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Figure 1.
Scanning electron microscopic views of stage 12 Macaca fascicularis embryo. A, Frontal view in which the frontonasal process, stomodeum, and developing pharyngeal (branchial) arches are evident. The mandibular prominence develops from the first pharyngeal arch (long, thin arrow). Two other pharyngeal arches are located caudal to the first arch. A large frontonasal groove can be seen medially on the embryo (short, thick arrow). B, Oblique view with some corpus damage evident. The pharyngeal arches are evident as is the wide stomodeum.

Scanning electron microscopic views of stage 12 Macaca fascicularis embryo. A, Frontal view in which the frontonasal process, stomodeum, and developing pharyngeal (branchial) arches are evident. The mandibular prominence develops from the first pharyngeal arch (long, thin arrow). Two other pharyngeal arches are located caudal to the first arch. A large frontonasal groove can be seen medially on the embryo (short, thick arrow). B, Oblique view with some corpus damage evident. The pharyngeal arches are evident as is the wide stomodeum.

Figure 2.
Scanning electron microscopic views of stage 14 Macaca fascicularis embryo. A, This frontal view of an approximately 29-day-old embryo (stage 14) shows that the nasal placodes have moved medially from their stage-12 positions. The nasal placodes consist of the medial (thinnest arrow) and lateral (medium-thickness arrow) nasal prominences. The mandibular groove is closing with the developing mandibular prominences (shortest, thickest arrow). The frontonasal prominence is located rostral to the frontonasal groove. B, This oblique view shows some corpus shrinkage. The nasal placodes are visible, and the mandibular prominences can be distinguished.

Scanning electron microscopic views of stage 14 Macaca fascicularis embryo. A, This frontal view of an approximately 29-day-old embryo (stage 14) shows that the nasal placodes have moved medially from their stage-12 positions. The nasal placodes consist of the medial (thinnest arrow) and lateral (medium-thickness arrow) nasal prominences. The mandibular groove is closing with the developing mandibular prominences (shortest, thickest arrow). The frontonasal prominence is located rostral to the frontonasal groove. B, This oblique view shows some corpus shrinkage. The nasal placodes are visible, and the mandibular prominences can be distinguished.

Figure 3.
Scanning electron microscopic views of early stage 16 Macaca fascicularis embryo. A, All major prominences are visible: the lateral nasal, medial nasal, maxillary, frontonasal, and mandibular prominences. The labial swellings (arrow), lateral to the closing mandibular groove, are prominent on the mandible. The frontonasal groove is present between the 2 medial nasal prominences and caudal to the frontonasal prominence. B, The pronounced maxillary nasal prominence (arrow) is clearly evident in this oblique view. C, A frontal view focuses on the right nasal placode and the migrating nasal prominences. There is evidence of epithelial bridging (arrow). D, Here, the mandible has been removed from the head. The photograph shows a ventral view up into the Rathke pouch (arrow) and at the nasal placodes. The frontonasal groove is present between the closing medial nasal prominences.

Scanning electron microscopic views of early stage 16 Macaca fascicularis embryo. A, All major prominences are visible: the lateral nasal, medial nasal, maxillary, frontonasal, and mandibular prominences. The labial swellings (arrow), lateral to the closing mandibular groove, are prominent on the mandible. The frontonasal groove is present between the 2 medial nasal prominences and caudal to the frontonasal prominence. B, The pronounced maxillary nasal prominence (arrow) is clearly evident in this oblique view. C, A frontal view focuses on the right nasal placode and the migrating nasal prominences. There is evidence of epithelial bridging (arrow). D, Here, the mandible has been removed from the head. The photograph shows a ventral view up into the Rathke pouch (arrow) and at the nasal placodes. The frontonasal groove is present between the closing medial nasal prominences.

Figure 4.
Scanning electron microscopic views of late stage 16 Macaca fascicularis embryo showing major epithelial bridging. A, The nasal pit has not yet pushed through into the choana. The medial nasal prominence is bridging to the lateral and maxillary nasal prominences. B, A close-up of the epithelial bridging (arrow), which is a result of dynamic fusion of the major prominences.

