'Membranous aplasia cutis' with hair collars. Congenital absence of skin or neuroectodermal defect?

The skin and the nervous system are both derived from ectoderm. Separation of neural ectoderm from epithelial ectoderm occurs concurrently with the closure of the neural tube. This chronologic association may explain the cutaneous abnormalities often found overlying neural tube defects. A ring of dark long hair encircling a congenital scalp lesion (the hair collar sign) is one such marker and is often associated with encephaloceles, meningoceles, and heterotopic brain tissue.

We describe six children with aplasia cutis who displayed the hair collar sign. Aplasia cutis is a relatively heterogeneous disorder; however, these lesions had a unique and strikingly similar appearance. This subtype of aplasia cutis, which we have termed membranous aplasia cutis, shares several clinical and histologic features with cranial neural tube defects.

We propose that membranous aplasia cutis is a form fruste of a neural tube defect and may be derived from a similar embryological defect. Recent advances in the understanding of cranial neural tube closure may provide support for this hypothesis.