Method of transferring tissue section from support to microscope slide

0001This invention relates to a method for transferring thin sections of tissue specimens, prior to staining and mounting processes, from a support to a microscope slide.

0002Very thin slices of animal and plant tissues are prepared for many different kinds of microscopic studies by sectioning using various kinds of micro- tomes. While the tissue may be cut fresh, the soft and compliant nature of most fresh tissue makes the cutting of undistorted thin sections very difficult. Often, the tissue is cut on a freezing microtome or in a cryostat at temperatures below 0°C. (32°F), the hardness of the frozen water within the tissue allowing sections as thin as a few micrometers to be cut relatively easily. As these frozen sections are brittle and friable, they are difficult to handle and process further. To simplify sectioning of tissue, a number of procedures have been developed which produce a block of supported tissue which has superior sectioning properties and produces high quality, relatively easy-to- handle tissue sections. Such procedures typically involve: (1) fixation of the tissue in a solution which insolubilises and hardens the natural polymers of which tissue cells are composed; (2) dehydration ofthetissuethrough a series of water- miscible (e.g. an alcohol) and then paraffin or plastic-monomer-miscible (e.g. toluene or xylene) solvents; (3) infiltration of the tissue by melted paraffin or monomer solution; and (4) embedding by freezing the paraffin or polymerising the monomer to form a solid polymer. Reference is made to Staining Methods, J. F. A. McManus and R. W. Mowry (P.B. Hoeber, Inc., N.Y. 1960) and to Techniques for Electron Microscopy, D. Kay, Ed. (Blackwell Sci. Publ., Oxford, England, 1965) pp. 166-212.

0003However, there are certain specimens which remain difficult to section. As the section is cut from the tissue, parts of the cut section tend to fragment and fall from the cut section, or fall from the section as it is removed from the microtome. Reference is made to the following which describes procedures for facilitating the cutting and handling of such difficult-to-cut tissue sections:

00041. A. Palmgren, Nature, Vol. 174, p. 46, (1954) describes the use of a pressure-sensitive adhesive tape as a sectioning aid for the cutting of very large, hard or brittle specimens. A piece of adhesive tape is applied to the surface of a specimen, either frozen or embedded in a paraffin block, supported in a microtome. Thus, the section, when cut, is supported by the applied tape. The quality of the uncompressed section of hard, brittle and friable tissue thus produced can be far superior to that of a conventional section of the same block of tissue. However, following Palmgren, processing such a section while it remains on the tape, or transferring itto a glass slide (to permit it to be processed thereafter in conventional fashion) involves elaborate, time-consuming and inconvenient methods which can also damage the section.

00052. W. E. Beckel, Nature, Vol. 184, p. 1584 (1959) describes the use of Scotch brand No. 810 cellulose-acetate-backed adhesive tape in the process described by Palmgren above. The tape-mounted sections are applied, section-side down, on wet conventionally albuminised glass slides. After thorough drying, which requires at least a few hours, the adhesive backing, the adhesive layer and the paraffin are all dissolved in tetrahydrofuran for 30 minutes, leaving only the section adhering to the glass slide and available for further processing by conventional techniques. Alternatively, chloroform for 2 minutes, followed by xylene for 30 minutes, can be used for dissolving the adhesive backing, the adhesive layer and the paraffin. Beckel also describes a "more rapid method" which uses a film of albumin and a solution of 2 per cent celloidin in methyl benzoate or ethyl alcohol to "cement" the section to the glass slide, followed by 1 minute in chloroform and 10 minutes in xylene to complete the treatment.

00063. D. S. Gowers and R. E. Miller Nature, Vol. 190, p. 425 (1961) attempted to repeat Beckel's method but found that, with available Scotch brand No. 810 tape, the adhesive could not be dissolved, and with the best alternate available tape, Tuck brand No. 200, safe removal of the tape without damaging the section in solvent took from 1 to 10 hours.

