Oral Anatomy and Embryology ORAL HISTOLOGY AND EMBRYOLOGY. EDn,ruNDAPPLEBAUM,
D.D.S.,” NEW YORK? N. Y.
Embryonic Development of Jaws and Teeth
HE oral surgeon needs fundamental knowledge in handling diseased tissues. Pathology is altered physiology and as Verworn sta.ted, “16 is to the cell that the study of every bodily function sooner or later drives us.” The cell was gradually established as the smallest structural and functional unit of life, as a consequence of a series of discoveries during the last three hundred years. Robert Hooke (1667) first saw and named the cellular units of cork using very crude lenses. Anton van Leeuwenhoek made many contributions to histology and bacteriology around this time. Other workers made more detailed studies with improved microscopes. Thus Robert Brown (1831) first observed the cell nucleus as a structure marked off from the surrounding protoplasm or cytoplasm. In 1838 Schleiden and Schwann enunciated the now accepted cell theory of the structure of both plants and animals. In 1861 Max Sehultze established the doctrine that cells are masses of protoplasm and that this substance is much the same in all living organisms. All this and other work laid the foundation of modern histology, embryology, and pathology, with the cell as the common structural and functional basis for vital phenomena such as metabolism, growth, reproduction and irritability. The modern conception of the living substance of a cell which Purkinje (1840) named protoplasm is that it is a little laboratory where various chemical reactions take place (Hofmeister, 1901). Protoplasm is an organized system of substances combined and interacting in certain ways with the environment and with one another. In cells the reactions are concerned with building up processes (anabolism) and destructive processes (catabolism). The former makes possible the growth and repair of t.he tissues, while the latter releases energy needed for muscular activity and maintenance of vital functions. The laws which govern the growth of normal human tissues and organs are very rigid. The form of normal cells is so constant and their relation to one another, in the architecture of the organ, so fixed that we become familiar with their appearance and instant,ly recognize any divergence in disease. Growth of the multicellular organism consists primarily in the synthesis of new protoplasm through the activity of that already present. As the protoplasm continues to grow, and the nuclei to multiply, the patterns assumed by This paper was presented at the flrst session of the class in Clinical the New York Institute of Clinical Oral Pathology. *Associate Professor of Dental Anatomy. Columbia University.
the cell cytoplasm, nuclei, and met~tl~attes may come to differ witlely in the various tissues ant1 organs. There is at1 increase itt the ttmr~l:t~t-of wlls through cell division. The details of ~11 tlirision, tttit,osis, oecurt%rg in all higher plants and animals can IW seen only with Ihv highest powr~~ 01 tile trtic*roscopc. The most significant feature of ttritosis is the rsaet (livisiott of the tllWlt31~ chromosomes ant1 the tlistt*il.utic)tt ol’ equivalent balvvs to
This large oval cell has granular Fig. l.--Photomicrograph (X800) of a mouse ovum. There is a central nucleus with cytoplasm surrounded by a cell membrane or zona pellucida. The ovum is surrounded by a double layer of small deep-staining chromatin and a nucleolus. follicular cells with large nuclei.
The ovum is a germ cell which, when fet%ilized, gives rise to all the cells of the l:otly hy cell division or mitosis. b’ip. 1 is a ~)hotot~tict,o~~it~~)h (ntagnification, x800) of a mouse ovum in an early stage of il-s tlevelopment, surt~oundetl 1)~ a double layer of follicular cells. The oral ovum has a large vesicnlar The cystoplasm is nucleus with deep-stainin, ~7 chromatin ant1 a rtucleolus. granular and surrounded by a, cell memltra.ne or ZOIM pellucida. The ~lucle~s is the reproductive center of the cell and cont,ains the chromosomes which are the units of inheritance. In practically the entire anittta,l kitrptlom t,here tttust Ite a union of two sesually different cells to ~~rotluw a new intlivitlual. I:efore this takes place. the sex cells must ~tass through a j)ret,aratjot*y process called maturation. The Jlutnl)er of chromosomes itt each ttlatut*e SC’S cell is t~rtlucetl to half the somatic number for the species. This reduction division is called miosis and differs from ordinary mitotic division of a cell.
