Book - Embryology of the Pig 13
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Patten BM. Embryology of the Pig. (1951) The Blakiston Company, Toronto.
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Patten BM. Developmental defects at the foramen ovale. (1938) Am J Pathol. 14(2):135-162. PMID 19970381
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Chapter 13. The Development of the Face and Jaws and the Teeth
I. The Face and Jaws
The Stomodaeum
In studying the early development of the digestive tract we saw that the primitive gut first appeared as a cavity which was blind at both its anterior and posterior ends (Fig. 37). Its opening in the future oral region is established by the meeting of an ectodermal depression, the stomodaeum, with the cephalically growing anterior end of the gut. The stomodaeal depression, even as late as the time the oral plate ruptures and establishes communication between the anterior end of the gut and the outside world, is very shallow (Fig. 40). The deep oral cavity characteristic of the adult is formed by the forward growth of structures about the margins of the stomodaeum. Some idea of the extent of this forward growth can be gained from the fact that the tonsillar region of the adult is at about the level occupied by the stomodaeal plate when it ruptures. The growth of the structures bordering the stomodaeum, then, not only gives rise to the superficial parts of the face and jaws, but actually builds out the walls of the oral cavity itself.
The Jaws
Because the face of a young embryo is pressed against the thorax it is difficult to study unless the entire head is cut off and mounted separately. Preparations of this kind observed under a dissecting microscope by strong reflected light show the surface configuration of the facial region very clearly. The most conspicuous landmarks are the stomodaeal depression, which in view of its fate we may now call the oral cavity, and the olfactory pits. In embryos as small as 7 mm. most of the structures which take part in the formation of the face and jaws are already clearly distinguishable (Fig. 168). In the mid-line cephalic to the oral cavity is a rounded overhanging prominence known as the Jrontal process. On either side of the frontal process are horseshoe-shaped elevations surrounding the olfactory pits. The median limbs of these elevations are known as the nap-medial processes and the lateral limbs are called the naso-lateral processes.
Growing toward the mid-line from the cephalo-lateral angles of the oral cavity are the maxillary processes. In lateral views of the head (Figs. 31 and 32) it will be seen that the maxillary processes and the mandibular arch merge with each other at the angles of the mouth. Thus the structures which border the oral cavity cephalically are: the unpaired frontal process in the mid-line, the paired nasal processes on either side of the frontal, and the paired maxillary processes at the extreme lateral angles. From these primitive tissue masses the upper jaw and the nose are derived.
Fig. 168. Face of 7 mm. pig embryo photographed X 15. Note especially the unmistakably paired character of the thickenings which later fuse in the mid-line to complete the mandibular arch.
The caudal boundary of the oral cavity is less complex, being constituted by the mandibular arch alone. In very young embryos (Fig. 168) the origin of the mandibular arch from paired primordia is still clearly evident. Appearing first on either side of the mid-line are marked local thickenings due to the rapid proliferation of mesenchymal tissue. Until these thickenings have extended from either side to meet in the mid-line there remains a conspicuous mesial notch. With their fusion, the arch of the lower jaw is completed (Figs. 1 69172).
In 10-12 mm. embryos (Fig. 169) very marked progress can be seen in the development of the facial region. The maxillary processes are much more prominent and have grown toward the mid-line, crowding the nasal processes closer to each other. The nasal processes have grown so extensively that the frontal process between them is completely overshadowed (cf. Figs. 168 and 169). The growth of the medial limbs of the nasal processes has been especially marked and they appear almost in contact with the maxillary processes on either side.
Fig. 169. Face of 11 ,5 mm. pig embryo photographed X 12. Fusion of the right and left components of the mandibular arch is practically complete. Both the medial and lateral limbs of the horseshoe-shaped nasal processes have undergone conspicuous enlargement. Note especially the approximation of each naso-medial process to the maxillary process of the same side.
The groundwork for the formation of the upper jaw is now well laid down. Its arch is completed by the fusion of the two nasomedial processes with each other in the mid-line, and with the maxillary processes laterally (Fig. 170), The premaxillary bones carrying the incisor teeth are formed, later, in the part of the upper jaw which is of naso-medial origin. The maxillary bones, carrying all the upper teeth posterior to the incisors, are developed in the part of the arch arising from the maxillary processes.
