An ultrastructural study of the melanophages in the cerebrospinal fluid of the tadpoles of Xenopus.

Melanin deposits in the brain ventricles of Xenopus tadpoles were studied with light and electron microscopy (TEM and SEM). They appeared to be aggregations of melanophages which accumulated free pigment granules excreted by ependymal cells into the cerebrospinal fluid. Whereas the meningeal melanophores contained oval melanosomes of various sizes, the melanosomes in the scavenger cells were all spherical, large (0.6-1.1 microns) and fairly uniform in size. Moreover, they were arranged in spherical groups which were never seen in the cytoplasm of the melanophores. The melanosomes within the cells were identical to the free melanosomes found in the cerebrospinal fluid and those which occurred within the ependymal cells in the young larva, suggesting a common origin from the egg cytoplasm. The number of the melanosomes in the melanophages increased with age. Fine cytoplasmic projections were involved in catching and engulfing the melanosomes. Some other features of the cytoplasm, e.g., large deposits of cell detritus, also indicated that the cells were macrophages. In the later stages, (48, 49) no projections were observed, but the cells were totally filled with melanosomes.