Abstract:
A culturing device having one or more culture containers adapted to receive and to discharge a culture medium or nutrient solution is disclosed. The device has a cell culture insert removably received in each of the one or more containers. The device also has at least one supply mechanism for introducing the culture medium into each of the one or more culture containers and for discharging the culture medium from the one or more containers. The device also has at least one level sensor that is arranged to sense a level of the culture medium for the one or more culture containers. The at least one sensor controls the supply mechanism as a function of an output signal from the level sensor that represents the level of the culture medium. The device can carry out a submerged nutrient supply and a basal nutrient supply of cell or tissue cultures in a single culturing device. The device can be adapted to periodically alternate between the submerged and the basal supply condition.

Description:
This patent is a continuation of related co-pending international application Serial No. PCT/EP99/00295, filed on Jan. 19, 1999, and published on Jul. 22, 1999 as international publication No. WO 99/36505, and claims priority from U.S. Provisional Application Ser. No. 60/082,817 filed Apr. 23, 1998. 
    
    
     FIELD OF THE INVENTION 
     The invention is generally related to culturing of cells or tissue components, and more particularly to a culturing device and to a method for changing the level of a culture medium within a culture vessel. 
     BACKGROUND OF THE INVENTION 
     Culturing devices are generally known and are used for culturing isolated cells or tissue components in a culturing media. These known culturing devices are limited for use in submerged culture systems. In a submerged system, cultivated cells are supported on the membrane of a cell culture insert and supplied with a culture solution with the appropriate nutrients by layering the solution over the cells. It is not feasible to vary the culture or growth conditions in such a submerged system. 
     For many experimental studies, such as for example, studies utilizing differentiated ciliary or lung epithelial cells, it would be desirable if the tissue growth conditions could be varied during the studies in order to evaluate and solve special problems. As an example, when using a conventional culturing device, culturing of cells of the respiratory tract cannot be carried out on an air-liquid interface, even if porous membranes are used, which are offered by some companies as so-called transwell systems. 
     SUMMARY OF THE INVENTION 
     In a disclosed example of an apparatus constructed in accordance with the teachings of the invention, a culturing device has at least one culture container adapted to receive and to discharge a culture medium. The at least one container has a cell culture insert removably received therein. The device has at least one supply mechanism for introducing the culture medium into the at least one culture container and for discharging the culture medium from the at least one culture container. At least one level sensor cooperates with the at least one culture container to sense a level of the culture medium for the at least one culture container. The sensor controls the supply mechanism as a function of an output signal of the level sensor representing the level of the culture medium such that a submerged culture medium supply condition and a basal culture medium supply condition can both be achieved by the device. 
     In another disclosed example, the at least one cell culture insert provides a horizontal culture surface within the at least one culture container. 
     In another disclosed example, the culturing device has at least one pair of discharge lines in fluid communication with the at least one culture container. In another disclosed example, the at least one culture container comprises a plurality of culture containers, and wherein each pair of the discharge lines is associated with a respective one of the individual culture containers. In an alternative disclosed example, the at least one culture container comprises a plurality of culture containers, and wherein at least one pair of the discharge lines is associated with more than one of the individual culture containers. 
     In another disclosed example, the at least one culture container is positioned so that a culture surface defined by the cell culture insert lies in a common horizontal plane. 
     In another disclosed example, the at least one culture container comprises a plurality of culture containers, and wherein the plurality of culture containers are connected to a common culture medium supply line. 
     In a further disclosed example, a supply line is connected to the at least one culture container and communicates with a riser on which the at least one level sensor is carried. In yet another disclosed example, the at least one level sensor is vertically adjustable relative to the riser. 
     In another disclosed example, the at least one level sensor comprises a plurality of level sensors, and wherein at least one of the plurality of level sensors includes a forked photoelectric barrier. 
     In another disclosed example, the at least one level sensor continuously measures the culture medium level. 
     In another disclosed example, the at least one level sensor comprises a plurality of level sensors, and wherein at least one of the plurality of level sensors includes a level switch that responds to a predetermined target level. 
     In another disclosed example, the culturing device also has an exterior housing and a plurality of discrete modules within the housing. At least one of the at least one culture container is arranged within each of the modules. A culture medium supply distribution system of the device is in fluid communication with a common culture medium supply line and with each of the modules. 