Nasal Chambers
The olfactory pits have by this time become much deepened, not only by the growth of the nasal processes about them, but also by extension of the original pits themselves which soon break through into the oral cavity (Figs. 93 and 97, C). We may now speak of the external openings of the nasal pits as the nostrils {external nares) and their new openings into the oral cavity as posterior nares or nasal choanae. The septum of the nose is formed by fusion in the midline of the original naso-medial processes'; the upper part of the bridge of the nose is derived from the frontal process; and the alae of the nose arise from the naso-lateral processes (Fig. 172).
Fig. 170. Face of 16 mm. pig embryo photographed X 10. The naso-medial processes have fused with the maxillary processes on either side, and with each other in the mid-line, thus completing the arch of the upper jaw.
Naao-Iacrimal Duct
Where the naso-lateral process and the maxillary process meet each other there is formed iot a time a well marked groove, which extends to the mesial angle of the eye (Fig. 169). This is known as the naso4acrimal groove. It soon closes over superficially (Fig. 171), and it is usually stated that the deep portion of the original groove is converted into a tube, the naso-lacrimal duct^ or tear duct, wjhich drains the fluid from the conjunctival sac of the eye into the nose. Recently Politzer has maintained that the nasolacrimal duct arises as an independent epithelial downgrowth from the conjunctival sac which follows closely along the line of closure of the old naso-optic furrow.
Fir;. 171. Face of 17.5 mm. pig embryo photographed X 10. The originally separate processes have now largely lost their identity in the series of fusions which have taken place in the formation of the face.
Tongue. While these changes are going on externally, the tongue is being formed in the floor of the mouth. Anatomically the tongue is usually described as consisting of a freely movable part called its body, and a less freely movable portion, called its root, by which it is attached in the oro-pharyngeal floor. The body of the tongue arises from a small median elevation, the tuberculum impar^ and paired lateral lingual primordia. These elevations appear very early in development on the inner face of the first branchial (mandibular) arch (Fig. 173, B). The tuberculum impar grows slowly and is soon crowded in on by the more rapidly growing lateral lingual primordia which form the great bulk of the body of the tongue (Fig. 173, c5.
Fig. 172. Face of 21.5 rum. pig embryo photographed X 10. The characteristic features of the adult face are even at this early stage clearly recognizable. The regions of the upper jaw and nose which have arisen from originally distinct primordia are differentiated by shading. Vertical hatching indicates origin from frontal process; stippling, from naso-lateral processes; small crosses, from naso-medial processes; horizontal hatching, from maxillary processes. The entire lower jaw is derived from the mandibular arch.
Arising in the pharyngeal floor at the bases of the second and third branchial arches is an elevation known as the copula (i.e., yoke) because of the way it joins these arches together (Fig. 173, B). The copula, supplemented by some tissue from the adjacent basal portions of branchial arches 2, 3, and 4, gives rise to the root of the tongue.
All the various elevations which thus take part in the formation of the tongue must be thought of as composed of an outer covering and the underlying mesodermal tissue which causes the covering to bulge into the lumen. The covering tissue arises in situ from the lining of the branchial arches involved. The sensory innervation of the surface of the tongue is, therefore, just what one would expect from the basic relations of the cranial nerves to the branchial arches. The epithelium of the body of the tongue gets its sensory supply from the lingual branch of the mandibular division of the trigeminal (V) nerve (Fig. 97, A, B), and from the chorda tympani branch of the seventh nerve. The root of the longue receives its sensory fibers from the glossopharyngeal (Fig. 94) and vagus nerves.
Fig. 173. Dissections of pig embryos made to expose the floor of the mouth and show the development of the tongue. (After Prentiss.) A, 7 mm.; B, 9 mm.; C, 13 mm. (All figures X 12.)
The skeletal muscle that makes up the main mass of the tongue beneath the mucosal covering is derived from mesodermal cell masses that are believed to migrate into the pharyngeal floor from the myotomes of the occipital somites. Ontogenetically, in mammalian embryos this migration is exceedingly difficult to trace, for the cells of myotomal origin early mingle indistinguishably with the local mesenchymal cells. Nevertheless the way the hypoglossal nerve (XII), which is the cranial nerve arising at the level of these occipital myotonies, grows in with the developing lingual muscles (Figs. 93, 94, and 97, A, B) and innervates them, furnishes strong circumstantial evidence for this interpretation of tongue muscle origin and migration.
