Book - Oral Histology and Embryology (1944) 1
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Orban B. Oral Histology and Embryology (1944) The C.V. Mosby Company, St. Louis.
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Chapter I - Development of the Face and Oral Cavity
1. Introduction
Knowledge of the embryology of an organ is essential for an understanding of its structure; it is also an important source of information in the field of malformations. This chapter deals with the development of the face, the palate, and the tongue.
2. Development of the Face
In the human embryo, 3 millimeters in length (3 weeks old), the rounded prominence formed by the forebrain (proseneephalon. the anterior of the three primary brain vesicles) constitutes the greater part of the face. It is covered by the ectoderm and a thin layer of mesoderm (Fig. 1).
First draft submitted by Harry Sicher.
Below this rounded prominence there is a deep groove, the primary oral groove or stomatodenm. Its caudal boundary is the first branchial arch or mandibular arch. Its lateral boundaries are formed by the maxillary processes which arise from the posterolateral ends of the mandibular arch. and are directed upward and slightly anteriorly. In early stages, the mandibular arch consists of three parts. On either side a smooth bulge protrudes on the lateral and anterior surfaces of the embryonic head. They are ‘united at the midline by the copula (Fig. 2. -1).
The oral groove lined by ectoderin extends inwards to meet the blind cranial end of the foregut. Here, the entoderinal gut and ectodermal oral groove are separated by a double layer of epithelium, the buccopharyngeal membrane (Fig. 1). Anterior to the cranial end of this membrane the primordium of the anterior lobe of the hypophysis develops as a shallow ectodermal pouch: Rathke's pouch. Rupture of the buccopharyngeal membrane, which occurs when the eu1br_vo is about 3 millimeters long, establishes the communication between the oral cavity and the foregut.
Fig. 1. Diagram of a. median section through the head of a human embryo of 3 mm.
Oraingxlxvboriengemrated from foregut by a double layer of epithelium, the bucco The Significant change in the development of the face is caused by rapid proliferation of the mesoderni which covers the anterior end of the brain, and a broad prommence is formed between the two maxillary processes (Fig. 2, it). This prominence constitutes the middle part of the upper face and is known as the frontonasal process. The next stage is the formation of shallow and ever deepening oval grooves, olfactorv (nasal) pits, which divide the caudal part of the frontonasal process into a single middle and two smaller lateral nasal processes (Fig. 2, C). The lateral nasal processes are adjacent to the maxillary processes, and are separated from them by shallow furrows, running upward and laterally. These furrows, the nasomaxillary groove, was formerly called the nasolacrimal groove, but it is now known that this groove has no relation to the de.
Fig. 2.—Development of the human face.
A and B. Embryo 3 mm. long, 3rd week: Frontonasal process (blue) undivided. Caudal to mandibular arch (yellow), the hyoid arch and the third branchial arch. Depression on top of figure is neuropore.
C. Embryo 6.5 mm. long. 4th week: Nasal Difs divide the frontonasal process into medial nasal process (blue) and lateral nasal processes (red).
D. Embryo 9 mm. long, 5th week: Fusion of medial nasal and maxillary processes has narrowed entrance into nasal pit.
E. Embryo 9.2 mm. long. 6th week: Fusion of medial and lateral nasal processes has further narrowed the nostrils. Medial nasal process reduced in relative width. Eyes at lateral edges of face. Medial nasal process
Lateral nasal process
Maxillary process
Mandibular arch
F. Embryo 14.5 mm. long, 7th week: Nasal area slightly prominent. Nasal septum further reduced in relative width. Eyes on anterior surface of face.
G and H. Embryo 18 mm. long, Sth week: Lidless eyes an anterior surface of face. Their distance relatively reduced, mandible short.
1 and .1. Embryo 60 mm. long, 12th week: Lids closed. Nostrils closed by epithelial proliferation. Relation of mandible to maxilla normal.
