TRANSCRIPT OF NARRATION FOR DEVELOPMENT OF THE FACE

Transcription

1 TRANSCRIPT OF NARRATION FOR DEVELOPMENT OF THE FACE Slide 1 (introduction) In this lecture, I ll be addressing two separate but interrelated topics. First, I ll be summarizing some of the most fundamental processes and events that contribute to the formation of the face during normal embryonic and early fetal development. Specifically, I ll be focusing in particular on the development of the superficial structures of the face before then moving on to look at the formation of two more deeply placed anatomical complexes the nasal cavities and the palate. After laying this basic foundation, we ll then move on to briefly examine some of the more common and clinically significant orofacial deformities that can sometime arise as consequences of genetically and/or environmentally based perturbations to the developmental processes that underlie the formation of various facial structures, most notably the upper lip and the palate. In a lecture about the face, it probably bears specific emphasis here at the outset that this term is commonly used in at least two somewhat different ways. Although it s sometimes used to refer specifically to the superficial structures that we clearly see externally when we look at one another, it s also commonly used by anatomists to refer to an entire region of the skull namely, that part that lies deep to the superficial face and that develops through intramembranous ossification of ectomesenchyme derived from cranial neural crest, especially that of the first pharyngeal arch. Thus, in the latter, more inclusive, sense, this entire lecture is about the development of the face, as it will focus entirely on the development of the facial skeleton and associated soft tissues. However, only the first third or so of the lecture will focus specifically on the development of the superficial soft tissues of the face. Finally, before moving on to the next slide, I d just quickly point out the photographs that I ve included at the bottom of this first slide, which show three actual human embryos preserved at three different stages of facial development. More than anything else, I include these photos here to help you orient yourselves to the very highly schematic illustrations that follow in the ensuing slides. Slide 2 (24 days/middle of 4 th week) The first externally visible indications of the onset of facial development are seen in the middle of the 4 th week of embryonic development, at approximately 24 days, with the appearance of five swellings surrounding the primordial mouth, or stomodeum, and it s this stage of development that s illustrated in this slide. These five primordial swellings represent the so-called facial prominences (sometimes referred to alternatively as facial processes, especially in older texts and those by British authors). One of these, the frontonasal prominence (shaded here in green), is unpaired and develops immediately superior to the stomodeum. The remaining prominences are paired, consisting of left and right maxillary prominences (shaded here in yellow), which develop lateral to the stomodeum, and left and right mandibular prominences (shaded here in blue), which develop caudal to the stomodeum. What gives these facial primordia their prominent swollen appearance is the rapid mitotic proliferation of ectomesenchyme immediately 1

2 deep to the surface ectoderm. This ectomesenchyme originated from neural crest cells that began migrating into these areas a few days earlier, in the case of the frontonasal prominence, from a population of these cells located just beneath the developing forebrain, or prosencephalon, and in the case of the maxillary and mandibular prominences, from the 1 st and 2 nd rhombomeres. You ll note in the left lateral view on the right side of this slide that both the maxillary and mandibular prominences are derivatives of the 1 st pharyngeal (or visceral) arch. Also, you ll note in the anterior view on the left that I ve shaded the stomodeum gray. This represents my feeble attempt to depict the fact that the oropharyngeal membrane a structure that you already learned about in a previous lecture remains intact at this earliest stage of facial development, and serves to partition the primordial oral cavity from the pharynx. As you can see, at this early stage, the facial prominences are little more than relatively featureless swellings. This will change relatively rapidly, however, as the embryo continues to develop. Slide 3 (28 days/end of 4 th week) In this slide, we re once again looking at the same anterior and lateral views of the developing facial region that we saw in the previous slide, but here we ve jumped ahead by about four days, to the very end on the 4 th week of development. Over the course of this period, you ll note that several changes have taken place: the oropharyngeal membrane has now ruptured, thereby establishing continuity between the developing oral cavity and the pharynx; the 2 nd pharyngeal arch (the hyoid arch) has begun to develop; and a lens placode the ectodermal precursor to the lens of the eye has appeared on either side of the frontonasal prominence. More significant for the purposes of this lecture however, is the conspicuous inferolateral expansion of the frontonasal prominence, and the localized thickenings of the surface ectoderm overlying this inferolateral portion of the prominence on both the left and right sides; these ectodermal thickenings are the paired nasal (or olfactory) placodes, which represent the primordia of the nasal cavities. Although these placodes are rather unremarkable in their gross appearance at this stage of development, we ll see in the next several slides that their morphology, and that of the tissues immediately surrounding them, is going to quickly become significantly more complex over the course of the coming days and weeks. Slide 4 (31 days/middle of 5 th week) As promised, here we ve moved ahead just a few short days now approaching the middle of the 5 th week of development and the nasal placodes that we first saw on the previous slide are already no longer really even recognizable as such. This is because they ve invaginated, transforming themselves from what were initially convex structures into what are now shallow concavities within the surface ectoderm that push into the underlying ectomesenchyme of the frontonasal prominence. These concavities are called the nasal pits, and the walls of these pits represent the primordia of the anterior nares what we more commonly refer to as the nostrils. 2

