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Developmental Biology and Morphogenesis of the Face,
Lip and Palate
Alphonse R. Burdi
Prologue. The “history of man for the nine months preceding his birth would, probably,
be far more interesting and contain events of greater moment than all the three score
and ten years that follow it.”
Samuel Taylor Coleridge, Miscellanies, Aesthetic and Literary. Circa 1800.
The developmental biology of the face, lip, and palate
is best understood against a backdrop of biological
paradigms and information drawn from the multidisciplinary worlds of classical embryology, developmental biology, and, today, from the exciting world of
molecular biology. The advent of many new and exciting clinical interventional strategies for the treatment
of birth defects now allows the clinician to treat the
most delicate of craniofacial abnormalities which
were beyond the realm of treatment even for the skillful clinician due to lack of appropriate technologies.
Even the details of craniofacial morphogenesis, however interesting to the clinician, were also seen as being beyond the everyday clinical realm, or esoteric,
prior to the advent of the new wave clinical interventional techniques, such as high resolution imaging
and information technologies, in the fields of craniofacial and maxillofacial surgery. Today, there is a
rekindled need to understand more of the details of
craniofacial morphogenesis, especially as that understanding increases our awareness of the etiology,
pathogenesis, and clinical features of a variety of
craniofacial defects. The goal of this chapter is to provide a highlighted update or working understanding
of the “developmental blueprint” followed in human
craniofacial morphogenesis, with a special focus on
defects of the face, lip, palate, and associated structures.
While recent advances in developmental and molecular craniofacial biology contribute heavily to the
picture of face and palate morphogenesis, there has
been an explosion in our fundamental understanding
of the very beginnings of these body regions [43].
Such new information clearly centers on the genesis,
behavior, and developmental outcomes of many craniofacial “building block “ cell types throughout their
life span. These fundamental phenomena include patterns of early DNA signaling, gene and biochemical
organizers, nuclear and cellular differentiation, proliferation, migration, and, importantly, patterns of inter-
active behaviors at intracellular, cell surface, and extracellular matrix levels. Complete or partial interruptions of any one or combination of these phenomena have been implicated in the identification of
etiologic and pathogenic causes of mammalian birth
defects, including those of the human craniofacial
regions.
Normal and abnormal morphogenesis of the craniofacial regions, and even that of rest of the body, is
dependent upon a myriad of cell types and tissues.
One of the most important cell types in understanding normal and abnormal craniofacial morphogenesis is the neural crest cell [2, 36].While the importance
of crest cells has been hypothesized for a century or
more, not until the advent of neural crest biological
markers – first with isotopic labels and later with specific markers as monoclonal antibodies, intracellular
dyes, and protein assays – did the neural crest become
so widely appreciated in a variety of studies of vertebrate embryogenesis in general, and human craniofacial morphogenesis, in specific. While the majority of
recent neural crest studies of have necessarily dealt
with chick and mouse embryos, there is ample evidence to show that basic information and associated
technologies gained from vertebrate embryos can be
directly applied to neural crest cells in mammalian
and human embryos.
With this in mind, prime consideration should be
given to neural crest cells because they contribute
heavily to craniofacial morphogenesis. These important “building-block” cells arise from the final stages
in formation of the embryonic neural tube. Neural
crest cell specificity is the result of an inductive action
by nonneural ectoderm adjacent to the developing
neural tube (mediated by bone morphogenetic proteins BMP-4 and BMP-7) on the lateral cells of the
neural plate as the plate transforms from a plate of ectoderm into the definitive neural tube. The induced
neural crest cells express the transcription factor slug
which characterizes cells which separate from an em-
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A. R. Burdi
bryonic epithelial layer and subsequently migrate as
mesenchymal cells away from the parent site [33].
The identification of the exact molecular mechanisms and cellular events linked to the differentiation,
proliferation and, and especially, of the migration of
crest cells into the facial and pharyngeal regions is not
yet fully known. What is known, however, is that, with
the variant OTX2 transcription factor, specific patterns of neural crest proliferation and migration into
the pharyngeal arches are controlled by four members
of homeodomain proteins called HOX genes (A-D).