K. Adult face: the derivatives of medial nasal process (blue), lateral nasal processes (red). maxillary processes (green). and mandibular arch (yellow). Modified after Sicher and 'l‘and1er.1°
Development of the nasolacriinal duct. The nasolacrimal duct begins its development with the forniation of an epithelial ingrowth. along a line parallel with, but medial to the nasoinaxillary groove (Fig. 3 .7
The medial nasal process grows downward more rapidly than the lateral nasal processes. its rounded and prominent infer-olateral corners are known as the globular processes (Fig. ‘2, C and D}. Later, the globular processes come in contact with the maxillary processes on both sides. Therefore, the lateral nasal processes do not take part in bounding the entrance
Fig. 3.—Photograph of human embryo of 10 mm. length. I\'asomaxilIa.ry and nasolacrimal grooves. (Courtesy Dr. P. Gruenwald.)
The subsequent changes are only partly due to fusion of primarily separated “processes.” A fusion takes place only during formation of the primary palate and, to some extent, during the development of the mandible. In all other regions the grooves separating the facial processes gradually become shallow by proliferation of the mesoderm, and finally disappear. The primary palate is a horseshoe-shaped rounded structure that will give rise to the upper lip and the anterior part of the upper alveolar process. The term “primary palate" for this tissue has been chosen because in the embryo it separates nasal duct from oral cavity and because a small anterior part of the palate is derived from the same tissue.
The formation of the so-called primary palate (Fig. 4) and the primary choana begins with a deepening of the nasal or olfactory pit (Fig. 4, A and A’). Here, an actual fusion takes place, beginning at the inferior border of the nasal groove. At first the lateral border of the medial nasal process fuses with the adjacent part of the maxillary process. (Fig. 4, B);
Fig. 4.—Six smges in the development of the primary palate (diagrams).
A and .—l'.—Face of a. human embryo of 6.5 mm. length (compare Fig. 2, 0). The zigzag line on the inferior border of the nasal pit marks the line or later fusion of medial ntasal procezs to maxillary and lateral nasal processes. The broken line marks the plane 0 section .
B and B’.-—-Human embryo of 9 mm. length (compare Fig. 2, D). The medial nasal process has fused with the maxillary process. By this tuslon _a.n epithelial wall has been formed which is visible in section B’. The nasal pit is closed in its interior part to form a short blind olfactory sac.
6‘ and b".—Human embryo, 9.5 mm. in length (compare Fig. 2, E). The medial nasal process is now fused to maxillary and lateral nasal processes. The epithelial wall has lengthened (0'). The arrow in 0' points to the area in which the epithelial wall separates olfactory san from oral cavity.
D.—Huxnan embryo of 12 mm. length. The plane or section is as in Figs. A’, B’, and C". The mesoderm has broken through the superior part of the epithelial _wa1l thus strengthening the primarily epithelial fusion of medial nasal process to maxillary and lateral nasal processes. The interior part of the epithelial wall has thinned out (arrow).
_E.-—_Human embryo of 14 mm. length. The destruction of the superior part or the epithelial wall by proliferating mesoderm (crosses) has advanced. The interior part of the epithelial wall is thinned out to form the nasobuccal membrane (arrow).
F.—Human embryo of 15 mm. length. The nasobuccal membrane has disappeared. Nasal cavity communicates with oral cavity through the primary choana (arrow): The superior part of the epithelial wall is entirely replaced by proliferating mesoderm forming the primary palate between nasal and oral cavities later, the medial nasal process fuses with the tip of the lateral nasal process, and, therefore, the maxillary process does not border the outer nasal opening or the nostril (Fig. -1, C).
