This document discusses the etiological factors of cleft lip and palate. It begins by reviewing normal embryonic facial development. Cleft lip and palate are considered to be multifactorial, caused by both genetic and environmental factors interacting. The precise roles of each factor are difficult to determine. The document then discusses several potential pathogenic mechanisms for cleft lip and palate formation, including anatomical obstruction, interference with cell differentiation or migration, and genetic susceptibility interacting with environmental teratogens. Both genetic and environmental factors are likely involved, though more research is still needed to fully understand the etiology.

1. Editorial
Dent. Med. Probl. 2012, 49, 2, 149–156 © Copyright by Wroclaw Medical University
ISSN 1644-387X and Polish Dental Society
Muhamad Abu-Hussein
Cleft Lip and Palate – Etiological Factors
Rozszczep wargi i podniebienia – czynniki etiologiczne
Pediatric Dentistry, Athens, Greece
Abstract
Congenital cleft-lip and cleft palate have been the subject of many genetic studies, but until recently there has been
no consensus as to their modes of inheritance. In fact, claims have been made for just about every genetic mecha-
nism one can think of. Recently, however, evidence has been accumulating that favors a multifactorial basis for
these malformations. The purpose of the present paper is to present the etiology of cleft lip and cleft palate, both
the genetic and the environmental factors. It is suggested that the genetic basis for diverse kinds of common or
uncommon congenital malformations may very well be homogeneous, while, at the same, the environmental basis
is heterogeneous (Dent. Med. Probl. 2012, 49, 2, 149–156).
Key words: clef lip, cleft palate, etiology, genetic, multifactorial.
Streszczenie
Wrodzone rozszczepy wargi i rozszczepy podniebienia były przedmiotem wielu badań genetycznych, nie usta-
lono jednak modelu ich dziedziczenia. Opisywano wiele mechanizmów genetycznych biorących udział w ich
powstawaniu. Obecnie wybiera się koncepcję wieloczynnikowej etiologii tych zaburzeń. Celem pracy jest wska-
zanie etiologii rozszczepu wargi i rozszczepu podniebienia przez pryzmat czynników genetycznych i środowisko-
wych. Sugeruje się, że podstawa genetyczna różnych rodzajów zaburzeń genetycznych może być homogenna, jak
również podstawa środowiskowa tego samego zaburzenia może być zróżnicowana (Dent. Med. Probl. 2012, 49,
2, 149–156).
Słowa kluczowe: rozszczep wargi, rozszczep podniebienia, etiologia, genetyczny, wieloczynnikowy.
A short review of the normal embryonic de- oral cavity where, in association with ectodermal
velopment of the facial primordia is necessary be- cells, they form the maxillary processes. Palatal
fore reviewing the factors that may interfere with shelves from these processes are evident at embry-
this development leading to clefts of the lip and onic day 45 in humans. An intrinsic force, main-
the palate. ly produced by the accumulation and hydration of
In the developing embryo, migration of cell hyaluronic acid-1, is progressively generated with-
masses, fusion of facial processes and the differ- in the palatal shelves and reaches a threshold level
entiation of tissues are three important events that which exceeds the force of resistance factors (e.g.
lead to an adult appearance. There is a certain pat- tongue). Synthesis and hydration of hyaluronic ac-
tern of development, but cells also respond to en- id by palatal mesenchyme is stimulated by epider-
vironmental signals. Since both factors are pres- mal growth factor and transforming growth fac-
ent and interact, it is difficult to ascertain the ex- tor beta. The erectile shelf elevating force is partly
act role of each of them. directed by bundles of type I collagen which run
The facial primordia (a series of small buds down the center of the vertical shelf from its base
of tissue that forms around the primitive mouth) to its tip. Moreover, the epithelial covering and as-
are made up mainly of neural crest cells that orig- sociated basement membrane of the palatal shelf
inate from the cranial crest (rev by Ferguson, exhibit differential traction, which serve to con-
1988). Neural crest cells migrate to the primitive strain and direct the swelling osmotic force. Al-

2. 150 M. Abu-Hussein
so the palatal mesenchymal cells are themselves In human embryos, palatal shelves elevate simul-
contractile and secrete various neurotransmit- taneously on day 43 (22–24 mm CRL), and the palate
ters that effect both mesenchymal cell contractili- is closed by day 55 (33–37 mm CRL). Mesenchymal
ty and glycosaminoglycan dehydration and there- fusion is complete by day 60 (45–46 mm CRL).
fore play a role in palate morphogenesis [1].
