Abstract:
The invention concerns an apparatus for preparing monolayers of cells. The apparatus comprises a container ( 6 ) for a cryosubstitution system and an insert ( 1 ) for the container, the insert ( 1 ) having a surface ( 1   a ) and a plurality of orifices ( 3 ). An SCS ( 2 ) together with a cellular monolayer ( 20 ) is insertable into each of the orifices ( 3 ). The SCS ( 2 ) is thus arranged perpendicular to the surface ( 1   a ) of the insert ( 1 ).

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
         [0001]    This application claims priority of the European patent application 01 114 433.4 which is incorporated by reference herein.  
         FIELD OF THE INVENTION  
         [0002]    The invention concerns a method for preparing monolayers of cells.  
           [0003]    The invention furthermore concerns an apparatus for preparing monolayers of cells. In particular, the invention concerns a container for a cryosubstitution system and an insert for the container.  
         BACKGROUND OF THE INVENTION  
         [0004]    According to a method of the existing art (see 1993 Leica brochure of Leica Aktiengesellschaft, Vienna, Austria), it is proposed to perform a cryosubstitution in special flow-through capsules (Reichert™ ). Embedding of the specimen for producing thin sections is then also accomplished in the same capsules. A method for the preparation of thin sections of cellular monolayers is not presented here. In addition, the container of the specimens for cryosubstitution disclosed in the existing art cannot successfully guarantee the cryosubstitution of cellular monolayers.  
         SUMMARY OF THE INVENTION  
         [0005]    It is the object of the invention to create a method with which cellular monolayers can be prepared in simple fashion, and thereby to make available an efficient way of producing thin sections of the cellular monolayer for subsequent microscopic examination.  
           [0006]    The object is achieved by a method comprising the following steps:  
           [0007]    culturing a cellular monolayer on at least one SCS;  
           [0008]    cooling each SCS together with the cellular monolayer in such a way that the formation of ice crystals in the cells of the monolayer is avoided;  
           [0009]    substituting the cellular water in the cells of the monolayer; and  
           [0010]    embedding the cellular monolayer together with the SCS in such a way that the cells of the cellular monolayer on the SCS are surrounded by embedding material, and the SCS is substantially free of embedding material.  
           [0011]    A further object of the invention is to create an apparatus which allows cellular monolayers to be prepared in such a way that efficient section production for a subsequent microscopic examination can be achieved. Moreover, the apparatus is also intended to ensure efficient utilization of the cellular monolayer for production of the thin sections.  
           [0012]    The object is achieved by means of an apparatus for preparing monolayers of cells, which comprises:  
           [0013]    a container for a cryosubstitution system,  
           [0014]    an insert for the container wherein the insert defines a surface and a plurality of orifices, and  
           [0015]    at least one SCS, carrying a cellular monolayer, is insertable into the plurality of orifices so that the SCS is arranged perpendicular to the surface of the insert.  
           [0016]    The invention has the advantage that the method according to the present invention makes possible reliable specimen preparation of cellular monolayers. For that purpose, a cellular monolayer is cultured on a sapphire coverslip (SCS). Culturing is performed in a conventional tissue culture dish. The cellular monolayer formed on the SCS is removed from the tissue culture dish. Cooling of each SCS together with the cellular monolayer is then accomplished in such a way that the formation of ice crystals in the cells of the monolayer is avoided. The cooled SCSs are introduced into a suitable apparatus that itself is transferred into a cryosubstitution system (CSS). In the CSS, the cellular water in the cells of the monolayer is substituted with a suitable medium. Lastly, the cellular monolayer together with the SCS is embedded in such a way that the cells of the cellular monolayer on the SCS are surrounded by embedding material, and the SCS is substantially free of embedding material.  
           [0017]    The embedding material is cured by irradiation with light of a suitable wavelength. The entire cured block is cooled in such a way that the SCS can be popped off from the embedding material as a result of mechanical stresses.  
           [0018]    It is particularly advantageous if the substitution is performed in a CSS, each SCS with the cellular monolayer being introduced into the container using an insert. The container itself is inserted into the CSS, the SCSs being arranged perpendicular to the surface of the insert. The insert possesses a central opening through which, using a hollow needle, the container in the CSS can be filled with an appropriate medium so that substitution of the water in the cells of the cellular monolayer can be performed.  
           [0019]    Methanol or acetone, for example, has proven to be a suitable medium for substituting the water in the cells of the cellular monolayer. Additions of uranyl acetate (UA) or OSO 4  are used for contrast and to fix the cells.  
           [0020]    The apparatus for preparing monolayers of cells proves to be particularly advantageous for the method if it comprises a container for the CSS and if the container itself is equipped with an insert. The insert possesses a plurality of orifices, an SCS together with a cellular monolayer being insertable into each of the orifices in such a way that the SCS is arranged perpendicular to the surface of the insert.  