00074. R. P. Wedeen and H. I. JernowAm. J. Physiol, Vol. 214, p. 776, (1968) used cyanoacrylate (East- man 910 "superglue") to attach adhesive-tape- supported frozen sections to radioautographic (photographic) plates. The cyanoacrylate is initially liquid, but polymerises to a solid when pressed into a thin film. As the cyanoacrylate polymer is soluble in xylene and other processing solvents which would cause the section to float free, it is not useful for conventional staining procedures.

0008U.S. patent specification no. 4269139 describes a method of transferring a biological cellular sample, such as a smear, from a first substrate to a second substrate (e.g. a microscope slide) by providing on the slide a transfer medium in which the sample becomes embedded and encapsulated. The slide, bearing the transfer medium, is brought into contact with the sample, and the adhesive properties of the transfer medium are such that the sample remains on the slide when the first substrate is removed.

0009We have now devised an improved method of transferring thin tissue sections from supports such as adhesive tape, on to microscope slides.

0010According to the present invention, there is provided a process for transferring a thin section of a tissue specimen, supported on a substrate by a first pressure-sensitive adhesive layer, from said substrate and mounting it on a microscope slide having at least a portion of one surface coated with a second pressure-sensitive adhesive layer which is sufficiently tacky to adhere to the specimen, comprising the steps of: bringing said supported tissue section into contact with said second layer to form a laminate comprising said substrate, said first layer, said tissue section, said second layer, and said microscope slide; polymerising said second layer, said second layer being formed of a polymerizable material containing no curable components having diffusion coefficients at the temperature of use large enough to permit significant diffusion into said tissue specimen prior to or during polymerization and having a viscosity at a temperature of use high enough to prevent significant flow into said tissue specimen prior to or during polymerization, and being such that, after polymerization, it is substantially insoluble and unswellable in reagents to which it is likely to be exposed in normal use; and subsequently removing said substrate and said first layer to expose substantially the entire internal structure of said tissue section bonded by the polymerised material to said surface portion of said microscope slide.

0011In the method of the present invention, a tissue specimen supported on, for example, a flexible piece of pressure-sensitive adhesive tape, is brought into contact with a microscope slide over whose contacting surface of film of pressure-sensitive adhesive, capable of further polymerization, has been layered. Techniques by which flexible tape can be applied to such specimens to aid sectioning have been described, for example, in the above-identified publications by Palmgren, Beckel, Gowers et al and Wedeen et al. After the specimen section has been contacted with the polymerisable layer, the layer is then polymerised, usually within about 1 to 3 minutes, and then the tape may, for example, be submerged in a solvent which weakens the supporting tape backing and/or supporting-tape adhesive-layer which are washed or peeled away, to fully expose the tissue specimen. The polymerisable layer and its bonds to the section have sufficient strength after curing to allow peeling of the insoluble tape backing without damaging the section and to thereafter firmly support the specimen on the microscope slide.

0012The polymerisable layer has only curable components whose respective diffusion coefficients, before and during cure, are sufficiently low that the diffusion thereof into the specimen during the curing process is minimal and a viscosity sufficiently large that flow into the specimen on application of finger-pressure is minimal, that is in both cases less than, for example, 1 micro- ; meter in 30 minutes. However, when cured, the layer forms a positive bond between the microscope slide and specimen section, to permit ease of handling during subsequent processing of the supported section. As diffusion and flow into the section is minimal, the substantially entire interior structure of the tissue section remains exposable to processing. Also, the polymerised bonding layer is not affected, in any way, by such processing. _<

0013The polymerised layer preferably has a refractive index matched to the refractive index of the unstained tissue, e.g. from about 1.53 to 1.57, and to the mounting medium conventionally used to envelop and to attach the coverslip over the < specimen at the completion of the processing. Conventionally, the specimen is stained during processing to highlight particular components thereof. As the respective refractive indices of the various components of the completed microscope slide are substantially the same, there is no optical interference introduced to degrade the image in the subsequent microscopic examination of the tissue.

