A new antigen retrieval technique for human brain tissue

Immunohistochemical staining of tissues is a powerful tool used to delineate the presence or absence of an antigen. During the last 30 years, antigen visualization in human brain tissue has been significantly limited by the masking effect of fixatives. In the present study, we have used a new method for antigen retrieval in formalin-fixed human brain tissue and examined the effectiveness of this protocol to reveal masked antigens in tissues with both short and long formalin fixation times. This new method, which is based on the use of citraconic acid, has not been previously utilized in brain tissue although it has been employed in various other tissues such as tonsil, ovary, skin, lymph node, stomach, breast, colon, lung and thymus. Thus, we reported here a novel method to carry out immunohistochemical studies in free-floating human brain sections. Since fixation of brain tissue specimens in formaldehyde is a commonly method used in brain banks, this new antigen retrieval method could facilitate immunohistochemical studies of brains with prolonged formalin fixation times.     

Importance of fixation in immunohistochemistry: use of formaldehyde solutions at variable pH for the localization of tyrosine hydroxylase

Adequate fixative in immunohistochemistry requires not only a rapid and total immobilization of the antigen, but also a sufficient preservation of its immunoreactivity and maintenance of its accessibility to the immunochemical reagents for localization. Thus, the optimal fixation condition for a specific antigen necessitates a compromise between these opposing variables and can be determined by the preparation of a series of tissues with a progressively increasing degree of fixation. Unless the results of localization using such a series is available, one must be satisfied with adequate but less than optimal results. In the present study, this principle is demonstrated using the localization of tyrosine hydroxylase in the dopaminergic system with formaldehyde as the fixative. The rate and degree of fixation with formaldehyde was shown to be highly pH dependent. By perfusing the tissue with formaldehyde at pH 6.5 (where the rate of fixation is extremely slow) it is possible to rapidly distribute the fixative homogeneously into the tissue. By suddenly changing to a formaldehyde perfusate of higher pH, the cross-linking reaction is rapidly increased. This two-step fixation procedure provides a means of obtaining a rapid and uniform immobilization of the antigen, so that its translocation can be avoided. The final degree of fixation is controlled by the duration and pH of the second fixative solution. The results obtained by increasing the pH of the second solution demonstrated that complete fixation of tyrosine hydroxylase in the dopaminergic system with formaldehyde maybe obtained using a very basic formaldehyde solution (pH 11) while still retaining immunoreactivity of the enzyme. The localization that was achieved at lower pH appeared adequate until it was compared to the results obtained by perfusion at pH 11 in the second step.     

Tissue Fixation for Immunohistochemical Detection of Proliferating Cell Nuclear Antigen with PC10 Monoclonal Antibody

PC10 is a monoclonal antibody to proliferating cell nuclear antigen, a nuclear protein associated with the cell cycle. We have evaluated the effects of tissue fixation on PC10 immunoreactivity in sections of paraffin embedded rat tissues. Immunoreactivity was well preserved in tissues after fixation with alcohol-based solutions for 3-24 hr. Fewer PC10-positive cells were detectable in samples fixed with formaldehydecontaining solutions compared with samples fixed with alcohol for the same time. Loss of PC10 immunoreactivity in formaldehyde fixed tissues was progressive, and quantifiable as early as after 3 hr fixation. Consequently, alcohol-based fixatives are strongly recommended for any immunocytochemical prospective study using PC10 antibody. In contrast, loss of PC10-immunoreactivity is always predictable, but difficult to quantitate, using formaldehyde fixed specimens. This aspect should be considered when using PC10 antibody in retrospective studies with routinely-processed archival material.     

Cedarwood Oil as a Clearing Agent with Acetic-Alcohol Fixatives

PC10 is a monoclonal antibody to proliferating cell nuclear antigen, a nuclear protein associated with the cell cycle. We have evaluated the effects of tissue fixation on PC10 immunoreactivity in sections of paraffin embedded rat tissues. Immunoreactivity was well preserved in tissues after fixation with alcohol-based solutions for 3–24 hr. Fewer PC10-positive cells were detectable in samples fixed with formaldehydecontaining solutions compared with samples fixed with alcohol for the same time. Loss of PC10 immunoreactivity in formaldehyde fixed tissues was progressive, and quantifiable as early as after 3 hr fixation. Consequently, alcohol-based fixatives are strongly recommended for any immunocytochemical prospective study using PC10 antibody. In contrast, loss of PC10-immunoreactivity is always predictable, but difficult to quantitate, using formaldehyde fixed specimens. This aspect should be considered when using PC10 antibody in retrospective studies with routinely-processed archival material.     