3 You ll also note that the maxillary prominences begin to expand here during the 5 th week; as they do this, each grows progressively closer toward the lateral margins of the ipsilateral nasal pit. However, throughout the early stages of this expansion, the former remains separated from the latter by a distinct furrow, called the nasolacrimal groove. As we ll see in later slides, the expansion of the maxillary prominences that begins in earnest here during 5 th week is one of the more important processes in the overall development of the face, and if this process becomes disrupted in any significant way, serious malformations of the face may occur. Slide 5 (33 days/middle of 5 th week [SEM]) In this slide we take a brief break from simplified schematic anatomy and return to reality. What we re looking at here is a scanning electron micrograph of an actual human embryo, once again near the middle of the 5 th week of development, but just a couple of days beyond the stage that we just looked at. As you probably know, the very high levels of contrast obtained through SEM make this an extremely useful tool for visualizing even the most subtle of surface structures, and this is precisely the reason that I ve included this image here. Specifically, it provides a very detailed view of the region immediately surrounding the nasal pits (labeled NP on the micrograph). From this, you ll note that these pits are in actuality not formed entirely through simple invagination of the nasal placodes. Rather, as the pits push progressively deeper into the underlying ectomesenchyme of the frontonasal prominence, there s a concurrent proliferation of the surrounding ectomesenchyme, which results in sort of a horseshoe-shaped elevation of the surface ectoderm immediately surrounding the nasal pits, thereby effectively accentuating their depth. In general, the elevation of the lateral margins of the pits precedes that of the medial margins by about a day, and this growth differential can indeed be seen here, as the lateral margins are somewhat more strongly elevated than the medial margins. Based on your study of anatomy thus far, you can probably well anticipate the fact that once these lateral and medial elevations surrounding the nasal pits develop to a point at which they can be clearly distinguished from adjacent portions of the frontonasal prominence, they re recognized as distinct entities unto themselves and, accordingly, are given their own names. However, as these structures aren t labeled on this micrograph, you ll have to wait until the next slide to learn what these names are. Before we forge ahead, however, I d also point out that this embryo quite nicely illustrates the fact that the nasal pits initially form rather far laterally on the frontonasal prominence; this should seem somewhat peculiar to you, as it is quite likely that your own nostrils lie immediately adjacent to the midline of your face. As you ll soon see, this change in the position of the developing nasal passageways begins toward the end of the 5 th week of embryonic development. Slide 6 (33 days/middle of 5 th week) Here we re just looking at a schematic view of an embryo at essentially the exact same stage of development as that shown in the scanning electron micrograph in the previous slide at day 33, just beyond the middle of the 5 th week post-conception. Really, my only reason for showing you this slide is simply to put names to those elevations that we just 3

4 saw flanking the nasal pits in the SEM on the last slide. These elevations are called, quite aptly, the medial and lateral nasal prominences. Slide 7 (35 days/end of 5 th week) As I already mentioned earlier, expansion of the maxillary prominences represents one of the more significant processes in the overall formation of the face during the 5 th week of development. This expansion, which again occurs through mitotic proliferation of ectomesenchymal cells that populate these prominences, occurs in all directions; however, the most conspicuous growth that occurs during this period is that of the distal, or anteriormost, ends of the prominences, which grow increasingly medially toward the midline as they expand. Given that the maxillary prominences originate lateral to the stomodeum, you might predict quite logically, in fact that the first structure encountered by the anterior tip of each maxillary prominence as it expands toward the midline would be the inferior end of the ipsilateral lateral nasal prominence. However, as you can see in this slide, this prediction would be incorrect: the anterior end of the maxillary prominence does not contact the inferior end of the lateral nasal prominence, but rather passes inferior to it to establish contact with the inferior end of the ipsilateral medial nasal prominence (which, as you ll note here, does indeed extend somewhat further inferiorly than its lateral partner). Slide 8 (40 days/middle of 6 th week) Although the maxillary prominences make their initial contact with the medial nasal prominences sometime near the very end of the 5 th week of development (or sometime shortly thereafter), this event in no way impedes the continued expansion of the maxillary prominences. They, of course, continue to grow, still with much of their expansion directed anteromedially. This continued growth effectively results in more forceful and extensive contact, on both sides of the developing face, between the distal end of the maxillary prominence and the inferior end of the medial nasal prominence, a force that, in turn, results in the two medial nasal prominences being pushed more closely together at the midline, as is illustrated in this slide. These represent rather important events in the formation of the face that we ll look at in more detail in just a moment. But before doing that, however, I d quickly call your attention to the fact that, even at this relatively early stage in embryological development around the middle of the 6 th week what we re staring at here is really already starting to look somewhat like a human face, and this is reflected in the fact that we re starting to see adult anatomical structures being labeled here. This should serve as your cue to start thinking about the adult derivatives of all these embryological facial prominences (if you haven t already started to do so). Indeed, at this point, you can already infer many of the derivatives of at least the frontonasal prominence, as well as those of the mandibular and lateral nasal prominences: with respect to the superficial soft tissues of the face, the frontonasal prominence gives rise to the skin overlying the forehead and the bridge of the nose, the mandibular prominences give rise the lower lip and lower portions of the cheek, and the lateral nasal prominences positioned immediately lateral to the developing nostrils will give rise to the wing-like lateral portions of the nose, which are called the alae. 4

