A text-book of dental histology and embryology, including laboratory directions

Front matter

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Preface

It is indispensable for the successful treatment of disease in the dental tissues that the dentist should acquire as intimate a knowledge of structure as is essential to the physician, and consequently that a parallel study of histology should be followed. The development of biology has placed histology at the basis of all the medical sciences, for as, in the last analysis, all physiology is cell physiology, and all pathology is cell pathology, a knowledge of structure and function is necessary for an intelligent conception of the workings of the animal body in health and for the restoration of normal function when impaired by disease.

Yet as recently as fifteen years ago, when the author began teaching Dental Histology in the Northwestern University, the subject was comparatively new in the curriculum, and was considered rather unimportant and as having little practical value. It would be impossible to give adequate acknowledgment to the help received from Dr. G. V. Black in developing the course. Every detail was worked out in the closest cooperation with him, and for years he guided and directed the work.

The object of a course in general and special histology suited to the needs of dental students is to convey a definite knowledge of the activities of these parts of the human body in terms of tissues and cells. This is the basis of every practical procedure. The structure of the enamel and dentine is obviously the starting point in handling these tissues and in the preparation of cavities, and the structure and function of the pulp, the bone, the periosteum, and the peridental membrane are similarly the basis for an understanding of

VI PREFACE

their pathology and treatment. The study of the enamel in relation to cavity preparation has proved to be of the greatest value not only in forming better cavity walls, but also in facilitating operation. For this purpose it is necessary to understand both the structure of the enamel in itself, and the arrangement of the structural elements in relation to the tooth crown. To accomplish this, sections cut through the crown in various planes must be studied and their relation to it kept in mind. The modern dentist while looking at the surface of a tooth must think of the enamel in terms of its structural elements, and use this knowledge in handling the tissue. In the following pages the enamel is studied primarily in relation to operative dentistry.

The chapters on the pulp, peridental membrane, and periosteum are likewise intended to emphasize the relation of structure to function, and to impress the idea that the treatment of disease in these tissues is in every instance a biological problem. In forming true conceptions of caries and necrosis, a knowledge of the intercellular substances and their relation to the cells in the structure and function of tissues is necessary. A chapter has, therefore, been devoted to this subject. The study of the structure and development of bone has very greatly modified treatment in orthodontia, as it is now recognized that in all movements of the teeth the results are accomplished by tissue changes under the influence of mechanical stimuli.

Though there are many good books on general histology, they are not fully adapted to the special needs of the dental curriculum. The author has accordingly felt that teachers and students of dentistry might find some advantage in a work covering the subject from their own standpoint, and embodying the results of his experience in teaching as well as in research. In a word, this volume has been planned primarily as a text-book for use in dental schools, and it aims to provide students with a didactic text and teachers with a course to follow. It contains directions for twentytwo days of laboratory work, and an appendix giving technical methods for preparation of material for the classes.

* PREFACE vn

It suggests many fields for original investigation, and technical directions which would enable any man to begin such work. Let us hope that the benefits certain to result from discoveries still to be made will lead some students and practitioners to interest themselves in this inviting field.

Most of the illustrations are from the author's own negatives. Those on the relation of enamel structure to cavity walls are new as well as original in plan. The drawings illustrating the periosteum and pathological conditions of the pulp were made by Dr. G. V. Black, and he has written the chapter on his machine for making ground sections and the technique of its use. Some illustrations are taken from other works, and these are duly credited in every instance. Thanks are also due to Dr. Louis Schmidt, of the Rockefeller Institute, who made the colored plates from the author's specimens, and to A. B. Streetdain, of the University of Chicago, for his work on some difficult diagrams.

Finally, the author wishes to thank his publishers for their pains and patience in carrying out his wishes.

F. B. N.

Chicago, 1912.