Palate
Coincidently also the palatal shelf is being formed in the upper jaw and separating off the more cephalic portion of the original stomodaeal chamber. Since it is into this cephalic portion of the cavity that the nasal pits break through (Figs. 93 and 97, C), the formation of the palatal shelf in effect prolongs the nasal chambers backwards so they open eventually into the region where the oral cavity becomes continuous with the pharynx.
The palate as well as the arch of the upper jaw is contributed to by both the naso-medial processes and the maxillary processes. From the premaxillary region a small triangular median portion of the palate is formed (Fig. 174). The main portion of the palate is derived from the maxillary processes. From them shelf-like outgrowths arise
Fig. 174. Photographs (X 6) of dissections of pig embryos made to expose the roof of the mouth and show the development of the palate. A, 20.5 mm.; B, 25 mm.; C, 26.5 mm.; D, 29.5 mm.
The diagrams of transverse sections are set in to show the relations before (E) and after (F) the retraction of the tongue from between the palatine processes.
on either side and extend toward the mid-line (Fig. 174, A— G). When these palatal shelves first start to develop, the tongue lies between them (Fig. 174, E). As development progresses the tongue drops down (Fig. 174, F); the palatal shelves are extended toward the mid -line and finally fuse with each other medially and with the premaxillary process anteriorly to complete the palate (Fig. 174, D). At the same time the nasal septum grows toward the palate and becomes fused to its cephalic face (Figs. 174, F, and 178). Thus the separation of right and left nasal chambers from each other is accomplished at the same time that the nasal region as a whole is separated from the oral.
Fig. 175. Transverse section of snout of 28 mm. pig embryo (X 12). The area included in the rectangle is shown in detail in the following figure.
II. The Development of the Teeth
The Dental Ledge
Local changes leading toward tooth formation can be made out in the jaws of embryos as small as 1 5 mm. or even less. By the time a size of 28-30 mm. has been attained, a definite thickening of the oral epithelium can readily be seen on both the upper and the lower jaw. This band of epithelial cells which pushes into the underlying mesenchyme around the entire arc of each jaw is known as the labi<Hlental ledge {labio-dental lamina) (Figs. 175 and 176). Shortly after its first appearance, cross-sections show this ledge of epithelial cells to be differentiating into two parts, a more distal part which by its ingrowth marks off the elevation which is to become
Fig. 176. Drawing (X 130) showing labio-dental ledge of 28 mm. pig embryo. For location of area represented see preceding figure.
Fig. 177. Drawing (X 130) showing differentiation of the labio-dental ledge into labio-gingival lamina and dental ledge.
The ingrowth of the labio-gingival lamina initiates the separation of the lip from the gum (gingiva). From the dental ledge a series of local bud-like outgrowths are formed, each of which gives rise to the enamel cap of a tooth.
The region shown is the same as that in the preceding figure but from a slightly older (37 mm.) embryo.
the lip from that which is to become the gum, and a more proximal part which is destined to grow into the gum and give rise to the enamel-forming organs of the teeth. The part of the original labiodental ledge which separates the lip from the gum (gingiva) is known as the labio-gingival lamina^ and the part of the original ledge which is to take part in tooth formation is known as the denial ledge or dental lamina (Fig. 177).
Enamel Organs. Soon after the dental ledge is established, local buds arise from it at each point where a tooth is destined to be formed. Since these cell masses give rise to the enamel crown of the tooth they are termed enamel organs. As would be expected, the enamel organs
Fig. 178. Drawing (X 10)tDf a transverse .section of the snout of a 71 mm. pig embryo. The area included in the rectangle is shown in detail in the following figure.
for the milk teeth are budded off from the dental ledge first, but the cell clusters which later give rise to the enamel of the permanent teeth are formed at a surprisingly early time (Fig. 180). They remain dormant, however, during the growth period of the milk teeth and begin to develop actively only after the jaws have enlarged sufficiently to accommodate the permanent dentition.
The histogenetic processes involved in the formation of milk teeth and permanent teeth are essentially the same. It is, therefore, sufficient to trace them in the case of the milk teeth only, keeping in mind that the same process is repeated later in life in the formation of the permanent teeth.