By the fusion of the epithelial covering of the processes an epithelial lamina, or epithelial wall, is formed, extending from the lower border of the nostril to the blind end of the nasal groove at the anterior part of the primitive oral cavity «Fig. -1. B and ('3. The epithelial wall is destroyed by a penetration of the adjacent mesotlerm which comprises the main body of the processes éFig. -l. D and E . Only the part close to the oral cavity persists: it thins out and forms the only boundary between the blind oral end of the nasal groove and the primitive oral cavity itself (Fig. 4, D and E’). This epithelial lamella is called the bucconasal membrane. When this membrane ruptures and disappears the nasal sac opens into the primitive oral cavit_v through the primary choana «Fig. 4, F 3. The bar of tissue between nasal duct and oral cavity at the edge between facial and oral surfaces is the primary palate ( Fig. 4. F ,1.
Fig. 5.—Mandible of a. human embryo of 6.5 mm. length (fourth week) showing the median and lateral groovm. (Sicher and Pohl.')
While these changes take place in the upper region of the face the mandible undergoes a peculiar transformation; at first it is an undivided arch. In embryos, approximately 5 to 6 millimeters in length, furrows appear on the mandible (Fig. 5): one, in the median plane, divides the mandible into halves. On either side of this sharp median groove, parallel to and not far from it, another furrow develops. The median groove disappears by coalescence of the medial prominenees. The lateral grooves, first reduced to rather deep pits, are eventually closed by fusion of their epithelial lining. These grooves and pits close and disappear simultaneously with the fusion of nasal and maxillary processes in the upper face; this occurs in embryos about 10 millimeters long (6 to 7 weeks).
Further development can be explained briefly by difierential growth of the regions of the embryonic face (Fig. 2). The most important change is caused by the fact that the derivatives of the medial nasal process grow
more slowly in breadth than those of the lateral nasal and maxillary processes. On the other hand, the middle region of the face between the eyes increases in an anterior direction and, thus, bulges over the surface of the face. Thereby. tl1e external nose is formed and, at the same time, the eyes, fiI’S't situated on the lateral surface of the head. come to lie on the anterior surface Fig. 2. E’, F, and G). The outer nasal openings are temporarily closed by proliferating epithelium. as are the openings of the eyes after development of the lids (Fig. 2, I and J).
The nose, even in a newborn infant, is not yet fully developed. This is illustrated by the fact that all children are born with a deeply saddled snub nose. Only at the time of puberty does the nose develop to its inherited size and shape.
The growth of the mandible follows a peculiar curve. At first it is small as compared with the maxilla: the growth in length and width shows a spurt coinciding with a certain stage in the development of the palate. Later, the mandible again lags in growth behind the maxilla (Fig. 2, G and H). A t'etns of ‘2 to 3 months still shows a physiological mierognathism which disappears before birth. The oral opening is, at first, very wide. In the lateral area the upper and lower lips fuse to form the cheeks and, thereby, the width of the mouth is considerably narrowed.
3. Development of the Secondary Palate
When the primary palate is formed the primary nasal cavity is a short duct leading from the nostril into the primitive oral cavity. Its outer and inner openings (primary choanae) are separated by the primary palate (Fig. 4), which develops into the upper lip, the anterior part of the alveolar process, and the premaxillary part of the secondary palate.
Fig. 6.-—Reconstruction of the root‘ of the primitive oral and pharyngeal cavities of a human embryo of 23 mm. length (8th week). Primary palate and internal surface of maxillary process form a. horseshoeshaped and incomplete root of the oral cavity. In the center the oral cavity communicates with the nasal cavity. At the edges of maxillary proceses the palatme processes develop. (Sieher and Tandlerl“)
When the primitive oral cavity increases in height, the tissue separating the two primitive ehoanae grows back and down to form the future nasal septum. At this stage, the oral cavity communicates freely with the nasal cavities. The oral cavity has an incomplete horseshoe—shapcd roof formed anteriorly by the primary palate and laterally by the inner horizontal surface of the maxillary processes ‘Fig. 6 . In the middle the oral cavity communicates with the nasal cavities to the left and right of the nasal septum (Fig. T).
Folds develop where the lateral part of the oral roof bends sharply into the vertical lateral wall of the nasal cavity. They grow downward almost vertically and lie to each side of the tongue which, in cross section, is high and touches the inferior edge of the nasal septum (Fig. 7). This vertical process, the posterior end of which can be traced to the lateral walls of the pharynx, is the palatine process (Figs. 6 and '7).