           [0021]    Further advantageous embodiments of the apparatus are evident from the dependent claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    The subject matter of the invention is depicted schematically in the drawings and will be described below with reference to the Figures, in which:  
         [0023]    [0023]FIG. 1 is a perspective view of the insert for holding the SCSs during substitution;  
         [0024]    [0024]FIG. 2 shows, in a detail view, the configuration of the insert for holding the SCSs;  
         [0025]    [0025]FIG. 3 is a cross section through the container for receiving the insert having SCSs;  
         [0026]    [0026]FIG. 4 shows an exemplary embodiment of an embedding mold;  
         [0027]    [0027]FIG. 5 is a schematic depiction after the embedding mold has been removed from the embedding material;  
         [0028]    [0028]FIG. 6 is a schematic depiction of the method for the preparation of cellular monolayers. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]    The monolayer of cells is already present on a sapphire coverslip (SCS). The method for preparing the monolayers of cells is disclosed in the description of FIG. 6. Without raising the temperature of the specimens above −100° C., they are transferred into a cryosubstitution system (CSS). The CSS is not depicted, since it is sufficiently known to anyone skilled in this art.  
         [0030]    [0030]FIG. 1 depicts an insert  1  that serves to hold SCSs  2  in the CSS. Insert  1  possesses a plurality of orifices  3  which are configured such that SCSs  2  can be inserted perpendicularly (see FIG. 2 for a more detailed depiction of this). SCSs  2 , together with cellular monolayer  20 , are arranged in insert  1  perpendicular to the latter&#39;s surface  1   a.  A container  6  is provided for the reception of insert  1 . Container  6  is in thermal contact with the CSS. Insert  1  is circular in shape, and fits into a receptacle  12  (also circular) that is configured on container  6 . A central opening  4 , through which a medium can be delivered centrally to container  6 , is configured in insert  1 . Container  6  is filled with medium by means of a hollow needle  5  until SCSs  2  are immersed. An exchange of media can also take place through this opening  4 . Container  6  is in good thermal contact with the temperature-controlled surface of the CSS, so that preselected temperature profiles can also be achieved in the medium that is present in container  6 .  
         [0031]    [0031]FIG. 2 shows, in a detail view, the configuration of insert  1  for holding SCSs  2 . In this exemplary embodiment, orifices  3  in insert  1  are circular in shape. This is not to be construed as a limitation. Configured at the periphery of a circular orifice  3  are oppositely located notches  13  that ultimately serve to receive and hold SCSs  2 . Orifices  3  are arranged around central opening  4 .  
         [0032]    [0032]FIG. 3 shows, in cross section, container  6  into which insert  1  with SCSs  2  is inserted. Container  6  is made up of a solid, highly thermally conductive block. Circular receptacle  12 , adjoining which is a conical diminution  14 , is configured in the container. Conical diminution  14  is configured in such a way that it constitutes a stop  15  for insert  1 . When insert  1  is inserted, its central opening  4  is arranged above cup-shaped depression  16 . As depicted in FIG. 3, hollow needle  5  can be guided through central opening  4  and ends in central cup-shaped depression  16 . Insert  1  possesses, at each orifice  3 , at least one stop  17  which, when insert  1  is inserted, prevents SCSs  2  from falling out in the direction of central cup-shaped depression  16 .  
         [0033]    After completion of the treatment of the cellular monolayers on SCSs  2 , a low-temperature embedding step is preferably added. FIG. 4 schematically depicts a portion of an embedding mold, commercially available Eppendorf vessels being used as the embedding mold. Cap  7  of the Eppendorf vessel has a recess  18  into which an SCS  2  is placed in such a way that cellular monolayer  20  present on SCS  2  faces away from base  19  of the cap. A cut-off end  9  of the Eppendorf vessel can be placed onto cap  7 . This allows sufficient space above cellular monolayer  20  so that pre-cooled embedding medium  10  can be placed therein. Polymerization of embedding medium  10  occurs under UV irradiation. After curing of embedding medium  10 , the temperature is raised to ambient. After removal of cap  7 , the cured block  22  is pushed out of cut-off end  9  of the Eppendorf vessel. Cellular monolayer  20 , which is still covered by SCS  2 , is present at end  24  of block  22 . In the next process step, SCS  2  is popped off from end  24 . This is done by brief immersion in liquid nitrogen; as a result of the mechanical stresses that build up, SCS  2  can be detached particularly easily. Cellular monolayer  20 , however, remains in embedding medium  10  at the desired end  24 . The production of sections for examinations of cellular monolayer  20  under the electron microscope is existing art.  
         [0034]    [0034]FIG. 6 schematically depicts the method for preparation of a cellular monolayer  20 . Culturing  30  of monolayers  20  of cells on SCSs  2  is accomplished in a conventional tissue culture dish  26 . For subsequent examination, monolayer  20  of cells, or at least a portion thereof, must be removed from tissue culture dish  26 . For that purpose, at least one SCS  2  was already introduced into tissue culture dish  26  before production of monolayers  20  of cells. To begin with, a certain number of SCSs  2  is placed into tissue culture dish  26 . It is important in this context that SCSs  2  be carefully cleaned and sterilized. The tissue cultures and an appropriate nutrient liquid are then introduced into tissue culture dish  26 . It is particularly important in this context to check that SCSs  2  do not begin to float in the nutrient liquid. The suspension of tissue cultures is deposited onto SCSs  2  in the nutrient liquid. The growth of cellular monolayer  20  on SCSs  2  is monitored with a light microscope (not depicted). When SCS  2  is 70%-80% covered, SCS  2  can be removed together with cellular monolayer  20  from tissue culture dish  26 . Removal of SCS  2  is accomplished with a forceps, and drying is performed with filter paper.  
