Evidence of membrane transformation during melanogenesis. Electron microscopic study on the retinal pigment epithelium of chick embryos.

Some characteristics of early premelanosomes (PM) suggest that primarily a continuous cisternal complex of the endoplasmic reticulum (ER) is transformed simultaneously to PM. These characteristics are: (i) the form and size, which are similar to ER cisternae; (ii) the localization in groups in the ER; )iii) the same stage of maturation within a group; (iv) the continuities between early PM, and (v) the lack of continuities between ER and PM. Comparative measurements reveal that the limiting membrane of PM, with a total thickness of 7.6 +/- 0.19 nm and a center-to-center distance of 5.2 +/- 0.06 nm, is significantly thicker than the ER membrane (6.3 +/- 0.15 nm and 4.3 +/- 0.04 nm, respectively) and the melanosome limiting membrane (6.5 +/- 0.22 nm and 4.4 +/- 0.05 nm, respectively). Therefore, during the formation of melanosomes, the limiting membrane must be transformed from a thin (ER) to a thick (PM) and again to a thin (melanosome)stage.