The Thirteenth and Sixteenth Centuries

The medical universities made no contributions to medical knowledge. During the 13^th and 14^th centuries the medical curriculum continued to be on the writings of the ancient Roman and Greek authors translated into rude Latin. The mythical toothworm was still blamed for toothache. Liquid prescriptions were applied as drops on an aching tooth to conquer the worm. Following the methods of the Arabic writers specialists applied acids taking care to shield the rest of the mouth from damage. One innovate method of protection was to build a small cofferdam of wax around a carious tooth before filling it with caustic liquid.

The Eighteenth and Nineteenth Centuries

The 18^th century saw profound changes in the practice pf dentistry. Dentistry became an independent scientific discipline. In 1699 the French parliament passed a law that dental practitioners along with such other specialists as oculists and bonesetters had to be examined by a committee of surgeons before being allowed to practice in Paris.

On march 6, 1840 the first dental college in the world, the Baltimore College of Dental Surgery, was chartered by the state of Maryland. Under the supervision of a board of visitors consisting of nine physicians, four ministers, and two dentists was a faculty of four. Two were dentists: Hayden, professor of dental physiology and pathology and the president of the College, and Harris, professor of practical dentistry and the dean. Two were physicians: professor of special pathology and therapeutics, and professor of anatomy and physiology.

Only five students were enrolled in the first class. The requirements for the newly designated D.D.S. degree were as high as, if no higher than those for the M.D. degree. The course of study lasted two years, the same amount of time required for medical degree, with instruction during four months of each year, The remainder of the time was spent receiving practical experience in a dental office. Only two members of the class graduated.

It was Greene Vardiman Black who truly brought dentistry into the modern world and put it on the solid, scientific foundation it now occupies. He authored more than five hundred articles and several outstanding books which became recognized classics in the field. His Dental Anatomy appeared in 1890 and in a908 his great two volume Operative Dentistry was issued.

Dentistry

The Mouth

Of all organs of the human body the mouth is the most readily available for observing the past, present and future history of an individual. Something of the characteristics of a man can be determined by examining his mouth. The words that flow from it, the degree of repair, decay and attrition of the teeth, and the color, the texture, and the temperature of the oral soft tissues all reveal an individual’s history and response to his conditions of life. In order to recognize these conditions one must know something of the form and function of oral structures. One must know the size and shape of the structures of the mouth and how they contribute to functions of chewing, swallowing, speech, facial harmony, and the general well-being of the human organism. One must know how these structures originate, grow, age, and respond to the human condition of health and disease. The value of knowledge concerning forms and functions of oral anatomy is to provide a basis for understanding the curious questions how can oral diseases be controlled and prevented.

Birth is the beginning of independent life. A great deal of growth and development takes place before birth.

But what is growth and what is development? Growth is simply an increase in size while development is the maturation of what has grown. growth implies the multiplication of cells, to get bigger, and development is the changing that the grown cells do to take on a special form and function. These changes to special form and function are maturation.

When an infant is first born he cannot see well, nor hear, walk, talk, smell, nor even grab something with his hands. He is an immature new being. But he eats well. This is necessary for his survival. Eating is a specialized, well-developed function. The mouth as compared to the rest of the body matures very early.

The mouth is the first organ of the newborn to perform voluntary and purposeful functions. the functions of sucking and swallowing are well matured before birth, they are the earliest independent acts of the young infant.

The infantile stage of life is the “mouth period”, that period of life when most sensations are received through the mouth. A person can feel better with his lips and tongue than he can with his fingers. This is true even for the adult. The tongue and lips contain a very large number of nerve endings enabling us to distinguish between hot and cold, rough and smooth, wet and dry. The mouth is the portal of pleasure and survival, eating and speaking. Frequently psychological problems manifesting themselves later in life find earlier expressions through actions of the mouth. Thumb and finger sucking habits, nervous tic-like movements of jaws and tongue, grinding of teeth, disorders of speech, and abnormalities in eating habits are examples of distorted oral functions. If these distorted oral functions become compulsive and persist into late childhood they are symptomatic of psychological problems.

