Abstract:
An observation apparatus includes a sample tray that holds a container housing a cultured cell and a culture medium and covered with a lid; an observation unit that serves for observation of the cultured cell; and a shifting unit that relatively shifts the sample tray and a light axis of the observation unit along a plane that is orthogonal to the light axis of the observation unit, wherein the sample tray includes a container holding unit that holds the container by utilizing an elastic force, and at least one of the lid and one portion of the lid is made detachable, with the container being held in the sample tray by the container holding unit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2004-361784, filed on Dec. 14, 2004, the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to an observation apparatus used for observing a cultured cell.  
         [0004]     2. Description of the Related Art  
         [0005]     In the field of biological research, various experiments have been conducted with the use of cultured cells for the study of dynamic changes in living organisms. The cultured cells are placed in a carbon dioxide incubator that is kept at an inner temperature of 37° C., a carbon dioxide gas concentration of 5%, and a humidity of 100%, together with a liquid referred to as a culture medium made from bovine serum or the like, so that the activities of the cultured cells are maintained. Moreover, observation apparatuses, which allow observation while the activities of the cultured cells are maintained with the use of a microscope equipped with functions of the carbon dioxide incubator, have been commercially available.  
         [0006]     Cultured cells and a culture medium are generally cultured in a dish that is made of plastic or glass and has a cylindrical form in its outer shape or a petri dish which is available in various shapes. Upon observation under the microscope, the use of an appropriate holder with a suitable shape for the container is required. Otherwise, the observable area may be limited or the container may fall onto the microscope.  
         [0007]     For this reason, Japanese Patent Application Laid-Open (JP-A) No. H11-202213 proposes a sample holder capable of holding petri dishes of various sizes and shapes with the use of a movable petri dish holder.  
         [0008]     Some experiments using such cultured cells last a long period of time, in particular, experiments on cell lineage, in which changes in specific cells are traced, is carried out over a long period of time.  
         [0009]     The culture medium, however, needs to be changed once approximately every three days. Hence, the long-term observation of a specific cell necessarily accompanies the exchange of culture mediums, which is carried out on a clean bench or the like after the removal of the container containing cultured cells from the microscope.  
         [0010]     In JP-A No. H11-202213, though the petri dish can be held with the use of the shiftable petri dish holder, it is difficult to position the petri dish within a range of approximately 5 μm that is required for a high-magnification observation under a microscope.  
         [0011]     Even if the petri dish is positioned within a range of approximately 5 μm with the use of the petri dish holder, it is impossible to position a cell located out of a center within a range of about 5 μm, because the round petri dish cannot controlled in the rotation direction.  
         [0012]     Thus, during the long-term observation requiring the exchange of the culture medium, the specific cell practically cannot be observed immediately before and after the exchange of the culture medium.  
       SUMMARY OF THE INVENTION  
       [0013]     An observation apparatus according to one aspect of the present invention includes a sample tray that holds a container housing a cultured cell and a culture medium and covered with a lid; an observation unit that serves for observation of the cultured cell; and a shifting unit that relatively shifts the sample tray and a light axis of the observation unit along a plane that is orthogonal to the light axis of the observation unit, and the sample tray includes a container holding unit that holds the container by utilizing an elastic force, and at least one of the lid and one portion of the lid is made detachable, with the container being held in the sample tray by the container holding unit.  
         [0014]     According to the present invention, the observation apparatus is provided, which allows for an observation of a specific cell even immediately before and after the culture medium exchanges with a good positional reproducibility.  
         [0015]     The above and other objects, features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is a schematic sectional view of a culture observation apparatus according to a first embodiment of the present invention;  
         [0017]      FIG. 2  is a plan view of a sample tray shown in  FIG. 1 ;  
         [0018]      FIG. 3  is a sectional view taken along line III-III of  FIG. 2 ;  
         [0019]      FIG. 4  is a partial enlarged view of the section shown in  FIG. 3 ;  
         [0020]      FIG. 5  is a sectional view taken along line V-V of FIG.  2 ;  
         [0021]      FIG. 6  is a sectional view taken along line VI-VI of  FIG. 2 ;  
         [0022]      FIG. 7  is a plan view of a sample tray employed in a culture observation apparatus according to a second embodiment of the present invention;  
         [0023]      FIG. 8  is a sectional view taken along line VIII-VIII of  FIG. 7 ;  
         [0024]      FIG. 9  is a schematic drawing of an observation apparatus according to a third embodiment of the present invention;  
         [0025]      FIG. 10  shows a stage and a sample tray shown in  FIG. 9 ;  
         [0026]      FIG. 11  is a plan view of a container holding mechanism installed in the sample tray; and  
         [0027]      FIG. 12  is a sectional view taken along line XII-XII of  FIG. 11 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]     Embodiments of the present invention will be described in detail below with reference to the accompanying drawings.  