At this precise developmental stage, the shelves
rapidly elevate to a horizontal position above the Pathogenesis of CL and CP
dorsum of the tongue. Self elevation probably oc-
curs within minutes or hours. The medial edge In studying different types of orofacial mal-
epithelia of the approximating palatal shelves fuse formation, animal specimens have been proved to
with each other, developing cell adhesion mole- be especially helpful because they permit observa-
cules and desmosomes to form a midline epithelial tion of embryological and fetal stages that lead to
seam. The epithelial seam starts to thin by expan- malformations found at birth.
sion in palatal height and epithelial cell migration The majority of congenital craniofacial mal-
onto the oral and nasal aspects of the palate [1] and formations occur during the first 5–12 weeks
then degenerates, establishing mesenchyme conti- of fetal development [2]. The embryonic period
nuity across the intact horizontal palate. Medial (from 3–9 weeks) is the most sensitive period dur-
edge epithelial cells cease DNA synthesis 24–36 hrs ing which teratogens can be particularly damag-
prior to shelf contact and this is referred to as pro- ing. This is especially true for midline morpholog-
grammed cell death (PCD). The basement mem- ic disorders such as cleft lip and palate. They are
brane on each side of the epithelial seam remains considered to be a polygenic multifactorial prob-
intact even when it has completely thinned. Epi- lem in which genetic susceptibility is influenced
thelial-mesenchymal recombination experiments by multiple and probably cumulative environmen-
have demonstrated that epithelial differentiation tal factors, interacting altogether to shift the com-
is specified by the mesenchyme and that medial plex process of morphogenesis of the primary and
edge epithelial cell death is rather a “murder” by secondary palates, toward a threshold of abnor-
the underlying mesenchyme than an intrinsic ep- mality at which clefting may occur (multifactori-
ithelial suicide (rev by Ferguson, 1988). The ways al/threshold model). Both the genetic and the en-
in which mesenchyme could signal epithelial dif- vironmental factors have not yet been established.
ferentiation is either through extracellular matrix Cell death is a normal phenomenon seen in
molecules (i.e. collagen molecules), through solu- the developing embryo (PCD). It is also a common
ble factors (i.e. growth factors), direct cell-to cell feature seen in embryos after exposure to a variety
contact, or a combinations of all the above. The ac- of teratogens that induce craniofacial malforma-
tual period of fusion of the mesenchymal shelves tions. There are three distinct types of PCD (rev
may be just a matter of minutes, but complications by Sulik, 1988). Type 1 is characterized by cellu-
in events leading up to and during fusion will re- lar condensation, fragmentation, phagocytosis
sult in a palatal clefting of varying severity. and finally lysosomal degradation. Type 2 is char-
Also, seam disruption occurs by migration of acterized primarily by the appearance of large ly-
a large number of epithelial seam cells (perhaps sosomes which initiate cellular degradation. Type
50%) into the palatal mesenchyme (rev by Fergu- 3 occurs without the involvement of lysosomes
son, 1988). These fragments very quickly become and without apparent phagocytosis.
undistinguishable from other palatal mesenchyme The sites of cell death vary depending up-
cells. The epithelia on the nasal aspect of the pal- on the teratogen (or genetic insult) and the expo-
ate differentiate into pseudo-stratified ciliated co- sure time (i.e. developmental stage of the embryo).