Origin and Structure of the Teeth

Teeth are the hard, bony structures jutting into the oral cavity from the jawbones. Teeth serve to seize and masticate food.

Teeth and their accessory structures arise from the three basic embryonic tissues. each part of a tooth originates from a different layer in the embryo. The mesoderm, middle embryonic layer, gives rise to the greater bulk of the tooth.

The tooth is mainly made up of dentin which surrounds a central chamber of pulp cavity containing a very sensitive pulp (nerve and blood supply). The dentin is coated on the outside by the enamel on the crown of the tooth and by cementum on the root or roots of the tooth.

Preparation of Food for Digestion

Let us see how the teeth work in the preparation of food for the act of swallowing.

A man carries the food to his mouth with his hands. The next act of food preparation is incision by his incisors tearing with the canines followed by crushing and cracking by the premolars and finally grinding with the molars. The next act is insalivation or mixing the food with saliva and the last act in preparation of food for digestion is swallowing.

Our teeth break food into small particles so that the food can be readily digested by our stomach. Our tongue, inner lips and inner cheeks roll the food mass from one side of our mouth to the other and from our palate to the floor of the mouth. These two processes - chewing and rolling of food within the mouth are called mastication.

Saliva

Saliva is produced by the salivary glands and emptied into the mouth. The food in the mouth is moistened by the saliva. By mastication the food is rolled into a soft mass or bolus.

Saliva is made up of about 99% water but it contains several important solid substances. One of these is mucin. It is secreted in the floor of the mouth by two of our three pairs of salivary glands, the submaxillary and sublingual glands. This complex chemical mucin makes the saliva sticky. It helps to form the small particles of food into a bolus and is semibacteriocidal.

Another important ingredient of saliva is an enzyme, salivary amylase, a substance which assists in the decomposition of starches into a sugar maltose. The source of salivary amylase is the third paired salivary gland, the parotid. The gland is located in the inner cheek near the back of the lower jaw at a level approximately in line with the molar teeth.

Another digestive enzyme found in the saliva but not a product of the salivary glands is maltase. This enzyme produced by oral bacteria decomposes some of the more complex sugar, maltose, to a simple sugar called glucose.

Saliva softens food and helps to clear the mouth of fine food particles. It also acts as a lubricant preventing friction between various parts of the mouth. The coating of saliva on food also makes the bolus easier to swallow.

Obviously saliva acts more thoroughly on a masticated food bolus than on food which is gulped down without chewing. If we continually swallow our food without much chewing we may suffer from stomachache, belching, and eventual malnutrition. The stomach has difficulty in digesting food that is not prepared by mastication and wetting with the saliva. The food adequately chewed and wetted with the saliva is ready for swallowing.

At the moment of swallowing food is different physically and chemically from what it was before we placed it into our mouth. Saliva plays an important role in maintaining the water balance of the body. So the teeth, their surrounding structures and saliva are significant in maintaining health over long periods.

Accessory Tooth Structures

The teeth must be attached to the bones of the jaws. Sound healthy teeth depend on three separate structures for support. The name periodontium (peri = around, odont = tooth) is given to those structures that surround and support the teeth, namely:

1. the periodontal membrane serves to attach the tooth to bone;

2. alveolar bone is the bone surrounding the roots of the teeth;

3. gingivae is the soft tissue or skin that covers and protects the neck of the crown and the periodontal structures.

Each of three structures can become affected by disease or other abnormal conditions with the result that the tooth loses its support, loosens, and may require removal. These teeth are lost because of “gum” diseases, or more correct “periodontal” diseases. There are just as many teeth lost for this reason as from tooth decay. Therefore, the dentist and his patient must pay as much attention to the tooth surrounding structures as they do to the teeth themselves.