0014Thus, in a preferred embodiment, the present invention provides a method for positively attaching a tissue to a microscope slide by a pressure-sensitive polymerisable layer, the curable components of such layer prior to and during polymerisation penetrating only very minimally into such specimen and, when polymerised, having a compatible index of refraction and neither affecting nor being affected by any subsequent chemical or physical treatment of the supported tissue.

0015In order that the invention may be more fully understood, reference is made to the accompanying drawings, wherein: <ul id="ul0001" list-style="none"><li>FIGURE 1 is an isometric view of a thin tissue section supported on the surface of a flexible tape or substrate;</li><li>FIGURE 2 is an isometric view of the substrate of Figure 1 juxtaposed with a microscope slide;</li><li>FIGURE 3 is an isometric view illustrating the super-positioning of the substrate over the microscope slide, so as to laminate the thin tissue section therebetween;</li><li>FIGURE 3B illustrates the removing of the substrate from over the microscope slide, to expose the thin tissue section bonded on such slide for further processing; and</li><li>FIGURES 4A and 4B, respectively, illustrate the final mounting of a thin tissue section on a microscope slide, with and without a coverslip.</li></ul>

0016As indicated in Figure 1, a thin tissue section 9 surrounded and infiltrated with paraffin 10 is supported on and adhered to the surface of a flexible tape or substrate 12, by a pressure-sensitive adhesive layer 14. Section 9 is located within a window or opening in a spacer 13 adhered to layer 14. As described in European patent publication no. 88549, spacer 13 facilitates the cutting of a thin section 9 by a microtome.

0017Referring to Figure 2, a microscope slide 16 is illustrated, which supports on its upper surface a pressure-sensitive adhesive layer 18. Layer 18 can be formed, of for example, a heat-curable polymerisable material or a photopolymerisable material. Preferably, layer 18 is photopolymerisable, e.g. by exposure to U.V. light of wavelengths in the range of 320 nm-390 nm. Figure 2 further illustrates the inversion of tape 12 so as to juxtapose specimen 9 and layer 18. Specimen 9 and layer 18 are subsequently contacted. Preferably, a slight pressure is applied, e.g. manually. Also, such pressure can be applied progressively along the specimen 9, to ensure a smooth contact thereof with layer 18.

0018Layer 18 is formed to have the characteristics of: <ul id="ul0002" list-style="none"><li>1. Being sufficiently tacky, when unpolymerised, to adhere to specimen 9.</li><li>2. Being polymerisable, so as to form a permanent bond between specimen 9 and slide 16.</li><li>3. Containing no curable components having diffusion coefficients large enough to permit diffusion of such components into tissue section 9 before polymerisation of layer 18 preferably by as much as 1 micrometer in 30 minutes. It is evident that significant diffusion of any such components would, after curing, prevent penetration of processing reagents throughout specimen section 9 during subsequent processing.</li><li>4. Having a sufficiently high viscosity, so as not to flow into pores of specimen section 9 when contacted therewith and while pressure is being applied during the smoothing process. Significant flow into the section would also interfere, after curing, with subsequent processing.</li><li>5. When cured, being insoluble and unswellable in any of the reagents to which specimen section 9 is to be exposed during subsequent processing.</li><li>6. Having a refractive index compatible with the refractive indices of the unstained tissue, e.g. from about 1.53 to 1.57, and of the mounting medium conventionally used to envelop specimen section 9 on microscope slide 16.</li><li>7. When cured, being effective to strongly bond to both the bottom of specimen section 9 and to the opposing surface of slide 16.</li></ul>

0019Accordingly, when specimen 9 has been contacted with layer 18 and smoothed, the resulting laminate 20 is exposed to U.V. light from source 21, as shown in Figure 3A, for a time sufficient to completely polymerise layer 18, typically 3 minutes. Source 21 may be a 4-watt (or larger) black-fluorescent lamp (with phosphor-peak- light-output near 350 nm). At this time, layer 18 and the adhesive layer 14 on tape 12, which is unpolymerised, are firmly adhered to the interposed specimen 9.