PART II (Twenty-four Periods in the Laboratory)

Period I 429

Period II 429

Period III 432

Period IV 435

Period V 435

Period VI 437

Period VII 439

Period VIII 440

Period IX 440

Period X 441

Period XI 442

Period XII 442

Period XIII 443

Period XIV 444

Period XV 445

Period XVI 445

Period XVII 446

Period XVIII 446

Period XIX 447

Period XX 448

Period XXI 449

Period XXII 450

Period XXIII 451

Period XXIV 442 xii CONTENTS

Introduction

The development in knowledge of the cell has had a most profound effect upon the entire practice of medicine; in fact, the progress of modern medicine has dated from the studies of cell biology, the germ theory of disease being only one of the phases of this development. In terms of the cell theory the functions of the body are but the manifest expression of the activities of thousands or millions of more or less independent but correlated centres of activity. If these centres or cells perform their functions correctly, the functions of the body are normal, but if they fail to perform their office or work abnormally, the functions of the body are perverted. In the last analysis, then, all physiology is cell physiology, all pathology cell pathology. To modern medicine, histology, or the cell structure of the organs and tissues of the body, together with cell physiology, is the rational foundation of all practice. This is as true for the dentist as for the physician in regard to the soft tissues of the mouth and teeth that he is called upon to handle. With caries of the teeth, the disease which most demands the attention of the dentist, the case is somewhat different. Caries of the teeth is an active destruction, by outside agencies, of a formed material which is the result of cell activity, the teeth themselves being passive. The cellular activities of organs and tissues of the body may have an influence, but this is only in producing those conditions of environment which render the activities of the destructive

18 DENTAL HISTOLOGY

agent efficient in their action upon the tooth tissues. Though the dental tissues are passive, the phenomena of caries can only be understood when the structure of the tissues is understood, and not only must the treatment be based upon knowledge of the structure of the tissues, but the mechanical execution of the treatment is facilitated by that knowledge of structure.

In the preparation of cavities, the arrangement of the enamel wall is determined by the knowledge of the direction of the enamel prisms in that locality, and to a certain extent the position of cavity margins must be governed by the knowledge of the structure of the enamel. In the execution of the work a minute knowledge of the direction of enamel rods becomes the most important element in rapidity and success of operation. The longer the author studies and teaches the structure of the enamel in its relation to the structure and preparation of enamel walls, the more he finds himself using this knowledge at the chair in daily operations. He believes that nothing will do more to increase facility, rapidity, and success of operation than a close study of the enamel structure.

All tissues are made up of two structural elements — cells and intercellular substances. The cells give the vital characteristics, the intercellular substances the physical character. The cells are the active living elements, the intercellular substances are formed materials produced by the activity of the cells, and more or less dependent upon them to maintain their quality, but they possess no vital properties. They surround and support the cells, and the physical characteristics are given by them. An understanding of the relation of cells and intercellular substances in the structure and function of tissues is absolutely fundamental to a study of dental histology, and should be acquired in a thorough study of general histology before the subject is undertaken.

Chapter I Homologies

Exoskeleton. — In studying the organization of animal forms they are found, very early in the evolutionary stages, to develop some sort of a framework, or skeleton, to support and protect the creature. In the lower and earlier forms this framework is formed entirely of some sort of shell upon the outside of the creature, and consequently is called an exoskeleton. This may be either horny or chitinous in nature, as in the insects, crabs, etc., or it may be calcified, as in the shell-fish, or it may be both. The exoskeleton serves not only as a supporting framework, but also as a protection.

Endoskeleton. — In the higher forms an internal framework, or endoskeleton, is developed, which forms the scaffolding to support the creature, but does not act as a protection. In the first place, this is of cartilage, but may be changed into bone. In lower forms of animals it remains always cartilage. In man the cartilage is partly converted into bone, all of the bones of the endoskeleton being preceded by cartilage.