5 Slide 9 (41 days/end of 6 th week [SEM]) Now I want to return to the developing contact between the expanding distal end of the maxillary prominence and the inferior end of the medial nasal prominence, in order to look more closely at what happens when such a contact develops. As we ll see, such contacts represent a recurring theme in the development of the face, so it s important to have at least some understanding of the cellular processes that underlie them. Here, of course, we re looking at another scanning electron micrograph, but this time one at a relatively high level of magnification, focusing in particular on the contact zone between the maxillary prominence (labeled MXP ) and the medial nasal prominence (labeled MNP ) immediately inferior to the right nasal pit (labeled NP ). As you can see, in relatively short order after contact is first made here, the ectodermally derived epithelium overlying the maxillary prominence begins to develop continuity with that of the medial nasal prominence, resulting in the externally visible epithelial bridges that I ve pointed out with the red arrow on this slide. To a lesser degree, you can also see a few such epithelial bridges forming further superiorly, in the floor of the nasolacrimal groove, between the maxillary prominence and the lateral nasal prominence (labeled LNP ). We ll see in the next slide just how this epithelial bridging develops, but before we do that, I d just take a moment to quickly emphasize the critical importance of the developmental stage that we re looking at here in the overall formation of the face; and that is, by the end of the 6 th week, each of the maxillary prominences has expanded to the point at which it s begun to merge with both of its ipsilateral nasal prominences. You won t fully appreciate the significance of these developing fusions until later in the lecture, but trust me these represent very important events indeed. Slide 10 (process of fusion of facial prominences) Obviously, this is a highly schematic cartoon, not really meant to illustrate any specific anatomy. Rather, it s meant to serve only as a very general representation of what happens when two facial prominences approach, contact, and merge with one another. However, given that we re concerning ourselves right now primarily with the contact made between the maxillary and medial nasal prominences, you might as well imagine the structure at the top left being the former and the structure at the top right being the latter, as this would be congruent with the general view that we had in the SEM on the previous slide. As you can see here, each prominence consists of an outer epithelial layer, derived from ectoderm, and a central core of embryonic connective tissue, which at this early stage of development consists largely of ectomesenchyme derived from cranial neural crest. When the two prominences come into contact with one another, it s the outer epithelia, of course, that first touch. Once this occurs, the epithelial cells in the contact zone between the two prominences begin to merge together through a process of intercalation: essentially, the epithelial cells begin to interweave with each other. Soon thereafter, the intercalated epithelial cells begin to break down, thereby allowing the ectomesenchyme 5

6 contained within the two prominences to intermingle. So in a very general sense, these are the basic cellular processes that are occurring when we see two or more facial prominences merging together. Slide 11 (48 days/end of 7 th week) As depicted in this slide, by the end of the 7 th week, this process of merging is well under way in fact, nearing completion at several different locations throughout the central portion of the developing face. First, on each side of the face, the anterosuperior margin of the maxillary prominence has merged with the inferior end of the lateral nasal prominence. I should mention that, as this process is taking place, an interesting chain of events is occurring within the nasolacrimal groove. Specifically, the ectoderm in the floor of the groove becomes conspicuously thickened to form a solid epithelial cord, which subsequently separates itself from the surrounding ectoderm to sink deeper into the underlying ectomesenchyme. This epithelial cord will eventually canalize that is, it ll develop a central lumen to become the nasolacrimal duct, which will ultimately serve as a conduit for secretions of the lacrimal gland, carrying them from the lacrimal puncta and canaliculi to the inferior nasal meatus. However, this process of canalization begins much later in development at about four months and, in fact, the nasolacrimal duct typically doesn t become fully patent until about 4 6 weeks after birth. As each maxillary prominence begins to merge with its ipsilateral lateral nasal prominence, it also continues to merge with its ipsilateral medial nasal prominence, a process that we already saw beginning in the previous week. Now that this process is nearly complete, you ll hopefully notice that this fusion between the maxillary and medial nasal prominences has one very important functional consequence, in that it s effectively partitioned the developing nostrils (the nasal pits) from the developing oral cavity (the stomodeum). Finally, you ll note here that as each medial nasal prominence merges with its ipsilateral maxillary prominence, it also begins to merge with its contralateral partner at the midline. Slide 12 (48 days/end of 7 th week [plane of section for Slides 13 14]) In the next two slides, we re going to be looking at this merging between the contralateral medial nasal prominences from a different perspective that seen in horizontal section and to help you orient to what you ll be seeing in these slides, I ve indicated here with the dashed red line the approximate plane of these sections. Slide 13 (formation of intermaxillary segment [early]) Alright, as promised, what we re looking here are two horizontal sections highly schematic ones, of course taken through the maxillary and medial nasal prominences as they begin to merge together. The section on the left depicts a somewhat earlier stage in 6

