Atrophic macules containing mesenchymal cells are precursor lesions of osteoma cutis in Albright hereditary osteodystrophy.

A newborn presented numerous small violaceous and atrophic macules randomly distributed over the body surface, with no other skin or relevant systemic signs. A skin biopsy showed mild hypercellularity with spindle-shaped cells that stained positively for alpha smooth muscle actin (SMA) and were negative for CD34 and S100. By the age of 18 months, the patient developed obesity, multi-hormone resistance and his skin lesions became indurated; a skin biopsy at this time showed osteoma cutis. The diagnosis of Albright hereditary osteodystrophy (AHO) was made and confirmed by GNAS sequencing. We characterize the skin lesions preceding osteoma cutis as atrophic violaceous macules containing a proliferation of SMA+ mesenchymal cells similar to myofibroblasts in the upper and mid dermis. We speculate that these cells may represent primitive mesenchymal osteoprogenitor cells, from which intramembranous ossification develops as direct conversion of mesenchyme into bone. This article is protected by copyright. All rights reserved.