Scanning electron microscopic views of late stage 16 Macaca fascicularis embryo showing major epithelial bridging. A, The nasal pit has not yet pushed through into the choana. The medial nasal prominence is bridging to the lateral and maxillary nasal prominences. B, A close-up of the epithelial bridging (arrow), which is a result of dynamic fusion of the major prominences.

Figure 5.
Frontal (A) and oblique (B) scanning electron microscopic views of stage 17 Macaca fascicularis embryo show the formed nasal pits and the formed maxillary median groove (arrows) located caudal to the 2 merged medial nasal prominences. The frontonasal groove is now located rostral to the merged medial nasal prominences and above the maxillary median groove. The maxillary, medial, and lateral nasal prominences and the mandibular prominence are all evident.

Frontal (A) and oblique (B) scanning electron microscopic views of stage 17 Macaca fascicularis embryo show the formed nasal pits and the formed maxillary median groove (arrows) located caudal to the 2 merged medial nasal prominences. The frontonasal groove is now located rostral to the merged medial nasal prominences and above the maxillary median groove. The maxillary, medial, and lateral nasal prominences and the mandibular prominence are all evident.

Figure 6.
Scanning electron microscopic views of stage 18 Macaca fascicularis embryo. A, Evident in this view are corpus shrinkage, the merging eyes, and the gull-wing-like appearance of the stomodeum. The comma shape of the nasal pits due to the rotation of the nasal prominences is also evident. B, In this oblique view, there is definite ocular development. The gull-wing-like appearance of the stomodeum is still evident. C, Close-up of the maxillary median groove. The comma-shaped nasal pits do not exhibit the strong appearance of the major nasal prominences. D, Ventral view shows the breakdown of the buccal nasal membrane within the site of the primitive choanae (arrows). The newly formed lip of the maxilla can be seen containing a decreased maxillary median groove.

Scanning electron microscopic views of stage 18 Macaca fascicularis embryo. A, Evident in this view are corpus shrinkage, the merging eyes, and the gull-wing-like appearance of the stomodeum. The comma shape of the nasal pits due to the rotation of the nasal prominences is also evident. B, In this oblique view, there is definite ocular development. The gull-wing-like appearance of the stomodeum is still evident. C, Close-up of the maxillary median groove. The comma-shaped nasal pits do not exhibit the strong appearance of the major nasal prominences. D, Ventral view shows the breakdown of the buccal nasal membrane within the site of the primitive choanae (arrows). The newly formed lip of the maxilla can be seen containing a decreased maxillary median groove.

Figure 7.
Scanning electron microscopic views of stage 20 Macaca fascicularis embryo. A, This frontal view shows the developed nostril and the developed stomodeum. The stomodeum contains a smooth transition through the decreased maxillary median groove. B, In this oblique view, the ocular movement can be seen relative to earlier stages. The developing auricle is visible caudal to the eye.

Scanning electron microscopic views of stage 20 Macaca fascicularis embryo. A, This frontal view shows the developed nostril and the developed stomodeum. The stomodeum contains a smooth transition through the decreased maxillary median groove. B, In this oblique view, the ocular movement can be seen relative to earlier stages. The developing auricle is visible caudal to the eye.

Figure 8.
Scanning electron microscopic views of stage 21 Macaca fascicularis embryo. A, The frontal view shows a definite humanlike appearance. The flattened maxillary median groove evens out the stomodeum. Major facial prominences have become less pronounced in visibility. B, The lateral view shows the medial movement of the developing eye as well as the developed nostrils.

Scanning electron microscopic views of stage 21 Macaca fascicularis embryo. A, The frontal view shows a definite humanlike appearance. The flattened maxillary median groove evens out the stomodeum. Major facial prominences have become less pronounced in visibility. B, The lateral view shows the medial movement of the developing eye as well as the developed nostrils.

Figure 9.
Scanning electron microscopic views of stage 23 Macaca fascicularis embryo. The frontal (A) and lateral (B) views of a late stage 23 embryo show humanlike physical characteristics.