The first trace of the endoskeleton is found in the lowest form of vertebrate, the Amphioxus or Lancit, the lowest form of fish, and appears as a rod or notochord in the dorsal region. There is also an important difference in the nervous organization (Figs. 1 and 2). In the invertebrate the nervous system is represented by a larger or smaller ganglion in the anterior or head end, corresponding to the brain; this is dorsal to the alimentary canal. From this a ring passes around the anterior end of the alimentary canal and unites with a chain of ganglia ventral to it. The nervous system of the invertebrate then is, with the exception of the brain ganglia, ventral to the alimentary canal, and corresponds to

22 HOMOLOGIES

the sympathetic system in higher animals. It will be noted that this arrangement puts the nervous system, which controls the activity of the individual, in the most protected position. The invertebrate crawling upon the ground is subject to attack or injury from above, but it may be cut almost in two before the nervous system is reached.

In the vertebrate the central nervous system appears as a chain of ganglia dorsal to the alimentary canal and notochord (Fig. 2) . This difference is significant, and may be expressed roughly in this way : The invertebrate framework is an outside protecting shell, upon which the creature depends for protection. The vertebrate framework is an internal structure to facilitate motion and give support, and is accompanied by a development of the nervous organization, so that the creature protects itself by more rapid motion . In the invertebrate the digestive system is above or dorsal to the nervous system, in the vertebrate the nervous system is in the upper position, both structurally and functionally.

In ascending in the scale of organization the endoskeleton increases in importance and development, while the exoskeleton decreases in importance and development.

From the standpoint of comparative anatomy the teeth are not a part of the osseous system, but appendages of the skin, and are to be compared with such structures in the body as the hair and the nails. The teeth aie a part of the exoskeleton, and their relation to the bones of the endoskeleton is entirely secondary for the purpose of strength, the bone growing up around the tooth to support it. When the skin of such an animal as the shark is examined, the entire surface is found covered with small calcified bodies, which are really small, simple, cone-shaped teeth. From the standpoint of development the mouth cavity is to be regarded as a part of the outside surface of the body which has been enclosed by the development of neighboring parts, and the dermal scales, or rudimentary teeth, which are found in the skin covering the arches forming the jaws, have undergone special development for the purpose of seizing and masticating the animal's food. In the simplest forms there is only

HOMOLOGY AND ANALOGY 23

a development in size and shape of these scales, and they are supported only by the connective tissue which underlies the skin. These teeth are easily torn off in the attempt to hold a resisting prey, and in the shark (Fig. 3) they are continually being replaced by new ones. In the more highly developed forms, the bone forming the jaw grows upward around the bases of these scale-like teeth to support them more firmly and render them more useful.

Fig. 3

Shark's skull (Lamna cornubiea), showing succession of teeth.

Homology and Analogy. — In biology structures that are similar in formation and origin are called homologous. Structures that are similar in function are called analogous. A structure or organ may be both homologous and analogous to another, but not necessarily so. For instance, the wing of a fly is analogous to the wing of a bird, because they are used for the same purpose, but they are not homologous. The wing of a bat and the wing of a bird are both analogous and homologous, being used for the same purpose, and

HOMOLOGIES having similar structure and origin. The arm of man is homologous to the wirig of a bird but not analogous to it. The jaws of a crab or beetle are analogous to the jaws of man, but they are not homologous structures, as the jaws of the crabs and insects are modified legs. The teeth are said to

Fig. 4

Development of the hair: £c, stratum corneum; SM, stratum malpighii; C, derma; F, follicle; Dr, sebaceous gland; CZ, central, PZ, peripheral zone of hair germ; HK, hair knob; P, beginning the formation of the hair papilla; P', same in a later stage of development when it has become vascular. (Wiedersheim, Comparative Anatomy of Vertebrates.)

be homologous to the dermal scales of certain fishes, and to the appendages of the skin, such as the hair and nails, because they are similar in structure and origin (Plate I).

