The purpose of this article is to discuss the main course of the normal development of human dentition, together with the concept of evolution of tooth development, clinical features of the dentition and the most common developmental disturbances.
Knowledge of the normal development of the dentition and an ability to detect deviation from the normal are essential prerequisites for pedodontic diagnosis and a treatment plan.
Dentition means a set of teeth. Teeth in the dental arch are used to designate the natural teeth in position in their alveoli.
During evolution several significant changes took place in the jaws and teeth. When the Reptilian evolved to mammalian, the dentition went from “polyphydont” (many set of teeth) to “diphydont” (only two sets of teeth) and then to “hornodent” (all of same teeth) to heterodent (different types of teeth like incisor, canines, premolars and molars).
There also arose a necessity for the teeth and bones to develop somewhat synchronously in order that the function of occlusion could be facilitated. Finally, The number of cranial and facial bones has been reduced by loss or fusion and the dental formula has is undergone changes.
Stages of tooth evolution
Graphically there are four stages of tooth evolution.
i) The reptilian stage (Haplodont)
ii) Early mammalian stage (Triconodont)
iii) Triangular stage (Tritubercular molars)
iv) Quadri tubercular molars
The Reptilian stage: This stage is represented by the simplest form of dentition, the single cone type. It includes many teeth in both jaws which limit jaw movement. Thus the jaw movement is confined to that of a single hinge movement.
Early mammalian stage: This stage exhibits three cusps in the line of development of the posterior dentition. The larger or anthropologically original cusp is centered with one smaller cusp located anteriorly and another posteriorly.
Tritubercular stage: According to the recognized theories explaining evolutionary dentition development, three triconodont lines are changed to three cone shaped with the teeth still by-passing each other more or less, when the jaw opened or closed. These types are found in dogs and other carnivorous animals.
Quathitubercular stage: The next stage of development created a projection on the triangular form that finally occluded with the antagonist of the opposing jaw. During the time as an accommodation to the changes in the dentition form and anatomy, the articulation of the jaws changed accordingly.
The animals with dentition similar to that of humans are anthropoid apes. These include Chimpanzee, Gibbon. Gorilla and Orangutan. The shapes of individual teeth in these animals are very close to their counterparts in human mouth. Nevertheless, development of canines, arch form and jaw development are quite different.
Common evolutionary trends in the primates
I. There was shortening of the jaw due to the decrease in the size of the olfactory organs, upright body position and wide angle of the head to the body.
2. There was decrease in tile tooth size to be accommodated in these jaws, with subsequent elimination of some teeth from the dentition.
3. There was progressive shortening of the arch (in front) and relative widening.
4. Canines reduced in size
5. Lower premolar crowns became more symmetrical from oval.
6. First molars became the dominant cheek teeth.
7. In the upper second and third molars, the distolingual cusp reduced and often disappeared.
8. Third molars, which were larger than the first molars, were reduced in size and often eliminated.
In modern man:
There is a decrease in the tooth-bearing portion of the face. This decrease is partly due to a reduction in tooth size.
• In some primitive and pre historic human skulls the second permanent molars usually succeeds the 1st molar.
• The occlusal length of the lower molars is reduced in modern man and the ramus width is even more reduced less than the occlusal length of the lower molars.
Characteristic of human dentition
Tooth of the vertebrates are characterized depending upon:
• Mode of attachment
• Number of successive sets
• Shape of teeth
The way teeth are attached to the jaws.
a) Acrodont: Teeth attached to the jaw by a connective tissue.
b) Pleurodont: Teeth are set inside the jaws
c) Thecodont: Teeth are inserted in a bony socket
Depending on the number of teeth of successive sets.
a) Polyphyodont: Teeth replaced throughout the life, e.g. shark.
b) Diphyodont: Two sets of teeth, e.g. human being.
c) Monophvodont: One set of teeth, e.g. sheep, goat.
According to type or shape of teeth
a) Hoiuodont: A single type of teeth
b) Heterodont: Various types of teeth, e.g. human being.
Origin of teeth
Theories: Each tooth whether primary or permanent is believed to develop from the epithelial Primary germ cell. Mirious theories regarding mammalian dentition are reported.
1. The theory of concrescence: The mammalian dentition was produced by the fusion of two or more primitive conical teeth and each tubercle with its corresponding root originated as a single tooth.
2. Theory of trituberculv: Each of the mammalian teeth was derived from a single reptilian tooth by a secondary differentiation of tubercies and roots. This theory is widely accepted.