Rapid and sensitive detection of enhanced green fluorescent protein expression in paraffin sections by confocal laser scanning microscopy

Various forms of green fluorescent protein (GFP) have become important reporters of gene transfer and expression after transfection or infection of cells in cell culture. Frequently, molecular biological assays (Northern blots, PCR) are applied to detect reporter gene expression in target organs. However, these methods are not suitable for evaluation of tissue- or cell-specific expression which would be of great interest especially in case of using tissue-specific promoters. Therefore, organs of transgenic mice with the enhanced green fluorescent protein (EGFP) gene under control of the cytomegalovirus (CMV) promoter were processed for histology by formaldehyde fixation and embedding in paraffin. Sections were deparaffinized, mounted and evaluated for fluorescence in a confocal laser scanning microscope. This method combines the advantages of direct exploitation of tissue sections without further staining procedures with evaluable tissue-, cell-, and even subcellular-specific distribution patterns of EGFP expression in tissues. Results obtained by direct evaluation of EGFP fluorescence in paraffin sections were confirmed by immunohistochemical staining with anti-EGFP. In the present report, we demonstrate that application of confocal microscopy on routinely processed histological preparations is very suitable for determining gene transfer efficiency and promotor activities.     

The effects of mercaptoethanol-formaldehyde on tissue fixation and protein retention

Summary The study compared the effects of mercaptoethanol-formaldehyde and formaldehyde alone, on tissue fixation and protein retention in human and mouse tissues. Shrinkage of tissues and the penetration rate of the fixatives were assessed. The cross-linking ability of the fixatives was determined by viscometry, sodium dodecyl sulphate-polyacrylamide gel electrophoresis, and spectrophotometry, using bovine serum albumin and human haemoglobin. Tissues fixed in buffered 0.0025% mercaptoethanol-4% formaldehyde showed good nuclear and cytoplasmic detail, better than those fixed in buffered 4% formaldehyde. There was no significant difference in shrinkage. A mixture of 0.0025% mercaptoethanol-4% formaldehyde penetrated faster into adult liver than 4% formaldehyde. The mean penetration rate (±SE) or coefficient of diffusibility of 0.0025% mercaptoethanol-4% formaldehyde into adult liver was 1.32±0.01 and that of 4% formaldehyde was 1.12±0.06 (p<0.04). Both fixatives diffused more rapidly into mouse liver than into human liver. The cross-linking ability of mercaptoethanol-formaldehyde depends on the concentration of the fixative and the time of fixation. Bovine serum albumin (15%) and 0.1% mercaptoethanol alone formed a gel, whilst electrophoresis showed monomers in the supernatant. Mercaptoethanol (0.1%) also rapidly decreased the absorption at 420 nm, suggesting denaturation. It seems that mercaptoethanol increases the number of thiol groups available to form cross-links with formaldehyde. This study demonstrated that mercaptoethanol-formaldehyde fixed and cross-linked tissues better than formaldehyde at 3 h and 4 h, but not at 1 h and 2 h. The most effective concentration of mercaptoethanol for tissue fixation in 4% formaldehyde is 0.0025%.     

Determination of optimal rehydration,fixation and staining methods for histological and immunohistochemical analysis of mummified soft tissues

During an excavation headed by the German Institute for Archaeology, Cairo, at the tombs of the nobles in Thebes-West, Upper Egypt, three types of tissues from different mummies were sampled to compare 13 well known rehydration methods for mummified tissue with three newly developed methods. Furthermore, three fixatives were tested with each of the rehydration fluids. Meniscus (fibrocartilage), skin, and a placenta were used for this study. The rehydration and fixation procedures were uniform for all methods. The stains used were standard hematoxylin and eosin, elastica van Gieson, periodic acid-Schiff, and Grocott, and five commercially obtained immunohistochemical stains including pancytokeratin, vimentin, alpha-smooth-muscle-actin, basement membrane collagen type IV, and S-100 protein. The sections were examined by transmitted light microscopy. Our study showed that preservation of the tissue is dependent on the quality and effectiveness of the combination of the rehydration and fixation solutions, and that the quality of the histological and histochemical stains is dependent on the tissue quality. In addition, preservation of the antigens in the tissues is dependent on tissue quality, and fungal permeation had no influence on the tissue. Finally, the results are tissue specific. For placenta the best solution combination was Sandison and solution III (both fixed with formaldehyde) while results for skin were best with Ruffer I (using formaldehyde and Schaffer as fixatives), Grupe et al. (using formaldehyde as a fixative) and solution III (in combination with formaldehyde and Bouin fixatives). Ruffer II (using formaldehyde as a fixative) and solution III (in combination with Schaffer fixative) gave the best results for fibrocartilage.     