7 this process, essentially picking things up at a point just a bit earlier than that depicted in the previous slide, probably somewhere in the 6 th week of development. Accordingly, you can see the earliest stages of these fusions here, but the four constituent processes all remain identifiable as at least partially distinct entities. In the section on the right, however, we see a somewhat later stage of development, likely toward the end of the 7 th week, in which the continued expansion of the left and right maxillary prominences (indicated in these figures by the black arrows) has forced the left and right medial nasal prominences together at the midline. At this point, the only remaining indications of the formerly independent medial nasal prominences are the globular projections present laterally on both the anterior and posterior surfaces of this newly formed central structure. Of course, having formed a new structure, we need to give it a new name, and we ll do that on the next slide. Slide 14 (formation of intermaxillary segment [late]) Here we re looking at the same horizontal sections that we were just looking at, only at two slightly later stages, and you can see that the new central structure formed through the fusion of the left and right medial nasal prominences is called the intermaxillary segment, so named because of its position between the distal ends of the two maxillary prominences. Perhaps more important than its name, however, in the later section on the right, you can see the structures that ultimately derive from the intermaxillary segment: the central portion of the upper lip, called the philtrum; the anteriormost part of the palate, called the primary palate; and in between these two structures, the premaxillary part of the maxilla, which supports both the central and lateral incisors. I would anticipate that my reference to a premaxillary part of the maxilla might seem confusing to many of you, especially those of you who have not taken a course in comparative anatomy. However, in many non-human tetrapods most, in fact the intermaxillary segment gives rise to an independent bone (or pair of bones) called the premaxilla (or premaxillae). The premaxillary part of the human maxilla is thus homologous the independent premaxillary bone of other tetrapods, and this in fact accounts for this seemingly confusing anatomical nomenclature. Finally, now that I ve told you that the central portion of the upper lip, or philtrum, is derived from the intermaxillary segment (itself derived from the fused medial nasal prominences), you can almost certainly surmise at this point that the remainder of the upper lip specifically, its lateral portions is derived from the maxillary prominences, as are the upper portions of the cheeks. Slide 15 (end of 10 th week) You all know how this story ends, as you see the final chapter every time you look in the mirror, so I won t belabor this process any more than I already have. I would just conclude our look at the development of the superficial structures of the face with two important points. First, most major events in the development of the superficial face basically, those that I ve summarized previously occur by the end of the embryonic 7

8 period (that is, by 60 days, when the embryo becomes considered a fetus); by the 9 th week, and certainly by ten weeks as we re looking at here, the face has taken on an unmistakably human appearance. Thus, the most critical window of time in the formation of the face is between the 4 th and 8 th weeks of development. Slide 16 (end of 14 th week) Second, looking here at a slightly older fetus, one at approximately 14 weeks, you ll hopefully notice great similarities between the domains of the frontonasal, maxillary, and mandibular prominences (in terms of their derivative structures) and distributions of the V 1, V 2, and V 3 dermatomes of the face the patches of skin innervated by the ophthalmic, maxillary, and mandibular divisions of the trigeminal nerve. This should serve as a clue to all of you that if your memory of facial dermatomes ever fails, you should be able to intuit the cutaneous innervation of a given facial structure by recalling its embryonic derivation. However, I would point out two noteworthy exceptions to this general rule: whereas the philtrum of the upper lip is derived from the intermaxillary segment and the side of the nose is derived from the lateral nasal prominence (both, in turn, ultimately derivatives of the original frontonasal prominence), neither of these parts of the face receive their cutaneous innervation from the ophthalmic nerve; both are instead innervated by branches of the maxillary nerve. Unfortunately, I can t offer a satisfactory explanation for just why this is; however, it seems likely that it must relate to the very close association between the maxillary prominences on the one hand and the medial and lateral nasal prominences on the other during the latter stages of early facial development, during which time, as I outlined earlier, there is at least some intermingling of the embryonic tissues within these prominences. Slide 17 (contributions of facial prominences to external soft tissues of the face) Before moving on to deeper structures, I d merely emphasize here the fact that at this point you should be able to look at your own face in the mirror and identify the embryonic origin of nearly any of the superficial soft tissues that you see. If you can already successfully do this, then you should probably proceed on to the next slide. If you can t yet do this, then this slide may serve as a useful at-a-glance review. So, briefly: the mandibular prominences give rise to the lower lip and the lower portions of the cheeks; the medial nasal prominences give rise to the crest and tip of the nose and, via the intermaxillary segment, the central portion of the upper lip (the philtrum); the maxillary prominences give rise to the lateral portions of the upper lip and the upper portions of the cheeks; the lateral nasal prominences give rise to the lateral parts of the nose (the alae); and the frontonasal prominence gives rise to the skin of the forehead and the bridge of the nose (as well as the medial and lateral nasal prominences themselves, of course). As we ll soon see, however, this does not represent an exhaustive list of the structures derived from these prominences, as nearly all give rise to various deeper structures as well, most notably, the numerous membrane bones that constitute the facial portion of the skull. So we ll now move on to look more closely at some of these deeper structures, focusing in particular on the development of the nasal cavities and the palate. 8