In the process of fertilization the full species number of chromosomes is restored. In this way both parents have contributed equally to the characters of the new individual. The fertilized ovum is encapsulated by the mucous membrane of the uterus, which later becomes the maternal part of the placenta. Immediately after its fertilization, the ovum enters upon a series of cellular mitotic divisions called segmentation or cleavage. Marked variations in the details of cleavage occur in eggs of different species, apparently largely dependent on the amount of yolk present and its distribution within the egg. Cleavage in all types of eggs is initiated by the mitotic division of the nucleus. In the case of simple marine forms such as the starfish, the egg develops with clocklike regularity and one can watch the entire process right under the microscope. In such spherical ova without yolk, the whole egg divides into hemispheres. These two cells divide into four, at right angles to the first plane. These four divide into eight, in a plane at right angles to the last. The process goes on until a morula is formed (named after its resemblance to a mulberry). The blastula stage is reached when a central cavity is established in the embryonic morula. Gastrulation of the embryo occurs by an invagination of the hollow blastula. The central cavity is called the archenteron or primitive intestine; the opening is called the blastopore. All during this process there has been a constant proliferation of cells and gastrula formation is probably related to the inturning of the cells at the blastopore rim. In a mammal such as man there are marked differences from simple marine forms during development. After rearrangement of the cells of the mammalian morula to form a sort of blastula, a cluster of cells is established at one pole. This is called the inner cell mass and is destined to form the embryo. The thin enveloping layer of the blastula forms certain membranes which become related to the uterus of the mother, for absorption of food and removal of waste. The inner cell mass soon differentiaties into ect,oderm, mesoderm, and entoderm. The ectoderm of the inner cell mass forms the amniotic cavity, and the entoderm forms the yolk sac. The area between the amniotic cavity and the yolk sac forms the embryo proper, the embryonic disc.
Development of External Body Form Any standard embryology text shows sequential changes in of the embryonic disc to its enveloping membranes. While the amniotic cavity is attached to the chorion, the embryonic disc only at its caudal end to the chorion, by means of the belly stalk. around 1’7 days.
the relation roof of the is attached This occurs
A few days later, what was originally the embryonic disc has assumed a cylindrical shape owing to the rolling under of its lateral margins, which curve downward and inward. There is also a curving of cranial and caudal
portions of the embryonic disc. Then there is a pinching off of a portion of the yolk sac which becomes the primitive gut. This occurs around 21 days and the embryo is about 1.5 mm. Later, the constriction between embryo and yolk sac becomes deeper. The developing neural groove has expanded at its ant,erior end, into the brain, and the posterior portion is developing into the spinal cord. As the embryonic axis grows, the paraxial mesoderm becomes divided successively into symmetrically arranged paired segments called somites, which give rise to vertebrae. etc.
Fig. 2.-Model of a 4 mm. human embryo (His). of its amniotic and chorionic sacs. Note the protrusion, Just above the heart is the stomodeum surrounded by the of the flrst branchial arch and the nasofrontal process of
This embryo has been dissected out the heart, just above the yoIk sac. mandibular and maxillary processes the developing face.
In a little older and larger embryo, about. 4 mm. (Fig. 2), t,he extreme end of the head region is bent ventrally almost at a right angle to the long axis of the body. On the ventral side of the body and cranial to the attachment of the yolk sac, there is a rather large protrusion, the heart. Between the heart and the bent part of the head there is a deep depression, the oral fossa or stomodeum. We also see the beginning of the branchial grooves and branchial arches which are exceedingly important in the development of the face and neck regions. They are the equivalents of the gills and gill slits in lower A strong process called the maxillary process has grown vertebrates. cranially from the dorsal part of the first arch. The main part of the arch is the mandibular process. The constriction between embryo and yolk sac has become deeper. The yolk sac is attached to the embryo only by a slender cord, the yolk stalk. In a somewhat later stage (11 mm.) (Fig. 3) further changes have OChave increased so that the tail almost touches the All the flexures curred.
head. The branchial arches and grooves are especially prominent. The site of the external ear is marked by the second branchial groove. In addition, the The optic vesicle is seen just anlage of other sense organs are apparent. cranial to the first arch. The nasal fossa is a distinct depression on the ventral side of the head, cranial to the first arch. Upper and lower limb buds are seen. The yolk stalk has become longer and more slender.
Fig. .3.-M$e170f ,an 11 mm. human embryo. dissected out- of its amnjoti,c and chorionic sacs. more mar nosy nexures nave mcreasea. ‘me opric vesicles 1s seen gust cranial to the first arch. The nasal fossa is a distinct depression on the ventral side of the head. The site of the external ear is marked by the second branchial groove. Upper and lower limb buds and somites are well shown.