lumnar cells while on the oral aspect of the palate There seems to be a selective sensitivity of cells;
they differentiate into stratified squamous non-ke- tissues with high proliferative activity are more
ratinized cells. Osteogenic blastemata for the pala- likely to show cell death than tissues that prolif-
tal processes of the maxillary and palatine bones erate more slowly. Other factors may also been in-
differentiate in the mesenchyme of the hard pal- volved, i.e. the state of cellular differentiation, dif-
ate while several myogenic blastemata develop in ferential drug distribution or other specific cel-
the soft palate. lular characteristics. Both the disappearance and
During the period of shelf elevation, there expansion of areas of PCD may have a role in tera-
is almost no growth in head width but constant togenesis (rev by Sulik, 1988).
growth in head height. This establishes a conduc- Pathogenesis is probably caused by one of the
tive orofacial environment that permits the ex- following mechanisms:
panding palatal shelves to occupy a position above 1) Anatomic obstruction, i.e. the tongue ob-
the dorsum of the tongue. struction hypothesis – only when associated with

3. Cleft Lip and Palate 151
mandibular underdevelopment [4]. In cases where an embryo to the disturbance of lip formation.
the chin is compressed against the sternum, the Where the size of the facial processes is reduced
tongue may interpose in the space between the as- and they are not in tight apposition, there is an in-
cending shelves. The resultant palatal deficiency is creased probability of cleft lip. Experimental sup-
U-shaped not V-shaped by disturbance in growth port of the previous is found in the work of Trasler,
potential not tissues that may have been affected 1968 and Brown, Hetzel, Harne & Long, 1985, re-
by disturbances of ecto-mesenchyme or other phe- viewed by Poswillo, 1988, where the spontaneous
nomena at the cellular level [5]; development of cleft lip and palate in A strain mice
2) Interference with cell differentiation or mi- is attributable to the pointed facial processes that
gration, either through hormonal defect, biochem- prevent wide areas of contact. On the other hand,
ical defect, or extrinsic biochemical interference. in the C57 black strain of mouse, the larger facial
Numerous studies have substantiated the associa- processes facilitate wider contact of the processes
tion between teratogens and clefting. Such terato- and therefore clefting doesn’t develop.
gens may be individually operative in a subgroup While anatomical variation is one potential
of individuals that is genetically and biologically predisposing factor in the development of cleft lip
susceptible. Conversely, several different terato- and palate, other factors also exist. It is well es-
gens may act together on a single mechanism con- tablished that at the time of consolidation of the
trolled by only a few genes. At present our knowl- facial processes, there is a concurrent program of
edge of the teratogens that are associated with spontaneous cell death (PCD). It is involved in the
clefting is very limited. Only a few substances such removal of the epithelial debris from the devel-
as retinoic acid (used in the treatment of acne and oping nasal placode. When this PCD is more ex-
psoriasis), have been confirmed as teratogens with tensive than necessary and repair of mesenchyme
direct effect on facial morphogenesis. is disturbed, a weakness develops in the forming
Several other factors that may influence genet- lip and alveolus. The continued action of growth
ic behavior and early morphogenesis have received traction forces may further disrupt the association
attention in the investigation of the etiology of CL of the facial processes with the lip margins being
and CP (rev by Amaratunga, 1989). pulled apart. Resultant clefts of the lip may vary
A seasonal variation in the incidence of CLP from a simple groove in the muscle to a complete
has been reported by several authors while oth- cleft that includes the nasal floor [7].
ers have reported the opposite. This phenomenon In regards to submucous cleft palate and bi-
has not been satisfactorily explained. One possible fid uvula, both can be considered microforms of
reason is viral infection, which may have a season- isolated palatal clefting. Probably, the disturbanc-
al trend. However, a correlation between clefts and es in the local mesenchyme occurred at the time
viral infections has not clearly been established. of the ossification of the palatal bridge and merg-
Also, some authors report that the instance of ing of the margins of the soft palate. These phe-
CLP is higher in earlier born children (in terms nomena occur between 7–10 weeks of human de-
of birth order) while others conclude the opposite. velopment [8].