9 Slide 18 (development of the nasal cavities [early]) At this point, we ve already seen the earliest stages of the development of the nasal cavities, with the formation of the nasal pits, which you ll recall first appeared during the 5 th week of development, primarily through invagination of the nasal placodes (but also accentuated to some degree by the concurrent elevation of the surrounding portions of the frontonasal prominence to form the medial and lateral nasal prominences). Now we re going to backtrack to this earlier stage of development and follow this process further, but from a somewhat different perspective. Up until now, we ve only seen the nasal pits from an external perspective. In this slide and the one that follows, however, we ll be looking at the further development of the nasal pits and their derivative structures in sagittal section, which will allow us to better visualize how they continue their expansion throughout the deeper portions of the face. In the image on the left-hand side of this slide, we return to the end of the 5 th week, and see that invagination of the nasal placode and subsequent deepening of the nasal pit has resulted in the formation of a blind sac called the nasal sac between the developing forebrain (superiorly) and the oral cavity (inferiorly). Here in sagittal section we re looking into the right nasal sac, but realize, of course, that a nasal sac has also formed on the left side of the head through deepening of the left nasal pit; we just can t see it here because it lies to the left of the plane of this section, that is, in front of the plane of your computer screen. Because the nasal placodes, you ll recall, initially arose as thickenings of the ectoderm on the inferolateral surfaces of the frontonasal prominence, the linings of these sacs created through their invagination are lined with surface ectoderm. As development continues into the 6 th week, as illustrated on the right, the nasal sac continues to expand. This really occurs in all directions, but is most conspicuous superiorly and inferoposteriorly. In the case of the former, the ectoderm lining the roof of the sac pushes more deeply into the ectomesenchyme underlying the developing forebrain. In the case of the latter, the ectoderm lining the posterior wall of the sac expands further posteriorly beyond the underlying intermaxillary segment and the anlagen of the primary palate that develop from its posterior surface, and the ectoderm of the newly expanded floor of the sac pushes closer toward the roof of the oral cavity, thereby creating a thin membrane of intervening ectoderm called the oronasal membrane. Thus, at this relatively early stage, the nasal sac remains completely separated from the oral cavity, anteriorly by the developing primary palate, and posteriorly by the oronasal membrane. As illustrated here, however, by the end of the 6 th week, the oronasal membrane begins to rupture. Slide 19 (development of the nasal cavities [late]) By the end of the 7 th week of development, as illustrated on the left-hand side of this slide, the oronasal membrane has disintegrated completely, thereby establishing an area of continuity between the developing nasal cavity and the cranial end of the foregut. This 9

10 newly established area of continuity is called the primary, primitive, or (as given on this slide) primordial choana. In addition, you ll note that two other important events have transpired during the course of the 7 th week. First, some of the epithelial cells in the roof of the nasal cavity have differentiated into olfactory neurons, the axons of which have begun to project superiorly toward the developing forebrain within small bundles called fila olfactoria. Second, the intermaxillary segment, which was previously rather globular in median sagittal section, has now become more triangular due to the posterior expansion of the primary palate. Note that, here at the end of the 7 th week, the primary palate is the only structure lying between the nasal and oral cavities, and because of its relative brevity, it serves only as a very incomplete partition between these two spaces. The final steps in the development of the nasal cavities at least the final steps that I m going to say anything about here are illustrated in the 12-week fetus on the right. Two of these are relatively minor in the grand scheme of this lecture, and I ll merely just point these out to you now. First, the axons of the olfactory neurons conveyed within the fila olfactoria have now reached their final destination, synapsing within the olfactory bulb on the inferior surface of the developing forebrain. Second, we re beginning to see at this stage the early development of three ridges that are raised up along the lateral wall of the nasal cavity, the inferior, middle, and superior nasal conchae. Far more important than these events, however, is the development of the secondary palate. I ll get into the details of just how this is formed in the coming slides. But for the time being, however, this sagittal view provides an excellent illustration of two of the functional consequences of this structure. First, it provides a far more complete separation between the nasal and oral cavities than that which was provided by the primary palate alone. And second, precisely because it extends so much further posteriorly than the primary palate, its formation has brought about a change in the position of the choana, which you ll note now lies much further posteriorly that it did at seven weeks. For this reason, some embryologists use the term secondary (or definitive) choana to distinguish between this newly formed aperture and the one formed earlier upon the initial rupturing of the oronasal membrane. This distinction is perfectly appropriate, even useful; in practice, however, the definitive choana is more commonly referred to simply as the choana. Finally, there s one important component of the nasal cavities that I ve ignored up until now, the nasal septum, which is a thin, midline partition between the left and right nasal cavities. We ll look at the origins of this structure next in our overview of palatal development, but for the time being, simply realize that this structure does indeed exist, and that it s been omitted from these schematic sagittal sections for the sake of clarity. Indeed, a precise median sagittal section would bisect the nasal septum, and thus if it was left intact, we would not be able to see the nasal cavity in these sections. Slide 20 (palatal development intermaxillary segment) Whereas the development of the face as a whole begins in the 4 th week, with the initial appearance of the five primordial facial prominences, and the development of the nasal cavities begins shortly thereafter, in the 5 th week, with the invagination of the nasal placodes to form the nasal pits, the palate doesn t begin to form until the 6 th week of 10