Fig. 7. Frontal section through the head of a human embryo of 24 mm. length (8th week). Tongue high and narrow between the vertical palatine processes. (Courtesy Dr. P. Gruenwald.)
The secondary palate which separates oral and nasal cavities is formed by a fusion of the palatine processes. after they have changed from a vertical to a horizontal position (Fig. 8). The anterior parts of the palatine processes fuse not only with each other but also with the inferior edge of the nasal septum (Fig. 9 ). In this area the hard palate develops. The posterior parts of the palatine processes which form soft palate and uvula have no relation to the nasal septum.
The transposition of the palatine processes can occur only when the tongue has moved down and. thereby has evacuated the space between these processes. This is made possible by a sudden growth of the mandibular arch in length and width, at this time. The growth of the mandible is
Fig. 8. Fronta1 section through the head of a. human embryo of 30 mm. length (9th week). Tongue has evacuated the space between the palatine processes and lies flat and wide wlthin the mandibular arch. The palatine processes have assumed a. horizontal POSHIOD. (Courtesy Dr. P. Gruenwald.)
Fig. 9. Frontal section through the head or a human embryo slightly older than that in Fig. 8. The honzontal palatine processes have fused with each other and with the nasal septum. Secondary palate separates nasal from oral cavity
accelerated to such an extent that a mandibular protrusion can be observed. The tongue drops into the wide arch of the mandible and assumes its natural shape, with its transverse diameter larger than its vertical (compare Fig. 7 with Figs. 8 and 9}. The transposition of the palatine processes is brought about by difierential growth. The mesodermal cells are densely grouped on the oral (lateral) surface of the vertical palatine processes, especially at the angle between the process itself and the lateral part of the oral roof 4‘Fig. '7). The dense arrangement of the cells and the presence of mitoses prove this area to be one of rapid proliferation. In other words, the oral surface of the fold grows more rapidly than the nasal; this, necessarily. leads to a rapid change in the position of the fold away from the faster growing side. Thus, the palatine processes turn into horizontal position immediately after the tongue has evacuated the space between them.
Fig. 10. Reconstruction of palate or a. human embryo or 28.5 mm. length. The palatine processes fused in the area of the hard palate. The fusion has not reached the soft palate and uvula. (Sicher and Tandlen”)
When the palatine processes have assumed their horizontal position, they touch the lower border of the nasal septum, but are still separated from each other by a median cleft (Fig. 8) which widens posteriorly. The cleft closes gradually in an anterior-posterior direction. At first an epithelial suture can be observed between the palatine processes and between those and the nasal septum (Fig. 9). Later, this epithelial wall is perforated and broken up by growing mesoderm; remnants of the epithelium may persist as epithelial pearls. The epithelium remains only at the anterior end where the palatine processes fuse with and partly overgrow the primitive palate on its oral side. Here, the epithelium forms two strands, beginning in the nasal cavity and uniting below the septum to connect with the oral epithelium. They are the primordium of the nasopalatine ducts, vestigial in man (see chapter on Oral Mucous Membrane).
Fig. 11. Advanced ta ‘ th d I 3 months old. 3. I-Iumzn gin: 4 1'1e1on§l‘;§ %Il’g.len(§. °Htut£:nh:‘e.‘ViVbI:)al.‘Il1atieIlfalAl-1E. Human fetus’
Note the changes in the face between the alveolar ridge eanxcle :sle‘l’xr¢|is;:I-l21l)l\':3)olal:'a‘r£l‘v.E1?¢:3. p(a§il::Il;a'era:;l1dtr.l‘ea:lIEg1l1<;nn1°§'nd those
It has to be stressed that the entire palate does not develop from the palatine processes. They give rise only to the soft palate and the central part of the hard palate. This portion of the hard palate. termed the tegmen oris, is surrounded by a horseshoe-shaped prominence. the tectal ridge (Fig. 10).
