The Dopa Reaction in Amphibia

Abstract

Considerable evidence has accumulated indicating that the dopa reaction is specific for distinguishing melanoblastic cells in mammals (Bloch 1 ). According to Bloch the dopa is oxidized to dopamelanin by a specific oxidase which is present only in cells capable of forming melanin normally. The theory further assumes that dopa (3,4:dioxyphenylalanin) which is an intermediate oxidation product of tyrosine 2 is the normal precursor of melanin in animals. In mammals the reaction occurs only in pigmented regions of the epidermis, never in white areas. However, in adult amphibia dopa reacts in all regions of the epidermis whether pigment is present or not. 3 The reaction is most intense in the outer layers of the epidermis in contrast to the conditions in mammals where the reaction is most intense in the basal cells (stratum germinativum).

In the present work the skin of 4 different urodeles was used with results which confirm those of Schmidt (Table I). Laidlaw's technic 4 was used with the exception that frozen sections were not made. Small pieces of skin were fixed for 2 hours in 5% formalin, washed 10 minutes in distilled water and placed in 0.1% buffered dopa solution at a temperature of 36°C. for 6-11 hours. The controls were put in the phosphate buffer solution without dopa. The pieces of skin were then imbedded and sectioned in the usual way. Apparently, the dopa solution penetrates the epidermis rapidly, since the reaction is the same at the cut edges as elsewhere.