Scanning electron microscopic views of stage 23 Macaca fascicularis embryo. The frontal (A) and lateral (B) views of a late stage 23 embryo show humanlike physical characteristics.

Figure 10.
A drawing showing the contributions of the maxillary (orange), lateral nasal (purple), and medial nasal prominences (green) as an embryo and a child. A, Late stage 17 embryo; B, stage 18 embryo. In the developed young boy (C), the medial nasal prominence has produced the dorsal ridge of the nose, columella, and the lip philtrum. The ala of the nose has been created by the lateral nasal prominence. The cheeks and lateral upper lip have been created by the maxillary prominence. The lower lip and chin have been formed from the mandibular prominences.

A drawing showing the contributions of the maxillary (orange), lateral nasal (purple), and medial nasal prominences (green) as an embryo and a child. A, Late stage 17 embryo; B, stage 18 embryo. In the developed young boy (C), the medial nasal prominence has produced the dorsal ridge of the nose, columella, and the lip philtrum. The ala of the nose has been created by the lateral nasal prominence. The cheeks and lateral upper lip have been created by the maxillary prominence. The lower lip and chin have been formed from the mandibular prominences.

Table 1 
Catalog of the Embryos Analyzed According to Stage Distribution, Estimated Age, and Crown-to-Rump Lengths
Catalog of the Embryos Analyzed According to Stage Distribution, Estimated Age, and Crown-to-Rump Lengths
Table 2 
Index of Terms
Index of Terms
1.
O'Rahilly  RMuller  FDevelopmental Stages in Human Embryos Lunenberg  Vt Meriden-Stinehour Press1987;
2.
Booher  CBPrahalada  SHendrickx  AGUse of a radioreceptor assay (RRA) for human luteinizing hormone/chorionic gonadotropin (hLH/CG) for detection of early pregnancy and estimation of time of ovulation in macaquesAm J Primatol. 1983;445- 53Article
3.
Cruz  GRDevelopmental Staging in the Cynomolgus Monkey (Macaca fascicularis) With Emphasis on the Metanephros [master's thesis]. Davis University of California1985;
4.
Hendrickx  AGEmbryology of the Baboon. London, England University of Chicago Press1971;
5.
Rahilly  O'.Developmental Stages in Human Embryos. Washington, DC Carnegie Institution of Washington1973;
6.
Wilson  DBSawyer  RHHendrickx  AGProliferation gradients in the inner ear of the monkey (Macaca mulatta) embryo.J Comp Neurol. 1975;16423- 30Article
7.
Streeter  GLDevelopmental Horizons in Human Embryology: Contributions to Embryology. 32 Baltimore, Md Lord Baltimore Press1948;
8.
Hinrichsen  K>The early development of morphology and patterns of the face in the human embryoAdv Anat Embryol Cell Biol. 1985;981- 79
9.
Shuler  CFProgrammed cell death and cell transformation in craniofacial developmentCrit Rev Oral Biol Med. 1995;6202- 217Article
10.
Sun  DBaur  SHay  EDEpithelial-mesenchymal transformation is the mechanism for fusion of the craniofacial primordia involved in morphogenesis of the chicken lip.Dev Biol 2000;228337- 349Article
11.
Moore  KLPersaud  TVBefore We Are Born 5th Philadelphia, Pa WB Saunders Co1998;
12.
Sadler  TWLangman's Medical Embryology 6th Baltimore,Md Williams & Wilkins1990;
13.
Bhaskar  SNedOrban's Oral Histology and Embryology 9th St Louis, Mo CV Mosby Co1976;
Citations 0
Original Article
January 2003

Development of the Upper Lip

Author Affiliations

From the University of California, Davis. Dr Cukierski is now with Merck Research Laboratories, West Point, Pa.

 

From the University of California, Davis. Dr Cukierski is now with Merck Research Laboratories, West Point, Pa.

Arch Facial Plast Surg. 2003;5(1):16-25. doi:
Abstract

Objectives  To affirm and reanalyze George L. Streeter's "merging theory" of upper-lip development in primates by observing progressive embryologic stages in facial development using scanning electron microscopy (SEM) and to further understand upper-lip development.