     In another disclosed example, the supply distribution system has a single connection in fluid communication with the at least one supply mechanism. The single connection is disposed at a lowermost elevation of the supply distribution system. 
     In another disclosed example, the exterior housing has a plurality of separate connectors each coupled to a withdrawal line of a respective one of the discrete modules. 
     In another disclosed example, each of the modules includes a discrete temperature-control housing surrounding the at least one of the at least one culture container disposed within the corresponding discrete module. Each temperature-control housing has a temperature-control medium inlet and a temperature-control medium discharge. 
     In another disclosed example, each temperature-control medium discharge is in fluid communication with an overflow apparatus lying in an upper region of the respective temperature-control housing. The overflow apparatus is positioned diametrically opposite to the temperature-control medium inlet within the respective temperature-control housing. 
     In another disclosed example, the temperature control medium inlet and discharge of the temperature-control housing of each of the discrete modules is connected in series relative to a flow path of the temperature-control medium. 
     In another disclosed example, the at least one culture container is disposed within an exterior housing that can provide a sealed environment within an interior of the housing. In another disclosed example, the outer housing has at least one connection for introducing a gaseous medium into the interior of the housing. 
     In another disclosed example, certain components of the culturing device that must be sterilized are fabricated from materials that can withstand sterilization. In another disclosed example, the sterilizable materials are selected from a group comprising at least one of glass and silicone. 
     In another disclosed example, the supply mechanism includes a bidirectional pump. In another disclosed example, the bidirectional pump is a peristaltic pump. 
     In another disclosed example, the at least one sensor comprises a pair of level sensors wherein one of the sensors controls the supply mechanism for the submerged supply condition and the other of the sensors controls the supply mechanism for the basal supply condition. 
     In another disclosed example, the culturing device also has a programmable controller that can control a culture medium target level transducer in a time-dependent manner. In another disclosed example, the programmable controller can adjust a target level value periodically between at least two values. One is a first level that corresponds to a liquid level that is a predetermined distance above a top side of a culture surface of the cell culture insert of the at least one culture container. The other is a second level that corresponds to a liquid level that is a predetermined distance below the first level. 
     In another disclosed example of the teachings of the invention, a method for culturing a culture material such as cells and tissue components includes first providing a culturing device having at least one culture container and at least one nutrient solution supply mechanism. A material to be culture is supported within the at least one culture container to expose a quantity of the culture material for culturing. Nutrient solution is then supplied for the culture material to the at least one culture container such that a liquid level of the nutrient solution can be adjusted between at least a submerged culturing level of the nutrient solution and a basal culturing level of the nutrient solution. 
     In another disclosed example, the method also includes the step of exposing the culture material to at least one other substance during periods where the liquid level is at the basal culturing level, and wherein the at least one other substance is selected from at least one of a solid, gaseous or aerosol-form substance. 
     In another disclosed example, the step of providing the supply mechanism further includes providing a sensor for sensing the liquid level and a pump controlled by the sensor for supplying the nutrient solution. 
     In another disclosed example, the step of supplying is carried out with the aid of a programmable controller to automatically adjust the liquid level over a time period. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 a  is a schematic view of a culturing device constructed in accordance with the teachings of the invention and utilizing only one culture container that has a controllable culture medium level; 
     FIG. 1 b  is an enlargement of the device taken from circle  1   b  in FIG. 1 a;    
     FIG. 2 a  is a schematic view of another culturing device constructed in accordance with the teachings of the invention; 
     FIG. 2 b  is an enlargement of the device taken from circle  2   b  in FIG. 2 a;    
     FIG. 3 is a top view of another culturing device constructed in accordance with the teachings of the invention and utilizing a number of culture modules placed parallel to one another, each having a number of culture containers; 
     FIG. 4 a  is a longitudinal section taken along line IV—IV through a culture module of the culturing device shown in FIG. 3; 
     FIG. 4 b  is an enlargement of the device taken from circle  4   b  in FIG. 4 a;    
     FIG. 5 is an angled transverse section taken along line V—V through the culture module shown in FIG. 4; 
     FIG. 6 is a longitudinal section taken along line VI—VI through the culturing device shown in FIG. 3; 
     FIG. 7 is a transverse section taken along line VII—VII through the culturing device shown in FIG. 3; and 
     FIG. 8 is a circuit diagram of a level height control which is used in connection with the culturing devices shown in FIGS.  1 - 7 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, FIG. 1 a  illustrates an entire culture unit  10  having a culture container  12 . The illustrated culture unit  10  is generally in the form of an inverted bottle. The upper end of the culture unit  10  defines a circular opening  14 . 