Determination of optimal rehydration, fixation and staining methods for histological and immunohistochemical analysis of mummified soft tissues.

During an excavation headed by the German Institute for Archaeology, Cairo, at the tombs of the nobles in Thebes-West, Upper Egypt, three types of tissues from different mummies were sampled to compare 13 well known rehydration methods for mummified tissue with three newly developed methods. Furthermore, three fixatives were tested with each of the rehydration fluids. Meniscus (fibrocartilage), skin, and a placenta were used for this study. The rehydration and fixation procedures were uniform for all methods. The stains used were standard hematoxylin and eosin, elastica van Gieson, periodic acid-Schiff, and Grocott, and five commercially obtained immunohistochemical stains including pancytokeratin, vimentin, alpha-smooth-muscle-actin, basement membrane collagen type IV, and S-100 protein. The sections were examined by transmitted light microscopy. Our study showed that preservation of the tissue is dependent on the quality and effectiveness of the combination of the rehydration and fixation solutions, and that the quality of the histological and histochemical stains is dependent on the tissue quality. In addition, preservation of the antigens in the tissues is dependent on tissue quality, and fungal permeation had no influence on the tissue. Finally, the results are tissue specific. For placenta the best solution combination was Sandison and solution III (both fixed with formaldehyde) while results for skin were best with Ruffer I (using formaldehyde and Schaffer as fixatives), Grupe et al. (using formaldehyde as a fixative) and solution III (in combination with formaldehyde and Bouin fixatives). Ruffer II (using formaldehyde as a fixative) and solution III (in combination with Schaffer fixative) gave the best results for fibrocartilage.     

New freeze-dry and vapor fixation method for immunohistochemistry of soluble proteins: subcellular location of the progesterone receptor

We describe a new application of freeze-drying and vapor fixation for immunohistochemical location of soluble proteins. The method avoids the liquid phase, which eliminates the possible diffusion of soluble proteins. Two vapor fixatives, paraformaldehyde and p-benzoquinone, were tested and p-benzoquinone was found to preserve antigenicity of progesterone receptor (PR) and ovalbumin better than paraformaldehyde. The method proved to be highly sensitive, since higher concentrations of antigen were found in some tissues and some tissues found to be antigen negative by earlier liquid fixation methods proved to contain antigen. The location of PR as a highly soluble protein was studied. With the present method, both unoccupied and occupied PR were located in the nuclei, a similar finding as with the earlier liquid fixation method. The results further support the concept that PR is an intranuclear protein independent of its ligand occupation. PR was detected in a few cells inside the follicles of the bursa of Fabricius and in the smooth muscle cell nuclei of the small intestine, observations not previously made owing to the insensitivity of the earlier methods.     

Application of the aluminum-formaldehyde (ALFA) histofluorescence method for demonstration of peripheral stores of catecholamines and indolamines in freeze-dried paraffin-embedded tissue,cryostat sections and whole-mounts

Summary This paper describes new procedures for highly sensitive visualization of monoamine stores in peripheral tissues, taking advantage of the recently introduced aluminum-catalysed formaldehyde (ALFA) reaction. The tissues are exposed to an aluminum sulphate solution (with or without formaldehyde fixation) in a perfusion and/or immersion step, followed by formaldehyde vapour treatment. Procedures are described for freeze-dried, paraffin embedded tissue, cryostat sections and whole mount preparations. For all these tissue preparations the ALFA method gives a highly sensitive and precise demonstration of catecholamine-containing neurons and 5-HT-containing cells in a variety of peripheral tissues. For freeze-dried tissue and cryostat sections the ALFA method represents an improvement in comparison with other available methods. This is particularly noticeable for the very delicate adrenergic nerves in such organs as the thyroid, ovary, pancreas and the gastrointestinal tract.     

Formaldehyde fixation and microwave irradiation

Summary Formaldehyde is the most commonly used fixative in pathology laboratories. However, due to time pressures, this fixative is often not optimally exploited. the majority of biopsies are only partly fixed when histoprocessing is started, with adverse effects. This paper reports how formaldehyde fixation is improved, by using 1.5 min of microwave irradiation of tissue previously soaked for four hours in the fixation solution. It is argued that this beneficial effect of microwave irradiation can be attributed to the acceleration of the reaction of formaldehyde to the tissue. Formation of free formaldehyde, by the dehydration of methylene glycol present in the tissue when the irradiation starts, is also enhanced. Five different formaldehyde-containing fixatives were evaluated, using five different working protocols. Spleen was taken as a suitable tissue for these tests. The technique described leads to uniform microscopical results. It is a simple method and is suitable for use in routine laboratories.     