Comparison of Structure. — If the tooth is compared with the hair in this way this will be better understood. The hair

COMPARISON OF ORIGIN may be considered as a horny structure composed of epithelial cells resting upon a papilla of connective tissue. The tooth may be considered a calcified structure, formed by epithelial cells, resting upon a papilla of connective tissue, which is also partially calcified.

Comparison of Origin. — From a study of the development of the tooth and the hair, the similarity of their origin and structure becomes more apparent.

The first step in the development of the hair is a thickening of the epithelium at a point, the epithelial cells multiplying and growing down into the connective tissue below, so as to make a two-layered bag or cap, the connective tissue growing up in the form of a cone-shaped papilla into the cavity of the cap (Fig. 4). The epithelial cells of the inner layer, next to the connective tissue, multiply rapidly and develop horny material and are pushed out from the surface of the skin as the shaft of the hair.

Diagram to illustrate development of a tooth: A, inner layer of enamel germ; B, outer layer; C, remains of intermediate cells; D, dentine; D. I, dental lamina; E, epithelium; E.G, enamel germ; En, enamel; F, dental furrow; L.D, labiodental furrow; M, connective-tissue cells; O, odontoblasts: P, dentine papilla: R.G, reserve germ; V, bloodvessel. (Cunningham's Anatomy.)

Fig. 5

IV Homologies

In the development of the tooth there is at first a thickening of the epithelium, and a mass of epithelial cells like that forming the hair, but larger, grows down into the connective tissue (Fig. 5). This becomes bulbous, then invaginated, forming a two-layered cap. The two layers are at first perfect and are farther from the surface than the epithelial

Fig. 6

Changes in the mandible with age; buccal and lingual view.

structure which develops the hair. A cone-shaped papilla of connective tissue, the dental papilla, grows up into the cavity of the epithelial organ corresponding to the bulb of the hair.

The inner layer of epithelial cells produce the enamel, the outer layer of connective-tissue cells, covering the connective-

RELATION TO THE BONE 27

tissue papilla, develop the dentine, leaving the pulp inside as the remains of the dental papilla.

Relation to the Bone. — The relation of the bones of the jaws to the teeth is entirely secondary and transient. They grow up around the roots of the teeth to support them, and are destroyed and removed with the loss of the teeth or the cessation of their function. In this way the development of the alveolar process appears around the roots of the temporary teeth. All this bone surrounding their roots is absorbed and removed with the loss of the temporary dentition, and a new alveolar process grows up around the roots of the permanent teeth as they are formed. This development of bone around the roots of the teeth leads to the changes in the shape of the body of the lower jaw, increasing the thickness from the mental foramen and the inferior dental canal upward (Fig. 6) . When the teeth are finally lost this bone is again removed and the body of the jaw is reduced in thickness from above downward. These phenomena have an important bearing upon the causes and treatment of diseased conditions of the teeth, particularly those which involve the supporting tissues.

Chapter II The Dental Tissues

Study of the structure of the teeth shows that all teeth, from the simplest to the most complex, are composed of but four tissues — enamel, dentine, cementum, and the pulp, or formative tissue of the dentine.

Even the simplest placoid scales, as found in the skin of the shark and dog-fish, contain these four tissues. In many of the specialized forms of teeth some of these tissues may be absent. For instance, in the bony fishes the teeth are fastened to the bone by an interlocking of bone and dentine, forming an ankylosed attachment, and the cementum is absent; but in some of these there is also a slight formation of cementum. In the tusks of elephants during the functional period the dentine is not covered by enamel, but when the tusk first erupted there was a slight enamel cap, which was at once broken or worn off. In many instances the enamel seems to be entirely absent, and for that reason it has sometimes been called the most inconstant of the dental tissues, but in every case in which the development of the tooth has been studied an enamel organ has been found. It is probably much more nearly correct to consider that in all cases enamel is formed, but that it may be so thin and transparent as to be very difficult to recognize, and very soon may be entirely lost.