When birth rank is raised, maternal age could also An association between clefts of the lip and
be raised. Mutations of genes can occur with ad- cleft palate has been observed. Animal stud-
vanced parental age. ies suggest that “following the failure of lip clo-
Monozygous twins discordant for clefting are sure there is an overgrowth of the prolabial tissues
interesting. Examinations of the developing fe- which then divert the tongue into the nasal cavity.
tus using ultrasound have shown that there are al- The mesenchymal obstruction of the tongue can
tered rates of fetal growth, both of the whole body delay the movement of one or both palatal shelves,
and of its parts, so that at any one time, twins may so that opportunities for palatal fusion are lost”
exhibit different stages of development. There- (rev by Poswillo, 1988).
fore the variable expression of clefting could re- Lip and palate formation lasts over 15 days in
sult from the same factor acting on both twins at humans. Therefore in many syndromes, cleft lip
the same time, but at relatively different stages of and palate may be accompanied by anomalies of
their early growth. different parts of the body. Many developing sys-
With regards to lip clefting, it seems that the tems can be disturbed simultaneously by terato-
crucial stage of lip formation is the moment when genic influences which operate over a long period
medial and lateral nasal processes contact each of morphodifferentiation. But despite the fact that
other and fuse. there are over 150 recognized disorders in which
Anatomical variations (differences in the size, CL, CP or both may represent one feature, it is
shape or position of the facial processes), possibly widely believed that the majority of affected indi-
based on ethnic or other factors, may predispose viduals are otherwise structurally normal (rev by

4. 152 M. Abu-Hussein
Jones, 1988). Recent studies [10] have emphasized well as for many common disorders, e.g. congeni-
the fact that a significant portion of children with tal malformations.
clefts have the cleft as one feature of a broader pat- The normal rate of development can be
tern of malformation. It is important to recognize thought of as a continuous distribution in which,
that structural defects are not, for the most part, if it is disturbed, a serious malformation may oc-
randomly associated. The presence of other ma- cur. This has been described as the multifactori-
jor and minor malformations in association with al/threshold model and several human congenital
a cleft implies that a single etiologic factor – ge- malformations reveal family patterns that fit this
netic, chromosomal or teratogenic – produced the model. CL with or without CP and CP alone are
pattern as a whole. included in this category.
Although CL is frequently associated with CP, CL with or without CP shows both geographi-
CL with or without CP and CP alone are distinctly cal and racial variations which means that it could
different in etiology. Subsequent studies have con- be explained by the multifactorial/threshold mod-
sistently confirmed that these two conditions indeed el. In contrast CP alone shows little variation in
differ in etiology and also in incidence, sex predis- different racial groups. This may mean that isolat-
position and their relationship to associated birth ed CP will not fit the purely multifactorial model.
defects. CL results from the non-fusion of the up- To date, there have been three pedigrees re-
per lip and the anterior part of the maxilla at the ported in which CP is clearly inherited as a single-
5–7 week of gestation and is observed in approxi- gene X-linked disorder [13–15].
mately 1/1000 births [11]. Isolated CP results from One of these pedigrees is described to a large
a failure of the mesenchymal masses of the palatine Icelandic family (293 individuals) that shows Men-
processes to fuse during weeks 7–12 and has an av- delian inheritance of X-linked secondary cleft palate
erage incidence of 0.7/1000 births. The incidence of and ankyloglossia [15]. Family analysis has shown
CL with or without CP varies from 2.1/1000 in Ja- that the frequency of CP among all the members of
pan to 0.4/1000 in Nigeria (rev by Moore, 1988), with this family was much higher among the male than
the geographical variation being less important than among the female CP probands. There was no in-
ethnic differences. In contrast, the incidence of CP cidence of male to male transmission in this large
alone shows little variation in different racial groups. family. The X-linked mode of inheritance of CP is
This may mean that CP alone will not fit the purely indicated by the family distribution. Also the large
multifactorial model which includes genetic and en- number of members of this family together with
vironmental factors that would increase the varia- the availability of many X-chromosome probes has
tion in incidence both geographically and to some made it possible to localize the defect subchromo-
extent racially. Generally CL with or without CP is somally (using RFLP-restriction fragment length
more frequent in males, whereas CP alone is more polymorphism studies for linkage) to the q13-q21.1
frequent in females. Therefore, due to both genetic region of the X chromosome [16]. Finer mapping
and environmental evidence, it seems that CL with and the use of cell lines from patients with dele-
or without CP and CP alone are separate entities. tions of the X chromosome have further localized
the defect to Xq21.31-q21.33 [17].