3. Theory of multituberculy: The mammalian dentition is the result of reduction and condensation of primitive tuberculate teeth.
In the developmental process specialized structures such as teeth, differentiate as part of a closely integrated pattern of events. These progress from the initial genetic potential of the fertilized ovum to be influenced later by the prenatal and postnatal environment.
The complex mechanisms, concerned with oro-dental development, involve a series of interactions not only between specific cell components, but also between the different varieties of cells which arise during organization of the various tissues
The formation of the primitive oral cavity or stomatodeum and the perforation of the bucco-pharyngeal membrane depend upon the contact between the oral ectoderm and the pharyngeal ectoderm. The odontogenic epithelium is derived from this ectoderm.
It is believed that ectoderm, eclo—mesenchyme which is contributed by the primitive streak through the notochord and adjacent tissue and the mesoderm are involved in dentition formation. The inductive differentiation of the cell layers of a tooth germ results from both ectodermal—ectomesenchymal and eclomcscnchyinal — mesodermal interactions.
In humans, odontogenic epithelium which is the anlage of the dentition, can be identified in 28-30 day (ovulation age) embryo.
28-30 days: The epithelium proliferates, giving the appearance of epithelial thickening located on the inferior border of the maxillary process and the superior borders of the mandibular arches in the area forming the lateral margins of the stomatodeum
30-32 days: The odontogenic epithelium is 3-4 cell thick, the cells being ovoid to cuboid with little cytoplasm.
32-34 days: The mesenchyme immediately beneath the odontogentic epithelium can be distinguished from the adjacent mesenchyme. The odontogenic epithelium becomes invaginated relative to the underlying mesenchyme and a dental lamina from which individual tooth buds arise.
4th week The formation of dental lamina commences around and the tooth buds for the deciduous dentition begin to form about two weeks later. The dental lamina marks out the position of the future dental arches. The tooth buds for the corresponding permanent teeth develop from the same arch.
Stages of tooth bud development
• Phase of deciduous tooth – 5th month in utero
• Phase of permanent tooth – 6th month
• Phase of accessional tooth – spaced from 4th month in utero to 4-5 years
A tooth germ (tooth bud) consists of three parts: An enamel organ, which is derived from the oral ectoderm, a dental papilla and a dental sac, both of the latter being derived from the mesenchyme. Each swelling of the lamina which is destined to be a tooth germ proliferates and differentiates, passing through various histological and morphological differentiation stages, namely bud, cap and bell stages.
This basic configuration of the future tooth crown is fixed at the morphological differentiation stage. The enamel organ produces the enamel by a process of cell proliferation, cell differentiation and later mineralization, and the dental papilla produces the dentin and pulp of the tooth in a similar way. The dental sac produces the cementum and the periodontal ligament.
Enamel formation ceases once the tooth crowns is complete, but dentition formation continues with root development. A layer of cementum is laid down on the surface of the root dentine. and incorporates periodontal fibers that support the tooth through its attachment to the bony wall of the tooth socket.
Once histo-differentiation of the cells has progressed sufficiently far, mineralization commences. This occurs in the deciduous dentition during the 14th intrauterine week on average and begins with the central incisors.
The permanent tooth buds appear in the fourth and fifth intrauterine months, at about the same age at which mineralization of the deciduous teeth commences. Mineralization of the permanent dentition is initiated around the time of birth on average, beginning with the first permanent molar.
Originally chronology for dentition is based on the data of Logan and Kronfeld reported in 1933. Later, a careful review of the data was done by various researchers like Massler and Shour in 1941, Moorrees and Fanning and Hunt 1963, Kraus and Jordan in 1965. and Nystrom in 1977 etc.
Factors affecting development of Dentition
A retarding effect: Delayed eruption in both the primary dentition and permanent dentition, but more especially in the later has been attributed to many diseases, syndromes and systemic factors, the most common are:
• Cleidocranial dysostosis
• Downs syndrome
• Hypovitaminosis (A and D)
• Arnelogenesis imperfecta
B. Local factors: Some local factors which may influence the developing dentition are
• Aberrant tooth position
• Lack of space in the arch
• Very early loss of predecessor
• Ectopic eruption
• Congenital absence of teeth
• Ankylosi of predecessor
• Retained tooth or persisting deciduous root remnants
• Arrested tooth formation (trauma)
• Supernumeran’ tooth
• Abnormal habit exerting muscular forces