Susan N. Meloan

On the chemistry of formaldehyde fixation and its effects on immunohistochemical reactions

SummaryFormalin has been recommended as an innocuous fixative for immunohistochemistry. However, several studies demonstrated impairment or blocking of antigenic activity of certain proteins. Formalin fixation was discovered accidentally by F. Blum in 1893 and its deleterious effects on various tissue structures were discussed extensively during the following decades. More recently, some authors assumed that formaldehyde bound to tissues can be largely or completely removed by washing and dehydration. According to chemical data, formaldehyde forms highly reactive methylols with uncharged amino groups. Such methylol groups yield methylene bridges with suitably spaced amides, arginine and aromatic amino acid sidechains. Only loosely bound formaldehyde is removed by washing for several hours. Residual bound formaldehyde cannot be dislodged by washing for weeks, but some formaldehyde is gradually removed when tissues are stored in water for an extended number of years. Methylene crosslinks resist treatment with high concentrations of urea, and can be broken only by drastic hydrolysis. It appears unlikely that such firmly bound formaldehyde is removed by conventional washing and dehydration procedures used in histochemistry. The superiority of methacarn, alcohol or acetone over formaldehyde fixation for immunohistochemical demonstration of prekeratin, myosin, type I and type IV collagen, laminin and fibronectin can be ascribed to the irreversible alterations of tissue proteins by formaldehyde.

Methacarn (methanol-Carnoy) fixation

SummaryAccording to chemical data, methanol raises the shrinkage temperature of collagen significantly more than ethanol (86° C versus 70° C). Since increase of shrinkage temperature appears desirable in tissues to be embedded in paraffin, methanol was substituted for ethanol in Carnoys fluid. This methanol-Carnoy mixture is referred to as methacarn solution. The fixation-embedding procedure was similar to that described in the study of Carnoy fixation. Methacarn-fixed sections showed little or no shrinkage and compared well with material fixed in Carnoys or Zenkers fluid. Myofibrils, especially in endothelial and epithelial cells, were more prominent in methacarn- than in Carnoy-fixed tissues.A review of the chemical literature showed that methanol, ethanol and chloroform stabilize or even enhance helical conformations of proteins, presumably by strengthening of hydrogen bonds. Interference with hydrophobic bonds causes unfolding and/or structural rearrangements in globular proteins. The twin-helical structure of DNA collapses in alcoholic solutions. Hence, methacarn fixation can be expected to preserve the helical proteins in myofibrils and collagen, but the conformations of globular proteins and DNA will be significantly altered. Literature on conformational effects produced by fixatives used in electron microscopy was also reviewed. Glutaraldehyde and OsO4 cause considerable loss of helix (22–29% and 39–66% respectively). KMnO4 and glutaraldehyde followed by OsO4 produce extensive transitions from helical to random-coil conformations similar to those seen in powerful denaturants such as 8 M urea. Evidently these fixatives are unsuitable for studies of helical proteins. In contrast ethylene glycol preserves helical conformations.

Application of current chemical concepts to metal-hematein and-brazilein stains

SummaryCurrent chemical concepts were applied to Weigerts, M. Heidenhains and Verhoeffs iron hemateins, Mayers acid hemalum stain and the corresponding brazilein compounds. Fe+++ bonds tightly to oxygen in preference to nitrogen and is unlikely to react with lysyl and arginyl groups of proteins. Binding of unoxidized hematoxylin by various substrates has long been known to professional dyers and was ascribed to hydrogen bonding. Chemical data on the uptake of phenols support this theory. Molecular models indicate a nonplanar configuration of hematoxylin and brazilin. The traditional quinonoid formula of hematein and brazilein was revised. During chelate formation each of the two groups of the dye shares an electron pair with the metal and contributes a negative charge to the chelate. Consequently, the blue or black 2:1 (dye:metal) complexes are anionic. Olation of such chelates affects the staining properties of iron hematein solutions. The color changes upon oxidation of hematoxylin, reaction of hematein with metals, and during exposure of chelates to acids can be explained by molecular orbital theory.Without differentiation or acid in dye chelate solutions, staining patterns are a function of the metal. Reactions of acidified solutions are determined by the affinities of the dye ligands. Brazilein is much more acid-sensitive than hematein. This difference can be ascribed to the lack of a second free phenolic −OH group in brazilein, i.e. one hydrogen bond is insufficient to anchor the dye to tissues. Since hematein and brazilein are identical in all other respects, their differences in affinity cannot be explained by van der Waals, electrostatic, hydrophobic or other forces.

