Accumulation of macrophages containing brown pigments in the lungs is a well-known spontaneous lesion found in cynomolgus monkey. However, its pathogenesis has not been clearly described. In our survey, brown pigment-laden macrophages were found in the lungs of 4 out of 43 cases. Brown pigments were mostly found in the macrophages of the perivascular interstitium, which proved to be hemosiderin. Some small- to medium-sized vessels that exhibited prominent accumulation of brown pigment-laden macrophages showed degeneration and necrosis of the smooth muscle cells of tunica media. Furthermore, ruptures of the internal and external elastic laminae were seen in some of the vessels. These findings suggested that partial fragmentation of the vascular elastic lamina followed by degeneration and necrosis of the tunica media caused blood leakage leading to the accumulation of hemosiderin-laden macrophages in the perivascular interstitium of the lungs.
Keywords: brown pigmentation; cynomolgus monkey; hemosiderin; lung; vascular lesions.