Tooth Development

Introduction

The development of the tooth is an example of organogenesis that involves many biological processes. As there is a gradient of activity an advantage the tooth germ has over many other organ systems is the temporal relationships between cells utilizing different processes. principal processes are mitosis (cell division), differentiation (the acquisition of a different phenotype amongst cells of the same genotype) growth (an increase in size),apoptosis (cell deletion), morphogenesis (the establishment of shape), and mineralization (the deposition of calcium phosphate salts). By the production of extracellular matrices and the incorporation of inorganic material the outcome of these widely diverse processes is preserved. Subsequently the teeth move into functional relationships. For convenience, the process of tooth development is divided into a number of stages, e.g., dental lamina, bud, cap, bell and apposision stages, but it must be remembered, however, that the process is continuous. Furthermore the adjacent bone and soft tissues develop concurrently.

In describing the growth the convention is followed that the crown of the tooth is above and the root below, therefore, although cells or tissues are described as growing “down” it is the relative position that is being described.

The processes associated with tooth development are the same as for other organ systems but what is unusual about teeth is that they first appear during the embryonic period but unlike other organs the processes from induction through to emergence are repeated several times for each tooth from the foetal period through to early childhood (the third molars).

Early Events

The stromatodeum or primitive oral cavity is lined by primitive, ectodermally derived epithelium rich in glycogen. Tooth development commences as a condensation of cells beneath the primitive epithelium due to the migration of cells that have their origin from the margins of the ectodermal neural crest. These cells are considered to be similar but distinct from those of the primary germ layers and since they behave as mesenchymal cells they are called ectomesenchyme.

Interaction between the ectomesenchyme and the overlying ectoderm leads to the thickening of the latter to form the primary epithelial band in each jaw. This is a more or less horse-shoe shaped continuum in each jaw. The nature of these interactions remain unknown. As many developmental events require reciprocal interactions between ectodermal and mesenchymal cells the process is generally referred to as epithelial-mesenchymal interaction (EMI). The mechanism underlying these early developmental events is unclear, however, there is some evidence of direct hererotypic cell contacts during tooth development.

from the primary epithelial band a vestibular lamina and dental lamina develop. These laminae are regions of thickened ectoderm. The vestibular lamina which is situated buccally leads to the formation of the sulcus between the cheeks and the tooth bearing areas of the jaws. The dental lamina gives rise to the teeth. Often it is difficult to distinguish which is which, the dental lamina is the one closely associated with the condensation of ectomesenchyme.

Shortly after the dental lamina has formed mitotic activity in the primary epithelial band results in thickening in four discrete regions in each quadrant (in lamina). Each of these give rise to an epithelial downgrowth or bud into the ectomesenchyme. The epithelial downgrowth is distinctive and characterizes the bud stage. The tissue changes are not exclusive to teeth common to many situations where epithelium and mesenchyme interact to produce an organ, e.g., hair follicles and mucous glands. A fifth thickening forms later distal to the fourth resulting in epithelial buds at sites that correspond to the positions of the future deciduous teeth.

As the epithelial bud proliferates into the ectomesenchyme the deeper peripheral cells divide more rapidly than those in the center. This difference in mitotic activity results in the formation of concavity. The epithelial mass now resembles a small cap, hence named the cap stage. Up to this point the principal processes have been mitosis and epithelial-mesenchymal interaction but the cap stage marks the beginning of the formation of distinct layers within the epithelium.

Enamel Organ

From now on the epithelial ingrowth is called the enamel organ. As tooth development progresses the epithelial cells lining the concavity of the cap-like structure become progressively elongated, the central epithelial cells become separated from each other while the cells lining the outer convex surface remain cuboidal. Eventually a stage is reached where there are four distinct epithelial layers and the enamel organ resembles a bell.

The tall columnar epithelial cells lining the concavity are called the inner enamel epithelium (IEE) while the cuboidal cells on the convex outer surface are the outer enamel epithelium (OEE). At the periphery of the cap the cells of the inner enamel epithelium and the outer enamel epithelium are continuous. This junctional region is called the cervical loop and delineates the anatomical crown.

