Electron microscopy of 11-day-old Notophthalmus viridescens retinal pigmented epithelium reveals particulate premelanosomes which are identical to the melanosomes found in the oocyte. These organelles, when found in the pigmented epithelium, are called premelanosomes because they undergo further maturation to form relatively homogeneous, spherical melanosomes. At this stage, oocyte melanosomes found in cells other than melanocytes have not undergone this subsequent maturation.

Elongated melanosomes which develop from fibrillar premelanosomes are also found in the pigmented epithelial cells.

Treatment with phenylthiourea blocks the maturation of both the fibrillar and particulate premelanosomes.

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