         [0029]     A first embodiment is directed to a culture observation apparatus for culturing and observing a cell in culture. The culture observation apparatus basically includes a culture device (incubator) used for culturing a sample and a microscope used for observing the sample, which are combined with each other.  FIG. 1  is a schematic sectional view of a culture observation apparatus according to the first embodiment of the present invention.  
         [0030]     As shown in  FIG. 1 , the culture observation apparatus  100  includes a culture device main body  190 , a culture device sub-main body  140 , and a main-body supporting base  110  that supports these.  
         [0031]     The main-body supporting base  110  has a plurality of leg members  114 .  
         [0032]     The culture device sub-main body  140  is provided with a lower base portion  142  that is supported by the leg members  114 , a side wall  144  that surrounds the upper periphery of the lower base portion  142 , and an upper base portion  146  that covers an opening on the upper side of the side wall  144 .  
         [0033]     The upper base portion  146  is supported by a plurality of supporting pillars  148  that stand on the lower base portion  142 . The upper base portion  146  and the side wall  144  are made in contact with each other through a seal member  150 , with a gap between these being kept in an air-tight state. The side wall  144  has a hollow structure including a heat-insulating space  152 , and a heater  154  is installed inside the heat-insulating space  152  of the side wall  144 .  
         [0034]     The lower base portion  142  has a through hole  142   a  having a diameter of about 30 mm, which allows the inner space of the culture device sub-main body  140  to communicate with outside air.  
         [0035]     The culture device main body  190  is provided with a box-shaped case member  192  with an opening on the bottom face. The case member  192  is attached to the side wall  144  by hinges  194  so as to be opened and closed with respect to the culture device sub-main body  140 . A seal member  200  is placed between the case member  192  and the upper base portion  146 , and when closed, the case member  192  is made in contact with the upper base portion  146  through the seal member  200 , with a gap between the case member  192  and the upper base portion  146  being kept in an air-tight state. The case member  192  has a hollow structure including a heat-insulating space  204 , and a heater  206  is installed inside the heat-insulating space  204  of the case member  192 .  
         [0036]     When the case member  192  is closed, the culture device sub-main body  140  and the culture device main body  190  are allowed to form a culture space  202  used for culturing a sample.  
         [0037]     A gas supplying flow path  208 , used for supplying a gas such as a carbon dioxide gas to the culture space  202 , is connected to the case member  192 . The gas supplying flow path  208  is connected to a gas supplying source  210 , and a valve  212  used for controlling the amount of supply of the gas is installed in the middle of the gas supplying flow path  208 .  
         [0038]     The culture observation apparatus  100  is provided with a tray attaching unit  252  to which a sample tray  550  is attached and a horizontal shifting mechanism  260  used for shifting the tray attaching unit  252  horizontally inside the culture space  202 .  
         [0039]     The tray attaching unit  252  has a tray receiving unit  254  that receives the sample tray  550 , a protruding portion  256  that protrudes upward from the tray receiving unit  254  and a rotation shaft  258  that extends downward from the tray receiving unit  254 . The rotation shaft  258  is rotatably supported by a mechanism not shown.  
         [0040]     The upper base portion  146  has a through hole  146   a , and the rotation shaft  258  of the tray attaching unit  252  extends through the through hole  146   a  of the upper base portion  146 . A gap between the upper face of the upper base portion  146  and the lower face of the tray receiving unit  254  is preferably set to 0.1 mm or less in order to preferably suppress a leak of moisture. Moreover, in order to further suppress the leak of moisture, an elastic member may be placed between the upper face of the upper base portion  146  and the lower face of the tray receiving unit  254 .  
         [0041]     The horizontal shifting mechanism  260  is provided with a motor  262  used for rotating the tray attaching unit  252 , a motor supporting member  264  that supports the motor  262 , a linear guide  266  that shiftably supports the motor supporting member  264 , a ball screw  268  that is engaged with the motor supporting member  264 , a motor  270  used for driving the ball screw  268 , and a motor supporting member  272  that supports the motor  270 .  