Development of the Face In an embryo at 3 weeks the facial primordia are still undifferentiated. At about 4 weeks of intrauterine life we see that the rapidly expanding neurocranial portion of the head and the facial primordia are beginning to develop. The site of the external ear is marked by the second branchial groove. In addition, the anlage of other sense organs are seen, the optic vesicle and nasal fossa. The facial primordia undergo a series of migrations, fusions, and outgrowths in order to form a normal human face. The fusions are like the healing of wounds. The development of the facial region during the fifth to eighth weeks (Fig. 4) is dominated by changes resulting in the formation of the nose. First t,he nasal placodes sink in to form the nasal pits. This sinking in of the
placodes is due not, so much to their own depression as to the elevation of the adjacent mesenchyme. The elevations resulting from mesenchymal growt,h are called medial and lateral nasal processes. The medial nasal processes together with the intervening area above the stomodeum, form the so-called frontonasal process. The upper portion of the front,onasal process becomes the forehead of t,he adult,. The line of fusion of the medial nasal processes becomes the philtrum or groove in the upper lip of an adult. The masillnry processes also contribute t,o the upper lip formation. At the end of t,he second month a transverse furrow appears between the frontonasal process and the forehead region. The nasal septum is also increased in length and reduced in relative width, so that the nose can be recognized. Thus, while the nose is still quite simian in breadth and flatness, both eyes and ears are well advanced and the embryo is beginning to look human.
Fig. 4.-Ventral view of developing head of processes together with intervening area above lateral nasal process separates the corresponding process gives rise to wing of nose in adult while Medial cheek and lateral portion of upper lip. upper lip of adult.
embryo at about 6 weeks. The medial nasal stomodeum form frontonasal process. Each nasal pit from developing eye. Lateral nasal maxillary process gives rise to major part of nasal processes become philtrum or groove in
Each lateral nasal process separates the corresponding nasal pit from the developing eye. Later the maxillary process on each side of the face fuses with the corresponding lateral and medial nasal processes. This fusion obliterates the naso-optic furrow and also shuts off the communication between the mouth slit and the nasal pit. The lateral nasal process gives rise to the wing of the nose in the adult. while the maxillary process gives rise to the major part of the cheek and the lateral portion of the upI>er lip. The anterior extremities of the mandibular processes fuse to form a comprocess grows plete lower jaw very early, at 5 mm. stage. Each maxillary anteriorly from the dorsal portion of the corresponding mandibular arch and fuses with its lateral nasal process. It then extends beyond this across the lower margin of t,he nasal pit to fuse with its medial nasal process. Later it fuses with its fellow of the opposite side, extending in front of the lower nor-
tions of the medial nasal processes, as the primary palate. This primary palate c’onstitutes the bar of tissue between the nasal pit and the oral cavity and it develops into t,he upper lip and the premasilla.. This region is the sit.e of harelip when these processes fail to unite (at about the second month). At this time the secondary palate begins to form, when the extensions of the maxillary processes, called the palatal ])rocesses. coalesce in the midline to shut off the mouth from the nasal fossae. There also occurs a union with t.he nasal septum. Failure of this fusion result,s in cleft palate. Fig. 5 is a fronta. section of a 21 mm. human embryo about this time. Note that the palate is not yet closed and the tongue protrudes up into the nasal cavity. One can also see an ingrowth of the oral epithelium for the dental lamina, the precursor of the enamel organ or tooth germ. Meckel’s cartilage is formed in the first gill arch and is the forerunner of the bony mandible.
Fig. 5.-Photomicrograph ( X30) of a histologic section of head of human embryo at about 7 to 8 weeks. Note the open palate with tongue protruding into nasal cavity. Note ingrowth of oral epithelium for dental lamina and signs of bong mandible forming around Meckel’s cartilage.
