A m e r i c a n Journal of Medical Genetics 35229-235 (1990)
Mirror Polydactyly: Pathogenesis Based on a Morphogen Gradient Theory Denis L. Viljoen a n d Susan H. Kidson Department of H u m a n Genetics, MRC Unit for Skeletal Dysplasias (D.V.), and Department of Anatomy and Cell Biology (SX.), University of Cape Town Medical School Obseruatory, Cape Town, South Africa
We report on a n infant with 7 toes of the left foot in a mirror configuration in association with ipsilateral duplication of the calcaneus a n d fibula, tibial aplasia, femoral hypoplasia, a n d a teratomatous sacrococcygeal tumour. The possible pathogenetic mechanisms leading to this limb abnormality are discussed with special emphasis on a field morphogen gradient hypothesis. KEY WORDS: limb malformation, sacrococcygeal teratoma, fibular duplication, calcaneal duplication, tibial aplasia,morphogen gradient hypothesis. INTRODUCTION Polydactyly is a common abnormality in man. It may occur as an isolated defect or a component of a wellestablished syndrome. The anomaly may be pre- or postaxial, unilateral or bilateral, hereditary or non-genetic. However, “mirror” polydactyly, where supernumerary digits are arranged in descending order of size from a single central digit is rare [Temtamy and McKusick, 19781. The formation of the limb bud and the control of growth and development has been investigated intensively using animals [Goetnick, 1966; Saunders, 19771. By these means, the tentative pathogenesis of various limb disruptions and anomalies has been elucidated. Several forms of polydactyly are known to be due to single gene defects [Cohen et al., 1987; Scott-Emuakpor and Madueke, 19761 or vascular disruptions at critical stages of limb formation [Van Allen et al., 1987a, 1987bl. Although the pathogenesis of many such anomalies is now clear, embryological mechanisms resulting in “mirror polydactyly” are ill-understood.
Received for publication March 7,1989; revision received July 6 , 1989. Address reprint requests to Dr D.L. Viljoen, Department of Human Genetics, University of Cape Town, Medical School, Observatory, 7925, South Africa.
0 1990 Wiley-Liss, Inc.
We describe a girl with an abnormal left leg, a sacrococcygeal tumour, and 7 digits in a “mirror” configuration on the left foot. The possible embryological mechanisms responsible for these anomalies are discussed. CLINICAL REPORT L.G., an 11-month-old black girl, was referred from a remote rural area of the Cape Province for surgical appraisal of the anomalies of her left limb and buttock. Her young parents were unrelated and she has 4 normal elder sibs. No other relatives were similarly affected. Her mother was well throughout pregnancy and there was no history of drug use, excessive alcohol intake, or exposure t o teratogenic agents. The baby was born by vaginal delivery in hospital, and was normal apart from the congenital anomalies affecting the left lower limb and pelvic area. Subsequent growth and development were normal. Clinically, the left leg is grossly abnormal. A soft cystic mass extends from the left gluteal region into the mid-thigh (Fig. 1).When the patient is supine, the left hip is dislocatable and the limb takes up a position of extreme external rotation. The right hip joint is normal. Two skin appendages and a dimple are evident on the anterior aspect of the left thigh. A contracture at the left knee with popliteal webbing prevents extension beyond 130”.The ankle, too, is fixed in extreme equinus (Fig. 2). “Mirror” polydactyly of the left foot is present. There is a central digit (21, followed by digits 3 , 4 , 5 on either side to make up the total configuration, 5432345 (Fig. 3). Radiographs of the left foot confirm the presence of 7 tarsal bones associated with the appropriate phalanges, but duplication of the calcaneus and a single tarsus is evident (Figs. 4,5). A surgical scar extended over the tendo achilles from previous unsuccessful attempts at tendon lengthening. Radiographically the left capital femoral epiphysis appears larger than that of the normal side (Fig. 6) while the left hemipelvis is underdeveloped with absence of the acetabulum. The left femur is hypoplastic in comparison with its normal counterpart, and there is ipsilateral tibial aplasia with duplication of the fibula (Fig. 7). Computerized axial tomography of the pelvis confirmed the absence of the left acetabulum and magnetic resonance imaging of the gluteal tumour was suggestive of a soft-tissue teratoma. The tumour was not
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Fig. 3. Mirror polydactyly of the left foot.