         [0042]     The motor supporting member  264  is attached to the upper base portion  146  through the linear guide  266 , and allowed to shift laterally with respect to the upper base portion  146 . Here, the motor supporting member  272  is secured to the upper base portion  146 . Moreover, the ball screw  268  converts a rotation movement of the shaft of the motor  270  to a linear movement of the motor supporting member  264 .  
         [0043]     The microscope, which is an observation unit of the culture observation apparatus, is provided with an objective optical unit  310  and an image-forming optical unit  340 . The objective optical unit  310  is housed inside the culture device sub-main body  140 . The image-forming optical unit  340  is placed on a lower outer portion of the culture device sub-main body  140 .  
         [0044]     The objective optical unit  310  is provided with an objective lens  312  and a focusing mechanism  320  used for shifting the objective lens  312  upward and downward.  
         [0045]     The focusing mechanism  320  is provided with an objective lens supporting member  322  that supports the objective lens  312 , a linear guide  324  that shiftably supports the objective lens supporting member  322 , a ball screw  326  that is engaged with the objective lens supporting member  322 , a motor  328  used for driving the ball screw  326 , and a motor supporting member  330  that supports the motor  328 .  
         [0046]     The motor supporting member  330  is secured to the upper base portion  146 . The objective lens supporting member  322 , which is attached to the motor supporting member  330  through the linear guide  324 , is capable of shifting upward and downward with respect to the motor supporting member  330 . The ball screw  326  converts a rotation movement of the shaft of the motor  328  to a linear movement of the objective lens supporting member  322 .  
         [0047]     The upper base portion  146  has a through hole  146   b , and the objective lens  312  extends through the through hole  146   b  of the upper base portion  146 . A gap between the through hole  146   b  of the upper base portion  146  and the objective lens  312  is preferably set to 0.1 mm or less in order to preferably suppress a leak of moisture. Moreover, in order to further suppress the leak of moisture, an elastic member may be placed between the through hole  146   b  of the upper base portion  146  and the objective lens  312 .  
         [0048]     The image-forming optical unit  340  is provided with an image-forming lens  342  and an imaging device  344 . The lower base portion  142  has a through hole  142   b , and the image-forming optical unit  340  is optically coupled to the objective lens  312  through the through hole  142   b  of the lower base portion  142 . Moreover, the lower base portion  142  has an image-forming optical unit attaching unit  142   c , and the image-forming optical unit  340  is attached to the image-forming optical unit attaching unit  142   c.    
         [0049]     Moreover, the microscope is provided with a transmissive lighting optical system used for providing transmissive lighting of the sample  510 . The transmissive lighting optical system is provided with an illuminating light source  372  that is attached to an outer wall  192   a  of the case member  192  in a tightly-sealed state, and an optical window  374  that is formed in an inner wall  192   b  of the case member  192  in a tightly-sealed state. Both of the illuminating light source  372  and the optical window  374  are located above the objective lens  312 . The illuminating light source  372  emits illuminating light, and the optical window  374  allows the illuminating light to pass therethrough.  
         [0050]     Moreover, the microscope is provided with an excitation lighting optical system used for exciting the sample  510 , though not shown in the drawings.  
         [0051]      FIG. 2  is a plan view of the sample tray shown in  FIG. 1 .  FIG. 3  is a sectional view taken along line III-III of  FIG. 2 .  FIG. 4  is an enlarged view of one portion of  FIG. 3 .  FIG. 5  is a sectional view taken along line V-V of  FIG. 2 .  
         [0052]     As shown in  FIG. 2 , the tray receiving unit  254  of the tray attaching unit  252  has a round shape, and the upper face thereof is orthogonal to the light axis of the objective lens  312 . The protruding portion  256  of the tray attaching unit  252  has a substantially pentagonal shape. The protruding portion  256  has two male dovetails  256   a  and  256   b  that are respectively formed on the two side faces adjacent with each other. Each of the male dovetails  256   a  and  256   b  has a slope that tilts outward at approximately 60 degrees. Moreover, the protruding portion  256  is provided with a pressing face  256   c  that is different from the two side faces respectively having the male dovetails  256   a  and  256   b . The pressing face  256   c  is substantially orthogonal to the bisector of each of the two side faces having the respective male dovetails  256   a  and  256   b.    
