Hypophysectomy and the Tail Darkening Reaction in Xenopus. ∗

Abstract

It has long been known that if the hypophyseal placode is removed from amphibian embryos, the resulting larvae will be very pale in appearance and vary in color in different species from white or silver, to golden yellow (1,2). Pallor generally results from persistent contraction of epidermal melanophores and expansion of both dermal and epidermal guanophores (2). It is therefore surprising to observe that in hypophysioprivic Xenopus larvae, melanophores of the tail are capable of considerable expansion or contraction under appropriate conditions. The experiments reported here were designed to study the influence of the hypophysis on the behavior of some of the chromatophores of Xenopus larvae, and to gain some insight into the peculiar activity of melanophores in the tail.

Methods. All tadpoles were raised from naturally spawned eggs of Xenopus laevis Daudin. At an early tailbud stage, the hypophyseal placode was removed from 378 embryos. Judging from the number of pale larvae resulting, removal of the hypophysis was accomplished in about 85% of the operated animals. Only about 25% of these larvae survived after 2 weeks, because of severe damage to their mouth parts.

Melanophore counts were made in the ventral fin of both normal and hypophysioprivic larvae of the same egg batch. An area about one-third the distance from the anterior margin of the pigmented region of the tail and midway between fin and somites was selected for the counts. These were made using an ocular micrometer on 6 larvae from each group. Under magnification of 45X, melanophores enclosed in a square, one-sixteenth of a square cm in size, were counted for 25 squares and the average number per square was then determined for each tadpole.