Vital Staining With Methyl Red.

Abstract

A generally accepted hypothesis advanced to explain the permeability of protoplasm to certain substances is that these substances penetrate because of the solubility of their associated molecules in the plasma-membrane of the living cell.

This hypothesis would account satisfactorily for the vital staining of the basic dye, neutral red, which is a chloride or iodide of a colored organic base. At the pH of the normal medium of living cells, viz., a pH greater than 7.0, this dye is presumably in its least dissociated state and it readily penetrates and accumulates within the cells. When the stained cells, e.g., marine ova, are placed in a medium having a pH of about 6.0 which is lower than that of the protoplasm 1 , 2 the color quickly washes out. Moreover, vital staining with this dye does not occur when the outside medium is more acid than the protoplasm. Apparently, therefore, the dye passes readily through the plasma-membrane from a medium of a higher to that of a lower alkalinity.

An interesting case which is explicable on the same hypothesis of permeability is the vital staining with methyl red. This dye which has not yet been recorded as a vital stain, except in the special instances which are noted below, is an amphoteric electrolyte possessing both a carboxyl and an amino group (carboxylic benzene azo dimethyl aniline).

Because of this it exists in a dissociated state both on the alkaline and acid side of its neutral pH range which lies between pH 5.0 and 5.5. In its alkaline range there is a low dissociation of the basic group while the acid group is highly dissociated. The dissociated state of the acid group would account for the fact that methyl red is not a vital stain at the pH of fluids which surround lving cells under normal conditions, their pH being well on the alkaline side of 6.0.