Some Observations on the Relation between Dyeing Properties and Fibre Structure

Are picro-dye reactions for collagens quantitative? Chemical and histochemical considerations

Previous studies of picro-dye reactions demonstrated wide variations in the binding of different dyes. Picro-Sirius Red F3BA was recommended because it colors all collagens intensely and is suitable for polarization microscopy. Recent publications on quantitative uses of this stain were surprising. To obtain further information on the chemical mechanisms of dye binding by proteins, 94 sulfonated azo dyes were tested under the conditions of the picro-Sirius Red F3BA reaction. Reaction patterns varied widely, from failure to compete successfully with picrate ions for binding sites to strong coloration of all tissue structures. Only a few dyes stained collagen, reticulum fibers and basement membranes intensely and selectively. The reactivity of dyes was determined by their molecular configuration and the nature and position of substituents. Correlation with physico-chemical data showed that dye binding is due to non-ionic interactions, i.e. van der Waals and dispersion forces and hydrophobic bonding. Coulomb forces do not impart affinity - increasing sulfonation actually decreases dye uptake - but draw dyes within reach of non-ionic sites. Bound dyes form aggregates with additional dye ions; the aggregation number can range from 2 to many powers of 10. Clearly, dye binding by proteins is not stoichiometric.

On the mechanism of Mallory's phosphotungstic acid-haematoxylin stain

The mechanism of Mallory's (1900) phosphotungstic acid-haematoxylin (PTAH) stain is not yet fully understood; staining properties vary with the age of the dye solution. In this study, a 1 year-old solution was employed as described previously. Paper chromatograms revealed blue, red and yellow components of phosphotungstic acid-haematoxylin. Modification of reactive groups as well as consecutive treatment of sections with haematoxylin and phosphotungstic acid indicated hydrogen bond formation by the blue component. Substitution of brazilin for haematoxylin proved that two free phenolic --OH groups per dye residue are essential for binding of the blue phosphotungstic acid-haematoxylin chelate. Uptake of the red fraction was determined by the polyacid moiety of the dye. Van der Waals forces presumably contributed to dye binding, but did not govern the selectivity of the red and blue components of PTAH for various tissue structures.

On the mechanism of Verhoeff's elastica stain: a convenient stain for myelin sheaths

Verhoeff (1908) recommended an iron-hematein formula containing Lugol's solution for demonstration of elastic tissue; sections are differentiated until desired staining patterns are obtained. Verhoeff's stain colored a variety of tissue structures and showed higher substantivity for myelin sheaths than for elastin. Addition of HCL or omission of Lugol's solution decreased or abolished coloration of pseudo-elastica and thus enhanced selectivity for elastin. Substitution of Fe++ for Fe+++ abolished dye binding by elastin. A review of chemical data indicated interaction of components of Lugol's solution with the dye. Hematein and Fe+++ form a variety of cationic, anionic and non-ionic chelates; the ratio of these compounds changes with time. Dye binding apparently occurs mainly via van der Waals forces and hydrogen bonds. Verhoeff's elastica stain is definitely not specific for elastin and is inferior to orcein and resorcin-fuchsin because of the required differentiation with its inherent bias to produce patterns which conform to expectations. However, Verhoeff's elastica stain is far superior to other metal-hematein technics for myelin sheaths. The combined Verhoeff-picro-Sirius Red F3BA stain can be performed in 30 min and does not require differentiation. It is therefore suggested to reclassify Verhoeff's elastica stain as a method for myelin sheaths.