Another factor thought to be critical in the migration
of crest cells is a loss of cell-to-cell adhesiveness which
is associated with the loss of cell adhesion molecules
(CAMs) characteristic of the neural tube and the migrating neural crest cells [91]. Following the completion of craniofacial crest cell migrations and differentiation into specific structures (such as bones of the
facial skeleton) CAMs are re-expressed. Migrating
craniofacial crest cells are thought to travel through
cell-free intercellular spaces and pathways that have
high levels of extracellular matrix molecules [3, 9, 10].
These migrations are determined by factors intrinsic
to the crest cells themselves and features of the external environment through which the crest cells migrate. While most available information on neural
crest migratory pathways comes from chick studies,
data suggest that such information is applicable to
mammalian and humans as well. Migrations are facilitated by the presence of such molecular substrates as
fibronectin, laminin, and type IV collagen. Attachment to and migration of crest cells is mediated by a
family of attachment proteins called integrins. It is important to note that other extracellular matrix molecules in the pathway (e.g., chondroitin sulfate-rich
proteoglycans) can impede or block the normal migration of neural crest cells, which may lead to a number of craniofacial malformations.
As will be expanded upon later in this chapter, neural crest cells have the remarkable capacity to differentiate into a wide variety of anatomical structures
throughout the body. Exactly what controls crest differentiation is still an open question. One hypothesis
is that all neural crest cells are equal in their developmental potential, and their ultimate differentiation is
entirely predetermined by the environment through
which the crest cells migrate and finally reside, i.e.,
“extrinsic determinants.” A second hypothesis favors
“intrinsic determinants” and suggests that premigratory crest cells are intrinsically programmed for different developmental fates. Recent studies indicate
that both hypotheses may be operative [1].While neural crest cells have a common site of origin during
neural tube formation, not all neural crest cells behave
alike. There are two families of crest cells, i.e., cranial
and trunk neural crest. Trunk neural crest cells extend
from lower cervical to the most caudal embryonic
somites in humans. Trunk neural crest cells appear to
have lessened migratory pathways and have fewer developmental outcomes than cranial neural crest cells,
including formation of spinal ganglia, sympathetic
ganglia, and adrenal medulla chromaffin cells. Interestingly, unlike cranial crest cells, trunk neural crest
cells do not have the capacity to differentiate into
skeletal tissues.
The life history of cranial neural crest cells, while
not of any more importance than trunk neural crest
cells, appears to be more complex. Cranial neural crest
cells are a major component of the embryo’s cephalic
end and differentiate into a wide variety of cell and
tissue types, including connective, skeletal, dentin,
and muscle tissues of the face [64]. Unlike trunk neural crest cells, which show diffuse migratory pathways,
cranial neural crest cells follow specific migratory
pathways into specific regions of the embryonic head.
They arise from the more cephalic neural tube regions
and migrate ventrally into the pharyngeal arches adjacent to the upper regions of the embryonic gut tube.
Such migrations are extensive and follow very definite
migratory paths away from the neural tube and into
the facial and pharyngeal regions. In hindbrain regions, neural crest cells arise from eight segmented regions on either side of the hindbrain (rhombencephalon) called rhombomeres (numbered R1-R8)
and subsequently migrate into specific pharyngeal
arches [12, 14].
Crest cells from R1 and R2 centers migrate into the
first pharyngeal arch and play important roles in the
formation of Meckel’s cartilage and the malleus and
incus ear ossicles developing from it. Crest cells from
R4 migrate into the second arch and contribute to the
formation of the stapes, styloid process, and lesser
horn of the hyoid bone. Crest cells from R6 and R7 migrate into the third arch, and those from R8 migrate
into the fourth and sixth pharyngeal arches. The one
variant noted above is that the first pharyngeal arch
also includes crest cells from midbrain levels which
express OTX2 transcription factors. Little evidence
exists to show that crest cells from rhombomeric centers R3 and R5 play any significant role in human
craniofacial morphogenesis. Crest cells initially express the HOX genes from their originating rhombomeric center, but maintenance of a specific expression is dependent upon interaction of the crest cells
with the arch-specific mesoderm in the pharyngeal
arches. While there is a specific linkage between given
HOX genes and pharyngeal arches, morphologic derivatives arising from these linkages are also dependent upon epithelial-mesenchymal interactions and includes molecular signaling from surface ectoderm
covering the arches, specifically, fibroblast growth factors (FGFs) which interact with underlying mes-
Chapter 1
enchymal cells. Crest cells alone do not establish or
maintain a specific pattern of morphologic expression. Several regulatory factors have been identified.