Protein fixation and antigen retrieval: chemical studies

Abstract

Fixation with formaldehyde is the first process to which most biopsy and necropsy specimens are exposed prior to dehydration and embedding in paraffin wax. Tissue specimens that have been fixed in formaldehyde have architectural characteristics that are familiar to virtually every pathologist and these facilitate routine diagnosis. Nevertheless, formaldehyde fixation has some deleterious effects including reduction in immunoreactivity and degradation of nucleic acids. Development of methods to counteract these deleterious effects requires an understanding of the chemical events that occur during tissue fixation and subsequent tissue processing. This short review illustrates some of the chemical consequences of formaldehyde fixation and ethanol dehydration. It also provides some insight into the molecular events accompanying heat-induced antigen retrieval.     

Immunocytochemical localization of nerve growth factor: effects of fixation

The fixation dependence of immunocytochemically demonstrable nerve growth factor (NGF) was investigated. Several commonly used fixation methods have been employed, including buffered formaldehyde, Bouin's fluid, and chloroform-methanol, as well as freezing and cryostat sectioning. The immunostaining technique was an immunoenzyme bridge procedure on either paraffin sections or frozen sections. Of those methods tested, fixation for 1 hr in a buffered formaldehyde appeared to provide optimal preservation and localization of immunoreactive material. Using this method, reaction product was localized in granules of the granular tubule cells of the male mouse submandibular gland. Prolonged fixation in buffered formaldehyde resulted in a diffuse background staining and loss of granule immunoreactivity. In frozen sections and in tissues fixed with either Bouin's solution, chloroform-methanol, or buffered paraformaldehyde-glutaraldehyde increased cytoplasmic background staining and loss of granule immunoreactivity were observed. It was concluded that, for the localization of NGF at the light microscopic level, a brief (1 hr) buffered formaldehyde fixation is optimal.     

Block staining of mammalian tissues with hematoxylin and eosin

Various mammalian tissues were stained en bloc with hematoxylin and eosin after fixation and prior to embedding in paraffin wax and sectioning. The choice of fixative is important and best results are obtained using Worcester's Fluid, a combination of saturated aqueous mercuric chloride, formaldehyde, and glacial acetic acid. After fixation, blocks of tissue up to 1.5 cm thick are stained for seven days in hematoxylin. Excess stain is removed by washing tissues in running water overnight. Tissue blocks then are dehydrated with graded concentrations of ethyl alcohols to 80% and counterstained, with further dehydration, in 0.5% spirit soluble eosin in 90% ethyl alcohol for five days. The tissue is subsequently transferred to 90% ethyl alcohol overnight to differentiate eosin staining; dehydration is completed in absolute ethyl alcohol. The blocks are cleared in in cedarwood oil and briefly in xylene prior to embedding, sectioning, and mounting. Following removal of wax by xylene, coverslips are applied. General morphological and histological features were particularly well differentiated and very selectively and reliably stained by this method.     

Block Staining of Mammalian Tissues with Hematoxylin and Eosin

Various mammalian tissues were stained en bloc with hematoxylin and eosin after fixation and prior to embedding in paraffin wax and sectioning. The choice of fixative is important and best results are obtained using Worcester's Fluid, a combination of saturated aqueous mercuric chloride, formaldehyde, and glacial acetic acid. After fixation, blocks of tissue up to 1.5 cm thick are stained for seven days in hematoxylin. Excess stain is removed by washing tissues in running water overnight. Tissue blocks then are dehydrated with graded concentrations of ethyl alcohols to 80% and counterstained, with further dehydration, in 0.5% spirit soluble eosin in 90% ethyl alcohol for five days. The tissue is subsequently transferred to 90% ethyl alcohol overnight to differentiate eosin staining; dehydration is completed in absolute ethyl alcohol. The blocks are cleared in cedarwood oil and briefly in xylene prior to embedding, sectioning, and mounting. Following removal of wax by xylene, coverslips are applied.

General morphological and histological features were particularly well differentiated and very selectively and reliably stained by this method.     

