Rudimentary meningocele: a variant of "primary cutaneous meningioma".

We studied 5 primary cutaneous meningiomas. All were congenital. Four were nodules or plaques on the scalp, and one was a lumbar polyp. Two were alopecic. A skull defect was present deep to one lesion, and the lumbar polyp was attached to dura. The tumors were concentrated in the subcutis, where strands of meningocytes were embedded in dense collageous tissue. Meningocytes wrapped around collagenous fibers, producing "collagen bodies". These formed the nidus for calcification that included psammoma bodies. Meningocytes also dissected between collagenous fibers, creating anastomosing spaces that mimicked a vascular tumor. Meningothelial-lined clefts, several milimeters in length, were present in 4 cases. Two lesions extended through dermal defects into the superficial dermis, where adnexa were reduced or absent. The meningocytes contained vimentin and epithelial membrane antigen. They lacked cytokeratin, S100 protein, and endothelial markers. The meningothelial lesions described herein lack the nodular and sheet-like growth patterns that typify meningiomas of the central nervous system and most primary ectopic meningiomas, including some that develop within the skin. They appear closely related to meningoceles and should be viewed as developmental abnormalities rather than neoplasms. The term "rudimentary meningocele" seems appropriate for these lesions.