Sonic hedgehog (Shh) genes and some retinoids have
been shown to regulate normal HOX gene expression
within the pharyngeal arches associated with a variety of developmental events, including neural plate
development and craniocaudal body pattern formation.
Defective differentiation, proliferation, and migration of cranial neural crest have been linked with a variety of developmental defects, i.e., the so-called neurocristopathies [24, 28, 29, 34]. Deficiencies and
excesses of retinoids, for example, can disrupt proliferation and migration on specific crest cells, resulting
in craniofacial defects, e.g., clefts of the lip and palate
[54, 62]. With reference to the structural abnormalities in the chromosome 22-deletion DiGeorge syndrome, the fundamental pathogenesis for this clinical
syndrome has been linked with defects in cranial neural crest cells of the third and fourth pharyngeal arches and cardiac outflow tract. Other cranial neurocristopathies include the wide range of craniofacial
abnormalities in the frontonasal dysplasia family,
Treacher Collins syndrome (mandibulofacial dysostosis), the Robin sequence, Waardenburg syndrome
(types I and III), and neurofibromatosis (von Recklinghausen’s disease).
The “building block “ cells for the head and face are
identifiable both premorphologically and morphologically as early as the second intrauterine week.
Once mapped out these cells continue with their peak
period of cell differentiation, proliferation, and migration through the second intrauterine month.While
the classical picture of craniofacial morphogenesis
can be framed upon the morphogenesis of primary
germ layer cells (i.e., ectoderm, mesoderm, endoderm), there is little doubt whatsoever that the current
understandings and excitement about mammalian,
including human craniofacial, morphogenesis have
been significantly advanced by a plethora of studies of
the origins and behavior of embryonic neural crest
cells. Morphogenesis of the facial regions depends
heavily on the timely differentiation, directed migration, selective proliferation of these crest cells which
arise as a product of neural tube formation as the neural tube progressively pinches off from the overlying
skin along the body’s dorsal axis. As will be discussed
later, cells and tissues within each of the embryonic facial primordia arise from neural crest cells begin their
migration (at about 21 postconception days) into the
facial regions, as cell clusters called rhombomeres,
from their sites of origin along the portions of the
neural tube which form the brain. The determinants
of crest cell migrations have been variously hypothesized as including intrinsic cell “targeting” factors and
Developmental Biology and Morphogenesis
chemical signaling from cells lining the extracellular
cleavage planes through which the crest cells migrate
[37, 42]. Crest cells from the developing midbrain regions migrate into upper facial regions, whereas crest
cells from hindbrain migrate selectively into the lower facial regions [45–47]. Importantly, once the crest
cells migrate into specific facial regions, they differentiate into mesenchymal cells that subsequently give
rise to connective tissue and muscle cells of those specific facial regions [41]. While the predominant neural
crest-derived mesenchymal cells in the facial regions
do co-mingle with mesodermally derived mesenchymal cells, the interactive nature of their co-mingling,
or lack thereof, remains uncertain. Consistent with
the tenets of the “developmental field concept” [48] in
human morphogenesis, both human and experimental studies generally have hypothesized that significant and early interference with normal differentiation, proliferation, and migration embryonic cells,
including especially the craniofacial neural crest cells,
can lead to isolated and syndromic craniofacial defects, called neurocristopathies, whose occurrence
and severity depend on a combination of environmental and genotypic factors specific to a given dysmorphic or phenotypic trait [46, 47, 52].
Having addressed the general developmental features of the craniofacial “building block” neural crest
cells, let’s turn our specific attention now to the key
events in the shaping of the human face, lip, and
palate. When the embryo’s cephalo-caudal axis is established at about 14 postconception days, the facial
developmental field is one of the first of the head regions to appear [48]. Centrally-located in this region
is a discrete bilaminar tissue plate, called the oropharyngeal membrane, whose structure and location
marks the junction between the oral ectoderm and
the endodermal digestive tube. This membrane progressively degenerates through the normal process of
apoptosis or “programmed cell death” which involves
increased phagocytic or lysosomal activity along the
inner and outer surfaces of the membrane. Once the
apoptosis of the oropharyngeal membrane is completed at 4 weeks, there is direct continuity between
the spaces of the early oral cavity and the pharyngeal
regions of the digestive tube. Only rarely does the
oropharyngeal membrane fail to degenerate. Interestingly, a similar ecto-endodermal membrane lies at the
depth of a groove which separates the first pharyngeal
from the second pharyngeal arch. As will be discussed
later, this membrane will have a very different and important developmental fate (i.e., does not undergo
apoptosis) than that of the oropharyngeal membrane
related to the fact that it has, unlike the oropharyngeal
membrane, a layer of mesenchyme interposed between its ecto- and endodermal layers.