0020As shown in Figure 3B, the laminate 20 is then immersed is a solvent solution 22, such as xylene, contained in beaker 24. Such immersion serves to rapidly soften the layer 14 on tape 12 and allows for a gentle manual peeling of tape 12 from over specimen 9 while laminate 20 is immersed in beaker 24. Exposed layer 14 and paraffin 10 dissolve as, or within one minute after, tape 12 is peeled away. The specimen section 9 is now fully exposed. When layer 14 has been dissolved, the remaining laminate is ready for subsequent processing and specimen section 9 is firmly bonded to the surface of slide 16. Also, it is evident that the solvent solution 22 may be selected, so as to be effective to dissolve both substrate 12 and adhesive layer 14, so as to avoid the need of peeling substrate 12, as described.

0021Preferably, the layer 18 on slide 16 consists of a mixture of very high molecular weight reactive oligomers (typically, each with two terminal acryl or methacryl groups). The high molecular weight ensures a high viscosity and a low diffusion coefficient. If the molecular weight is too high, there will be insufficient tack. The acryl or methacryl groups permit radical-initiated polymerisation to cross-link the oligomers into a highly insoluble polymer.

0022Typically, the mixture is composed of one oligomer with a cured refractive index of greater than 1.560 (e.g. an epoxy-acrylate oligomer with many bisphenol A or other high aromatic residues to raise the refractive index) and another oligomer with a cured refractive index less than 1.530 (e.g. a urethane acrylate oligomer). Both cured resins must be extremely resistant to xylene, alcohol and water. By mixing them in appropriate proportions, any desired cured refractive index, between 1.530 and 1.560, can be imparted to layer 18, when polymerised.

0023It is clear that other combinations of reactive oligomers and/or polymers with other initiators and solvents can be formulated by those skilled in the art to fill the requirements in the listed specifications. However, for a detailed description of compositions suitable for forming layer 18, reference is made to our European patent specification no. 104825 filed on even date herewith.

0024Matching of the refractive index of the layer 18 to the respective refractive indices of both specimen section 9 and the mounting medium to be applied to envelop the specimen on slide 16 is important. Unless such indices are properly matched any surface imperfections on the cured upper surface of layer 18 for example, left by any texture on the now-removed adhesive layer 14 or paraffin 10, will be visible under microscopic examination as a distracting, phase-contrast detail. Therefore, proper matching of such indices is necessary to give a very high quality finished microscope slide. The mounting medium for encapsulating specimen section 9 should preferably have a refractive index near 1.55, and layer 18, when'cured, a refractive index of between 1.545 and 1.555.

0025Following peeling of tape 12, the now-exposed specimen section 9 can be processed for storing and permanent encapsulation, according to conventional histological techniques. During such processing, specimen section 9 is securely and permanently bound to slide 16 by now- polymerised layer 18, which neither affects nor is affected by exposure to such techniques. Such encapsulation process may be manually effected, for example, as fully described in McManus and Mowry above. Alternatively, a number of tissue specimen sections 9, each supported on an individual slide 16, may be processed in automated fashion, for example, as clearly shown and described in an AUTOTECHNICON system as marketed by Technicon Instruments Corporation, of Tarrytown, New York, and described in Technical Publication No. TA 1-0225-10, June 1977, pages 3-10-3-13.

0026Figure 4 illustrates a completely mounted specimen section, ready for microscopic viewing. Figure 4A illustrates a specimen section 9 mounted by conventional techniques; such specimen is covered with a mounting medium 26 and a coverslip 28 is located on such medium and over such specimen.

0027Alternatively, Figure 48 shows the envelopment of a specimen 9 in a U.V. curable mounting medium 27, as results from the technique disclosed in U.S. Patent 4,120,991. As shown, a coverslip is not provided. Rather the upper surface of mounting medium 27 is formed so as to be optically flat.

0028For the purposes of this process, the adhesive tape used to capture the sections must at least have an adhesive layer which is soluble in a solvent, such as xylene. A silicone rubber adhesive as used by Wedeen and Jernow referenced above is satisfactory for frozen sections, and an adhesive tape, like Permacel No. 925 (Registered Trade Mark) is satisfactory for paraffin sectioning.