Fig. 1. Baby L.G.; polydactyly and swelling of the left thigh are evident.
Fig. 4. Radiographs demonstrating polydactyly with duplicated calcanea and fibulae. Fig. 2. The left leg is foreshortened with contracture a t the knee and the ankle is fixed in equinus.
Mirror Polydactyly
I
2 Calcanea
231
, 2 Fibulae
Fig. 5 . Line drawing of radiograph demonstrating polydactyly with duplicated calcanea and fibulae.
I
\
Fig. 6. Line drawing of radiograph showing a hypoplastic left femur with enlargement of the capital epiphysis.
Fig. 7. Line drawing of radiograph showing tibia1 aplasia and duplication of the fibula are confirmed.
232
Viljoen and Kidson
biopsied as cosmetic and restorative surgery will be undertaken at a later stage. DISCUSSION Normal Development In order to explain the cause of the mirror image duplication described in this patient, it is necessary to briefly examine normal limb pattern development. In vertebrates, limb development is first visible as a thickening of the somatic (parietal) layer of the lateral plate mesoderm (the somatopleure). The mesenchymal cells in this region loose their connections with the lateral plate and migrate t o a region immediately below the epidermis (Fig. 8). The epidermis over the mesenchyme then becomes slightly thickened and bulges outward, forming a limb bud. In higher vertebrates, a sharply defined ridge, the apical ectodermal ridge (AER),develops along the lateral aspect of the limb bud. Interactions between the AER and mesenchyme are essential for normal development, though the molecular nature of the interactive signals and the method of communication remains obscure [Saunders, 19771. Formation of the limb buds occur in 2 distinct areas, firstly, from behind the branchial region (forelimb),and a little later, in front of the anus (hind limb). In the fetus, the limb buds appear late in the fourth week of develop-
ment, whereas in the chick embryo they arise after 52-64 hours of incubation (stage 17). Once the limb bud has grown such that its length exceeds its breadth, differentiation begins and shape changes occur (Fig. 8).The distal portion becomes broad and flattened, forming the hand or foot plate. Digits slowly appear as the interdigital soft tissues undergo necrosis. Internal differentiation of the cartilage and muscle tissues is concomitant with growth and shape changes. Experimental evidence from chick embryos indicates that although overt differentiation of mesenchyme into muscle and cartilage begins once the limb bud has formed, the mesenchymal precursor cells are determined prior to their migration into limb bud stage [Rudnick, 1945;Hornbruch and Wolpert, 1986;Wilde et al., 19871. Chondrocyte precursors arise from the somatopleure, whereas myocyte precursors originate in the somites [Christ et al., 1977; Dienstman et al., 19741. The Experimental Production of Mirror Image Limbs in Chicks Saunders and Gasseling 119681 first produced mirror image digit duplications in chicks. Grafting a small piece of the posterior border mesoderm into an anterior site in the limb bud resulted in limbs with digit patterns 432234 or 43234 (Fig. 9). In order t o explain such dupli-
LB
+
4
P
a
b
Fig. 8. Normal limb development. During early development,cells from the parietal layer of the lateral plate mesoderm migrate to a region just below the epidermis (a),which begins to bulge outward (b).The bud continues to lengthen (c),while the distal portion becomes broad and flattened forming the foot plate (d).The interdigital tissue starts t o undergo necrosis (e),a process which continues as the digits continue to lengthen (f,g),ultimately resulting in fully separated toes (h).S‘l, myotome; E, endoderm, LR, limb rudiment; PLP, parietal layer of the lateral plate mesoderm; SC, spinal cord; LB, limb bud.