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A. R. Burdi
Clearly as much developmental “shaping” occurs
on the laterals aspects of the young embryo’s head as
in its the frontal regions. At four weeks, a series of lateral surface elevations, called pharyngeal arches, becomes quite prominent on the lateral side of the head.
In fact, the appearance of the embryo’s head region at
this time closely resembles the gill slit anatomy seen
in a comparably-staged fish embryo; however, unlike
in the fishes, the surface gill appearance in human embryos is short-lived, except as noted above, in the case
of the development of the ear drum (or tympanum).
The pharyngeal arches contribute significantly to the
formation of the face, palate, and associated structures. Most congenital malformations of the head and
neck have their beginnings during the cellular transformation of the pharyngeal arches into their adult
derivatives. For example, branchial cysts and fistulae
can occur in those rare instances in which human
pharyngeal (or gill) clefts fail to smooth over on the
lateral side of the neck. As mentioned earlier, cell
masses which contribute to the bulging prominence
of the arches are the neural crest cells that have migrated into the pharyngeal arches from specific brain
regions, and which eventually differentiate into mesenchymal cells and give rise skeletal and muscular
structures specific to a given pharyngeal arch.
The first pair of pharyngeal arches are most important in shaping the human face and associated structures and will receive most attention in this chapter.
The first pharyngeal arch, often called the mandibular
arch, develops as two elevations around the oral opening which was filled in earlier by the oropharyngeal
membrane. The larger and lower regions of this arch
form much of the mandibular anatomy and the
malleus and incus middle ear ossicles, whereas the
smaller and upper regions of the first arch on either
side of the oral opening give rise to the anatomy of upper lip, teeth, maxilla, zygomatic bone, and squamous
portions of the temporal bone. The second pharyngeal arch is located beneath the first arch and is often
called the hyoid arch in that it contributes significantly to the formation of the hyoid bone and one of the
three middle ear ossicles, called the stapes. These two
pharyngeal arches, like each of the other four pharyngeal arches, are separated from each other by a surface
pharyngeal l groove which grows inwardly to meet an
endodermal-lined outpocketing from the developing
pharyngeal region, i.e., the first pharyngeal pouch. As
is the case with most pharyngeal grooves and pharyngeal pouches, the contact zone between a pharyngeal l
groove and a pharyngeal pouch is a bilaminar plate of
ectoderm and endoderm which eventually degenerates, again through the process of “programmed cell
death” and increased phagocytic activity. In the case
of the first arch, however, this bilaminar plate is separated by invading crest-derived mesenchymal cells
which have been linked with the failure of that specific plate to degenerate and persist normally throughout life as the adult eardrum, or tympanum. The elevated margins around the first pharyngeal groove
develop through the selective proliferation of mesenchymal cells beneath the skin into six separate mesenchymal swellings, called auricular hillocks. These
auricular hillocks progressively (from both the first
and second pharyngeal arches) enlarge, migrate, and
consolidate through programmed cell activity and
eventually give rise to the external ear, or auricle. Failure of the auricular hillocks to develop normally can
result in auricles of abnormal size, shape, and position
as seen in a variety of isolated and syndromic craniofacial birth defects, e.g., first and second branchial
arch syndrome, hemifacial microsomia, and microtia.
The complete absence of the auricle (anotia) is a rare
event.
To complete this picture of the pharyngeal arches,
it is important to note that cells within the arches are
supplied by pairs of blood vessels, called aortic arches, that distribute blood from the embryonic heart upward through the tissue of each arch toward the brain
and then down to the body [49]. As with the pharyngeal l arches themselves, not all of the aortic arches
persist in humans. The aortic arches of the third,
fourth, and sixth aortic arches do persist and become
greatly modified throughout the embryonic period as
they are reconstituted as the common carotid arteries
which supply the neck, face, and brain. Especially important in this dynamic development of the craniofacial vasculature is the shifting of the primary arterial
supply to the embryonic face prior to, during, and following the formation of the secondary palate. Unlike
in the adult, prior to the seventh week, the primary
source of blood to both the superficial and deep head
tissues is the internal carotid artery and its branches.