FUNCTIONS OF THE DENTAL TISSUES

The Enamel. — The enamel forms a hard protecting surface or cap especially adapted to resist abrasion. It is the hardest animal tissue, but brittle and inelastic, and dependent upon

PLATE I

Comparison of Structure of Tooth and H air.

FUNCTIONS OF THE DENTAL TISSUES 29

the support of the elastic dentine for strength. Its function is to resist the abrasion of friction. Its arrangement in many instances is found specially modified for this purpose.

The Dentine. — The dentine is the strong elastic tissue forming the great mass of the tooth, and gives to it its strength. Teeth that are subjected to stress and force are often made up of dentine without enamel. If, for instance, the tusks of the elephant used for such purposes as tearing down branches, spading up the ground, and so on, were made up entirely of enamel, they would break off the first time they were locked in the branches or driven into the ground, but the elastic dentine gives and bends and will stand great stress. The teeth of many animals which use their tusks in fighting are constructed on the same plan.

The Cementum. — The cementum furnishes attachment for the connective-tissue fibers which fasten the tooth to the bone or surrounding tissues. It is formed on the enamel and dentine both before and after the eruption of the teeth. The formation of the cementum on the surface of the root fastens the surrounding connective-tissue fibers to the tooth. The fibers are calcified along with the matrix of the cementum which is built up around them. These fibres in man and the higher animals extend to the bone and the surrounding tissues and support the teeth against the forces of occlusion, and hold the surrounding tissues in proper relation to the teeth. The function of the cementum is, therefore, to attach the connective tissue fibers to the surface of the root.

The Pulp. — The pulp is the remains of the formative organ of the dentine. In teeth of continuous growth it remains actively functional throughout the life of the tooth, but in teeth of limited growth, after the typical development of dentine, it becomes functional again only in response to irritations, which, however, may be local or reflex. The pulp performs two functions — a vital function, the formation of dentine, and a sensory function, the response to thermal change.

Summary. — The dental tissues, i. e., enamel, dentine, cementum, and pulp, are so called not simply because they

30 THE DENTAL TISSUES

are found in the human teeth, but because all teeth are composed of these four tissues.

It is true that in comparative dental histology considerable difference exists in the microscopic structure of these tissues from the teeth of different animals, but certain characteristics are very persistent and quite characteristic of each.

DISTRIBUTION OF THE DENTAL TISSUES

The arrangement and distribution of the dental tissues in the structure of the human teeth is best studied in ground sections cut longitudinally through the entire tooth (Plate II), and series of transverse sections cut through the roots. For this purpose the sections should not be too thin (from 10 to 20 microns). For the study of the arrangement of the cementum and dentine in the roots at least three transverse sections should be ground from each root, one from the gingival, one from the middle, and one from the apical third.

The Enamel. — The enamel forms a cap over the exposed portion of the tooth. Its function is to resist the abrasions of mastication. It gives the detail of crown form to the tooth. It extends to the gingival line, and, except in old age, is covered in the gingival portions by the epithelium of the gingivus, which lies in contact with it but is not attached to it. It is thin in the gingival portion and is normally overlapped slightly by the cementum at the gingival line. It extends farther apically on the labial and lingual, and buccal and lingual, than upon the proximal surfaces, especially on the incisors, cuspids, and bicuspids. It is thickest in the occlusal third of the axial surfaces, and on the occlusal surfaces of the molars and bicuspids, especially over the cusps. In the incisors and cuspids it is thickest in the occlusal third on the labial and over the marginal ridges on the lingual and the dento-enamel junction, which, though not parallel with the surface of the enamel, is usually curved in the same direction.

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DISTRIBUTION OF THE DENTAL TISSUES 31

the occlusal thirds on the buccal and lingual is usually curved in the opposite direction. That is, while the surface

Fig. 7

Dento-enamel junction.