11 embryonic development. Once again, the first step of this process you ve already seen. You ll recall that in the 6 th week, the left and right medial nasal prominences merge together to form the intermaxillary segment, and that this segment soon begins to expand posteriorly to form a roughly triangular process, sometimes referred to as the median palatine process, which is the first structure to insinuate itself between the developing nasal and oral cavities. For this very reason, the median palatine process is said to form the primary palate. However, as I ve already emphasized, the primary palate is rather small; as you can see in the figure on the right-hand side of this slide depicting an inferior view of a child s palate it really only extends a relatively short distance posteriorly beyond the alveolar portion of the premaxillary part of the maxilla (the part that accommodates the incisors), to reach the level of the incisive fossa. The portions of the definitive palate that form posterior to the primary palate those portions shaded in sort of a salmon color and labeled palatal plates in the figure on the right don t begin to form in earnest until a bit later; for this reason, their formation and subsequent fusion with the original primary palate is said to constitute the development of a secondary palate. It s the formation of this secondary palate that we ll now be looking at in the next several slides. Slide 21 (palatal development lateral palatine processes [appearance]) The first stage in the formation of the secondary palate begins with the appearance of shelf-like expansions along the medial surfaces of the maxillary prominences. These expansions, alternatively recognized as palatal shelves, palatal plates (as in the previous slide), or lateral palatine processes (as in this slide), first begin to form during the 6 th week of development. However, most of their subsequent elongation occurs during the 7 th week and early part of the 8 th week, the latter being the approximate stage depicted here. You ll note here, however, that this expansion of the lateral palatine processes does not occur quite in the manner that you might expect. That is, they don t grow directly medially toward one other; rather, much of their initial growth is directed inferiorly, such that by the beginning of the 8 th week, they ve come to effectively bracket the developing tongue, as seen in the coronal section of the left. Consequently, at this stage of development, these processes form only a very incomplete partition between the oral and nasal cavities, a fact that s particularly evident in the inferior view on the right, in which we re still able to look up into the developing nasal cavities. This is actually rather fortuitous for the purposes of this lecture, because it affords a glimpse at a structure that I ignored earlier when I was describing the development of the nasal cavities the nasal septum. As I mentioned earlier, and as you can clearly see on this slide, the nasal septum forms as a midline structure between the developing left and right nasal cavities. It forms initially as a central sheet of mesoderm derived from ectomesenchyme of the medial nasal prominences and that of their parent structure, the frontonasal prominence enveloped by an ectodermally derived epithelium. At this stage in its development, it projects inferiorly only to a relatively moderate degree, but as indicated by the black arrow in the illustration on the left, it continues to expand inferiorly as it grows. Related to this, it probably bears emphasis that in the image on the right, the inferior margin of the nasal 11

12 septum lies at this point somewhat superior to the transverse plane in which the median palatine process, or primary palate, has developed, a relationship that may not be readily apparent to you from this illustration. Slide 22 (palatal development lateral palatine processes [elevation]) At some point toward the end of the 8 th week of development give or take the lateral palatine processes undergo a rather rapid and remarkable transformation. Over the course of just a few short hours, they rotate markedly, such that their medial ends elevate, thereby bringing the processes into a nearly horizontal orientation, as is illustrated here. It s generally believed that this rapid rotation occurs largely through an intrinsic so-called shelf-elevating force, generated through the synthesis, accumulation, and subsequent hydration of hyaluronic acid and glycosaminoglycans within the ECM, or extracellular matrix, of the ectomesenchyme within these processes. Whatever the precise mechanism, however, you ll note in the coronal section on the left that once this rotation has taken place, the medial ends of the processes lie relatively close to one another near the midline, and that they, in turn, are being closely approached superiorly by the inferior end of the newly expanded nasal septum. In addition, in the palatal view on the right, you can see that at this point, the anterior margins of the lateral palatine processes have begun to merge with the posterolateral margins of the median palatine process, or primary palate. Thus, at this stage, the secondary palate itself (rather that just its constituent parts) has started to form. I d also point out one other significant event that s occurring here toward the end of the 8 th week, and that is that we re now beginning to see the first signs of ossification within the maxillary prominences. I m sure that you can well anticipate that this ossification will ultimately result in the formation of the maxillae themselves. What you might not anticipate, however, is that, in fact, many of the membrane bones of the facial skeleton actually ossify within the maxillary prominences; these include the vomers and palatines, intramembranous portions of the sphenoid (such as the medial and lateral pterygoid plates), as well as the zygomatic bones and the squamous portions of the temporal bones. Although it s not illustrated here, I ll just say that, by this stage, ossification has also begun within other facial prominences as well: ossification centers for the frontal, nasal, and lacrimal bones have appeared within the deep portions of the frontonasal prominence, and ossification centers for the mandible have appeared within the mandibular prominences. Slide 23 (palatal development lateral palatine processes [fusion]) The final step in the formation of the secondary palate is simply one of fusion fusion between the now closely apposed medial ends of the left and right lateral palatine processes. This final fusion takes place primarily between the 9 th and 12 th weeks of development, and proceeds from anterior to posterior; thus, it s almost as if the secondary palate zips itself up, starting at the incisive fossa and continuing on back from there. 12