Mirror Polydactyly
____)
a
* 4
233
the duplication would depend on the precise position of the graft (Fig. 9). In addition to substantial indirect evidence for this theory, direct evidence arises from the recent finding that retinoic acid mimics the effect of the polarizing zone [Tickle et al., 1982, 19851 and that endogenous retinoic acid is distributed in an anterior-posterior gradient in the developing chick limb [Thaller and Eichele, 19871. This observation has led to claims that retinoic acid is the morphogen synthesized by polarizing cells.
Mirror Polydactyly in Human Limbs The mirror-image digit duplications documented in 4 3 this paper appear similar to those produced experimen2 tally in chicks (either by the grafting of an extra polariz2 3 ing zone or by applying retinoic acid to the anterior limb 3 margin). It is interesting to note that in addition to 4 similarities in digit duplications, the other limb abnormalities reported in this case were similar to those reported in the chick experiments. For example, in experimentally produced chick limbs, other changes included size and number of tarsalsicarpals, duplicated fibulae with an absent tibia, 2 normal fibulae and one tibia, or 4 frequently, a solitary fibula only [Wilde et al., 19871. 3 The femur was never duplicated, though changes of its 3 shape and size as well as that of the pelvic girdle some4 times occurred. Similarly, in our patient, the tarsal pattern was abnormal (2 calcanei) and the tibia was replaced by an extra fibula. The femur and pelvis were foreshortened and thickened with an absent acetabulum. In order to explain the naturally occurring mirror image duplication in this patient according to the morphogen gradient theory, it is necessary to postulate the 2 presence of an extra, anteriorly positioned polarizing 3 region and to explain how a duplicated polarizing region 4 might have arisen. Two possibilities exist; either some 3 3 predetermined polarizing cells migrated to the inap4 propriate position within the limb bud, or undifferentiated anteriorly positioned cells became polarizing cells after the limb bud had formed. For reasons described Fig. 9. A graft ofpolarizingcells causes mirror image duplications in chick limbs. Normal chick wing development; distance of the grafted below, we favour the former alternative, which suggests that the normal migratory pathway of predetermined polarizing zone (G) from the host polarizing zone determines the number and pattern of digits in the developing wing. (For a precise polarizing cells was disrupted resulting in their inapexplanation of digit pattern see Saunders and Gasseling [1968]; Tickle propriate anterior positioning, while other (polarizing) et al. [19751.) cells migrated to their normal posterior position. It has been established that in chicks polarizing cells are predetermined prior to the limb bud stage [Rudnick, cations, or indeed, to explain normal limb development, 1945; Hornbruch and Wolpert, 1986;Wilde et al., 19871. the theory of positional information was put forward The most obvious reason for cells moving to an inap[Wolpert, 1971; Tickle et al., 19751. According t o this propriate position in our patient would be the positionhypothesis, normal limb development depends on the ing of the sacrococcygealteratoma which may have parproduction of a diffusible morphogen from a specialized tially blocked the normal pathway of migration (Fig. group of cells located on the posterior border of the limb 10). In this way cells forced from their normal route of bud (the polarizing zone). In this way a concentration migration by the teratoma would continue to migrate gradient of the morphogen is set up within the mes- along the path of least resistance, eventually coming to enchymal mass and, by interpreting their position lie below the AER on the anterior aspect of the limb bud. within this gradient, the cells organize themselves and These cells would now be in a position to set up a second differentiate appropriately. An additional polarizing morphogenetic gradient, resulting in the limb-compozone on the anterior margin of the limb bud would alter nent duplications present in our patient (Fig. 10). An alternative origin of the duplicated polarizing zone the normal concentration gradient of the morphogen and result in extra digit formation. The exact nature of might have been the teratoma itself. It has been shown
234
Viljoen and Kidson a.