         [0053]     The sample tray  550  is provided with an opening  554  that has a substantially rectangular shape and is formed in the center, and a depressed plane  556  that has a round shape (see  FIG. 3 ) and is formed on the bottom face outside of the opening  554 . The opening  554  is larger than the protruding portion  256  so that the protruding portion  256  is allowed to pass through the opening  554 . The depressed plane  556  is larger than the tray receiving unit  254  so that the upper face of the tray receiving unit  254  can be made in face-contact with the depressed plane  556 .  
         [0054]     As shown in  FIGS. 3 and 4 , the sample tray  550  is provided with a female dovetail  558  that is made in face-contact with the male dovetail  256   a  of the protruding portion  256 , and formed on the side face of the opening  554  that faces the male dovetail  256   a  of the protruding portion  256 .  
         [0055]     As shown in  FIG. 5 , the sample tray  550  also has a depressed portion  562  that receives a ball  560  that is placed in contact with the male dovetail  256   b , and formed on the side face of the opening  554  that faces the male dovetail  256   b  of the protruding portion  256 .  
         [0056]     As shown in  FIGS. 2 and 3 , the sample tray  550  is provided with a female screw  564  that extends between the outer peripheral side face and the side face of the center opening  554 , and a fixed screw  566  that is meshed with the female screw  564 . The female screw  564  and the fixed screw  566  form pressing means that press the pressing face  256   c  of the protruding portion  256 .  
         [0057]     The sample tray  550  is attached to the tray attaching unit  252  in the following manner.  
         [0058]     The sample tray  550  is mounted on the tray attaching unit  252  with the fixed screw  566  drawn therein. In this state, the bottom face of the sample tray  550  is made in face-contact with the tray receiving unit  254 , with the protruding portion  256  being positioned inside the opening  554  of the sample tray  550 . The fixed screw  566  is bolted to be pressed against the pressing face  256   c . The sample tray  550  is shifted by the resulting reaction force, and pushed against the protruding portion  256 .  
         [0059]     The fixed screw  566  is bolted appropriately so that the sample tray  550  is secured onto the tray attaching unit  252 . In this state, by an interaction between the male dovetail  256   a  and the female dovetail  558  as well as by an interaction among the male dovetail  256   b , the ball  560 , and the depressed portion  562 , the depressed plane  556  of the sample tray  550  is positively pushed against the upper face of the tray receiving unit  254  so that the sample tray  550  is maintained horizontally. In other words, the sample tray  550  is placed in such a manner that the upper face thereof is made in parallel with the plane that is orthogonal to the light axis of the objective lens  312 . Moreover, since the sample tray  550  is made in contact with the tray attaching unit  252  through one face of the female dovetail  558  and the ball  560 , the sample tray  550  is always secured onto the tray attaching unit  252  with good reproducibility.  
         [0060]      FIG. 6  is a sectional view taken along line VI-VI of  FIG. 2 .  FIG. 6  includes the sample  510  drawn together with the apparatus.  
         [0061]     As shown in  FIG. 6 , the sample  510  includes a container  512  that houses a cultured cell and a culture medium, and a lid  518  that covers the container  512 . The container  512  is optically transparent.  
         [0062]     The container  512  includes a container main body  514  and a cover glass  516 . The container main body  514  is a Schale-shaped plastic container having a diameter of 35 mm, with an opening  514   a  having a diameter of about 10 mm formed in the bottom portion. The cover glass  516  is a cover glass having a thickness of 0.17 mm, which has been widely used in the microscope field, and closes the opening  514   a  of the bottom portion of the container main body  514 .  
         [0063]     The objective lens  312  requires a high NA for brightness and resolution. In general, an objective lens with a high NA is optimally designed to be used with a cover glass having a thickness of 0.17 mm. Since the container  512  has a cover glass of 0.17 mm in thickness at a portion facing the objective lens  312 , a generally-used objective lens with a high NA can be used as the objective lens  312 .  