Light-microscopic demonstration of myoid fibrils in renal epithelial, mesangial and interstitial cells

In kidney sections treated with the tannic acid‐phosphomolybdic acid‐Levanol fast cyanine 5RN procedure, myoid fibrils were observed in tubular and glomerular cells. The myoid fibrils in tubular epithelial cells were delicate in normal kidneys, but became conspicuous under pathological conditions. Under certain conditions myoid fibrils were found in capsular epithelium and mesangial cells of glomeruli; in some cases podocytes also contained prominent myoid fibrils. There seemed to be a continuous system of myoid fibrils from the media of the afferent arteriole into the mesangium and through the capsular into the tubular epithelium. These myoid fibrils could not be identified with certainty in sections treated with conventional staining methods.

Application of thiazole dyes to amyloid under conditions of direct cotton dyeing: correlation of histochemical and chemical data

SummaryThe fluorescent brightening agent Phorwhite (Blankophor) BBU imparts intense selective fluorescence to amyloid, but this modern reagent is no longer readily available on the biological dye market. Conventional Thioflavine S and T stains require differentiation and are not specific. To improve selectivity, direct and cationic thiazole dyes were substituted in the alkaline Congo Red and the Phorwhite BBU procedure. With the former technic Diphenyl Brilliant Yellow 8G, Clayton Yellow, Thiazol Yellow, Thioflavine T and Seto Flavine T imparted strong to intense selective fluorescence to amyloid. Under the conditions of the Phorwhite BBU reaction these dyes were suitable only for formalin-fixed amyloid. Several thiazole dyes did not fluoresce. Fluorescence is a function of the molecular orbital system, the thiazole rings per se cannot induce fluorescence. Paper chromatograms indicated two or more fractions in the dyes studied. Different samples of the same dye can vary significantly in their staining and fluorescence properties. This heterogeneity is inherent in the mode of synthesis. In some cases the cationic thiazole dyes rendered certain amyloid deposits, e.g. in vessel walls, intensely fluorescent; other amyloid deposits in the same sections showed only weak fluorescence. Further studies are required to correlate these peculiar patterns with immunological data on amyloid types.

Light microscopic distinction between elastin, pseudo-elastica (type III collagen?) and interstitial collagen

SummaryDistinction between elastin and collagen in arteriosclerotic lesions is difficult because the so-called elastica stains are bound also by collagen fibers which resemble collagen of premature infants. Investigations of effects of organic solvents on dye binding led to the development of methods for selective demonstration of pseudo-elastica, and for simultaneous visualization of elastin and pseudo-elastica in contrasting colors.Paraffin sections of human autopsy material were stained with solutions of resorcin-fuchsin, orcein or aldehyde fuchsin in absolute ethanol. In other series, sections pretreated with this resorcin-fuchsin solution were counter-stained with tannic acid-phosphomolybdic acid (TP)-dye technics.Solutions of these “elastica stains” in absolute ethanol colored only pseudo-elastica; elastin, e.g. elastic membranes of aorta, remained unstained. In sections counterstained with TP-dye technics elastin was colored red; pseudo-elastica retained the purplish blue coloration imparted by resorcinfuchsin. Other collagens were stained yellow.A review of the literature showed that until the 1920s elastin was classified as a gelatinoid of the collagen group. Elastic fibers were identified by mechanical properties, not a particular chemical composition. Hence, the elastic fibers of classical histology cannot be equated with the elastin of modern chemistry. Correlation of histochemical observations with chemical data indicates that the collagenous pseudo-elastica corresponds to [α1(III)]3 collagen.