The palate is separated from the lip by a shallow sulcus. From its depth two epithelial laminae arise: an outer vestibular and an inner dental lamina. Later, the alveolar process forms from the mesoderm between these laminae.
The palatine papilla develops very early as a round prominence in the anterior part of the palate. Irregular transverse folds. the palatine rugae. cross the palate in its anterior part. At this stage the lips show a definite division into an outer smooth zone, pars glahra, and an inner zone, beset with fine villi, pars villosa. In the upper lip the middle part of this inner zone is prominent, forming the tubercle of the upper lip. A fold connects the palatiue papilla with this tubercle: the tectolabial frenulnm (Fig. 11, .4 and B).
When, in later stages, the growing alveolar process bulges between palate and lip the tectolabial frennlum is separated from the palatine papilla and persists as the upper labial frenulum. connecting the 'anterior surface of the alveolar ridge with the upper lip (Fig. 11. C’).
The development of the maxillary alveolar ridges is complicated by the appearance of a bulge in the molar region which may be easily confused with the alveolar process. It has been called the pseudo-alveolar ridge and disappears gradually when the alveolar process expands posteriorly (Fig. 11).
The development of the alveolar ridge in the mandible is simple. No pseudo-alveolar ridge is present and the alveolar process bulges gradually into the oral cavity, inside the labial sulcus. The labial sulci open up and form the oral vestibule which extends posteriorl_v into the region of the checks.
4. Development of the Tongue
Before describing the development of the tongue a few words should he said about the development of the branchial arches (Fig. 2). A prominence similar to the mandibular or first branchial arch develops parallel and caudal to it. This, the second or l1_void arch, is separated from the first by a sharp and deep furrow. Caudal to the second branchial arch a third, fourth, and fifth develop, each smaller and less prominent than the preceding. The last three arches do not reach the surface at the midline but are confined to the lateral region of the neck.
The furrows which separate the arches on the outer surface are the branchial grooves. Corresponding deep furrows develop as lateral pockets on the pharyngeal wall; these are the pharyngeal pouches (Fig. 12, A).
The epithelium of the pharyngeal pouches gives rise by complicated processes to a variety of organs. From the first pouch are derived the auditory tube and the cavities of the middle car. In the region of the second pouch the palatine tonsil is laid down. The third gives rise to one parathyroid gland and the thymus; the fourth to the second parathyroid and the ultimo-branchial body.
Fig. Development of the tongue. Anterior wall of pharynx and floor of the 01-31
A. Human embryo of 3.5 mm. length (3rd week). B. Human embryo or 6.5 mm. length (4th week).
On the outside the third, fourth, and fifth arches are overgrown by a. caudal outgrowth of the second arch, the ope:-culum (Fig. 20). Thus, third, fourth, and fifth arches are placed in a deep recess, the cervical sinus. Later, this is closed by fusion of the operculum with the lateral Wall of the neck. The cavity of the recess is soon obliterated although remnants may give rise to branchial or cervical cysts.
Fig. 12.——Coutinued.
C. Human embryo of 8 mm. length (5th week). D. Human embryo of 11 mm. length (6th week). (Sicher and Tandlerfi.)
The tongue is derived from the first, second, and third branchial arches. Development The dividing line between the derivatives of the first and the more ¥,,::, caudal arches is marked throughout life by the terminal sulcus in the area of the vallate papillae. The body and apex of the tongue originate as three prominences on the oral aspect of the mandibular arch (Fig. 12, A B, and C). The lateral lingual prominences are two in number, one on each side; the third, unpaired, appears between these two and somewhat posteriorly; it is the tuberculum impar. The base of the tongue develops later as a bulge on the middle part (copula) of the second and the third arches. The unpaired tubercle. prominent and large at first, is soon reduced in relative size (Fig. 12, C) and, later, almost disappears (Fig. 12, D).