In a section of the developing mandible which cuts the dental ledge at a point where an enamel organ is being formed, the shape of the enamel organ suggests that of an irregularly shaped, inverted goblet, the section of the dental ledge appearing somewhat like a distorted stem (Fig. 178). The epithelial cells lining the inside of the goblet early take on a columnar shape. Because they constitute the layer which secretes the enamel cap of the tooth, they are called ameloblasts (enamel
formers) (Fig. 179). The outer layer of the enamel organ is made up of closely packed cells which are at first polyhedral in shape but which soon, with the rapid growth of the enamel organ, become flattened. They constitute the so-called outer epithelium of the enamel organ (Fig. 179). Between the outer epithelium and the ameloblast layer is a loosely aggregated mass of ceils called collectively, because of their characteristic appearance, the enamel pulp or the stellate reticulum (Fig. 179).
Fig. 180. Developing tooth from lower jaw of a 120 mm. pig embryo (X 14). The small sketch including half of the tongue (left) and part of the lip (right) gives the relations of the region drawn. The area in the rectangle is shown in detail in the following figure.
Fig, 181. Projection drawing (X 350) of segment of enamel organ and adjacent pulp from a 120 mm. pig embryo to show ameloblast and odontoblast layers. For location of area represented see preceding figure.
The Dental Papilla
Inside the goblet-shaped enamel organ there is caught a mass of mesenchymal cells which are said to constitute the dental papilla (Fig. 179). The cells of the dental papilla proliferate rapidly and soon form a very dense aggregation. The outer cells of this mass are destined to secrete the dentine of the tooth and the inner cells to give rise to the pulp of the tooth.
A little later in development the enamel organ begins to assume the shape characteristic of the crown of the tooth it is to lay down (Fig. 180). At the same time the outer cells of the dental papilla take on a columnar form similar to that of the ameloblasts (Fig. 181). They are now called odontoblasts (dentine formers) because they are about to become active in secreting the dentine of the tooth.
In the central portion of the dental papilla vessels and nerves arc beginning to make their appearance so that the picture is already suggestive of the condition seen in the pulp of an adult tooth. Meanwhile the growth of the dental papilla toward the gum has crowded the stellate reticulum of the enamel organ in the crown region so it is nearly obliterated (Fig. 180). This brings the ameloblasts of this region much closer to the many small blood vessels which lie in the surrounding mesenchyme. The approach of the ameloblasts to the neighboring vascular supply would appear to be significant, since it is precisely here at the tip of the crown where the ameloblasts first begin to secrete enamel (Fig. 182).
By the time the enamel organ has been well established the dental ledge has lost its connection with the oral epithelium, although traces of it can still be identified in the mesenchyme at the lingual side of the tooth germ (Fig. 180). The cluster of cells which is destined to give rise to the enamel organ of the permanent tooth of this level can be seen budding off from the ledge close to the point from which the enamel organ of ffie milk tooth arose (Figi >^).
Formation of Dentine. With these preparatory developments complete, the tooth-forming structures are, so to speak, ready to go about the fabrication of dentine and enamel. As is the case with bone, enamel and dentine are both composed of an organic basis in which inorganic compounds are deposited. We may use the same comparison that was used in describing bone : that of the familiar use in construction operations of a steel meshwork into which concrete is poured, the steel giving the finished structure some degree of elasticity and increasing the tensile strength while the concrete gives body and solidity. In the case of such hard structures in the body as bone, dentine, and enamel, these interlacing organic strands in the matrix give the tissue its resilience and tensile strength, and the calcareous compounds deposited in the organic framework give form and hardness.
Fig. 182. Developing tooth from lower jaw of a 130 mm. pig embryo (X 30). The small sketch gives the relations of the regions drawn. The area in the rectangle is shown in detail in the following figure.
Fig, 183. Projection drawing (X 350) of small segment of developing
Although bone, dentine, and enamel are similar in having both organic and inorganic constituents in their matrix they are quite different in detail, both as to composition and microscopical structure. Bone has approximately 45 per cent of organic material while dentine has but 30 per cent and adult enamel 5 per cent or less. There is also considerable difference in the kind and proportion of inorganic compounds present in each. Structurally they are totally unlike. Bone matrix is formed in lamellae and has cells scattered through it. Dentine is formed without lam^llation and has its cellular elements lying against one face and sending long processes into tubules in the matrix. Enamel is prismatic in structure and the cells which form it lie against its outer surface while it is being deposited, but are destroyed in the eruption of the tooth.