Design  The study was conducted at the California Regional Primate Research Center, Davis. Twenty primate embryos (Macaca fascicularis) and 2 fetuses were examined with SEM. The development of the frontonasal prominence, maxillary prominence, medial nasal prominence, and lateral nasal prominence were sequentially observed. The contribution of these prominences to the formation of the upper lip and nose were carefully analyzed.

Results  The maxillary prominence and medial nasal prominence form the upper lip, whereas the lateral nasal, medial nasal, and maxillary prominences form the nose. There is fusion of the maxillary prominence with the medial nasal prominence. This fusion has not been previously described. This has resulted in a modification of the current theory of upper-lip development into one we refer to as the "dynamic fusion theory."

Conclusions  The dynamic fusion theory explains the merging process of the mesenchymal and ecotodermal layers of the facial prominences that contribute to the upper-lip formation. The dynamic fusion theory of facial prominence movement details the interaction between epithelial layers: both epithelial layers must fuse properly to avoid cleft-lip deformities.

OUR QUEST for knowledge and understanding has led to an exploration of the embryologic development of the upper lip. The late Franklin P. Mall ignited the flame by founding the department of embryology at the Carnegie Institution of Washington, Washington, DC, which today contains one of the most well-respected human embryo collections. He bequeathed the collection to the late George L. Streeter, MD, who devoted his life to studying the development of the embryo.1 It is the great work of Streeter, who discovered and developed many of the insights into embryologic staging and development, that stands as the foundation for many of today's inquiries. Facial development has always been difficult to study because of the translucency of the embryo and limited visualization caused by the head-flexed position of embryos.

Streeter explored new techniques to overcome obstacles.1 He studied embryonic histologic sections and used these sections as a framework. Streeter hand-created 3-dimensional reconstructions by a best-fit method. His 3-dimensional reconstructions of the face are remarkable, especially when one considers that his work was done without the assistance of computer-aided digital analysis.

Our work used scanning electron microscopy (SEM), which has the advantage of allowing a full analysis of facial development at greater resolution of surface structures. A primate model was used because of the lack of human embryonic tissue. All embryos were used in multiple research projects. Although Streeter's work is remarkable, SEM analyses were used to confirm and alter certain aspects of his work.

METHODS

Adult female cynomolgus monkeys (Macaca fascicularis) were housed individually in aluminum cages at the California Regional Primate Research Center, Davis, and maintained in accordance with standards established by the Federal Animal Welfare Act and the Institute for Laboratory Animal Resources. Animal rooms were maintained on a 12-hour light-dark cycle (lighted from 7 AM to 7 PM) with a year-round temperature of approximately 22°C and 60% relative humidity. Monkeys were fed twice daily with Purina Monkey Chow (15% protein) (Ralston Purina Company, St Louis, Mo), and water was provided ad libitum by automatic Lixit devices (Lixit Animal Care Products, Napa, Calif). Menstruation was detected by visual examination of the external genitalia or by detection of blood on a cotton tip swab after insertion into the vaginal canal. Females were mated by natural insemination by housing with a single fertile male for approximately 2 hours every other day over a 5-day period at midcycle. The middle of the mating period was designated as day 0 of pregnancy. Pregnancy was confirmed by positive radioreceptor assay tests for chorionic gonadotropin in serum.2

Embryos were removed by hysterotomy, placed in phosphate-buffered saline, and dissected free of the placenta and developmental adnexa. The embryos were fixed by immersion in 2% glutaraldehyde, 2% paraformaldehyde in 0.1M cacodylate, or phosphate buffer at pH 7.3 and then measured and photographed intact. The Carnegie system for staging human embryos1 was used to stage the embryos externally, and subsequent histologic analysis of several internal organs was used for confirming of stage.3 The fixed heads were removed using a razor blade and stored in refrigerated buffer until processing. Other investigators used other tissues. The tissues were dehydrated in graded series of ethanol and critical-point dried using liquid carbon dioxide as a transition fluid. The dried specimens were mounted on stubs with double-stick tape or conducting silver paint and coated with palladium gold in a Denton Vacuum evaporator (Moorestown, NJ) or sputter-coated with gold in a Polaron E51 coater (Ladd/Polaron, Williston, Vt). The specimens were examined in an ETEC Autoscan U-1 (Applied Materials, Santa Clara, Calif) or an ISA SS60 (ISA Inc, Louisville, Ky) scanning electron microscope. After photographing the heads intact, we made further dissections (using microdissecting knives) on the dried specimens. The heads were then recoated and examined as above. A total of 22 specimens ranging from stage 12 to fetus were examined (staged according to Hendrickx,4 O'Rahilly,5 and Wilson et al6) (Table 1).