     A cell culture insert  15  is placed inside the culture container  12  and is produced from a porous synthetic material, such as for example, polyethylene terephthalate. The cell culture insert  15  has a liquid-permeable carrier structure or membrane  16 , which can be produced from the different synthetic materials, depending on the requirements of the cells to be cultured, again such as for example, polyethylene terephthalate. As is shown in FIG. 1 b  taken from the enlarged portion of FIG. 1 a , the membrane  16  supports and carries a cell culture  18 . 
     The culture container  12  is carried by a holding device  20 . The structure of the device can vary considerably and is therefore only indicated schematically in FIG. 1 a.    
     A connector  22  on the bottom of the culture container  12  is connected by conventional tubing  23  to the working connection of a bidirectional pump  24 . The pump can optionally be a peristaltic pump. The other connection of pump  24  is connected by conventional tubing  26  to the inside of a storage container  28 . The storage container contains a reservoir or source of culture liquid  30 . The culture liquid contains the particular nutrients which are to be made available to the cell culture  18  for its growth. 
     Two control terminals of the pump  24  are respectively connected through power amplifiers  32  and  34  to the outputs of an operating circuit  36 . The operating circuit  36  produces a signal at a first, a second or neither of its outputs in order to actuate the pump  24 . The pump can introduce additional culture liquid from the storage container  28  to the inside of the culture container  12  or can remove culture liquid from the inside of the culture container  12  and return it to the storage container  28 . The pump function depends on the output signal of a continuously operating level sensor  38  that is shown as a sensor mounted to the outside surface of the culture container  12 . Alternatively, the sensor can also be mounted on the inside surface of the culture container  12 . The sensor  38  can be an optical sensor in practice and operate as a function of a target value transducer  40 , which is shown as an adjustable resistor. The target value transducer  40  can be adjusted by a programmed controller  42  as indicated in FIG. 1 a  by a dotted line. 
     In one practical example discussed herein, the programmed controller  42  switches the target value transducer  40  between two positions. One position of the target value transducer  40  corresponds to a level height of the culture liquid  30  inside the culture container  12 . As shown in the drawing, the culture liquid  30  just touches the bottom side of the cell culture insert  15 . Under these conditions, the cell culture  18  receives culture liquid only from underneath. This condition is known and is described herein as basal nutrient supply. 
     The target value transducer  40  is switched to the second position by the programmed controller  42  to correspond to a liquid level shown in FIG. 1 a  by the dotted line  44 . In this position, the liquid level is above the peaks of the cell culture  18 . This condition is a submerged nutrient supply system. 
     A practical example shown in FIG. 2 a  corresponds largely to the example shown in FIG. 1 a . The corresponding components have the same reference number and will not be explained further in detail. 
     Instead of the bidirectional pump  24 , a unidirectional transport pump  24 ′ is provided and is connected to the connector  22  of the culture container  12  through a check valve  46 . Furthermore, a 2/2 magnetic valve  48  connects the culture container  12  to a discharge line  50 . The magnetic valve  48  is biased to a closed position with a spring and can be adjusted to an open position. The magnet of the magnetic valve  48  and the pump  24 ′ are again connected to the operating circuit  36  through the power amplifiers  32  and  34 , as described above, to form the control circuit. 
     FIGS. 3 to  7  illustrate a further practical example having a culturing device with a large number of cell cultures being cultivated simultaneously. The culturing device shown in FIGS. 3 to  7  has an outside housing  52  having a lower box-like housing part  54  that in one embodiment is assembled from a plurality of plates. The housing  52  also has a cover  56  that can be tightly placed on the housing to close or seal the device. Three glass bridges  57  extend from the housing cover, along lines  55 - 1 ,  55 - 2  and  55 - 3 , perpendicularly downward, to the upper edge of the culture containers such as the container  12 , in order to fix or retain the cell culture inserts  122  within the containers. 
     Four culture modules  58 - 1 ,  58 - 2 ,  58 - 3  and  584  are arranged within the external housing  52 . Each of these modules will be identified as a module  58  herein when there is no need to distinguish among the modules. 