         [0064]     As shown in  FIG. 2 , the sample tray  550  has a plurality of openings  568  that allow observation through the objective lens  312  from below and depressed portions  570  formed around respective openings  568 . The openings  568  are positioned along a circumference with a diameter of approximately 160 mm substantially from a center of the sample tray  550 . The opening  568  has, for example, a shape close to a square, with each side being set to approximately 25 mm, and is designed to such a size that, with respect to relative shifts of the sample tray  550  and the objective lens  312 , the two members are prevented from interfering with each other. The depressed portion  570  has a shape close to a square with each side being set to approximately 40 mm, and the container  512  can be mounted thereon. The container  512  is placed inside the depressed portion  570  so that the depressed portion  570  supports the container  512 . Moreover, each depressed portion  570  is provided with two protrusions  571 , which, when the container  512  is mounted on the depressed portion  570 , are made in contact with the container  512  to serve as stoppers that perform positioning.  
         [0065]     The sample tray  550  is provided with a container holding mechanism that holds the container  512  and is located between the protruding sections  570 . The container holding mechanism is provided with a container holding member  572  made of a plate spring, and fixed screws  574  used for attaching the container holding member  572  to the sample tray  550 .  
         [0066]     Moreover, the container holding member  572  is secured to the upper portion of a holding member attaching unit  551 , which is provided between the protruding sections  570 , with the fixed screws  574 . The holding member attaching unit  551  is allowed to protrude from the surface of the sample tray  550  so as to maintain the container holding member  572  at an appropriate height so as to secure the container  512  and the lid  519 . Here, the container holding member  572  made of the plate spring and the surface of the sample tray  550  are made substantially in parallel with each other.  
         [0067]     As shown in  FIG. 6 , the lid  518  includes an edge portion  519  and a center portion  520 , and the edge portion  519  and the center portion  520  can be separated from each other. The center portion  520  has an opening  520   a  with a cover glass  521  being fixed thereon in a manner so as to cover the opening  520   a . Moreover, the edge portion  519  has an opening  519   a  so that, when the center portion  520  is attached to the edge portion  519 , the center portion  520  plugs the opening  519   a.    
         [0068]     The peripheral portion of the opening  519   a  of the edge portion  519  is processed into a tapered shape that expands downwards, and the container holding member  572  is designed to press the portion of the edge portion  519  formed into the tapered shape. With this arrangement, the container holding member  572  allows the container  512  and the lid  518  to be maintained on the sample tray  550  in a stable manner.  
         [0069]     Upon securing the container  512  and the lid  518 , in a state where the lid  518  is mounted on the container  512 , the container  512  and the lid  518  are inserted onto the depressed portion  570  from the outer peripheral side of the sample tray  550 . In this case, the container  512  and the lid  518  are inserted onto the depressed portion  570  until the container  512  comes into contact with the protrusions  571 . In the state where the container  512  is in contact with the protrusions  571 , the container holding members  572  on two sides are allowed to press the edge portion  519  of the lid  518  in a diameter direction so that it becomes possible to prevent the container  512  and the lid  518  from jumping outside of the depressed portion  570  or from moving on the depressed portion  570 . Here, when the container  512  and the lid  518  are inserted onto the depressed portion  570 , the center portion  520  may be attached to the edge portion  519  or may be removed therefrom.  
         [0070]     The container holding member  572  is designed so that the center portion  520  of the lid  518  can be removed, when the edge portion  519  of the lid  518  is pressed with the container  512  and the edge portion  519  of the lid  518  being held on the sample tray  550 . For this reason, for example, upon exchanging culture mediums, the culture mediums can be exchanged with only the center portion  520  of the lid  518  being removed. In other words, operations such as the exchanging operation of culture mediums can be carried out without shifting the position of the container  512 . Here, the container holding member  572 , shown in  FIGS. 2 and 6 , has a shape extending toward the depressed portions  570  on two sides from one holding member attaching unit  551 ; however, the container holding member  572  is not intended to be limited to this structure.  
         [0071]     As shown in  FIG. 1 , upon using the culture observation apparatus  100 , the sample tray  550  holding a plurality of samples  510  is attached to the tray attaching unit  252 , and a moisturing pad  214  containing pure water is placed in the culture space  202 . The culture space  202  is controlled to 37° C. in its inside temperature by the heater  206 , and also to 5% in its carbon dioxide concentration by the valve  212 . The inner space of the culture device sub-main body  140  is controlled to 37° C. in its inside temperature by the heater  154 .  