At about 7–8 weeks when the embryonic palatal
shelves are experiencing their most critical stages of
elevation and closure, an important shift occurs in the
primary blood supply to the face and palatal tissues
from the internal carotid to the external carotid arterial system. This transition involves a temporary vascular shunt between internal and external carotid systems provided by the stapedial artery. Failure of either
the stapedial artery to form or failure of a complete
and timely transition to occur has been hypothesized
in identifying the pathogenesis of such conditions as
palatal clefting and mandibulofacial dysostosis [51].
Considerably dependent upon the timely set morphogenic events that occur from the time of implantation through the fourth week, the embryonic face continues through its “developmental critical period,”
which spans the fifth through seventh intrauterine
weeks. It is that time period during which in human
craniofacial morphogenesis generally is most suscep-
Chapter 1
tible to either known or suspected birth defect-producing agents, or teratogens [66]. Arising from the
first pharyngeal arch are four primordial or “building
block” tissue masses that surround the large central
depression of the primitive oral cavity. Continued
morphogenesis of the facial prominences depends
heavily upon the continuing migration, proliferation,
and differentiation of the neural crest cells, under the
direction of developmental morphogens, to a point in
time when the facial prominences, or primordia, are
clearly identifiable as the single median frontonasal
prominence, paired maxillary prominences on either
side of the frontonasal process, and two mandibular
prominences beneath the oral opening. The shape and
size of these prominences as well as development of
the specific skeletal and muscular structures of each
pharyngeal arch are critically dependent upon the
continued viability and differentiation of the neural
crest cells which are especially sensitive to teratogens,
e.g., cortisone and retinoic acid [50, 61].
Continuing further with our focus on the developmental “blueprint” for the face, specifically the lip, it is
important to note that the outcomes of several distinct brain-skin interactions in placode formation are
also essential in early facial morphogenesis. By the beginning of the fifth week, oval patches of skin ectoderm lateral to the median frontonasal prominence
interact with brain tissue to set off an ecto-ectodermal
interaction resulting in the development of the two
thickened nasal placodes located at the ventrolateral
regions of the frontonasal prominence. Neural crestderived mesenchymal cells along the margins of the
nasal placodes proliferate rapidly to produce horseshoe shaped elevations around the placode, called the
medial and lateral nasal prominences, whose continued rapid growth gradually forms the nasal pits, or
early nostrils. The forward growth of each lateral
nasal process forms the ala of the nose, whereas the
medial nasal process contributes to the formation of
the nose tip, columella, the philtrum, tuberculum, and
frenulum of the upper lip, and the entire primary
palate. Through the process of relative growth in this
area, the nasal placodes gradually “sink” to the depth
of each nasal pit. Failure of the nose to develop completely is associated with failure of both nasal placodes to develop. A second important skin-brain interaction gives rise to localized thickenings of surface
ectoderm on each side of the embryo’s head which
will form the optic lens, retina, and nerve. Importantly, and as will be discussed later, these eye fields are
first located on the lateral aspects of the embryo’s
head and progressively migrate to the frontal midline
at about the time the facial prominences are consolidating into the complete face [8].
Selective differentiation and proliferation of mesenchymal cells cause the maxillary prominences to
Developmental Biology and Morphogenesis
enlarge and migrate medially toward each other and
the lateral and medial nasal prominences [12]. This
migration is associated not only with patterns of cellular growth within the maxillary prominences, but
also with timely migration of the eye fields from the
lateral to the frontal regions of the embryo’s face during the fifth through eighth weeks [8]. Disturbances in
normal eye field migration have been suggested as
one possible cause of median facial clefting and the
conditions of hypo- and hypertelorisms. Continued
medial migration of the maxillary prominences on
both sides also moves the medial nasal prominences
toward the midline and each other. By the end of the
sixth week, each maxillary prominence blends, or
merges, with the lateral nasal prominence along a line
which demarcates the future nasolacrimal groove and
duct. This event then establishes the continuity between the side of the nose, or alar region, formed by
the lateral nasal prominence with the cheek region
formed from the maxillary prominence. A combination of reduced cell numbers and abnormal migration
of mesenchymal cells can lead to the abnormal merging or consolidation of the maxillary and lateral nasal
prominences. Although seen infrequently, this can
lead to facial defects involving oblique facial clefts,
persistent nasolacrimal grooves, and failure of the nasolacrimal duct to develop.