A Modified One-Step Trichrome Stain for Demonstration of Fine Connective Tissue Fibers

Gomoris one-step trichrome procedure was modified to improve coloration of fine connective tissue fibers. Paraffin sections from tissues fixed in alcohol, acetone, Zenkerformol, 10% formalin, Kaiserlings or Carnoys fluid were mordanted 1 hr at 56 C in Bouins solution, stained 1 min in a trichrome solution (chromotrope 2R-phosphomolybdic acidaniline blue WS) adjusted to pH 1.3 with HCl, rinsed in 1% aqueous acetic acid, dehydrated and covered. Collagen, reticulum fibers, basement membranes, ring fibers around splenic sinuses, intercalated discs in cardiac muscle and cartilage were colored blue. Nuclei, cytoplasm, fibrin, muscle fibers and elastic fibers were stained red. Pretreatment of sections with Bouins solution enhanced the affinity of tissues for chromotrope 2R and was found essential for satisfactory coloration of material fixed in alcohol, acetone, formalin or Carnoys fluid. Because this method does not require differentiation, it gave uniform results even in the hands of inexperienced labora...

Myoid fibrils in epithelial cells: Studies of intestine, biliary and pancreatic pathways, trachea, bronchi, and testis

SummaryCytoplasmic filaments have been studied extensively by electron microscopy, but the histochemical nature of such fibrils in non-keratinizing epithelia has not been systematically investigated. During studies of early arterial lesions we observed structures with the staining properties of myosins in epithelial cells of various organs. The configurational staining, polarization and fluorescence microscopic properties of these myoid structures were compared with those of myofibrils in smooth muscle and classical myoepithelial cells. The following structures showed the characteristics of myofibrils: the terminal web in columnar epithelial cells of intestine, trachea, bronchi, bile ducts, pancreatic ducts and ductus epididymidis, the pericanalicular layer of bile and pancreatic canaliculi, fibers in the caudal tube of spermatids and the flagella of spermatozoa. Cilia, e.g. of respiratory epithelium, tonofibrils in squamous epithelium and nerve axons did not react.These studies indicate significant histochemical differences between cytoplasmic filaments. Different types of intracellular fibrils can be found in the same cell, e.g. in respiratory epithelium.

On the structure of carminic acid and carmine

SummaryThe chemical mechanism and histochemical significance of carmine stains are not yet understood. To determine possible effects of dye configuration on staining patterns we built models of dye molecules with the Stuart-Briegleb-type of atomic models. However, steric hindrance prevented construction of carmine according to the formula suggested by Harms. A review of recent chemical literature showed that the widely accepted formula of carminic acid is incorrect; the carboxyl group is not in the 5 but in the 7-position, and the side-chain is not a methylpentose but a hexose. Models based on the revised structural formula could be combined to 2∶1∶1 carminic acid-Al-Ca complexes. But formation of the central Al-O-Ca-O-Al bridge of the conventional 4∶2∶1 carminic acid-Al-Ca formula of carmine was still impossible. It is suggested that carmine may be a 2∶1∶1 compound analogous to the 2∶1∶1 alizarin-Al-Ca complex established by Kiel and Heertjes. Investigations of carmine were rendered difficult by wide variations in the staining properties of dye samples and the lack of data concerning the composition of various batches of carmine.

Aldehyde-fuchsin: Historical and chemical considerations

SummaryThe staining mechanisms of Gomoris aldehyde-fuchsin are not yet fully understood. It seemed therefore timely to review the history of this dye class in context with current dye and aldehyde chemistry. In 1861 Lauth treated basic fuchsin with acetaldehyde. This dye became known as Aldehyde Blue, but consisted of violet and blue dyes. Schiff (1866) studied several aldehyde-fuchsins; these compounds contained two molecules of dye and three molecules of aldehyde. Acetaldehyde-fuchsin prepared according to Schiffs directions showed staining properties similar to those of Gomoris aldehyde-fuchsin. This dye class was soon superseded by new dyes more suitable for textile dyeing, and chemical investigations of aldehyde-fuchsins ceased around the turn of the century. Gomoris aldehyde-fuchsin has been regarded as a Schiff base. However, according to chemical data, low molecular aliphatic aldehydes and aromatic amines tend to form condensation products. Correlations of chemical and histochemical observations suggest such processes during aging of dye solutions. Models of dimers and polymers of aldehyde-fuchsin could be built without steric hindrance. The nature of the bonds formed by various components of aldehyde-fuchsin solutions is not clear. However, cystine in proteins, e.g. in basement membranes, apparently does not play a role in the binding of aldehyde-fuchsin by unoxidized Carnoy- or methacarn-fixed sections.