In the case of CL with or without CP, Melnick
Genetic Factors et al, 1980 [18] reviewed worldwide CL/P recur-
rence risk data and found that both a multifactori-
Polygenic inheritance refers to conditions de- al-threshold model and a monogenic with random
termined by a large number of genes. Each of them environment component model fitted poorly.
has a small effect, but they act additively (i.e. hair Farrall and Holder, 1992 [19] refer to the work
color) [12]. of several investigators. According to their report:
Multifactorial inheritance refers to conditions Marazita, 1984, in his analysis of a subset of Dan-
determined by a combination of factors each with ish CL/P families, found no support for a MF/T
a minor but additive effect (i.e. blood pressure) [11] model but suggested the possibility of a major gene.
and has been developed to describe the observed Also Marazita et al, 1986, have reported an anal-
non-Mendelian recurrences of common birth de- ysis of ten English multigenerational CL/P fami-
fects. It includes both polygenic origin and un- lies collected by Carter, 1982. They were able to re-
defined environmental factors that will increase ject an MF/T model and demonstrated that a ma-
the variation in incidence depending on race and jor locus acting on a multifactorial background
geographic region. The multifactorial inheritance (mixed-model) gave a reasonable fit. Chung, 1986,
pattern is more difficult to analyze than any oth- analyzed a series of Danish and Japanese CL/P
er type of inheritance but it is thought to account families and concluded that the best fitting model
for much of the normal variation in families, as predicted a recessive major gene acting on a multi-

5. Cleft Lip and Palate 153
factorial background (mixed-model). Chung et al, Vitamin A
1989, analyzed Hawaiian families from several ra-
cial groups and found that the data was consistent By introducing human teratogenic agents (e.g.
with a major-gene/multifactorial model (mixed excess vitamin A) into the maternal diet of A strain
model). Ardinger, 1988, has provided additional mice, it has been observed that 100% of offspring
evidence for an association between the locus for are born with the expected deformity [7]. Renewed
transforming growth factor alpha (TGFA) and the interest in retinoic teratogenicity has followed the
CL/P locus. TGFA is believed to be the embryon- introduction of 13-cis-retinoic acid as an effective
ic form of epidermal growth factor, which is be- treatment for severe cystic acne. Inadvertent use
lieved to regulate the proliferation and differenti- of 13-cis-retinoic acid during the first trimester of
ation of palatal epithelial cells both in vitro and human pregnancy has been reported to result in
in vivo. Hecht et al, 1991, analyzed mid-western a spectrum of malformations termed retinoic ac-
U.S. Caucasian families and showed consisten- id embryopathy (RAE) [20] and includes microtia
cy with a major-locus model. He found that the or anotia, micrognathia and in some cases CP. The
dominant or codommant models with decreased induction of CP following administration of excess
penetrance fitted the best. Both the MF/T mod- vitamin A to pregnant laboratory animals is well
el and the mixed model with a dominant major documented [21]. Most of the early animal stud-
gene effect were found to provide an explanation ies involved exposure to forms of vitamin A that
for the familiar clustering pattern. Marazita et al, are stored in the maternal liver and that, therefore,
1992, analyzed almost 2000 Shanghai families and have a relatively long half-life; they also involved
found that the best fitting model was that of an au- multiple administrations of the drug or examined
tosomal recessive major locus. the developmental end-point only, thereby exclud-
Farrall and Holder [19], in their own analysis, ing study of the developmental changes that lead
have shown that “the extensive published recur- to CP.