RESULTS
STAGE 12

Figure 1 shows the 3 prominences of stage 12, the frontonasal, maxillary, and mandibular (see Table 2). The frontonasal prominence forms the forehead and the rostral boundary of the stomodeum (primitive mouth). The mandibular prominence forms the caudal boundary of the primitive mouth. The maxillary prominence is ventral to the developing eye. The mandibular arch has a median groove (mandibular groove), which separates the developing branchial arches. The nasal placodes are barely discernible. The mandible is separated by the stomodeum, which is disproportionate to the size of the corpus.

STAGES 13 AND 14

The nasal placodes have become more pronounced in stages 13 and 14 (Figure 2) and have moved slightly medially from their stage-12 positions. The maxillary nasal prominence and lateral nasal prominence have become visible and are positioned more medially. The maxillary prominence is more distinct. The nasolacrimal groove is visible separating the maxillary and the lateral nasal prominences. The mandibular groove has become shallower. The frontonasal groove is located on the ventral surface of the stomodeum, which is still disproportionate to the size of the corpus but has become smaller with the merging of the mandibular prominences.

EARLY STAGE 16

A major advance in stage 16 (Figure 3) is the appearance of the nasal pits, which appear to end blindly and are located more medially than their precursors, the nasal placodes. The nasal pits begin as rounded spheres and develop into "comma" shapes, pointing slightly laterally by late stage 16. The maxillary prominence is well developed, much like the lateral and medial nasal prominences. As the maxillary prominence develops toward the median plane, the caudal portion of the frontonasal groove starts to move cranially. The lateral and medial nasal prominences are well developed and easily distinquished caudally, but the separation is ill-defined cranially. The medial nasal and maxillary prominences merging caudally form the paired labial furrows, which are located lateral to the frontonasal groove along the upper lip. The mandibular groove remains prominent. Lingual swellings are very evident just lateral to the mandibular groove. The median mandibular groove continues to become shallower. The nasolacrimal groove is very distinct. The lateral and medial nasal prominences and the maxillary prominence are merging at the nasal sill. The lateral and medial nasal prominences form the nasal sill, whereas the maxillary prominence supports the nasal sill caudally. The stomodeum is still elongated across the corpus but is achieving a more distinct shape with medial movement of the maxillary prominences.

LATE STAGE 16

Small bridges of epithelial tissue span the nasal sill region between the lateral and medial nasal maxillary prominence by late stage 16 (Figure 4). Epithelial bridging is even more evident than before between the maxillary prominence and the medial nasal prominence. Looking rostrally at the underside of the palate, one can see the forming of the paired labial furrows and frontonasal groove. In some cases, the epithelial bridging is seen in the labial furrows. The persistent labial furrows have been pushing caudally since early stage 16 as the frontonasal groove has been decreasing due to the medial movement of the medial nasal prominences. The stomodeum is becoming more proportional to the size of the corpus.