         [0072]     Since the culture space  202  is hardly influenced by the outside air by the heat-insulating space  204  of the case member  192 , and since the inner space of the culture device sub-main body  140  is kept at 37° C., the inner temperature of the culture space  202  is favorably maintained at 37° C. Moreover, since the moisture generated in the culture space  202  is hardly leaked outside, the inside of the culture space  202  is maintained at a high moisture state close to 100%.  
         [0073]     Since the through hole  142   a  that connects the inner space of the culture device sub-main body  140  to the outside space has a small diameter, only a little outside air is allowed to flow into the culture device sub-main body  140 . Moreover, since the inner space of the culture device sub-main body  140  is enclosed by the heat-insulating space  152 , it is hardly influenced by the outside air. For this reason, the objective lens  312  and the focusing mechanism  320 , placed inside the culture device sub-main body  140 , are desirably maintained at 37° C. without being influenced by the outside air. When the objective lens  312  and the focusing mechanism  320  are influenced by temperatures, defocusing tends to occur easily; however, since this structure maintains the temperature of the objective lens  312  and the focusing mechanism  320  at a constant temperature, it is possible to favorably prevent the occurrence of defocusing.  
         [0074]     Even if slight moisture invades inside the culture device sub-main body  140  from the culture space  202 , since the moisture is diffused into the outside air through the through hole  142   a , the inside of the culture device sub-main body  140  is maintained at a low level of moisture. Consequently, it becomes possible to prevent the objective lens  312  from dew condensation and also to prevent the focusing mechanism  320  from rusting.  
         [0075]     Upon observation, the sample  510  located above the objective lens  312  is observed. The sample  510  to be observed can be switched with the substantial rotation of the sample tray  550  by the horizontal shifting mechanism  260 . The observation site within the sample  510  is adjusted by the shift of the sample tray  550  along the plane orthogonal to the light axis of the objective lens  312  by the horizontal shifting mechanism  260 . This adjustment is achieved through the combination of the rotation and the translational shift of the sample tray  550 . The rotation and the translational shift are carried out within a range in which the tip of the objective lens  312 , located inside the opening  568 , is kept from contacting the sample tray  550 .  
         [0076]     In the first embodiment, the diameter of the container  512  is 35 mm, and eight samples  510  are arranged on the sample tray  550  along the circumference having a diameter of 160 mm. Since the switching process of the samples  510  is carried out by the rotation of the sample tray  550 , no translational shift of the sample tray  550  is required to switch the samples  510 . Although the sample tray  550  is translation-shifted so as to adjust the observation position, the amount thereof is limited to approximately 10 mm. Therefore, there is only a little space that allows the sample tray  550  to shift.  
         [0077]     The following description will discuss a comparative example in which nine samples  510  are arranged in a lattice format with longitudinal and lateral positions of 3×3, and switched by using X and Y stages. In this case, translational shifts of the X and Y stages of 80 mm or more are required for the respective X and Y directions. In contrast, in the first embodiment, the sample tray  550  requires no translational shift for switching the samples  510 , and only the translational shift of approximately 10 mm is required for the adjustment of the observation position. Therefore, in comparison with the device using the X and Y stages, the space required for the switching of the samples  510  and the adjustment of the observation position is reduced greatly. This is advantageous in achieving a small-size apparatus at low costs.  
         [0078]     Moreover, upon carrying out a long-term observation, an exchange of culture mediums is required. The exchange of culture mediums is carried out with each of the sample trays  550  being removed from the tray attaching unit  252 . In other words, during the exchange of culture mediums, the container  512  and the edge portion  519  of the lid  518  are maintained on the sample tray  550 , and firmly secured by the container holding mechanism so that no positional deviations occur. After the exchange of culture mediums, the sample tray  550  is attached again to the tray attaching unit  252  in the same state as the state before the exchange of culture mediums.  
         [0079]     In this manner, the first embodiment makes it possible to exchange culture mediums without the necessity of removing the container  512  and the edge portion  519  of the lid  518  from the sample tray  550  and also to attach the sample tray  550  to the tray attaching unit  252  with good positional reproducibility; therefore, the container  512  is properly placed at the original position. Thus, it becomes possible to observe a specific cell for a long time.  
         [0080]     In the first embodiment, the case member  192  can be opened and closed with respect to the culture device sub-main body  140 ; however, not limited to this structure, the case member  192  may be secured to the culture device sub-main body  140 . In this case, the case member  192  is required to have an opening formed on a side face and a lid for closing the opening of the side wall so as to bring in and take out the sample tray  550  and a moisturing pad  214 . Moreover, the case member  192  may be designed so that one portion of the bottom has an opening with the opening being closed when it is attached to the culture device sub-main body  140 .  