In the midline, between the derivatives of the first and Second aI‘0hCS which contribute to the development of the tongue, the thyroid anlage develops. This gives rise to the thyroid gland by progressive downward growth. The beginning of the traiisitory thyroglossal dllct is 111aI'k€d by the forameu cecum of the tongue which persists in the adult. In this region thyroglossal duct cysts may develop.
The later stages of tongue development are characterized by a mushroom-like growth of the organ, and by gradual differentiation of the various lingual papillae (see chapter on Mucous Membrane). The skeletal (extrinsic) muscles of the tongue grow into its mesodermal primordium; the intrinsic muscles differentiate in situ from the mesenchyme of the tongue.
The described development of the tongue explains two malformations. A lack of fusion between the two lateral mandibular tubercles may cause a bifid tongue. A persistence of the tuberculum impar is said to be the cause for the rhomboid glossitis.
5. Clinical Considerations
Some Malformations of the Face
Introductory
The most frequent malformations of the face are known as clefts. Clefts mm“ of the lip, jaw, or palate may occur once in about eight hundred births.‘ The complete harelip is a cleft, lateral to the midline cutting through the upper lip and continuing as cleft jaw or gnathoschisis through the anterior part. of the alveolar process. It may be unilateral or bilateral. The cleft palate may be unilateral or bilateral, complete or partial, involving the uvula only or extending into the soft and hard palate. The oblique facial cleft is a defect which begins in the upper lip and can be traced through the nostril. or lateral to it, over the cheek to the eye. More or less deep pits in the lower lip not far from the midline, on one or both sides, are known as labial fistulae.
Harelip, Cleft In its complete form the harelip and cleft jaw is a cleft which extends 3;; ifadme from the lower border of the nostril, lateral to the midline, through the upper lip and upper alveolar process, to the region of the foramen incisivum. In the past, the development of this malformation was attributed to a lack of fusion of the medial nasal process with the lateral nasal, and the maxillary process. However, recent investigations“ have revealed that an epithelial fusion does occur in cases of harelip but that the epithelial wall is not perforated by mesoderm. Therefore, the epithelial union of these processes ruptures. This explanation is borne out by the occurrence of thin strands of tissue which Imite, in some cases, the medial and lateral walls of the harelip. These bridges develop if the mesoderm perforates the epithelial wall only in a restricted area. Ilarelip and cleft jaw would be evident at 6 to 7 weeks in utero.
The relation of cleft jaw to the bone and to the teeth varies considerably. In some cases the cleft corresponds to the suture between premaxilla and maxilla: in other cases the cleft cuts through the premaxilla itself, dividing it into a medial and a lateral part. Frequently, the lateral incisor is found medial to the cleft jaw. in some cases lateral to it. The lateral incisor is. in some instances, medial to the harelip. and a supernumerary lateral incisor lies lateral to it. In other cases the lateral incisor is missing. The explanation for this variability is that the skeletal parts appear long after fusion of the facial processes has been completed. Thus, the bones develop in a uniform tissue and with no regard for the primary boundaries between the processes.
The dental lamina, the matrix of the tooth germs, is likewise independent of the facial processes. Harelip and cleft jaw occur in the general region of the lateral incisor. I11 some cases. it may cut the matrix medially or laterally to the prospective primordium of the lateral incisor. or it may go right through it. In the latter cases. by a process of regeneration. each part of the divided primordium may produce a complete lateral incisor or, on the other hand, the development of the lateral incisor may be suppressed altogether.
Cleft palate results from a lack of fusion of the palatine proeeses, with each other and the nasal septum. In unilateral defects one process fuses with the lower border of the septum so that one nasal cavity is completely separated from the oral cavity. Palatal fusion is usually completed at the end of the fourth month in utero.
The fusion of the palatine processes commences at their anterior ends and proceeds backward. The process of fusion ma_v be interrupted at any time. This explains the different types of cleft palate. The cleft may be limited to the uvula, may extend through a portion of, or the entire soft palate, or may involve parts of. or the entire hard palate.