STAGE 17

A greater degree of development of the nasal pits is apparent in stage 17 (Figure 5). The paired labial furrows are less prominent than they were in stage 16. The maxillary median groove can be distinguished from the frontonasal groove after the medial nasal prominences have merged medially. The merged medial nasal prominences separate the frontonasal groove from the maxillary medial groove, which is located caudally. The nasal pits have moved medially and have become smaller and more comma shaped. However, the maxillary and medial nasal prominences remain separate from each other. The frontonasal and medial nasal prominences are in the process of merging together and in some instances have almost completely limited the appearance of the frontonasal groove. The maxillary and lateral nasal prominences have remained in a relatively consistent position relative to earlier stages. There is less distinction between the medial and lateral nasal prominences as well as the maxillary prominences. The fusion of these prominences at the nasal sill appears complete. The labial furrows have moved inferiorly without any remnant of epithelial bridging. Epithelial bridging of the maxillary medial groove is not seen. The paired labial grooves have achieved a less distinctive appearance. The mandibular arch still contains a median groove that separates the labial swellings but is becoming less distinct. The stomodeum has a very curvaceous appearance with the well-defined mandibular labial swellings and the merging nasal prominences.

STAGE 18

The most distinguishing aspect of stage 18 (Figure 6) is the humanlike appearance and lack of distinction between prominences. The lip continues to show the maxillary medial groove. The paired labial furrows are evident near the sill of the nose only. Laterally, the lip has a smooth appearance. The central lip (prolabium) is made up of the fused medial nasal prominences. The lateral lip is formed from the maxillary prominences. The apex of the maxillary median groove has moved caudally, although a median groove persists. Epithelial bridging is not seen. The mandibular median groove has filled in, leaving a smooth lower lip. The appearance of the gum line on the inside of the lower lip has become evident. The primary palate has taken form ahead of the nasal septum. The stomodeum has a gullwinglike appearance owing to the disappearance of the mandibular labial swellings and the almost complete merging of the medial nasal, lateral nasal, and maxillary prominences. The stomodeum has become narrower and more proportional to a human mouth.

STAGE 20

The stomodeum has a gullwinglike appearance and is gradually narrowing by stage 20 (Figure 7). The premaxilla and nasal complex have moved ventrally. The premaxilla has a ventral position relative to the mandible. The nasal complex is in its medial location. The nostrils each have a commalike appearance where the point of the comma points laterally and caudally at about a 45° angle. The maxillary median groove remains inconspicuous and less prominent. The paired labial furrows are barely discernible. The site of future midline fusion between the lateral palatine shelves and the septum is evident. The developing upper lip and gum can now be distinguished.

STAGE 21

The nostrils have remained comma shaped with a similar angulation in stage 21 (Figure 8). The nose has increased in length. The developing premaxilla still contains a maxillary median groove, which is decreasing in subsequent stages. With the decrease in the maxillary median groove, the gullwing appearance has become more flattened. The paired labial furrows have disappeared. This is the last stage in which the maxillary median groove is evident in the developing embryo. The palate has begun its zipperlike closing of the palatine shelves from anterior to posterior.

STAGE 23

Many distinctive aspects of the future fetus appear in stage 23 (Figure 9). The nostrils still appear in a comma shape but with the curved end pointing ventrally. The nostrils have stopped moving medially. The palatal shelves are completely fused. The stomodeum has lost much of its gullwinglike appearance and continues to narrow. With the change in appearance of the stomodeum, the maxillary median groove has also been smoothed out and looks almost human in appearance.

FETUS

The fetal stage shows the greatest humanlike appearance (no figure available). The nostrils have retained a comma shape. The developing eyes have become more pronounced. The developed upper lip appears smooth without any paired labial furrows. There is no distinction between the medial or lateral nasal prominences and the maxillary prominence. The stomodeum has lost the gullwing appearance in its smooth and even development.

COMMENT

Facial prominence movement guides anatomic facial development. The medial nasal, lateral nasal, and maxillary prominences all contribute to the formation of the nose and upper lip. George L. Streeter7 hypothesized his views regarding the developing mesenchyme of the upper lip. Today we have a clearer view of embryologic development through the use of SEM that has allowed us to confirm and modify Streeter's hypothesis. We have reinvestigated Streeter's views of human lip development using a nonhuman primate.