rence risk data, which have been interpreted to be A study by Kochhar and Johnson, 1965, re-
consistent with an MF/T pattern of inheritance, viewed by Sulik, 1989, describes palatal clefts for
are equally compatible with an oligogenic model which the shelves were very small or entirely ab-
with perhaps as few as four genes.” sent; these resulted from insufficient maxillary
In conclusion, the extensive recurrence risk prominence mesenchyme. These investigators al-
data, which has been widely interpreted as pro- so found that size reduction of the palatal shelves
viding evidence of a polygenic multifactorial trait, occurred only posteriorly in most cases.
are now thought to be consistent with a mod- The use of all-trans-retinoic acid, which is of
el with a major-gene effect contributing to about short half-life, has shown that incidence of cleft
1/3 of CL/P and acting on a multifactorial back- palate peaks at more than one developmental stage
ground. For CL/P, the observed decline in risk in both hamsters and mice (Kochhar, 1973, rev by
with decreasing relatedness to the proband is in- Sulik, 1989).
compatible with any generalized single-major-lo- The changing incidence and severity of sec-
cus (gSML) model of inheritance and is suggestive ondary palatal malformations that may be in-
of multilocus inheritance. duced within a narrow window of time (over
a 16 hour period) appear to be related to a cor-
responding change in the pattern of PCD in the
Teratogenes first visceral arch. It has been shown that 13-cis-
-retinoic acid increases the amount of cell death in
Palatal shelf elevation and fusion depends on regions of PCD in C57B1/6J mice, a strain which
fetal neuromuscular activity, growth of the crani- is particularly prone to spontaneous craniofacial
al base and mandible, production of extracellu- malformations [3, 22]. The distribution of exces-
lar matrix and contractile elements in the palatal sive cell death in regions of PCD provides a basis
shelves, shelf adhesion, PCD of the midline epithe- for understanding the composition of syndromes
lial seam and fusion of the ectomesenchyme be- in which malformation appears to be unrelated by
tween one shelf and another. All these phenomena tissue type or location [22].
must act in perfect harmony over a short period What has been described as vitamin A cell ne-
of time in order to produce normal palatogene- crosis is consistent with type-2 cell death (rev by
sis. Factors that interfere with any of these events Sulik, 1988). On the other hand, it has been not-
could lead to a cleft. ed that not all lysosomal membranes of all cells
lyse. Only those membranes that are at a particu-
lar stage of differentiation or which have been per-
turbed in some other way lyse. Membrane destabi-

6. 154 M. Abu-Hussein
lization by the retinoids may interfere with many ethanol when the embryos have approximate-
cellular functions. For example, blebbing of the ly 7–10 somites results in a pattern of malforma-
neural crest cell membrane was noticed follow- tion that is consistent with the DiGeorge sequence
ing retinoic exposure. This may interfere with the (midline clefts in the nose and cleft palate are fea-
migratory ability of these cells. Recovery follows tures of this sequence. The DiGeorge sequence has
removal of the retinoic acid in vitro. In vivo, re- been described in the offspring of alcoholic moth-
covery from a brief interference with cell migra- ers.
tion might also be expected but sufficient recovery Among the cellular effects of ethanol are in-
probably does not follow the excessive cell death of creased peroxidase activity, interference with cy-
progenitor cells. toskeletal components, diminished DNA synthe-
Treatment of female C57B1/6J with 13-cis-ret- sis and suppressed rates of cell division, direct ef-
inoic acid at the early stage of pregnancy (8d14hr fects on membranes resulting in excessive fluidity
to 9d0hr) has a more severe effect on the second- (reviewed by Sulik, 1988). Recent investigations il-
ary palate [22]. 12 hours after the 8d14hr treatment lustrate excessive cell death within 12hr follow-
time, embryos have 13 to 20 somites. Extensive ex- ing maternal treatment. The rates of cell death are
pansion of cell death at this time would be expect- similar to the normal rates of cell death seen in
ed to have a major effect on almost the entire sec- PCD, but the areas of cell death are expanded. The
ondary palatal shelf complex, thereby resulting reason for this excessive cell death is not yet clear.