The first dentine is deposited against the inner face of the enamel organ, the odontoblasts drawing their raw materials from the small vessels in the pulp and secreting their finished product toward the enamel organ. It is significant in this connection that in an active odontoblast the nucleus, which is the metabolic center of the cell, has gravitated toward the source of supplies and come to lie in the extreme pulpal end of the cell (Fig, 183). Also, the end of the odontoblast toward the enamel organ, where the elaborated product of the cell is being accumulated preparatory to its extrusion, can be seen to take the stain especially intensely. Although our knowledge of intracellular chemistry is as yet exceedingly fragmentary and we do not know the exact chemical nature of the product in this stage, the staining reaction is clearly indicative of the presence of calcium compounds of some sort.
If attention is turned now to the recently formed dentine, two zones distinctly different in staining reaction can be seen. The zone nearer the cells is pale, taking but little stain (Fig. 183). This zone consists of the recently deposited organic part of the matrix not as yet impregnated with calcareous material. The zone nearer the enamel organ will be found, by contrast, very intensely stained. This is the older part of the dentine matrix which has had the organic framework impregnated with calcareous material.
As the odontoblasts continue to secrete additional dentine matrix the accumulation of their own product inevitably forces the cell layer back, away from the material previously deposited. Apparently strands of their cytoplasm become embedded in the material first laid down and are then pulled out to form the characteristic processes of the odontoblasts known as the dentinal fibers (Fig. 183). As the layer of secreted material becomes thicker and the cells are forced farther from the material first deposited, these dentinal fibers become progressively longer. Even in adult teeth where the dentine may be as much as 2 mm. in thickness they extend from the odontoblasts which line the pulp chamber to the very outer part of the dentine. These dentinal fibers are believed to be concerned with maintaining the organic portion of the dentine matrix in a healthy condition. When the pulp is removed from a tooth, taking with it the odontoblasts, we know that the dentine undergoes degenerative changes which involve, among other things, increase in brittleness. This would seem to be attributable to the degeneration of the organic framework of a matrix no longer nourished by the odontoblasts.
Formation of Enamel. While the dentine is being laid down by the cells of the odontoblast layer, the enamel cap of the tooth is being formed by the ameloblast layer of the enamel organ. As was the case with the odontoblasts, the active cells of the ameloblast layer are columnar in shape and their nuclei, too, lie in the ends of the cells toward the source of supplies, in this case the small vessels in the adjacent mesenchyme (Fig. 183). The amount of organic material laid down as the framework of enamel is much less than is the case with either bone or dentine, and it is therefore more difficult to make out its precise character and arrangement. It is, nevertheless, possible to see in decalcified sections, delicate fibrous strands projecting from the tips of the ameloblasts into the areas of newly formed enamel (Fig. 183). It seems probable that these strands {Tomes processes) are in some way involved in the formation of the organic matrix of enamel. The problem of tracing the relations of Tomes’ processes to the organic framework of enamel is greatly complicated by the fact that where the ameloblasts have deposited calcium compounds the calcium has rendered the organic part of the matrix so avid in its affinity for stains that it is not possible to discern fine structural details because of the very density of the resulting coloration (Fig. 183). This reaction of the tissue to stains persists even after the inorganic calcium compounds have been removed by decalcification, indicating that the organic framework itself has been chemically altered by the calcium deposited in it.
In spite of these difficulties in getting at the exact nature and arrangement of the organic matrix of enamel, it is quite possible to see the genesis of its fundamental prismatic structure. Each amcloblasl builds up beneath itself a minute rod or prism of calcareous material. These prisms are placed with their long axes approximately at right angles to the dento-enamel junction. Collectively these enamel prisms form an exceedingly hard cap over the crown of the tooth which in its structural arrangement suggests a paving of polygonal bricks laid on end. There is sufficient difference in the rate at which the different ameloblasts work so that in actively growing enamel the surface is jagged and irregular due to the varying extent to ^<^hich the different prismatic elements have been calcified (Fig. 183).