When the three parts of the palate unite, their epithelial covering disappears in order to insure a bony union. Small islands of epit.helium may remain in the line of closure. These ’ ‘ epithelial rests ’ ’ may be stimulated to proliferate in later years and then undergo cystic change. This is the origin of an anterior median maxillary cyst. While we see signs of tooth formation (Fig. 5) at the second month of intrauterine life, it is in the fetal stages and even after birth that we see the functioning enamel organ active in tooth formation. Although the fetal head is only the size of a walnut at the stage sectioned in (Fig. 6), one can readily recognize the various organs, in miniature, in this low-power picture. Included in the plane of this central sagittal section, in an anteroposterior direction, are the nose, lips, and chin. MTe see the enamel organs of the max-
iila,ry a,nd mandibular deciduous central incisors, the muscles of the tongue and the floor of the mouth, hard and soft pa.lates: hyoid bone, and epiglottis. We also note the sphenoid kone, pans. and odontoitl process of thr second caervical vertebra,.
Fig. 6.-Low-power photomicrograph of a central sagittal section of a human fetal head. 1, Nasal bones; 2, maxilla; 3, pala e; 4. lips: 3. tongue: 6. enamel organ of mandibular deciduous central incisor : 7, mandible ; 8, muscles of the floor of the mouth; 9, sphenotd bone ; 10, pituitary body: II. pans; 1Z. soft palate: 1.3, epiglottis: 14 hyoid bone: 15. odontoid process of second cervica.1 vertebra.
It is noteworthy that all the elementary tissue groups are seen in the fetal head, at slightly higher magnification. A tissue is defined as a group Thus epithelium is specialized of cells specialized for a particular function. for covering surfaces or lining cavities. Another group of cells, muscle, is specialized for contraction. The connective or supporting tissues are evident in the head. These tissues, together with blood and nervous tissue, are the elementary tissue groups which arise from the three primary gerrn layers of the embryo. Obviously no tissue exists in pure form. Thus artery, nerve, and vein run together to every tissue and organ.
Development of Teeth and Jaws Much of the difficulty in understanding the growth of the teeth and jaws is removed by the observation of M. IIiamond that the teeth and jaws have This certainly applies to prenatal independent (genetic) patterns of growth. phenomena where there is a self-diff’erentiation of the parts of the body, independent of, and prior to, function, yet the organs are roughly formed in anticipation of function (Roux j.
of the condyle consists of new sorption
Fig. 7.--Low-power photomicrograph of g-month human fetus in plane of ramus and body mandible. Bone outlines lower border of mandible and part of the ramus. Head of Interior of body of mandible with part of ratnus and coronoid process are also seen. Appositgey of loose connective tissue with spacing between developing tooth germs. bone is seen on posterior border of ramus and lower border of body of mandible. of bone is seen on inner aspect of mandible adjacent to expanding tooth follicles.
Fig. 8.-Low-power photomicrograph of horizontal section of maxilla of g-month fetus. Five deciduous maxillary and flrst permanent molar tooth germs are seen in their respective Lateral incisor germ appears in lingual relation to the central incisor states of development. A connective tissue follicle surrounds each germ. Varied thickness of bone surand cuspid. rounds each follicle. The proper positioning of the lateral incisor occurs as the growth of bone widens the perimeter of the anterior curve of the arch.
There is an inherent correlation of tooth ant1 jaw patterns of growth in space and time. Normally, the growth of t,eeth arltl ,jaws is synchronized 1)~ iI “St~agger” methotl of develolnnent, allowing room f’or the JIWJW~ alignment of the teeth. Thus the alternating growth of the ramus of the mandible and the clinical eruption of teet,h int,o the intermaxillary space crca,teti are responsible for the tlevrlopmenb of the (lental heighl ( I)iamontl). Fig. 7 is a
Fig. I).-Low-power photomicrograph of $%-month fetus cut in frontal plane through maxillary deciduous tooth germs and nose. Tooth germs are surrounded by loose connective tissue. Bone is present at fundic area of floor of nose as well as on mesial and distal aspects of tooth germs ; bone is absent incisaljy, above, tooth germs. Maxillary suture, is seen bet.ween U~;u~~isors: new bone is being deposIted on YK~Otoward the nose as well as tn the maxillary
low-power view of a sagittal section of a 6n1ont.h human fetus, cut in the plane of the ramus and body of the mandible. The condyle of the mandible is a very active center of intracartilaginous bone formation, essentia.1 in the growth of the ramus of the mandible. The entire interior of the body of t.he mandible consists of loose connective tissue with generous spacing between developing crown germs. Resorption of bone is seen adjacent to the expanding tooth follicles. Apposition of new bone is evident on t,he posterior border of the ramus and at the lower borcler of the body of the mandible. r\;o matter what plane is studied. we see that, the growth of bone of the fetal skull occurs by apposition on t,he outer surface. and resorption on the
inner surface. Fig. 8 is a low-power view of a horizontal section of the maxilla of a 6-month human fetus. Five deciduous maxillary and the first permanent molar crown germs are seen in their respective stages of development. The lateral incisor crown germ appears in lingual relation to the central incisor and cuspid. A connective tissue follicle surrounds each crown germ, and a varied thickness of bone surrounds each follicle. The proper lateral or horizontal positioning of the lateral incisor occurs as the growth of bone widens the perimeter of the anterior curve of the arch. The lateral incisor does not migrate to its normal position in the arch until the crown of the tooth has completed its formation.
