DIGNOSIS AND DISCUSSION:
Diagnosis: LIPOMYELOMENINGOCELE WITH NEPHROGENIC RESTS
The differential diagnosis of this pathology which contains tissues arising from (neuro)ectoderm and mesoderm is teratoma and lipomyelomeningocele with nephrogenic rests. We favor the latter because renal tissue is both very rare in teratomas and the only unexpected element in this lesion which otherwise contains heterotopic tissues that are recognized components of lipomas in association with neural tube defects (2). Further support for this choice is provided by the finding that nearly all sacrococcygeal teratomas in infants over 4 months old are malignant (6). However, the frequent presence of multiple tissues in midline lipomas attests to the indistinct borderline between malformations (including hamartomas) and teratomas in the lumbosacral region (2,3,13).
Benign ectopic immature renal tissue is a rare finding, and the combination of immature or dysplastic renal tissue and hamartomas or choristomas of the lumbosacral region has been documented only a few times (1,4,10). One of these four cases was associated with spina bifida, and one with myelomeningocele. None was associated with malignant behavior. However, cases of Wilms' tumor that arise in the lumbosacral region in association with spina bifida and pre-existent masses have been described (9,12,14).
The presence of nephrogenic rests in a lipomyelomeningocele might be explained if neural tube defects interfere with the migration of renal tissue. In support of this is the frequent association of neural tube defects with renal anomalies (2). Alternatively, aberrant induction of renal tissue might explain this lesion. Normal development of the nephron is induced in metanephric blastema by the ureteric bud; however, central nervous system tissue is capable of inducing nephron formation in cultured tissue taken from undifferentiated Wilms' tumor (5), and of inducing tubules in metanephric mesenchyme (8). The expression of nerve growth factor receptor in developing nephrogenic tissue is required for the formation of the kidney tubules (11).