13 I should point out here that the precise timing of this fusion as well as that of the earlier elevation of the lateral palatine processes that brought them into initial contact with one other in the first place varies somewhat between males and females. Specifically, these events begin several days to as much as a week earlier in males than in females. This fact will become particularly significant when we look at palatal cleft deformities, which are about twice as prevalent in females as they are in males. Also, you ll note that we now see bone forming more widely throughout the maxillary prominences, and that this ossification has already begun to spread into the lateral palatine processes. It s important to recognize, however, that this ossification is restricted to the anterior halves of these processes; thus, their anterior halves contribute to the formation of the hard palate, whereas their posterior halves form the soft palate. Obviously, the central core of the hard palate will ultimately consist of bone specifically, the palatine processes of the maxillae and the horizontal plates of the palatines. In contrast, as the soft palate is developing, it s invaded by myogenic mesenchyme, streaming in primarily from the 4 th pharyngeal arch, but also some from the 1 st pharyngeal arch as well; consequently, the soft palate will become largely muscular, with most of its muscles innervated by the nerve of the 4 th arch, the vagus nerve, but one (the tensor veli palatini) innervated by the nerve of the 1 st arch, the trigeminal nerve. Finally here, I d just point out in the coronal section on the left that as the medial ends of the lateral palatine processes are fusing together with one another at the midline, they re also becoming fused with the inferior end of the stucture that overlies them along the midline, the nasal septum. Slide 24 (facial prominence summary of key soft- and hard-tissue derivatives) You should realize that at this point I ve now mentioned most of the key derivatives both superficial and deep of the embryonic facial prominences. I won t bother to read through all of these again, but if you haven t yet committed these to memory, you may wish to pause here briefly in order to do this. As you do this, please keep in mind that all of the individual bones, or parts of bones, that I ve listed here are ultimately derived from neural crest, and that nearly all arise through intramembranous ossification. The two exceptions to the latter generalization are the ear ossicles, the incudes and mallei, which are also neural crest derivatives, but which arise through endochondral ossification of the dorsal part of Meckel s cartilage, a cartilage derived from the 1 st pharyngeal arch. The fact that, in mammals such as ourselves, these bones lie within the middle ear, and don t contribute to the formation of the face per se, may help you to remember this developmental difference. Slide 25 (general categories of cleft deformities) Now that you have a basic understanding of some of the more fundamental processes and events that underlie normal facial development, we can now move on to look at what happens when some of these processes and events become disrupted. In general, what we 13

14 often see when such disruptions occur are orofacial clefts in essence, gaps in various places throughout the mouth and/or face that persist beyond the embryonic and early fetal stages of development. As you can see in this figure, such clefts can occur at many different locations, and in fact some would seem to defy simple embryological explanations. However, the most common of these clefts can be readily understood with a basic knowledge of facial embryology, and it s these more common deformities that we ll be focusing on for the most part here. You ll no doubt be relieved to know that you aren t responsible for learning all of the numbered clefts shown on this slide; this very detailed system of categorization is really only used by maxillofacial surgeons. However, you should be aware of two far simpler systems for categorizing orofacial clefts, one based on anatomy, and the other on their relative prevalences. Slide 26 (general categories of cleft deformities anterior vs. posterior clefts) Based on location, anatomists often categorize orofacial clefts as being either anterior or posterior. The relative landmark here is the incisive fossa the point at which, under normal circumstances at least, the median and two lateral palatine processes meet. Thus, quite simply, an anterior cleft deformity is one occurring anterior to the incisive fossa, and a posterior cleft deformity is one occurring posterior to the incisive fossa. However, in practice, anterior cleft deformities are most commonly referred to as cleft lip and posterior cleft deformities are most commonly referred to as cleft palate. Although the latter makes perfectly good sense, the former is actually a bit confusing, because whereas some anterior cleft deformities are indeed restricted to the upper lip itself, others extend more deeply, through the alveolar process of the maxilla and indeed in some cases through the anteriormost part of the palate. Nevertheless, this represents common usage of these terms. Slide 27 (general categories of cleft deformities typical vs. atypical clefts) As you see here, another common way of categorizing orofacial clefts is based on their relative prevalences. Because cleft lip, cleft palate, and the combination of the two (known, logically enough, as cleft lip-cleft palate) collectively account for approximately 80% or more of all orofacial clefts, they re often categorized together as representing typical cleft deformities. In contrast, other types of clefts, such as median clefts of the upper and lower lips, oblique facial clefts, and transverse facial clefts, are obviously far less common, and for this reason, they re known collectively as atypical cleft deformities. I ll have just a few words to say about atypical clefts toward the very end of the lecture, but for the most part we ll be concerning ourselves here with typical clefts those of the lip and palate. Slide 28 (typical cleft deformities underlying causes and functional effects) 14