Cleft palate is frequently (8-1 per cent) associated with a unilateral or bilateral harelip. In the latter case the tissues between the two clefts are, sometimes. protruding as a knohlike growth in the midline. whereas. in other cases. the tissues may fail to grow. The latter results in the formation of the so-called false median cleft of the upper lip.
Heredity probably is the major etiologic factor in clefts of the face, lip, jaw, and palate. Members of one family sometimes show. in addition to, or instead of. harelip. the so-called fistula of the lower lip. This tends to show that these fistulae develop from causes similar to those responsible for harelip. In these cases. the pits between the medial and lateral parts of the mandibular arch (Fig. 5) remain open and even enlarge.
When the fusion between upper and lower lips remains incomplete, the cheeks do not develop to their full extent and the mouth is abnormally wide (macrostoma).
The oblique facial cleft, extending from the upper lip to the region of the eye, was, in the past, explained by failure of fusion between the maxillary process and the nasal processes. It has been stressed that such fusion occurs only below the nostril. The explanation for the oblique cleft between nostril and eye has been sought for in a traumatic injury to the face of the embr_vo. Adhesions of the amnion to the face may, according to this theory, cause this cleft by producing actual tears or cuts which have a variable relationship to the nasolacrimal duct.
Dermoid or epidermoid cysts may be located at any fusion point of the body, including those in the oral region. Median cysts are formed in the median fissure of the palate from embryonic epithelial inclusions. Incisive canal (nasopalatine duct) cysts form in or at the incisive canal from remnants of the nasopalatine ducts. Globulomaxillary cysts form from epithelial inclusions between the globular and maxillary processes. Branchial fistulae result most frequently from the first branchial groove or pharyngeal pouch, and branchial cysts result from proliferation of epithelial rests in the region of branchial clefts or the cervical sinus.“
References
1. Arey, L. B.: Developmental Anatomy, ed. 5, Philadelphia, 1946, W. B. Saunders (10Ex’)
Burket, L. W.: Nasopalatine Duct Structures and Peculiar Bony Patterns in the Anterior Maxillary Region, Arch. Path. 23: 793, 1937.
Grace, L. 6%.: Frequency of Occurrence of Cleft Palate and Bare Lips, J. Dent. Research 22: 495, 19-13.
Hochstetter, F.: Beitriige zur Entwicklungsgeschichte des menschlichen Gaumens (Contributions to the Development of the Human Palate), Morphol. J ahrb. 77: 179-272, 1936.
Keibel, F., and Mall, F. P.: Manual of Human Embryology, Philadelphia and London, 1910, J. B. Lippincott Co.
Peter, K.: Die Entwicklung des Saugetiergaumens (Development of the Mammalian Palate), Ergebn. d. Anat. Entwcklngsgesch. 25: 448-564, 1924.
7. Politzer, Gr.: Die Grenzfurche des Oberkieferfortsatzes und die Tranennasenrinne beim Menschen (The Limiting Sulcus of the Maxillary Process and the Nascgafgcrixgal Groove in Man), Ztschr. f. Anat. u. Entwcklngsgesch. 105: 329 . , 1 36. 8. Robinson, H. B. G.: Classification of Cysts of the Jaws, Am. J. Orthodont. 85 Oral Surg. 31: 370, 1945.
9. Sicher, H., and Pohl, L.: Zur Entwicklung des menschlichen Unterkiefers (Development of the Human Mandible—A Contribution to the Origin of the Fistulae of the Lower Lip), Ztschr. f. Stomatol. 32: 552-560, 1934.
10. Sicher, H., and Tandler, J.: Anatomic fur Zahniirzte (Anatomy for Dentists), Vienna and Berlin, 1928, Julius Springer.
11. Veau, V., and Politzer, G.: Embryologie du bec-de-liévre; le palais primaire (Embryology of the Harelipz The Primary Palate. Formation and Anomalies), Ann. d ’ anat. path. 13: 275-326, 1936.
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