12 L3 L4 L5
s1 s2 s3
L1
L2
L3 L4
L5
s1 s2
s3
I I
pz
Fig. 10. a: In normal limbs invaginating cells from somites S1 to S3 give rise to polarizing cells (arrows), which become posteriorly positioned in the limb bud, forming a single polarizing zone (PZ). b: The development of a sacrococcygeal teratoma (ST)blocks normal pathway of predetermined polarizing zone (PZP).The morphogen gradient set up by these two polarizing zones causes mirror image duplicationsin the leg.
that the cells in a sacrococcygeal teratoma arise from persistence of the primitive streak [Moore, 19881. It is also known that primitive streak cells give rise to mesoderm and may display polarizing activity in chick embryos [Hornbruch and Wolpert, 19861.Thus, it is possible that the teratoma originated in or included a larger mass of predetermined polarizing cells, which were inappropriately positioned and synthesized excess morphogen. Since mirror image duplications depend on the precise positioning of 2 separated polarizing zones (one anterior, one posterior), it must be assumed that somehow the mass of polarizing cells became split into 2 distinct groups. This theory is supported by evidence that diverse limb abnormalities can arise in association with sacrococcygeal teratomas, although these anomalies are rare [Durkin-Stamm et al., 1978; Lubinsky, 19871. In order t o explain the limb abnormalities in our patient, we have invoked a theory of pattern formation derived from studies of chick limb development. We think that this comparison is justified for 2 reasons: a)
the known similarities in early limb development between birds and mammals, and b) the changes in limb structure in our patient are remarkably similar to those induced by experimental manipulation of chick embryo. It is important to note that a second theory of limb patterning, the polar coordinate model, has not been considered in this paper. There is currently much debate between the proponents of the morphogen gradient and polar coordinate theories [Bryant and Muneoba, 19861. Although the pathogenesis of the limb defects in our patient fits the former model, it should be considered speculative until these arguments are resolved.
ACKNOWLEDGMENTS We are grateful to Mrs. E. Fuller, who prepared the illustrations; to Sister Sue Beatty for co-ordinating the patient material; and to Mr. Colin Clow of Medical Graphics, Groote Schuur Hospital, for preparation of the photographic prints. Phillippa Webster kindly typed the manuscript. The work was supported by grants from the South
Mirror Polydactyly
African Medical Research Council, the Mauerberger Foundation, the Harry Crossley Fund, and the University of Cape Town Staff Research Fund.
REFERENCES Bryant SV, Muneoba K (1986): Views of limb development and regeneration. Trend Genet 2:153-159. Christ B, Jacob HJ, Jacob M (1977):Experimental analysisofthe origin of the wing musculature in avian embryos. Anat Embryol 150:171-186. Cohen DM, Green JG, Miller J, Gorlin FiJ,Reed JA (1987): Acrocephalypolysyndactylytype I1 - Carpenter syndrome: Clinical spectrum and a n attempt at unification with Goodman and Summitt syndromes. Am J Med Genet 28:311-324. Dienstman SR, Biehl J, Holtzer S, Holtzer H (1974): Myogenic and chrondrogeniclineages in developing limb buds grown in vitro. Dev Biol 39:83 -95. Durkin-Stamm MV, Gilber EF, Ganich DJ, Opitz JM (1978): An unusual dysplasia-malformation-cancer syndrome in two patients. Am J Med Genet 1979-289. Goetnick PF (1966): Genetic aspects of skin and limb development. In Monroy A, Moscana AA (eds): “Current Topics in Developmental Biology.” New York: Academic Press, pp 253-283. Hornbruch A, Wolpert L (1986):Positional signalling by Hensen’s node when grafted to the chick limb bud. J Embryol Exp Morphol 94:257-265. Lubinsky M (1987): The origins of supernumerary limbs: lessons from teratology. Dysmorph Clin Genet 1 2 - 2 3 , Moore KL (1988): “The Developing Human; Clinically Orientated Embryology.” Fourth Ed. Philadelphia: WB Saunders Company. Rudnick D (1945): Limb forming potencies of the chick blastoderm: Including notes on associated trunk structures. Trans Conn Acad Arts Sci 36:353-377.