Between the fourth and eight weeks, the medial
nasal prominences merge with each other, small lower portions of the lateral nasal prominences, and with
cells in the larger maxillary prominences. This subsurface merging of cells, especially between the medial nasal and maxillary prominences, results in the
continuity of upper jaw and lip. As part of this consolidation of the medial nasal and maxillary prominences in upper lip formation, two important morphologic events need to occur. First, there is a
deepening and downward growth of the nasal pit toward the oronasal cavity as a blind-ending sac whose
floor eventually degenerates through programmedcell death resulting in the formation of the primitive
choanae, which allows a continuity between the
spaces of the primitive nasal cavity and the common
oronasal cavity. An event occurring concomitantly
with nasal pit morphogenesis is the formation of the
seam between the intermaxillary segment and the
maxillary prominence. As these two segments come
together in the sixth week, the developmental surface
seam of cells between them also elongates as the nasal
pit elongates, deepens, and moves downward. This developmental seam, called the nasal fin, essentially
forms the floor of the nasal pit and progressively degenerates by increased activity among phagocytic
cells on either side of the seam [67]. Once “programmed cell” death of the nasal fin is essentially
completed at about the seventh week, mesenchymal
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A. R. Burdi
cells from both the intermaxillary and maxillary
prominences intermix, leading to fusion of the upper
lip segments into the upper lip and its cupid’s bow.
The completion of the embryonic lips generally occurs about 1 week earlier that the formation of the
palate. Thus, the lips and palate have different “developmental critical periods” and, as such teratogens
might affect either the lips or palate separately, or in
combination. The intermixture of mesenchymal cells
within the consolidated lip segments give rise to connective tissue components and muscle fibers within
the orbicularis oris ring of the upper lip. Complete or
incomplete failure of the nasal fin to degenerate have
been associated with unilateral and bilateral clefts of
the upper lip which variously involve abnormalities of
the orbicularis oris muscle in terms of the numbers
and distribution of its muscle fibers as part of the orbicularis ring.
The incidence of orofacial clefts varies in accordance with the variances reported for differing population groups [22]. Examples of such population-specific differences, called polymorphisms, include cleft
lip with or without cleft palate, which is one of the
most common craniofacial birth defects in human
with a reported incidence as high as 1:1000 in whites
and higher in Asian and lower in black populations
[13, 38, 56]. Another example of population polymorphisms shows that isolated cleft palate occurs more
often in females (67%) than in age-matched males.
This gender difference has been associated with a
longer period of time for palatal closure in females
which essentially increases the time during which female embryos might be affected by palatogenic teratogens [6]. Lateral clefts of the lip may or may not be
associated with clefts of the palate. Mesenchymal cell
deficiency that results in partial or complete failure of
the two medial nasal prominences to consolidate into
a philtrum can contribute to the formation of such defects as a bifid nose, or the rare median cleft (“hare
lip”) of the upper lip, as characteristically seen in the
autosomally recessive Mohr syndrome [23, 25, 30].
As the consolidation of facial processes progresses
through the embryonic period, crest-derived mesenchymal cells within the maxillary prominences rapidly proliferate and differentiate into tissues which
form mesenchymal cell fields from which the muscles
of facial expression develop, and whose myofibers are
innervated by the cranial nerve to the second arch,
i.e., the facial nerve. Similarly, crest-derived mesenchyme in the maxillary and mandibular portions of
the first pharyngeal arch differentiate predominately
into the muscles of mastication, which are innervated
by the trigeminal nerve of the first pharyngeal arch.
Cells within the mandibular prominence give rise to
muscle and connective tissue structures of the lower
lip, chin, and lower cheek regions. With the reshaping
and consolidation of the five major facial prominences, a recognizable human face is evident by the
end of the eighth prenatal week [11, 40, 55, 58, 59].
Morphogenesis of the mammalian palate is an even
more complex process which depends heavily upon
the balance of genetic, hormonal, and various growth
factors. As the face nears the completion of its “developmental critical period,” lateral palatine processes
which form the secondary palate grow out from the
walls of the still common oronasal cavity. The “developmental critical period” for the palate is from the
end of the sixth week through the eighth intrauterine
week, or 1 week longer in duration than that of the lip.