         [0035]    A micropipette tip  28 , or another sharp-pointed object with low thermal conductivity, is equipped with grease  27  (e.g. Vaseline). Removal  40  of SCS  2  from the filter paper is accomplished using the grease. In the center of SCS  2 , SCS  2  is touched on the cell layer with grease  27  on micropipette tip  28 , and adheres. SCS  2  is transferred to freezing step  50 . Liquid ethane is used, as is known, for freezing  50 . As a liquid close to its melting point of approx. −183° C., ethane has the property of rapidly withdrawing heat from the immersed SCS  2 . Rapid cooling rates are necessary in order to minimize ice crystal formation in aqueous specimens, such as cellular monolayers  20 . In this application, SCS  2  is immersed parallel to liquid surface  34  of the ethane, cellular monolayer  20  facing away from the ethane. This has the advantage that cellular monolayer  20  is protected upon immersion of SCS  2 . The reduction in cooling rate is very small, since sapphire possesses good thermal conductivity. Immersion of SCS  2  perpendicular to the surface of the coolant medium yields poor results in specimens with good thermal conductivity, since upon first contact with the coolant medium, parts of SCS  2  that have not yet been immersed are already being pre-cooled.  
         [0036]    The cooled SCSs  2  are transferred into the CSS without allowing their temperature to rise above −100° C. Each SCS  2  is introduced vertically into an orifice  3  of the insert. Insert  1  and container  6  are then assembled and transferred into the CSS for further treatment of cellular monolayers  20  on SCSs  2 . As already described in FIG. 1, upon substitution  60  of the water in the cells of cellular monolayer  20 , various media that substitute for the water in the cells of cellular monolayer  20  are delivered to container  6 .  
         [0037]    After the completion of substitution  60 , low-temperature embedding is performed; from this, the user obtains a cured block  22  at whose end  24  cellular monolayer  20  is located. Curing  70  is performed with UV light. Production  80  of thin sections of cellular monolayer  20  can then be commenced as the next step. Since cellular monolayer  20  is arranged directly at the end, sectioning immediately yields usable thin sections for subsequent electron-microscope examination  90 . The preparation method just described makes it possible to obtain, immediately at the beginning of thin-section production  80 , usable thin sections for microscopic examination  90 .  
         [0038]    The tables below present exemplary embodiments for the preparation of cellular monolayers  20 . Additions of uranyl acetate (UA), OSO 4 , and glutaraldehyde (GA) serve to stabilize and enhance the contrast of the cells, and can be added to the substitution medium (methanol or acetone).  
         [0039]    A) Substituting water in the cells of the cellular monolayer with methanol containing 0.5 vol % uranyl acetate (UA).  
                                                                                 Temperature           Process step   Reagent   Time (min.)   (° C.)   Slope                                Begin   Methanol   12   −85           substitution   containing 0.5           vol % UA       Change       9   to −40   +5       temperature       Temperature       30   −40       equilibrium       Substitute   Mixture of 3 parts   180   −40           and 1 part for 60           min.           Mixture of 2 parts           and 1 part for 60           min.           Mixture of 1 part           and 1 part for 60           min.       Introduce   Lowicryl HM20   4   −40       embedding       medium       Cure   Irradiate Lowicryl   36   −40           HM20 with UV           light                  
 
         [0040]    B) Substituting water in the cells of the cellular monolayer with acetone containing 0.5 vol % uranyl acetate (UA), 2 vol % OSO 4 , and 0.25 vol % GA.  
                                                                                 Temperature           Process step   Reagent   Time (min.)   (° C.)   Slope                                Begin   Acetone containing   12   −90           substitution   0.5 vol % uranyl           acetate (UA), 2           vol % OsO 4 , and           0.25 vol % GA       Change       6   to −60   +5       temperature       Temperature       2   −60       equilibrium       Change       2   to −20   +20       temperature       Temperature       2   −20       equilibrium       Introduce   Epon 812, Spurrs,   30   +4       embedding   and Araldite       medium                  
 
         [0041]    C) Substituting water in the cells of the cellular monolayer with acetone containing 0.5 vol % uranyl acetate (UA) and 2 vol % OSO 4  
                                                                             Time   Temperature           Process step   Reagent   (min.)   (° C.)   Slope                                Begin   Acetone containing   12   −90           substitution   0.5 vol % uranyl           acetate (UA) and 2           vol % OsO 4         Change       6   to −60   +5       temperature       Temperature       2   −60       equilibrium       Change       2   to −20   +20       temperature       Temperature       2   −20       equilibrium       Introduce   Epon 812, Spurrs,   30   +4       embedding   and Araldite       medium                  
 
         [0042]    The invention was described with reference to a particular embodiment. It is self-evident, however, that changes and modifications can be made without thereby leaving the range of protection of the following claims.