         [0081]     A second embodiment is directed to a sample tray  650  that can replace the sample tray  550  according to the first embodiment.  FIG. 7  is a plan view of a sample tray in accordance with the second embodiment of the present invention.  FIG. 8  is a sectional view taken along line VIII-VIII of  FIG. 7 . In  FIG. 8 , a sample is drawn together with the sample tray. In  FIGS. 7 and 8 , the same elements as the elements according to the first embodiment are denoted by the same reference characters as in the first embodiment.  
         [0082]     As shown in  FIG. 8 , a sample  610  includes a container  612  that houses a cultured cell and a culture medium and a lid  618  that covers the container  612 . Both of the container  612  and the lid  618  are optically transparent.  
         [0083]     The container  612  includes a container main body  614  and a cover glass  616 . The container main body  614  is a Schale-shaped plastic container having a diameter of 35 mm, with an opening  614   a  having a diameter of about 10 mm formed in the bottom portion. The cover glass  616  is a cover glass having a thickness of 0.17 mm, which is widely used in the microscope field, and is allowed to plug the opening  614   a  of the bottom portion of the container main body  614 .  
         [0084]     As shown in  FIG. 7 , the sample tray  650  has a plurality of openings  668  that allow observation through the objective lens  312  from below and depressed portions  670  formed around respective openings  668 . The openings  668  are positioned along the circumference with a diameter of approximately 160 mm substantially from a center of the sample tray  650 . The opening  668 , which has a diameter of approximately 25 mm, has such a size that, with respect to relative shifts of the sample tray  650  and the objective lens  312 , the two members are prevented from intervening with each other. The diameter of the depressed portion  670  is approximately 40 mm, which is slightly larger than the diameter of the container  612 . The container  612  is placed inside the depressed portion  670  so that the depressed portion  670  supports the container  612 .  
         [0085]     Moreover, the sample tray  650  is provided with a container holding mechanism used for holding the container  612  on each of the depressed portions  670 . The container holding mechanism is provided with two fixed container holding members  672 , a shiftable container holding member  674 , and a coil spring  682  that is pressing the container holding member  674 .  
         [0086]     Two container holding members  672  and the single container holding members  674  are placed around the depressed portion  670  at equal intervals, that is, with an angular interval of 120 degrees. The container holding member  672  has a contact portion  672   a  that is made in contact with the container  612 , and the contact portion  672   a  has a sharp tip. The tip of the contact portion  672   a  has, for example, a diameter of about 50 μm. Moreover, the container holding member  674  has a contact portion  674   a  that is made in contact with the container  612 , and the contact portion  674   a  has a sharp tip. The tip of the contact portion  674   a  has, for example, a diameter of about 50 μm.  
         [0087]     The container holding member  672 , which has a column shape, is housed into a hole  676  that is formed in the sample tray  650 , and secured by a fixed screw  678 . The securing position of the container holding member  672  is changeable, and adjusted in accordance with the size of the container  612 .  
         [0088]     The container holding member  674  is provided with a column-shaped main-body portion  674   b , and an end portion  674   c  having a column shape that has a size larger than the main-body portion  674   b . The container holding member  674  is housed in a hole  680  formed in the sample tray  650 , and allowed to freely shift inside the hole  680 . The hole  680  has a small-diameter portion  680   a  and a large-diameter portion  680   b  so that the main-body portion  674   b  of the container holding member  674  is housed in the small-diameter portion  680   a  and the end portion  674   c  of the container holding member  674  is housed in the large-diameter portion  680   b . A coil spring  682  is arranged inside the large-diameter portion  680   b , and the coil spring  682  presses the end portion  674   c . A knob  684  is secured to the main-body portion  674   b  of the container holding member  674 , and the knob  684  protrudes from the upper face of the sample tray  650 . With the manipulation of the knob  684 , the container holding member  674  can be shifted against the elastic force of the coil spring  682 .  
         [0089]     Upon securing the container  612 , the container holding member  674  is retreated by the manipulation of the knob  684 , and after the container  612  is placed on the bottom of the depressed portion  670 , the knob  684  is released. Thus, the container  612  is pressed by the container holding member  674 , and made in contact with the two container holding members  672 . Since the container holding member  674  continues to press the container  612  by a predetermined force that is determined by the coil spring  682 , the container  612  is held by the two container holding members  672  and the single container holding member  674 .  