in severe hypoplasia and clefting. Minor effects One possible explanation is that exposure to etha-
would be expected to involve only the posterior nol results in lipid peroxidase/formation that leads
portion of the maxillary prominences, thereby re- to rupture of lysosomal membranes and release of
sulting in deficiency in the posterior aspect of the hydrolytic enzymes (type-2 cell death).
secondary palate.
12 hours after the 9d6hr treatment time (late
treatment), embryos have approximately 30 to
Hyperthermia
34 somites. Expansion of cell death in embryos Hyperthermia has teratogenic effects and the
at this stage of development results primarily in facial malformations induced include, among oth-
foreshortening of the secondary palate, which oc- ers, cleft lip and/or cleft palate. CNS is particular-
curs at the expense of its posterior portion. Ma- ly sensitive to hyperthermia. Facial abnormalities
jor effects on the entire palatal shelves would not have been associated with human maternal hyper-
be expected at this treatment time. Later treatment thermia at 4–7 weeks (rev by Sulik, 1988). The type
times are mostly associated with induction of limb and extent of damage depend on the duration of
malformations [23]. temperature elevation and the extent of elevation.
Also, low sustained temperature elevations appear
to be as damaging as repeated spikes of higher ele-
Phenytoin vation. Elevations of 1.5–2.5 degrees Celsius above
Also, “under the influence of teratogenic dos- normal body temperatures represent the threshold
es of phenytoin, the lateral nasal process fails to for teratogenesis in humans. Such elevations can
expand to the size necessary for tight tissue con- result from excessive exercise, the use of hot baths
tact with the medial nasal process” (rev by Poswil- and saunas and febrile episodes.
lo, 1988). Abnormal differentiation of the cellular Again in the case of heat-induced teratogen-
processes of the ectomesenchymal cells is probably esis, cell death is considered to play a major role,
associated with this condition, where the lip and with the mitotic cells being the most susceptible.
primary palate form. The pathogenesis of heat-induced malformations
in areas other than the CNS have not been studied
yet. It has been suggested though that hyperther-
Ethanol mia could result in intra and extracellular leakage
Ethanol (alcohol) is an important human te- of lysosomal enzymes which could lead to type-2
ratogen. IT is estimated to severely affect 1.1/1000 cell death.
live births and have lesser effects in 3–4/1000 chil-
dren born. Its abuse during pregnancy results in fe-
tal alcohol syndrome (FAS) which involves a wide
Ionizing Radiation
variety of malformations in many organs. Abnor- Ionizing radiation acts as a direct insult to the
malities that are not diagnostic of FAS, but are as- embryo. The malformations induced are similar to
sociated with maternal ethanol abuse are termed those noted following exposure to ethanol, retino-
fetal alcohol effects (FAE) (rev by Sulik, 1988). ic acid or hyperthermia. The cellular mechanisms
Treatment of C57BL/6J female pregnant mice with of radiation-induced teratogenesis are not com-

7. Cleft Lip and Palate 155
pleted understood. They vary from sublethal in- The fiber diameters were also found to be much
juries affecting differentiation and cellular inter- smaller. NADH stain and electron microscopy re-
actions, to effects on rates of proliferation and cell vealed a large accumulation of mitochondria at the
death (rev by Sulik, 1988). Response of the cells to central portion of the fiber, giving a star shaped
the radiation is dependant on cell cycle. Also, in appearance to the fiber. The mitochondria are
some instances cell death is linked to chromosom- more variable in shape than normal, and the cris-
al damage. Some studies have shown that irradia- tae are more densely packed than expected.