Fixation of cryo-sections under HIV-1 inactivating conditions: integrity of antigen binding sites and cell surface antigens

Summary Cryostat-sections of biopsies from HIV-infected patients or HIV/SIV-infected experimental animals pose a biohazard risk to laboratory workers. The objective of this study was to select a procedure that appropriately fixes cryo-sections and reduces the risk of HIV-1 infectivity. This inactivation procedure should preserve antigen binding capacity of host-produced antibodies and the antigenic structure of epitopes present in these tissues, while retaining sufficient morphologic detail. We tested the effect of seven different established fixation-inactivation procedures for HIV-1 on the detection of specific antibodies and membrane markers, compared to acetone fixation as a reference. Frozen sections of spleens from mice immunized with trinitrophenyl (TNP)-Ficoll were incubated with TNP-alkaline phosphatase to detect specific antibody-forming cells and follicular immune complexes containing TNP-specific antibodies. In addition, sections were stained with monoclonal antibodies directed against IgM (187-1), T-cells (anti Thy-1), and marginal metallophilic macrophages (MOMA-1). Five procedures proved useful as they gave results similar to regular acetone fixation. In contrast, two procedures with a methanol-containing fixative obscured both antigen binding sites and membrane antigens. Subsequently, these five selected procedures were tested on glass slide preparations of HIV-1 infected cell lines, expressing HIV-1 determinants defined by monoclonal antibodies. Finally, the procedures were tested on sections of an HIV-1 infected human lymph node. for detection of HIV-specific B-cells. We show that fixation-inactivation in 0.37% (v/v) formaldehyde in PBS for 10 min at room temperature and 0.5% paraformaldehyde (w/v) in PBS for 10 min at room temperature are the methods of choice, combining preservation of antigen binding sites (Fab), membrane antigens, and HIV-1 determinants with good tissue morphology.Abbreviations AFC antibody forming cell - AP alkaline phosphatase - MAb monoclonal antibody - HIV-1 human immunodeficiency virus type 1 - HRP horseradish peroxidase - TNP trinitrophenyl     

Histomorphometric comparison after fixation with formaldehyde or glyoxal

Abstract

Formaldehyde has long been the fixative of choice for histological examination of tissue. The use of alternatives to formaldehyde has grown, however, owing to the serious hazards associated with its use. Companies have striven to maintain the morphological characteristics of formaldehyde-fixed tissue when developing alternatives. Glyoxal-based fixatives now are among the most popular formaldehyde alternatives. Although there are many studies that compare staining quality and immunoreactivity, there have been no studies that quantify possible structural differences. Histomorphometric analysis commonly is used to evaluate diseased tissue. We compared fixation with formaldehyde and glyoxal with regard to the histomorphological properties of plantar foot tissue using a combination of stereological methods and quantitative morphology. We measured skin thickness, interdigitation index, elastic septa thickness, and adipocyte area and diameter. No significant differences were observed between formaldehyde and glyoxal fixation for any feature measured. The glyoxal-based fixative used therefore is a suitable fixative for structural evaluation of plantar soft tissue. Measurements obtained from the glyoxal-fixed tissue can be combined with data obtained from formalin-fixed for analysis.     

Reactions of aldehydes with unsaturated fatty acids during histological fixation

Synopsis Formaldehyde reacts with unsaturated fatty acids in tissues during histological fixation. For example, the reaction of formaldehyde with oleic acid gives rise to compounds with the structures shown below. The structures of the major reaction products have been confirmed, but those of the minor products have not been conclusively demonstrated. Esterified unsaturated fatty acids react more slowly, but the reaction is otherwise similar. Acrolein has been found to react in a similar fashion, but the reaction is far more complex.The occurrence of such compounds in tissues partly explains the loss of lipids during fixation, and raises some interesting possibilities regarding new fixatives and new histochemical reactions. The experimental work described in this paper was performed in the Departments of Pathology and Applied Biology, University of Cambridge.     

Bi-functional cross-linking reagents efficiently capture protein-DNA complexes in Drosophila embryos

Chromatin immunoprecipitation (ChIP) is widely used for mapping DNA-protein interactions across eukaryotic genomes in cells, tissues or even whole organisms. Critical to this procedure is the efficient cross-linking of chromatin-associated proteins to DNA sequences that are in close proximity. Since the mid-nineties formaldehyde fixation has been the method of choice. However, some protein-DNA complexes cannot be successfully captured for ChIP using formaldehyde. One such formaldehyde refractory complex is the developmentally regulated insulator factor, Elba. Here we describe a new embryo fixation procedure using the bi-functional cross-linking reagents DSG (disuccinimidyl glutarate) and DSP (dithiobis[succinimidyl propionate). We show that unlike standard formaldehyde fixation protocols, it is possible to capture Elba association with insulator elements in 2–5 h embryos using this new cross-linking procedure. We show that this new cross-linking procedure can also be applied to localize nuclear proteins that are amenable to ChIP using standard formaldehyde cross-linking protocols, and that in the cases tested the enrichment was generally superior to that achieved using formaldehyde cross-linking.