         [0090]     Since both of the contact portion  672   a  of the container holding member  672  and the contact portion  674   a  of the container, holding member  674  have a tip having a diameter of about 50 μm, these contact portions cut into the container  612 . For this reason, though the container  612  has a shape expanding upward from the bottom face, it is positively held without being push and displaced upward. With this arrangement, upon removing the lid  618  from the container  612  for the exchange of culture mediums or the like, the container  612  is prevented from unexpectedly moving and rotating. In other words, the operations such as exchanging culture mediums can be carried out without the necessity of shifting the position of the container  612 . Moreover, since the portion close to the bottom face of the container  612  is pressed, the lid of the attached container may also be utilized.  
         [0091]     A third embodiment is directed to an observation apparatus used for observing a cultured cell. The observation apparatus is formed by an inverted microscope system.  
         [0092]      FIG. 9  schematically shows an observation apparatus in accordance with the third embodiment of the present invention.  FIG. 10  shows a stage and a sample tray that are shown in  FIG. 9 .  FIG. 11  is a plan view that shows a container holding mechanism that is installed in the sample tray.  FIG. 12  is a sectional view taken along line XII-XII of  FIG. 11 . In  FIG. 12 , a sample is also drawn together.  
         [0093]     As shown in  FIG. 9 , a microscope system  700  includes an objective lens  712 , a focusing mechanism  714  used for shifting the objective lens  712  upwards and downwards, and a stage  720  on which a sample tray  750  holding a sample  730  is mounted. The stage  720  can be shifted horizontally. The stage  720  is provided with a depressed portion  722  that has a size larger than the sample tray  750  and an opening  724  that has a size smaller than the depressed portion  722 .  
         [0094]     As shown in  FIG. 10 , the depressed portion  722  of the stage  720  has contact faces  726  at three positions. When placed in the depressed portion  722  of the stage  720 , the sample tray  750  is pressed onto the contact faces  726  at the three positions by the operator to be positioned, and secured onto the stage  720  with four fixed screws  728 .  
         [0095]     As shown in  FIG. 12 , the sample  730  includes a container  732  and a lid  734 . The lid  734  is provided with a lid main-body  736  having a round opening and a glass plate  738  used for covering the opening of the lid main-body  736 . The container  732  has the same structure as the containers  512  and  612  described above in the first embodiment.  
         [0096]     As shown in  FIG. 10 , the sample tray  750  has a rectangular shape, and is provided with openings  752  that allow observation through an objective lens  712  from below. The openings  752  are arranged into a lattice format. Moreover, as shown in  FIGS. 11 and 12 , the sample tray  750  is provided with depressed portions  754  each formed around the respective openings  752 , and a lid mounting face  756  that is formed on the periphery of each depressed portion  754  for each of the openings  752 . The container  732  is placed inside the depressed portions  754  so that the depressed portions  754  support the container  732 . Here, the lid  734  is mounted on the lid mounting face  756 .  
         [0097]     Moreover, the sample tray  750  is provided with a container holding mechanism used for holding the container  732 , for each of the depressed portions  754 . The container holding mechanism includes a plate spring  758  used for pressing the container  732 , and a fixed screw  760  for securing the plate spring  758  onto a spring mount face  762  of the sample tray  750 . The spring mount face  762  is located at a position lower than the lid mounting face  756 . The plate spring  758  is bent into a crank shape, and extends between the container  732  and the lid  734  so that the resulting pressing force is applied onto the upper face of the container  732 . Thus, the container  732  is secured onto the sample tray  750 .  
         [0098]     In the third embodiment, since the lid  734  is opened and closed without the application of a force onto the plate spring  758 , there is no possibility of relative positional deviations of the sample  730  with respect to the sample tray  750  when culture mediums are exchanged. Moreover, since the sample tray  750  is pressed onto the contact faces  726  at the three positions formed in the depressed portion  722  of the stage  720  for the positioning, the sample tray  750  is accurately secured to the original position before the culture medium exchange.  
         [0099]     The third embodiment makes it possible to provide an apparatus having a simple shape, and consequently to achieve an inexpensive apparatus.  
         [0100]     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.