tion results in altered permeability of intracellular These abnormalities in mitochondrial size, lo-
structures and enzyme release, i.e. rupture of ly- cation, cristae and number suggest a form of met-
sosomal membranes, and suggest that this results abolic defect that could underlie cleft lip deformi-
from lipid peroxide formation. ties. The suggested explanation is that a defect in
energy production could result in insufficient cel-
lular migration and proliferation and thus be the
Hypoxia pathophysiological basis for cleft lip. As mentioned,
Of particular interest is hypoxia-induced cleft in addition to the mitochondria, the cleft lip mus-
lip. Hypoxia in the human embryo may result cles were found to be abnormal. However, no signs
from cigarette smoking, reduced atmospheric ox- of group denervation or reinnervation were found
ygen levels and also placental insufficiency. Pre- and the motor end plate structure appeared nor-
vious studies had shown size reduction and ab- mal. These findings argue against denervation or
normal apposition of the facial prominences as abnormal innervation as a cause of the abnormal-
possible pathogenetic mechanisms. The presence ities. Since the innervation was normal, the mus-
of cellular debris resulting from cell death in the cle atrophy was attributed to an inability of these
deepest aspects of the invaginating nasal placodes, muscles to function properly, which was further-
as well as overall growth retardation of the facial more attributed to the mitochondrial energy pro-
prominences, leads to inability of the facial prom- duction abnormality or to the lack of normal fiber
inences to contact and fuse (rev by Sulik, 1988). orientation. If the causative factor is the fiber ori-
There are also direct effects suggested of oxygen entation, one would expect this to improve follow-
deficiency on the cells which lead to glycolysis fol- ing adequate surgical reconstruction of the muscle
lowed by acidification of intercapillary spaces and at the time of lip repair. On the other hand, chang-
subsequent necrosis resulting from intra and ex- es secondary to cellular energy problems would
tracellular leakage of lysosomal enzymes. It is in- not be expected to improve following surgery.
teresting to note that chemicals that interfere with In general, a common target for some terato-
oxidative enzymes such as phenytoin induce cleft genes (i.e. cells in regions of PCD that represent
lip in mice. a developmental weak point) provides reason to
expect interactive effects. Repeated exposure of
teratogenes in subthreshold doses of more than
Antimetabolites one agent could result in potentiation. Potentia-
Methotrexate and aminopterin are two un- tion indeed occurs after repeated exposures to vi-
common antimetabolites that can induce crani- tamin A and hyperthermia.
al dysplasia and cleft palate in humans. Their ac-
tion is inhibitory of DNA synthesis through com-
petitive folic acid antagonism. The pathogenesis Conclusions
of methotexate involves fluid imbalance, result-
ing perhaps from interference with osmoregulato- At present, our knowledge of the teratogenes
ry cells in extraembryonic capillary beds, which is that are associated with clefts is not very exten-
partially responsible for the malformation. sive. Some of these substances (such as retinoic ac-
id) have been confirmed to have direct effects on
facial morphogenesis but many more await iden-
Metabolic Disorders tification.
Metabolic disorders, inherited or not, may
An interesting finding associated with lip play a role in the pathogenesis of clefting.
clefting was that of mitochondrial myopathy of Our knowledge of cell biology is increasing
cleft muscles [24]. Facial muscle specimens from rapidly and may eventually lead to the under-
the cleft site were characterized by disorganized fi- standing and possibly prevention of clefts of the
bers, going in many different directions. The num- lip and palate. This can particularly apply in cas-
ber of fibers appeared to be decreased and there is es with monogenic etiology and in chromosom-
more connective tissue between the muscle fibers. al disorders.

8. 156 M. Abu-Hussein
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Address for correspondence:
Muhamad Abu-Hussein
Pediatric Dentistry
123 Argus Street
10441 Athens
Greece
E-mail: muham001@otenet.gr
Received: 23.03.2012
Accepted: 16.04.2012
Praca wpłynęła do Redakcji: 23.03.2012 r.
Zaakceptowano do druku: 16.04.2012 r.