The microscopic findings reported fulfill Stout's criteria for "myxoma". Stout observed ten myxomata of bone and collected sixteen cases from the literature. Most of them occurred in the jaws, mainly in the mandible, and the rest in the long and flat bones, metatarsals, and phalanges. As to age distribution, Stout's table showed that the tumor may develop at any age.

The mucoid material in the tumor described is of connective-tissue origin, as the stain proved. Even a negative reaction, however, would not necessarily militate against this explanation, since the mucoid substance may be "hyaluronic acid and not mucus"⁴. Primitive mesenchyme contains a large amount of hyaluronic acid. As regards the argyrophil fibrils in the mucoid stroma, the authors agree with Sponsel, McDonald, and Ghormley that these elements should be called "silver-staining fibrils", in order to avoid the controversy over whether they are precollagenous fibers or reticulin fibers. The amount of fibrosis was not such as to warrant the diagnosis of a myxofibroma. No cartilaginous or bony elements were found. However, the small collagenous and fibrous areas indicated a slight tendency toward higher differentiation. This trend supports our opinion that occasionally a myxoma may locally show chondroid and fibrous tissue and so became a chondromyxoid fibroma. The latter has been described by Jaffe and Lichtenstein as a more mature tumor, and Lichtenstein pointed out that it "is sometimes misinterpreted as a myxoma of bone". One of the authors (Bauer) observed such a transition in three bone tumors of primitive mesenchymal origin, one in the proximal metaphyseal area of the fibula of an eleven-year-old girl, one in the rib of a forty-year-old male, and one in the rib of a female sixty-two years old. Neoplasms of primitive mesenchymal tissue may locally differentiate into cartilage and bone.

As to the nature of the myxoma of bone reported here, the healing process and the present excellent health of the boy confirm Stout's statement that the myxoma is a benign neoplasm.

Stout's opinion that there are no histopathological criteria to distinguish between myxoma and myxosarcoma was not shared by Sponsel and his associates, who stressed the greater vascularity, perivascular accumulation of cells, and variety of silver-staining fibrils in myxosarcoma of the soft tissue.