Streeter believed that ectoderm was never absorbed (fusion) between 2 prominences. Rather, he believed that the mesoderm from growth centers within a prominence merged with neighboring prominences. Initially, this created a furrow (groove) that was filled in by additional mesodermal growth. Streeter described this process as the "merging theory":

The furrows that lie between them [prominences] on the surface are smoothed out as the proliferation and fusion of the growth centers [mesoderm] fill in beneath. Under these circumstances no ectoderm requires absorption; it is simply flattened out in adaptation to the changed surface.7

Streeter indicated that the furrow between merging prominences became shallow and eventually smooth as the increase in mesoderm produced a new surface level. He hypothesized that a cleft lip occurred when mesoderm failed to fill in the furrows and the epithelium broke down.7-8

Our study clearly demonstrates epithelial bridging between prominences, which is inconsistent with Streeter's merging theory. The present study supports a new hypothesis of lip development, the "dynamic fusion theory," which acknowledges the fusion between prominences that are brought into opposition by the growing mesoderm. This is a dynamic process that begins beneath the future sill of the nose. As the prominences grow, there is epithelial bridging and fusion. Behind this advancing fusion there is merging of the mesoderm between the 2 prominences. This dynamically moves from the sill of the nose to the caudal margin of the lip.

Through the use of new investigative techniques, research into craniofacial development has led to the improvement of theories previously hypothesized. The discovery of epidermal growth factors, cell adhesion molecules, and epithelial-mesenchymal transformation has provided mechanistic explanations of lip and palatal fusion.9 Sun et al10 found that in respect to the palate, the periderm of the 2-layered embryonic epithelium sloughs prior to the primordial fusion, involving a cell adhesion molecule, cadherin. The inner layer of the epithelium undergoes epithelial mesenchymal transformation.10

Several processes of lip development and merging prominences are well accounted for by the dynamic fusion theory. With epithelial bridging and fusion there is not an exposed outward ridge of tissue. A nonaggregation of tissue suggests that programmed cell death or epithelial mesenchymal transformation occurs as epithelial cells close over other epithelial cells. Either process would allow mesenchyme to develop under the newly formed epithelial cells. Fusion must occur properly in both epithelial layers as well as the mesenchyme to avoid cleft deformities. In later stages when there is less visible epithelial bridging, Streeter's observation of epithelial smoothing from mesenchymal ingrowth applies.7

Although our study used a primate model, in comparing the SEM photographs of human embryos taken by Hinrichsen,8 one is easily able to compare areas of potential discrepancies. Two areas that have brought up questions across species are epithelial bridging and fusion. The figures presented in Hinrichsen's article show fusion with evidence of programmed cell death and epithelial mesenchyme on histologic analysis. The SEM figures also show epithelial bridging at stage 16, just as our study shows. Hinrichsen's work supports using nonhuman primates for studying the embryology of the lip and face.8

The dynamic fusion theory combines the mesodermal contributions of the merging theory with epithelial fusion of the prominences. This dynamic fusion was likely difficult for Streeter to recognize with his best-fit 3-dimensional reconstruction method of studying embryologic development. Furthermore, the fusion is dynamic and occurs quickly (within 1-2 days) as the prominences of the lip develop. The paired labial furrows as described by Streeter and shown in the SEM photographs demonstrate the mesenchymal merging process (Figure 5A). With further development and mesenchymal growth, the furrows are smoothed out (Figure 6B).

The study of normal development of the lip can help to elucidate abnormal growth. The major area of abnormal lip development is in the form of clefts. The prevalent theory (Streeter's merging theory) of lip development tried to explain cleft formation by suggesting that without proper mesenchymal growth and support, the epithelial layers would break down and a cleft would form.7, 11 The dynamic fusion theory requires reanalyzing the etiology of cleft lips. Today we see 2 possible causes of cleft lip anomalies: lack of epithelial/mesenchymal fusion and mesenchymal hypoplasia. If epithelial fusion is blocked, a cleft will result.

Mesenchymal growth may be the driving force behind dynamic fusion. Without the mesenchyme pushing the 2 prominences together, there may be no need for epithelial fusion. Clinically, children with a cleft lip often have some lack of mesenchymal tissue, but we cannot tell whether the cleft causes the lack of mesenchymal tissue or the lack of tissue causes the cleft.

