Stromal cells of the fibroadenoma of the human breast

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Abstract
Fourteen fibroadenomas of the human breast were examined by light and electron microscopy, and by immunohistochemistry for actin. They were classified into 3 groups according to their stromal patterns; myxoid, fibrous-cellular and sclerotic. Actin immunohistochemistry revealed that the stromal areas were strongly positive in the fibrous-cellular group and weakly positive in the myxoid and sclerotic groups. By electron microscopy the stromal cells in most cases of the myxoid and fibrous-cellular groups were fibroblasts, containing varying amounts of microfilaments, 5–7 nm in diameter (actin type filaments). However, a dense body was not usually present suggesting these stromal cells were variants of myofibroblasts. The amount of microfilaments in fibroblasts was greater in the fibrous-cellular group than in the myxoid group. This was consistent with the results of actin immunohistochemistry. In 3 cases of the fibrous-cellular group peculiar structures simulating Z-lines of striated muscles were noted in some stromal cells. Since no myosin filaments were detected, they were regarded as intermediate structures between Z-lines of striated muscles and dense bodies of smooth muscles. In the sclerotic group, stromal fibroblasts were sparse and had fewer organelles.