Extradermal Melanin Transfer? Lack of Macroscopic Spleen Melanization in Old C57BL/6 Mice with De-Synchronized Hair Cycle

The study investigated the possibility of melanin transfer from skin to the spleen in C57BL/6 mice, hypothesizing that melanin could be deposited in the spleen, especially since young black C57BL/6 mice often have melanized spleens. The research involved young female mice up to 17 weeks old and compared them with older mice up to 2.5 years old, whose hair cycles were no longer synchronized. The presence of melanin in the spleens was assessed macroscopically and histologically using Fontana-Masson staining, while hair cycle synchronization was evaluated through histomorphometric analysis of back skin hair follicles. The findings indicated that approximately 40% of the spleens in older mice contained black Fontana-Masson-stainable "debris" upon closer histological examination. The study concluded that splenic melanosis in C57BL/6 mice is at least partially correlated with synchronized skin melanization that occurs with the hair cycle, and that splenic melanin degrades gradually over the mouse's lifespan.