Fig. 184. Schematic diagram showing the topography of a tooth and its relations to the bone of the jaw. The numbered zones indicate empirically the sequence of deposition of the dentine and enamel. The so-called growth lines in the dentine and enamel follow the general contours indicated by the dotted lines in the figure but arc much more numerous.
Both enamel formation and dentine formation begin at the tip of the crown and progress toward the root of the tooth (Figs. 182 and 184). But the entire crown is well formed before the root is much more than begun. The progressive increase in the length of the root is an important factor in the eruption of the tooth, for as the root increases in length the previously formed crown must move closer to the surface of the gum. Even when the crown of the tooth begins to erupt the root is still incomplete, and it does not acquire its full length until the crown has entirely emerged.
The Formation of Cementum
The so-called cementum of the tooth is virtually a bone encrustation of its root. No cementum is formed until the tooth has acquired nearly its full growth and its definitive position in the jaw. But the first indications of specialization in the tissue destined to give rise to it can be seen long before the cementum itself appears.
Outside the entire tooth germ, between it and the developing bone of the jaw, there occurs a definite concentration of mesenchyme. The concentration becomes evident first at the base of the dental papilla and extends thence crownwards about the developing tooth, which it eventually completely surrounds.
This mesenchymal investment is known as the dental sac (Fig. 182). In the eruption of the tooth the portion of the dental sac over the crown is destroyed, but the deeper portion of the sac persists and becomes closely applied to the growing root. At about the time the tooth has acquired its final position in the jaw, the cells of the dental sac begin to form the cementum. Histologically and chemically cementum is practically identical with subperiosteal bone. When we consider the manner of origin of the dental sac and of the periosteum of the bone socket (alveolar socket) in which the root of the tooth lies, and see how they arise side by side from the same sort of tissue, this seems biit natural. The dental sac is essentially a layer of periosteal tissue facing the root of the tooth and back to back with the periosteal tissue of the alveolar socket (Fig. 182).
The Attachment of Tooth in the Jaw
The attachment of the tooth in its socket is brought about by the development, between the dental sac and the periosteum of the tooth socket, of an exceedingly tough fibrous connective tissue. As the periosteum of the alveolus adds new lamellae of bone to the jaw on the one side, and the dental sac adds lamellae of cementum to the root of the tooth on the other, the fibers of this connective tissue are caught in the new lamellae. Thus the tooth comes to be supported by fibers which are literally calcified into the cementum of the tooth at one end and into the bone of the jaw at the other (Fig. 184). The mechanism involved is precisely the same as that which occurs in the burying of tendon fibers in a growing bone, where the buried ends of the fibers are known as the penetrating fibers of Sharpey.
Replacement of Deciduous Teeth by Permanent Teeth
The replacement of the temporary or "milk" (deciduous) dentition by the permanent teeth is a process which varies in detail for each tooth. The general course of events is, however, essentially similar in all cases. The enamel organ of the permanent tooth arises from the dental ledge near the point of origin of the corresponding deciduous tooth (Fig. 180). With the disappearance of the dental ledge, the permanent tooth germ comes to lie in a depression of the alveolar socket on the lingual side of the developing deciduous tooth (Fig. 185).
When the jaws approach their adult size the hitherto latent primordia of the permanent teeth b<-gin to go through the same histogenetic changes we have already traced in the case of the temporary teeth. As the permanent tooth increases in size, the root of the deciduous tooth is resorbed and the permanent tooth comes to lie underneath its remaining portion (Fig. 186). Eventually nearly the entire root of the deciduous tooth- is destroyed and its loosened crown drops out, making way for the eruption of the corresponding permanent tooth.
Fig. 185. Photomicrograph (X 5) of upper jaw of 160 mm. pig embryo showing the milk cuspids just breaking through the gum.
Fig. 186. Photomicrograph (X 6) of section through the jaw of a puppy showing a deciduous tooth nearly ready to drop out and the developing permanent tooth deeply embedded in the jaw below it. The space about the crown of the permanent tooth was occupied in the living condition by enamel. Fully formed enamel, being approximately 97 per cent inorganic in composition, is almost completely destroyed by the decalcification with acids which must be carried out before such material can be sectioned. (From a preparation loaned by Dr* S. W. Chase.)
Cite this page: Hill, M.A. (2024, April 25) Embryology Book - Embryology of the Pig 13. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Embryology_of_the_Pig_13
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