32 THE DENTAL TISSUES

of the enamel is convex, the surface of the dentine is concave. It will be seen that this not -only gives a greater thickness to the enamel in the region which will resist abrasion, but also gives it a firmer seat upon the dentine. (Study illustrations in Chapter X.) The dento-enamel junction is seldom a smooth, even surface, but will appear scalloped in sections, projections of dentine extending between projections of enamel (Fig. 7). In three dimensions this means that rounded projections of the enamel rest in rounded depressions of the dentine surface, and pointed projections of the dentine extend between the rounded projections of the enamel. This is similar but much less marked than the interlocking of the papilla of connective tissue with the projections of the Malpighian layer of stratified squamous epithelium of the skin and mucous membrane. In some cases these projections of dentine into the enamel may be quite marked. This scalloping of the dento-enamel junction gives a stronger attachment of the enamel to the dentine, and accounts, partially at least, for the difference that is observed in the ease with which enamel can be removed from the dentine in the preparation of roots for crowns. Where the two tissues join with smooth surfaces the enamel can be comparatively easily cleaved away ; where the scalloping is marked it is removed with much greater difficulty.

The Dentine. — The dentine gives the strength to the tooth. This should never be lost sight of in operations, and sound dentine should always be conserved to the greatest possible extent in the preparation of cavities. That the function of the dentine is to give strength will be seen more clearly from a comparative study of teeth modified for special functions. The dentine forms the greatest mass of the tooth, the type form being determined by it. The cusps and ridges, although different in form, are still represented in the dentine as well as the number and shape of the roots, while the detail of the form of the roots is modified by the addition of the cementum on the surface.

The dentine forms a layer of comparatively even thickness

DISTRIBUTION OF THE DENTAL TISSUES 33

surrounding the central cavity or pulp chamber, which is occupied by the formative organ. From this cavity a great number of small tubules extend through the calcified dentine matrix to the surface under the enamel and cementum. In the crown portion the course of these tubules is characteristically curved like the letter S or /, so that the tubules tend to enter the pulp chamber at right angles to the surface and to end under the enamel at right angles to the dento-enamel junction (Plate II). On closer study these tubule directions will be found to be more complicated, but in studying the distribution of dentine they should be noted. In the root portion the tubules are usually comparatively straight, that is, without the double curve, and are at about right angles to the axis of the canal.

The outer layer of dentine under low magnification presents a peculiar granular appearance, which is specially apparent under the cementum. This is known as the granular layer of Tomes, and is caused by irregular spaces in the dentine matrix which communicate with the dentinal tubules.

The Cementum. — The cementum covers the dentine in the root portion, and in most cases slightly overlaps the enamel at the gingival line. This is not always true, for in some cases it just meets the enamel, and in others there is a space where the dentine is uncovered between the enamel and the cementum (Fig. 8). It has not been positively determined whether this can ever be considered a normal condition, and the author has some reason to suppose that the sections showing this condition were from teeth from which the gums had receded and the cementum was destroyed. The sensitiveness which is so marked in some cases, where the gums have receded beyond the gingival line, is probably due to the loss of cementum and the uncovering of the granular layer of Tomes.

The cementum is thin and structureless in appearance in

the gingival portion when viewed with low powers, but

becomes thicker in the apical third. In the thicker portions

irregular spaces (lacunae) with radiating canals (canaliculi)

THE DENTAL TISSUES are seen. In life these spaces contain living cells (the cement corpuscles), which correspond to the bone corpuscles found in the lacuna of bone. Upon the convex surfaces of the root the cementum is thin; upon the concave surface it is thicker. This increases with age, and so the continuous formation of cementum tends to round the outlines of the roots and to

Fig. 8

Gingival line, showing the relation of enamel and cementum.

unite them where they approach each other. The fibers which are built in the cementum are often imperfectly calcified, especially where the layers are thick, so that in the ground sections they may often be easily mistaken for canals, because the imperfectly calcified fiber has shrunken in the preparation.