The mass between the inner and outer enamel epithelia is largely made up of polyhedral cells that appear to form a network of star-shaped cells - the stellate reticulum. Their appearance is due to the presence of intercellular fluid rich in acid mucopolysaccharides (glycolsaminoglycans). A fourth, less conspicuous, layer or flattened cells found between the stellate reticulum and the inner enamel epithelium is called the stratum intermedium.

As the epithelial mass increases in size and it envelops most of the condensed ectomesenchymal cells beneath it and now resembles a bell as shown in the model. With continued development the IEE adjacent to the ectomesenchymal cells will differentiate into enamel forming cells (ameloblasts) while the juxtaposed ectomesenchymal cells will differentiate into cells that form dentine (odontoblasts). The shape of the crown is traced by the IEE. Permanence is only obtained when mineralization occurs. Initially the two cell layers are adjacent to each other separated by the basement membrane but as each forms their respective extracellular matrix they become separated moving in opposite directions away from each other. two important developmental processes occur at the bell stage, the appearance of four distinct epithelial layers (histodifferentiation) and the development of form, i.e. determination of crown shape (morphodifferentiation).

The shape of the future crown is largely determined by the underlying ectomesenchyme. The mechanism by which this occurs is not known. A number of cusps appear first and once the crown has mineralized root development commences as the tooth erupts. A close relationship between the number of conglomerates of small vessels, crown form, and root form has been demonstrated. The exact relationship between vasulature and morphogenesis during tooth development remains a mystery but it is evident that the vasculature provides more than nutrient supply.

The ectomesenchyme beneath the epithelial downgtowth becomes partially enclosed by the enamel organ during the bell stage. The tissue enclosed by the epithelium is called the dental papilla while that continuous with it and surrounding the enamel organ is called the dental follicle. The cells of the dental papilla will give rise to odontoblasts, dentine and the dental pulp whereas those of the dental follicle will give rise to the supporting tissues of the tooth, the periodontium. Collectively the enamel organ, the dental papilla, and the dental follicle constitute the tooth germ. The outline of the crown is determined by the inner enamel epithelium moving into the stellate reticulum to approximate the outer enamel epithelium in the region of the cusp tips. Mineralization later commences in these regions.

At the ultrastructural level the enamel organ at the bell stage is relatively uncomplicated. All the cells show the unusual cytoplasmic organells such as free ribosomes, endoplasmic reticulum, mitochondria, and a few scattered tonofilamentes. As with other epithelia, modifications in the plasma membrane forming junctional complexes allow for communication between neighbouring cells and also with cells of the dental papilla. Desmosomal attachments are particularly evident among the cells of the stellate reticulum as these cells are widely separated and attached to their neighbours by fine cytoplasmic extentions.

The cells of the dental papilla also have a relatively uncomplicated ultrastructure with all the usual organells associated with secretory activity being separated from the enamel organ by the basal lamina.

During the bell stage the connection of the enamel organ with the overlying oral epithelium is lost. The epithelial cells of the dental lamina undergo apoptosis resulting in fragmentation forming discrete islands of epithelial cells called “the epithelial glands” of Serres that later disappear. As the permanent teeth develop in bone the connection with the overlying oral epithelium passes through the bone, by contrast the deciduous teeth develop in a bony trough. The bundles of collagen fibres adjacent to the dental lamina were considered by the early anatomists responsible for the eruption of teeth by the mechanism similar to the retraction of the testis, hence this tissue was called the gubernaculum. The gubarnacular canal is thought to provide a path along which the tooth moves from its site of development into the oral cavity. As the number of cusps of a tooth is dependent on the folding of the inner enamel epithelium it is possible that the force vectors from the vasculature (conglomerates) in the papilla act on the epithelium causing it to fold into the stellate reticulum and the latter provides the eruptive force for tooth eruption.

Some of the teeth of the permanent or secondary dentition also arise from the dental lamina as a result of lingual extentions of the dental lamina of each of the deciduous tooth germs to give rise to the permanent incisors, canines, and premolars. These teeth, therefore, form from a successional dental lamina. The permanent molars, however, arise from a distal extention from the second deciduous molar. As there is no deciduous predecessor it is called an accessional dental lamina.