Specific facial features develop from distinct prominences (Figure 10). The lower lip develops from the fusion of the mandibular prominences. The formation of the upper lip is formed from the maxillary prominence fusing medially with the paired medial nasal prominences (Figure 3C). The philtrum is formed from the fused paired medial nasal prominence. The vermilion, which is the transitional zone between the skin of the lip and the mucous membrane of the lip, also called the red zone, likely develops from the maxillary prominence.12 This is suggested by the late filling of the maxillary median groove and by a lack of prolabial vermilion observed in patients with bilateral cleft lip. Our analysis suggests that the cleft blocks the maxillary prominence's contribution to the vermilion.

The nose is formed by 3 major prominences. The dorsal ridge of the nose is formed by the medial nasal prominence (Figure 5A). The lateral nasal prominence forms the lateral ala and lateral aspects of the nose (Figure 3C). The cheeks and epithelial tissue leading to the nose are formed by the maxillary prominence. The nasolacrimal groove and lacrimal ducts are formed by the fusion of the maxillary prominence and the lateral nasal prominence. It is this groove that separates these 2 prominences, and the latter forms the lacrimal duct. The columella and nasal tip develop from the medial nasal prominence.12-13

George L. Streeter's work has been a guiding light in the field of human embryology. Through reaffirming his discoveries and challenging them also, we honor the work that he and his predecessors have achieved. Embryologic investigation requires a detective approach that challenges present theories and creates new hypotheses. The use of available nonhuman primate material has suggested a refinement of the mechanism by which the upper lip forms, which can be compared with Streeter's pioneering work on human embryos. Through continued improvements in research techniques, the dynamic fusion theory will be strengthened or modified.

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Article Information

Corresponding author and reprints: Craig W. Senders, MD, 2521 Stockton Blvd, Suite 7200, Sacramento, CA 95817 (e-mail: cwsenders@ucdavis.edu).

Accepted for publication April 10, 2001.

This study was funded by grant RR 00169 from the National Institutes of Health, Bethesda, Md.

We would like to thank Laura Wilson for her SEM assistance.

References
1.
O'Rahilly  RMuller  FDevelopmental Stages in Human Embryos Lunenberg  Vt Meriden-Stinehour Press1987;
2.
Booher  CBPrahalada  SHendrickx  AGUse of a radioreceptor assay (RRA) for human luteinizing hormone/chorionic gonadotropin (hLH/CG) for detection of early pregnancy and estimation of time of ovulation in macaquesAm J Primatol. 1983;445- 53Article
3.
Cruz  GRDevelopmental Staging in the Cynomolgus Monkey (Macaca fascicularis) With Emphasis on the Metanephros [master's thesis]. Davis University of California1985;
4.
Hendrickx  AGEmbryology of the Baboon. London, England University of Chicago Press1971;
5.
Rahilly  O'.Developmental Stages in Human Embryos. Washington, DC Carnegie Institution of Washington1973;
6.
Wilson  DBSawyer  RHHendrickx  AGProliferation gradients in the inner ear of the monkey (Macaca mulatta) embryo.J Comp Neurol. 1975;16423- 30Article
7.
Streeter  GLDevelopmental Horizons in Human Embryology: Contributions to Embryology. 32 Baltimore, Md Lord Baltimore Press1948;
8.
Hinrichsen  K>The early development of morphology and patterns of the face in the human embryoAdv Anat Embryol Cell Biol. 1985;981- 79
9.
Shuler  CFProgrammed cell death and cell transformation in craniofacial developmentCrit Rev Oral Biol Med. 1995;6202- 217Article
10.
Sun  DBaur  SHay  EDEpithelial-mesenchymal transformation is the mechanism for fusion of the craniofacial primordia involved in morphogenesis of the chicken lip.Dev Biol 2000;228337- 349Article
11.
Moore  KLPersaud  TVBefore We Are Born 5th Philadelphia, Pa WB Saunders Co1998;
12.
Sadler  TWLangman's Medical Embryology 6th Baltimore,Md Williams & Wilkins1990;
13.
Bhaskar  SNedOrban's Oral Histology and Embryology 9th St Louis, Mo CV Mosby Co1976;
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