Pig. 9 is a low-power view of a frontal section of a 4yz-month human fetus. In this section, cut through the premaxillary suture between the two central incisor crown germs, we see new bone formation in the region of the suture. As growth of bone occurs labially, the perimeter of the anterior curve of the premaxilla is steadily increased, thus widening the suture area, into which bone grows from the rnesial terminal ends of the bones. New bone is also being deposited on the side toward the nose. Epithelium is the primary formative tissue of the human tooth. Starting with a .proliferation of the stratum germinativum covering the future
jaw ridges, the rpitheliunl b~omes thicker in places correspontliug to the positions of the future deciduous teei.h. This ha,s beeu observed RS early as the sixth week of intrauterine life when the embryo is about I 1 mm. The c$thelial thickening is commonly known as the dental lamina.. I.$. 5 is :I front,al section of a. hunlan embryo ahout, 21 mm. crown-rump length. The palate is still open and there is an epithclial ingrowth or latninn. for the tooth ge tms.
Fig. Il.-High-power photomicrograph of area X in Fig. 10 showing beginning of enamel Arrows indicate direction of movement of ameloblasts and odontoand dentine formation. blasts during incremental formation of enamel and dentine. SE, Stellate reticulum : XI, stratum intermedium ; A, ameloblasts ; I’, papi!la ; I,>, fwamel : II, dentinr ; 0, odontoblasts.
Fig. 6 is a sagittal section 01’ a hun~;rx~ fetus (ul)ollt, 1 tnouths) showing the enaniel organs of upper and lower central incisors in t,he “bell ” stage, just before the beginning of hard tissue formation. The enamel organ must Under high magnification we can enlarge many times to form a tooth crown.
see mitotic figures in the growing enamel el)ithelium as well as in t,he connective tissue papilla of the tooth germ. Fig. IO shows a later stage of enamel organ development during which the formation of enamel and dentine takes place. ln this stage we see clearly differentiated the four layers of the epithelial enamel organ. The inner enamel epithelium consists of a single layer of columnar cells (ameloblasts) which will be concernecl with the formation of enamel rods. Several layers of low squamous cells called the stratum intermedium appear between the stellate The cells of the outer enamel reticulum and the inner enamel epithelium. epithelium are of low cuboid form. The cells of the stellate reticulum are separated by an increase of intercellular fluid. In Fig. 10, the differentiation of ameloblasts is most advanced in the region of the incisal edge, and least advanced in the region of the future cervical loop at C. Fig. 11 is a high-power photomicrograph of the region cells; SR, marked X in Fig. 10. A, ameloblasts; SI, stratum intermedium stellate reticulum ; 0, odontoblasts; Y, dental papilla; E, enamel ; D, dentine. In the organizing stage of development, the ameloblasts seem to exert an influence upon the adjacent connective tissue cells of the dental papilla which causes them to differentiate into odontoblasts. During this process the formation of dentine by odontoblasts begins. The ameloblasts enter their formative stage only when the first layer of dentine ha.s already been formed by the odontoblasts. The line separating these two g&ups of cells is the future dentinoenamel junction. The ameloblasts progress outward in the direction of the &row in Fig. 11 while depo!iting layers of enamel, and the odontoblasts move inward while depositing layers of dentine. In the enamel the increments of growth are called the striae of Retzius. In the dentine the corresponding increments are called the contour lines of Owen. Calcificati6n of dentine occurs along the direction of the lines of Owen. Gut calcification or maturation of enamel starts at the top of t,he crown and progresses in transverse relation to the incremental pattern of the enamel matrix, as shown by Diamond and Weinmann. Nutrition, during enamel formation and calcification, affects its structure, which in turn affects the rate of caries after the tooth erupts and is exposed to the action of bacteria in the mouth.