Melanin Formation by Deplanted Fragments of Thalamus in Amphibian Larvae

Abstract

In the course of experiments on the functional deplantation of brain fragments into the dorsal fin of urodele amphibian larvae described in earlier communications, 1 a striking local pigmentation effect was observed, whenever thalamic grafts were used.

Donors and hosts were larvae of Amblystoma tigrinum, between 3 and 4 cm in length. The brain part which produced positive effects is shown as shaded area in Fig. 1. One-half of this portion, either with or without an adjacent slice of midbrain, was inserted into the gelatinous connective tissue of the fin. The grafts were positively free of hypophyseal and adjoining infundibular tissue.

Within a few weeks after the implantation, the vicinity of the grafts turned deep black (Fig. 2), indicating the deposition of vast amounts of newly formed melanin. The host chromatophores were not appreciably affected. Microscopically, the new pigment appears in two forms: (a) as granules inside of richly arborized cells, which give the appearance of hypertrophied chromatophores and which are found both in the interior and along the surface of the grafts; (b) as large compact clumps interspersed among the epidermis cells of the overlying skin (Fig. 4).

Whether the melanotic cells are all of metaplastic thalamic origin, or whether some surrounding host cells have likewise been induced to form pigment, has not yet been determined. However, the fact that deplants into animals which had been deprived of their hypophysis in the embryonic stage and were consequently very lightly pigmented, gave rise to local pigment clusters, points to the graft as the main source of the melanotic cells. Whether the large intercellular pigment clumps of the host epidermis arise as diffuse interstitial pigment deposits, or represent residues of degenerated chromatophores with excessive pigment content, remains to be seen.