Development of amelanotic retinal pigment epithelium in eyes with a tapetum lucidum: Melanosome autophagy and termination of melanogenesis☆☆☆

摘要

Bovineeyesofembryosandfetuseswereexaminedtodeterminethedevelopmentalprocessesinvolvedinestablishmentoftheamelanoticretinalpigmentepithelium(RPE)whichoverliesthetapetumlucidum.Melanogenesiswasdetectableattheopticvesiclestage(Day28);premelanosomeswerevisiblebyelectronmicroscopyinneuroepitheliumtemporaltothelensplacode.Pigmentationoftheeyewasvisiblebylightmicroscopyattheopticcupstage(aboutDay30)andspreadfromthelipoftheopticcupthroughouttheentirefundusbythe40thday.Thereafter,pigmentationofthesuperiortemporalfundusdiminishedandbythe65thdaytheadultpatternofamelanoticandmelanoticRPEwasestablished.Calculationsshowedthatafterthe40thday,growthoftheeyeballbroughtabouta16-folddilutionofthosemelanosomeswhichhadbeensynthesizedbyRPEcellsofthepresumptiveamelanoticzoneduringtheinitialwaveofpigmentation.Enzymecytochemicalstudiesshowedthattheremainingmelanosomesbecamesequesteredinautophagicvacuoles.Also,individualpremelanosomesoftheseRPEcellsbecamepositiveforacidphosphataseandarylsulfatase.Thecontentsoftheseautophagosomeswerelaterconsolidatedintoasinglemacromelanosomewhichwaspresentinadulteyesandwasgenerallypositiveforacidhydrolases.Incontrast,melanosomesofmelanoticareasofRPEwerenegativeforacidhydrolases.Thus,theRPEoverlyingthetapetumlucidumbecomesamelanoticbyatleastthreeprocesses:(1)prematureterminationofmelanogenesis,(2)dilutionofpreexistingmelanosomes,and(3)autophagicdigestion(melanolysis)andcentralizationoftheresiduaofpreexistingpremelanosomesandmelanosomesintoamacromelanosome.