DERMATOMYOFIBROMA
This is a benign skin tumor composed of fascicles of spindle cells having combined fibroblastic and myofibroblastic features. The spindle cells are separated by small collagen fibers, and these characteristically form closely spaced 'sine wave figures'. Similar fascicles of fibroblasts/myofibroblasts can be found in many examples of fibrous hamartoma of infancy. The fascicles interconnect, and there has been a reported propensity of these to be found parallel to the epidermal surface though I have seen examples wherein the orientation was more random. This tumor is mentioned in recent editions of most dermatopathology textbooks.

From the base of the neck of an 8 year old female.
Scan power view. The lesion is composed of fascicles of spindle cells of the type illustrated below. The lesion spares the papillary dermis and the most superficial part of the reticular dermis. It extends to the junction of the reticular dermis and subcutis. .
A medium power view showing a longitudinally oriented fascicle traversing the field plus fascicles above it that are cut in cross-section. 
A  high power view showing  small collagen fibers having a characteristic 'sine wave' pattern within the longitudinally cut fascicle.

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