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Saunders JW J r (1977): The experimental analysis of chick limb development. In Ede DA, Hinchliffe JR, and Balls M (eds): “Vertebrate Limb and Somite Morphogenesis.” Cambridge: Cambridge Univ Press, pp 1-24. Saunders JW, Gasseling MT (1968): Ectodermal-mesenchymal interactions in the origin of limb symmetry. In Fleishmajer R, Billingham RE (eds): “Epithelial-Mesenchymal Interactions.” Baltimore: Williams and Wilkins, pp 78-97. Scott-Emuakpor AB, Madueke EDN (1976):The study of genetic variation in Nigeria 11. The genetics of polydactyly. Hum Hered 26:198-202. Temtamy SA, McKusick VA (1978):The Genetics of Hand Malformations. New York: Alan R Liss, Inc., for the National FoundationMarch of Dimes. BD: OAS XIV(3):364-392. Thaller C, Eichele G (1987): Identification and spatial distribution of retinoids in the developing chick limb bud. Nature 327:625-628. Tickle C, Summerbell D, Wolpert L (1975): Positional signalling and specification of digits in chick limb morphogenesis. Nature 254:199-202. Tickle C, Alberts B, Wolpert L, Lee J (1982): Local application of retinoic acid to the limb bud mimics the action of the polarizing region. Nature 296:564-566. Tickle C, Lee J, Eichele G (1985): A quantitative analysis of the effect of all-trans-retinoic acid on the pattern of chick wing development. Dev Biol 109:82-95. Van Allen MI, Curry C, Gallagher L (1987a):Limb body wall complex: I. Pathogenesis. Am J Med Genet 28529-548. Van Allen MI, Curry C, Walden CE, Gallagher L, Patten RM (1987b): Limb body wall complex: 11. Limb and spine defects. Am J Med Genet 28:549-565. Wilde SM, Wedden SE, Tickle C (1987): Retinoids reprogramme preband mesenchyme to give changes in limb pattern. Development 100:723-733. Wolpert L (1971>:Positional information and pattern formation. Curr Top Dev Biol 6:183-224.
Mirror Polydactyly: Pathogenesis Based on a Morphogen Gradient Theory Denis L. Viljoen a n d Susan H. Kidson Department of H u m a n Genetics, MRC Unit for Skeletal Dysplasias (D.V.), and Department of Anatomy and Cell Biology (SX.), University of Cape Town Medical School Obseruatory, Cape Town, South Africa
We report on a n infant with 7 toes of the left foot in a mirror configuration in association with ipsilateral duplication of the calcaneus a n d fibula, tibial aplasia, femoral hypoplasia, a n d a teratomatous sacrococcygeal tumour. The possible pathogenetic mechanisms leading to this limb abnormality are discussed with special emphasis on a field morphogen gradient hypothesis. KEY WORDS: limb malformation, sacrococcygeal teratoma, fibular duplication, calcaneal duplication, tibial aplasia,morphogen gradient hypothesis. INTRODUCTION Polydactyly is a common abnormality in man. It may occur as an isolated defect or a component of a wellestablished syndrome. The anomaly may be pre- or postaxial, unilateral or bilateral, hereditary or non-genetic. However, “mirror” polydactyly, where supernumerary digits are arranged in descending order of size from a single central digit is rare [Temtamy and McKusick, 19781. The formation of the limb bud and the control of growth and development has been investigated intensively using animals [Goetnick, 1966; Saunders, 19771. By these means, the tentative pathogenesis of various limb disruptions and anomalies has been elucidated. Several forms of polydactyly are known to be due to single gene defects [Cohen et al., 1987; Scott-Emuakpor and Madueke, 19761 or vascular disruptions at critical stages of limb formation [Van Allen et al., 1987a, 1987bl. Although the pathogenesis of many such anomalies is now clear, embryological mechanisms resulting in “mirror polydactyly” are ill-understood.
Received for publication March 7,1989; revision received July 6 , 1989. Address reprint requests to Dr D.L. Viljoen, Department of Human Genetics, University of Cape Town, Medical School, Observatory, 7925, South Africa.
0 1990 Wiley-Liss, Inc.
