as the cuticular membrane which covers the enamel at birth and is soon afterwards worn off. The enamel of the milk teeth is completely formed before birth ; and that of the first permanent molar is already partly deposited enamel of milk incisor.
Fig. 48. Section through the lip and mandible of a foetus in the third month, showing the down-growth of the Dental Shelf.
Fig. 49. Showing the stage of development in an incisor tooth of a foetus of six months.
Origin of the Dentine
The dental papilla, formed from the mesoblast, 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 odonto-blasts a substance is deposited which becomes calcified into dentine or ivory. It is deposited round the processes of the odonto-blasts. 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 odonto-blasts 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. It is the formation of the root that forces the crown of the tooth through the gum. The roots grow continuously in Eodents. If from any accident to their teeth the normal wear does not take place, the incisors grow into long tusks which may ultimately prevent mastication.
The pulp is the remnant of the dental papilla enclosed by the dentine. It is made up of a matrix of branching cells and is said to have no lymphatics. Thus, like the tissue of the umbilical cord and vitreous humour of the eye, it retains the embryonic form of the mesoblast (Berry Hart). It contains the ramifications of the artery, vein and nerve of the tooth.
The Dental Sac
The foetal tooth, as may be seen from Fig. 49, lies imbedded in the alveolus within the dental sac. When the enamel bud is inyaginated on the dental papilla, the invaginated layer forms the enamel, while the invaginating or parietal layer becomes surrounded by a dense layer of mesoblast and forms the dental sac. Between the enamel (invaginated) and parietal (invaginating) layers, filling the cavity of the sac, lies a mass of epithelium corresponding to the corneous epithelium of the skin.
The Peridental Membrane
The peridental membrane (Kg. 47) 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 destroyed by the eruption of the tooth (Fig. 49).
The Crusta Petrosa
The peridental 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. It may inflame and give rise to an abscess or keep on discharging pus ; the tooth then becomes loosened in its socket and drops out.
Origin of the Permanent Teeth
From the dental shelf, besides the buds for the milk teeth, there grow inwards, so as to lie on the lingual aspect of the milk buds, processes of epiblast which form later the enamel of the ten teeth which replace the milk teeth (Figs. 48 and 49). The three permanent molars of each side arise from a process which prolongs the dental shelf backwards behind the part from which the enamel buds of the milk teeth arise. 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. 49), and if the milk teeth be roughly extracted the permanent bud may also be torn out.
Dentigerous and other Cysts of the Jaw
Cysts with epithelial walls, containing fluid, teeth or other dermal contents, occasionally develop in the jaw. They are formed from epithelial remnants of the dental shelf, which normally breaks up and disappear completely, or from detached parts of the enamel buds (see Figs. 48 and 49).
Nature of Teeth
A tooth must be regarded as an ossified dermal papilla which has received a coating of enamel from the epidermis covering it. In nature they correspond to the placoid scales of the shark's skin. The placoid scales and teeth of the shark are similar in structure, the one series becoming continuous with the other at the margin of the mouth. The dental papilla and enamel bud represent an invaginated or depressed part of dermis and epidermis.
Number of Dentitions
In many lower vertebrate forms, such as sharks, the dental shelf gives off constantly a series of buds, so that as soon as one tooth is lost another springs up from behind in its place. In mammals generally, as in man, the dental shelf 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.
Morphology of Human Teeth
The crowns of all the human teeth seem to be modifications of the same type, the tritubercular a form undoubtedly evolved from the simple conical tooth found in fishes and reptiles (see Fig. 50). The conical peg-like tooth is to be regarded as the most primitive type, and in man vestigial teeth of this type occasionally occur. In the incisor teeth the two outer or labial cusps are represented by the cutting edge of the crown ; the inner remains as the heel at the base of the crown. Secondary divisions of the two outer cusps into two or three cuspules may be seen in newly-erupted incisors. In the canine the outer two cusps of the tritubercular type are fused into one while the inner remains slightly marked as a rule, but it may rise up and form a prominent cusp as in the premolars (Farmer). In the premolars or bicuspids the outer cusp, as may be seen in many of the lower primates, is really double.
Fig. 50. The tritubercular Type of Tooth (A) 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. 50^). The two outer or buccal cusps are distinguished as the A.E. cusp (anteroexternal), the P.E. cusp (postero-external) ; the two inner as the A.I. (antero-internal) and P.I. (postero-internal). 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. 5 E). In the 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.
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 biscusps 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.
Eruption of the Teeth
The eruption 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 molars. In civilized races the third molars or wisdom teeth frequently remain imbedded in the alveolus and may give rise to an abscess. The upper wisdom tooth is developed in the posterior border of the superior maxilla, which bounds the spheno-maxillary fissure in front. The growth backwards of the maxillary antrum converts part of the posterior border of the superior maxilla into the alveolar border, thus bringing the wisdom teeth into position (see page 12 and Fig. 11). 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. A supernumerary incisor or premolar is very rare. The upper lateral incisor may be very small or even absent. If the teeth are too large for the jaw, a not uncommon condition in civilized races, they appear in irregular positions.
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Human Embryology and Morphology (1902): Development or the Face | The Nasal Cavities and Olfactory Structures | Development of the Pharynx and Neck | Development of the Organ of Hearing | Development and Morphology of the Teeth | The Skin and its Appendages | The Development of the Ovum of the Foetus from the Ovum of the Mother | The Manner in which a Connection is Established between the Foetus and Uterus | The Uro-genital System | Formation of the Pubo-femoral Region, Pelvic Floor and Fascia | The Spinal Column and Back | The Segmentation of the Body | The Cranium | Development of the Structures concerned in the Sense of Sight | The Brain and Spinal Cord | Development of the Circulatory System | The Respiratory System | The Organs of Digestion | The Body Wall, Ribs, and Sternum | The Limbs | Figures | Embryology History
Cite this page: Hill, M.A. (2021, April 15) Embryology Book - Human Embryology and Morphology 5. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Human_Embryology_and_Morphology_5
- © Dr Mark Hill 2021, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G