15 Before we move on to actually look at some of these typical cleft deformities, I ll just quickly summarize some key facts that you should be aware of concerning their underlying causes, effects, and prevalences. Most typical clefts result from multiple factors, both genetic and nongenetic. Among the former, more than 500 Mendelian syndromes are now known to include cleft lip, cleft palate, or both among their potential clinical manifestations, and thus cases of these deformities are often categorized as being either syndromic or nonsyndromic. Only about 30% of cases of cleft lip, with or without cleft palate, are syndromic; however, about 50% or more of all cases of cleft palate by itself are thought to be syndromic. Among nongenetic factors, maternal smoking and drinking are both recognized as risk factors, as is the use of corticosteroids and some anticonvulsants. Folic acid deficiency has also long been recognized as a risk factor, and a good deal of evidence suggests that even high altitude may contibute (this based, at least in part, on the fact that some of the highest prevalences of these deformities are seen among populations living in Bolivia and Tibet, high up in the Andes and Himalayas, respectively). Concerning their functional effects, typical cleft deformities can be quite debilitating indeed if not repaired surgically. Depending on their location and severity, they can potentially impair feeding, drinking, speech, breathing even ear function not to mention the difficulties that they present with regard to socialization, which are unfortunately quite common with almost any type of facial deformity. Slide 29 (typical cleft deformities prevalence) With respect to relative prevalences, cleft lip-cleft palate that is, both occurring together is the most common type of typical cleft deformity, accounting for approximately half of all cases. Among the remaining half, cleft palate by itself accounts for about 30% of cases, whereas cleft lip by itself accounts for about 20%. The prevaleneces of all these types of clefts vary among populations, and in general, they re most common in Asian-derived populations, least common in African-derived populations, with Europeans and their descendants lying somewhere in between. Overall that is, across all populations cleft lip, with or without cleft palate, occurs at a rate of about 1 case per 1000 births, and is roughly twice as prevalent in males than in females. Cleft palate by itself is somewhat less common, occurring at a rate of about 1 case per 2500 births, and is roughly twice as prevalent in females than in males. Finally, as we ll see in a minute, clefts of the lip and palate can occur either unilaterally or bilaterally. In cases of unilateral cleft lip, with or without cleft palate, there s a very strong left-right bias, with clefts on the left side being roughly twice as common as those on the right. In addition, there s a significant difference between unilateral and bilateral clefts of the lip with respect to the likelihood of them occurring is association with cleft palate, with this being the case about 68% of the time with unilateral cleft lip, but about 86% of time with bilateral cleft lip. Slide 30 (cleft palate general) 15

16 Alright, we ll begin our look at these typical cleft deformities by first looking at cleft palate, given that, as I already mentioned earlier, it may occur either by itself, or in association with other clefts, especially those of the lip. As you can probably infer for yourself at this point, clefts of the palate, despite their numerous underlying genetic and environmental causes, all ultimately stem from a common downstream developmental abnormality, namely, the failure of the left and right lateral palatine processes to properly merge together at the midline. Some such failures may affect only the final process of fusion, whereas others may stem from incomplete development of the processes themselves, thereby preventing them from even approaching one another at the midline; the latter can generally attributed to an inadequate founder population of neural crest cells migrating into the maxillary prominences during the earliest stages of facial development. You ll note here that this failure of the lateral palatine processes to fuse may be either complete or incomplete, and you ll see what I mean by this in the next slide. Slide 31 (cleft palate degrees of severity) As you can see in these four photographs, depending on what stage or stages of development are affected, clefts of the palate can range greatly in their severity. They may manifest themselves as nearly imperceptible furrows restricted to the posteriormost structure of the palate, the uvula, as seen in the photo in the upper left, or as complete clefts extending throughout the entirety of both the soft and hard portions of the secondary palate, as seen in the photo in the lower right. And, of course, as seen in the other two photos here, clefts of intermediate severity also occur with some frequency as well. As you might expect, the more severe or complete a cleft of the palate is, the greater its potential functional consequences. Often the most immediate problem that arises for infants born with severe palatal clefts is that breast milk tends to get aspirated up into the nasal cavities during their attempts to suckle. Slide 32 (cleft palate unilateral vs. bilateral) In addition to varying in their anteroposterior extent, clefts of the palate that stem from inadequate growth of one or both of the lateral palatine processes (as opposed to simpler defects in the final process of fusion), can occur either unilaterally or bilaterally. You ll note on right-hand side of this slide that, in cases of bilateral cleft palate in particular, the nasal septum is left with nothing to fuse to inferiorly, and thus its inferior margin can be seen terminating freely just superior to the plane of the cleft itself. Slide 33 (cleft lip unilateral [general]) In contrast to cleft palate, which as we just saw results from the failure of the lateral palatine processes to merge together, all typical forms of cleft lip result from failures of the maxillary and medial nasal prominences to properly merge together, thereby resulting 16