We describe a girl with an abnormal left leg, a sacrococcygeal tumour, and 7 digits in a “mirror” configuration on the left foot. The possible embryological mechanisms responsible for these anomalies are discussed. CLINICAL REPORT L.G., an 11-month-old black girl, was referred from a remote rural area of the Cape Province for surgical appraisal of the anomalies of her left limb and buttock. Her young parents were unrelated and she has 4 normal elder sibs. No other relatives were similarly affected. Her mother was well throughout pregnancy and there was no history of drug use, excessive alcohol intake, or exposure t o teratogenic agents. The baby was born by vaginal delivery in hospital, and was normal apart from the congenital anomalies affecting the left lower limb and pelvic area. Subsequent growth and development were normal. Clinically, the left leg is grossly abnormal. A soft cystic mass extends from the left gluteal region into the mid-thigh (Fig. 1).When the patient is supine, the left hip is dislocatable and the limb takes up a position of extreme external rotation. The right hip joint is normal. Two skin appendages and a dimple are evident on the anterior aspect of the left thigh. A contracture at the left knee with popliteal webbing prevents extension beyond 130”.The ankle, too, is fixed in extreme equinus (Fig. 2). “Mirror” polydactyly of the left foot is present. There is a central digit (21, followed by digits 3 , 4 , 5 on either side to make up the total configuration, 5432345 (Fig. 3). Radiographs of the left foot confirm the presence of 7 tarsal bones associated with the appropriate phalanges, but duplication of the calcaneus and a single tarsus is evident (Figs. 4,5). A surgical scar extended over the tendo achilles from previous unsuccessful attempts at tendon lengthening. Radiographically the left capital femoral epiphysis appears larger than that of the normal side (Fig. 6) while the left hemipelvis is underdeveloped with absence of the acetabulum. The left femur is hypoplastic in comparison with its normal counterpart, and there is ipsilateral tibial aplasia with duplication of the fibula (Fig. 7). Computerized axial tomography of the pelvis confirmed the absence of the left acetabulum and magnetic resonance imaging of the gluteal tumour was suggestive of a soft-tissue teratoma. The tumour was not
230
Viljoen and Kidson
Fig. 3. Mirror polydactyly of the left foot.
Fig. 1. Baby L.G.; polydactyly and swelling of the left thigh are evident.
Fig. 4. Radiographs demonstrating polydactyly with duplicated calcanea and fibulae. Fig. 2. The left leg is foreshortened with contracture a t the knee and the ankle is fixed in equinus.
Mirror Polydactyly
I
2 Calcanea
231
, 2 Fibulae
Fig. 5 . Line drawing of radiograph demonstrating polydactyly with duplicated calcanea and fibulae.
I
\
Fig. 6. Line drawing of radiograph showing a hypoplastic left femur with enlargement of the capital epiphysis.
Fig. 7. Line drawing of radiograph showing tibia1 aplasia and duplication of the fibula are confirmed.