These palatine shelves first grow medially, then become oriented inferolaterally to lie on either side of
the tongue, which is quite precocious in its own development as a muscle-filled epithelial sac that fills much
of the oronasal cavity. Nearing 8 weeks, the vertically
oriented palatine shelves are progressively repositioned above the tongue mass. This repositioning of
the shelves is thought to involve a combination of concurrent events, including a downward contraction of
the tongue, an ameboid-like reshaping of the shelves
which gradually places them over the tongue surface,
an increase in extracellular shelf “forces” (or shelf fluid turgor) which reposition the shelves in a horizontal
position, and a downward repositioning of the lower
jaw [7, 18, 19]. In reality, normal or abnormal horizontalization of the palatine shelves is related to a combination of these three events. Palatal shelf elevation begins in the posterior regions of the shelves and
depresses the tongue downward and forward and this
allows the more anterior regions of the shelves to first
contact one another near the posterior edge of the primary palate, or in the region of the future incisive
canal [4]. Once the shelves are in a horizontal position, the shelves contact each other, and essentially
stick together by a combination of interlocking shelf
surface microvilli and a proteoglycans surface coating
along the medial epithelial edge (MEE) of each shelf
[39]. The elimination of the MEE is crucial for normal
morphogenesis of the anterior regions of the secondary palate [15, 20, 26, 32]. Once the shelves make contact, there is a degeneration (i.e., apoptosis or programmed cell death) of epithelial cells along the
abutting shelf linings, and a directed movement of
crest-derived mesenchymal cells from one shelf to the
other. This process of epithelial degeneration along
with intershelf bridging of mesenchymal cells is
called fusion.
As is the case for craniofacial morphogenesis in
general, several categories of factors (e.g., chromosomes, genes, signaling proteins, transcription factors, specific proteins) have either been identified or
hypothesized as important in normal and abnormal
palatogenesis [57, 64]. Fusion of the palatal shelves
Chapter 1
has been linked to a variety of growth factors like
those in the TGF-Beta-3 growth factor family [32, 53,
63] and protein activities [16, 31]. Complete or regional failures in programmed cell death MEE along the
lengths of the palatal shelves can lead to various forms
of palatal clefts.While great strides have been made in
identifying genetic, cellular, and molecular controls of
normal palatal development (mostly in mice and
chick embryos), the identification of the exact balance
between intrinsic and environmental controls of
palatal morphogenesis remains elusive [44, 60]. The
embryonic palatine raphe, or future midpalatine suture, marks the line of fusion between the palatine
shelves. From the site of first shelf contact and fusion
near the future incisive foramen, fusion of the more
posterior regions of the shelves takes place over the
next two weeks. Fusion also occurs between the
shelves and the inferior edge of the nasal septum, except in the more posterior regions where the soft
palate and uvula remain free. Once fusion of the
shelves of the secondary palate is complete, their mesenchymal cells differentiate into osteogenic cells
which form the skeletal elements of the premaxillary,
maxillary, and palatine portions of the palate.
Formation of the soft palate and uvula takes a
slightly different course than that of the regions of the
secondary palate which give rise to the hard palate [5].
The soft palate and uvula develop from two separate
masses found at the most posterior portions of the
secondary palatine shelves. Unlike the fusion mechanism which is in place along much of the length of the
palatine shelves, the consolidation of these two separate masses is brought about by a selective proliferation of mesenchymal cells located deep in the valley
between the masses. As that proliferation, called
merging, continues the valley between the two distal
shelf masses is obliterated, which results in a
smoothening of the contour of the soft palate and
uvula. Failure of the merging process in soft palate
and uvula development can result in complete or partial clefts of the soft palate and uvula.
Clefts of the palate, with or without clefts of the lip,
are relatively common depending on the population
group of the individual [23, 65]. Whereas occurrence
figures for nonsyndromic cleft lips (with or without
cleft palate) are about 1 in 1,000 live births, clefts of
the palate (with or without cleft lip) occur in 1 in 2,500
live births, again depending on the population group
of the individual, i.e., highest incidence in Asian, intermediate in white, and lowest in black populations
[60]. Most clefts of the lip and palate generally are related to an interplay of genetic and environmental factors, i.e., multifactorial inheritance [21]. While animal
studies have provided some insight into the molecular
and cellular bases of these defects, precise etiologic
explanations, especially involving teratogens in the
Developmental Biology and Morphogenesis
etiology of clefts of the human lip and palate, are still
wanting. Crucial faulty chromosomes (see, for example, ref. 17) and genes linked to inherited forms of cleft
lip and palate have recently been identified. This candidate gene is known as Interferon Regulatory Factor
6 [IRF6] [35]. Other candidate genes have been assigned some importance in palatal morphogenesis,
including MSX1, LHX8 6p24 genes (associated with
palatal shelf growth and differentiation); TGFA, EGFR
and HOXA2 (associated with elevation and depression of tongue);TGFB3 and PVRL1 (associated with
associated with sequential fusion stages along the
midpalatal seam), and TGFA and EGFR (associated
with actual apoptosis, or “programmed cell death”
events along the midpalatal seam).
Some clefts of the lip with or without cleft palate
are seen regularly in a number of single mutant gene
syndromes. Other clefts are associated with chromosomal syndromes, especially in trisomy 13. A complete cleft palate represents a maximum degree of
clefting and is a birth defect in which the cleft extends
from the incisive foramen region through the soft
palate and uvula. The incisive foramen region is the
demarcation used in distinguishing the two major
groups of cleft lip and palate. Anterior cleft types include cleft lip, with or without a cleft of the alveolar region of the maxilla. A complete anterior cleft extends
through the lip and alveolar region to the incisive
foramen region. The pathogenesis of anterior clefts is
related to a deficiency of neural crest-derived mesenchymal cells chiefly within the intermaxillary segment of the lip. The posterior cleft type of birth defect
generally include clefts of the secondary palate that
extend from the incisive foramen through the soft
palate and uvula. The observation that the female secondary palate has a longer “developmental critical period” [6] than the male embryo by approximately
1 week offers some explanation why isolated cleft
palate is more prevalent in females (66%) than males
(34%). In general, the pathogenesis of posterior
palatal clefts is related to abnormalities in a combination of events ranging from deficiencies in mesenchymal cell numbers to perturbations in the shelf extracellular matrices to abnormal elevation and fusion of
the shelves, or lack thereof, as associated with a number of hypothesized teratogens, including excess doses of retinoic acids, glucocorticoids, and dioxins.
1.1
Summary
The understanding of the natural history, clinical delineation, and clinical management of birth defects
involving the face, lip, and palate has progressed significantly over the last 20 years and continues to do as
we move further into the 21st century. Although hu-
9
10
A. R. Burdi
man craniofacial morphogenesis is clearly the culmination of a very complex series of diverse and overlapping developmental events, all of these events can
be categorized into four fundamental processes which
span mammalian development and are evident in the
earliest beginnings of the face and palate – normal
and abnormal: (1) cell differentiation – the process
through which the myriad of “building block” cell
types invoked in facial morphogenesis are generated
from the single-celled zygote; (2) morphogenesis – the
process or set of processes through which the complex
form of the face and its constituent cells, tissues, and
organs will emerge in a timely fashion along patternable individual and population lines; (3) growth – the
collective results of differentiation and morphogenesis; and (4) dysmorphogenesis and abnormal growth –
this is the most exciting of the challenges we face today as we strive to understand how environmental influences interact with and cause changes in the expression of the genetic factors governing the behavior
of those cells which will give rise to the entire human
body, and especially the face and palatal regions. The
treatment of defective genes is very much a part of the
current clinical agenda dealing with craniofacial defects. The basic scientist, the dysmorphologist, the clinician, and, importantly, those with natural or acquired craniofacial defects have gained significant
advantage from the critical use of available information coming from classical and experimental studies
of human morphogenesis. These advantages will continue to increase as laboratory scientists and clinician
scholars move rapidly together into the world of molecular and gene biology. These approaches should
and will increase our knowledge base on the patterns
and underlying causes of normal and abnormal craniofacial morphogenesis – and our patients will be all
the better for it. However, most researchers and treatment providers well realize that the practical transfer
of new biological information on normal and abnormal development flowing from the laboratory bench
to the clinical bedside may be neither easy nor timely
to achieve in the effective treatment and management
of craniofacial abnormalities.
Epilogue. “And the end of all of our exploring will
be to arrive where we started and know the place for
the first time.” T.S. Eliot, 1918
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