232
Viljoen and Kidson
biopsied as cosmetic and restorative surgery will be undertaken at a later stage. DISCUSSION Normal Development In order to explain the cause of the mirror image duplication described in this patient, it is necessary to briefly examine normal limb pattern development. In vertebrates, limb development is first visible as a thickening of the somatic (parietal) layer of the lateral plate mesoderm (the somatopleure). The mesenchymal cells in this region loose their connections with the lateral plate and migrate t o a region immediately below the epidermis (Fig. 8). The epidermis over the mesenchyme then becomes slightly thickened and bulges outward, forming a limb bud. In higher vertebrates, a sharply defined ridge, the apical ectodermal ridge (AER),develops along the lateral aspect of the limb bud. Interactions between the AER and mesenchyme are essential for normal development, though the molecular nature of the interactive signals and the method of communication remains obscure [Saunders, 19771. Formation of the limb buds occur in 2 distinct areas, firstly, from behind the branchial region (forelimb),and a little later, in front of the anus (hind limb). In the fetus, the limb buds appear late in the fourth week of develop-
ment, whereas in the chick embryo they arise after 52-64 hours of incubation (stage 17). Once the limb bud has grown such that its length exceeds its breadth, differentiation begins and shape changes occur (Fig. 8).The distal portion becomes broad and flattened, forming the hand or foot plate. Digits slowly appear as the interdigital soft tissues undergo necrosis. Internal differentiation of the cartilage and muscle tissues is concomitant with growth and shape changes. Experimental evidence from chick embryos indicates that although overt differentiation of mesenchyme into muscle and cartilage begins once the limb bud has formed, the mesenchymal precursor cells are determined prior to their migration into limb bud stage [Rudnick, 1945;Hornbruch and Wolpert, 1986;Wilde et al., 19871. Chondrocyte precursors arise from the somatopleure, whereas myocyte precursors originate in the somites [Christ et al., 1977; Dienstman et al., 19741. The Experimental Production of Mirror Image Limbs in Chicks Saunders and Gasseling 119681 first produced mirror image digit duplications in chicks. Grafting a small piece of the posterior border mesoderm into an anterior site in the limb bud resulted in limbs with digit patterns 432234 or 43234 (Fig. 9). In order t o explain such dupli-
LB
+
4
P
a
b
Fig. 8. Normal limb development. During early development,cells from the parietal layer of the lateral plate mesoderm migrate to a region just below the epidermis (a),which begins to bulge outward (b).The bud continues to lengthen (c),while the distal portion becomes broad and flattened forming the foot plate (d).The interdigital tissue starts t o undergo necrosis (e),a process which continues as the digits continue to lengthen (f,g),ultimately resulting in fully separated toes (h).S‘l, myotome; E, endoderm, LR, limb rudiment; PLP, parietal layer of the lateral plate mesoderm; SC, spinal cord; LB, limb bud.
Mirror Polydactyly
____)
a
* 4
233
the duplication would depend on the precise position of the graft (Fig. 9). In addition to substantial indirect evidence for this theory, direct evidence arises from the recent finding that retinoic acid mimics the effect of the polarizing zone [Tickle et al., 1982, 19851 and that endogenous retinoic acid is distributed in an anterior-posterior gradient in the developing chick limb [Thaller and Eichele, 19871. This observation has led to claims that retinoic acid is the morphogen synthesized by polarizing cells.
Mirror Polydactyly in Human Limbs The mirror-image digit duplications documented in 4 3 this paper appear similar to those produced experimen2 tally in chicks (either by the grafting of an extra polariz2 3 ing zone or by applying retinoic acid to the anterior limb 3 margin). It is interesting to note that in addition to 4 similarities in digit duplications, the other limb abnormalities reported in this case were similar to those reported in the chick experiments. For example, in experimentally produced chick limbs, other changes included size and number of tarsalsicarpals, duplicated fibulae with an absent tibia, 2 normal fibulae and one tibia, or 4 frequently, a solitary fibula only [Wilde et al., 19871. 3 The femur was never duplicated, though changes of its 3 shape and size as well as that of the pelvic girdle some4 times occurred. Similarly, in our patient, the tarsal pattern was abnormal (2 calcanei) and the tibia was replaced by an extra fibula. The femur and pelvis were foreshortened and thickened with an absent acetabulum. In order to explain the naturally occurring mirror image duplication in this patient according to the morphogen gradient theory, it is necessary to postulate the 2 presence of an extra, anteriorly positioned polarizing 3 region and to explain how a duplicated polarizing region 4 might have arisen. Two possibilities exist; either some 3 3 predetermined polarizing cells migrated to the inap4 propriate position within the limb bud, or undifferentiated anteriorly positioned cells became polarizing cells after the limb bud had formed. For reasons described Fig. 9. A graft ofpolarizingcells causes mirror image duplications in chick limbs. Normal chick wing development; distance of the grafted below, we favour the former alternative, which suggests that the normal migratory pathway of predetermined polarizing zone (G) from the host polarizing zone determines the number and pattern of digits in the developing wing. (For a precise polarizing cells was disrupted resulting in their inapexplanation of digit pattern see Saunders and Gasseling [1968]; Tickle propriate anterior positioning, while other (polarizing) et al. [19751.) cells migrated to their normal posterior position. It has been established that in chicks polarizing cells are predetermined prior to the limb bud stage [Rudnick, cations, or indeed, to explain normal limb development, 1945; Hornbruch and Wolpert, 1986;Wilde et al., 19871. the theory of positional information was put forward The most obvious reason for cells moving to an inap[Wolpert, 1971; Tickle et al., 19751. According t o this propriate position in our patient would be the positionhypothesis, normal limb development depends on the ing of the sacrococcygealteratoma which may have parproduction of a diffusible morphogen from a specialized tially blocked the normal pathway of migration (Fig. group of cells located on the posterior border of the limb 10). In this way cells forced from their normal route of bud (the polarizing zone). In this way a concentration migration by the teratoma would continue to migrate gradient of the morphogen is set up within the mes- along the path of least resistance, eventually coming to enchymal mass and, by interpreting their position lie below the AER on the anterior aspect of the limb bud. within this gradient, the cells organize themselves and These cells would now be in a position to set up a second differentiate appropriately. An additional polarizing morphogenetic gradient, resulting in the limb-compozone on the anterior margin of the limb bud would alter nent duplications present in our patient (Fig. 10). An alternative origin of the duplicated polarizing zone the normal concentration gradient of the morphogen and result in extra digit formation. The exact nature of might have been the teratoma itself. It has been shown
234
Viljoen and Kidson a.
12 L3 L4 L5
s1 s2 s3
L1
L2
L3 L4
L5
s1 s2
s3
I I
pz
Fig. 10. a: In normal limbs invaginating cells from somites S1 to S3 give rise to polarizing cells (arrows), which become posteriorly positioned in the limb bud, forming a single polarizing zone (PZ). b: The development of a sacrococcygeal teratoma (ST)blocks normal pathway of predetermined polarizing zone (PZP).The morphogen gradient set up by these two polarizing zones causes mirror image duplicationsin the leg.
that the cells in a sacrococcygeal teratoma arise from persistence of the primitive streak [Moore, 19881. It is also known that primitive streak cells give rise to mesoderm and may display polarizing activity in chick embryos [Hornbruch and Wolpert, 19861.Thus, it is possible that the teratoma originated in or included a larger mass of predetermined polarizing cells, which were inappropriately positioned and synthesized excess morphogen. Since mirror image duplications depend on the precise positioning of 2 separated polarizing zones (one anterior, one posterior), it must be assumed that somehow the mass of polarizing cells became split into 2 distinct groups. This theory is supported by evidence that diverse limb abnormalities can arise in association with sacrococcygeal teratomas, although these anomalies are rare [Durkin-Stamm et al., 1978; Lubinsky, 19871. In order t o explain the limb abnormalities in our patient, we have invoked a theory of pattern formation derived from studies of chick limb development. We think that this comparison is justified for 2 reasons: a)
the known similarities in early limb development between birds and mammals, and b) the changes in limb structure in our patient are remarkably similar to those induced by experimental manipulation of chick embryo. It is important to note that a second theory of limb patterning, the polar coordinate model, has not been considered in this paper. There is currently much debate between the proponents of the morphogen gradient and polar coordinate theories [Bryant and Muneoba, 19861. Although the pathogenesis of the limb defects in our patient fits the former model, it should be considered speculative until these arguments are resolved.
ACKNOWLEDGMENTS We are grateful to Mrs. E. Fuller, who prepared the illustrations; to Sister Sue Beatty for co-ordinating the patient material; and to Mr. Colin Clow of Medical Graphics, Groote Schuur Hospital, for preparation of the photographic prints. Phillippa Webster kindly typed the manuscript. The work was supported by grants from the South
Mirror Polydactyly
African Medical Research Council, the Mauerberger Foundation, the Harry Crossley Fund, and the University of Cape Town Staff Research Fund.
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