     Each culture module  58  includes a discrete temperature control housing  60  which can also be assembled from a plurality of plates and can each individually support different temperatures or environments. Three culture units  10  are arranged in each of the culture housings  60 , as can be seen from the drawing, each including a culture container  12 . The connector  22  of each culture container  12  of each culture unit is connected to a common supply line  62  for each module  58 . 
     The supply lines  62  of the various culture modules  58  are each connected through tubing  64  to a corresponding discharge connector  66  of a culture medium distribution system  68 . The tubing  64  is shown only schematically in FIG. 3, and can optionally be produced from a material such as silicone. The culture medium distribution system  68  is connected to a supply of culture medium by a supply connector  70 . Similar to the practical examples shown in FIGS. 1 a  and  2   a , the supply connector  70  is connected to the discharge of a culture medium pump  24  or  24 ′, also with conventional tubing such as silicone tubing (not shown). 
     Each of the supply lines  62  has a branch  72  disposed at the end connected to the tubing  64  that lies outside the corresponding temperature-control housing  60 . Each branch  72  is connected through a tubing  74  to a corresponding discharge connector  76  of the outer housing  52 . In this way, culture liquid  30  can be extracted separately and independently from the various modules  58  for test purposes. 
     A temperature-control medium, such as for example, temperature-controlled water, is introduced via a temperature-control medium connecting piece  78  into each temperature control housing  60 . The temperature-control medium flows equally through the culture containers  12  of a culture module  58 . The temperature-control medium is removed through an overflow apparatus  80  that is provided near the corner of the temperature-control housing  60  and is positioned diametrically opposite the connecting piece  78 . 
     An angled section of tubing  82  connects the overflow apparatus  80  to a discharge connector  84  of the temperature-control housing  60 . The discharge connector  84  of a given culture module  58  is connected to the connecting piece  78  of the next adjacent culture module  58  with a tubing  86 . In this way, the temperature-control medium flows through the temperature-control housings  60  in succession. 
     The first connecting piece  78 - 1  is joined to a temperature-control liquid feed connector  88  which is carried by the outer housing  52 . The discharge connector  84 - 1  of a module is connected to the connecting piece  78 - 2  of the next module through a tubing  86 . The last discharge connector  84 - 2  of the module arrangement is connected through tubing sections  90  and  92  to a temperature-control medium discharge connection  94 . 
     As shown in FIGS. 3 and 7, a riser  96  is connected to the end of the supply line  62 . This stands perpendicularly to the plane of the drawing of FIG. 3, as can be seen clearly in FIG.  7 . Furthermore, as shown in FIG. 7, the culture medium distributor  68 , when viewed from the front, is in the form of a “V” with a wide opening. The supply connector  70  is disposed at the lowest point or elevation of the culture medium distributor  68  and functions both to deliver culture medium and to accept returned culture medium. 
     Two arms of a forked photoelectric barrier  98  span the outer wall of the riser  96 . The forked photoelectric barrier  98  is supported by a holder  100  in such a way that the barrier can be vertically adjusted relative to the riser. The holder  100  extends in the vertical direction parallel to one of the outside plates of outer housing  52 . 
     An infrared (IR) light source  102  and an IR detector  104 , as shown in FIG. 8, can be provided across from one another in the respective arms of the forked photoelectric barrier  98  defining an axis of the barrier. For example, the IR light source can be an IR diode, among other possible devices, and the detector can be an IR-sensitive photo transistor, among other possible devices. Other types of detector devices can be utilized as well that are suitable to perform the functions of the barrier as described below. 
     The liquid level in the riser  96  corresponds to the liquid level in the various culture containers  12 , which are each precisely horizontally aligned. The photoelectric barrier is adjusted to a vertical position that corresponds to a desired liquid height. If the liquid level or column in the riser  96  falls below the axis of the IR photoelectric barrier, the photo transistor  104  provides a signal in the form of electric current to perform a level control function. If the liquid column in the riser  96  rises to the height of the axis of the photoelectric barrier, the output signal of the photo transistor ceases. 
     A differential amplifier  106  is coupled to the output of the barrier  98  and receives its signals. The amplifier  106  also receives, at a side input, an output signal of a target value transducer  108 , which may optionally be implemented by an adjustable resistor. An output signal produced by the differential amplifier  106  activates a switching transistor  110 . A relay coil  112  of a reed-relay  114  is then activated via the switching transistor  110 . When activated, a reed contact  116  of the reed-relay is closed. The reed contact  116  may optionally be implemented by a potential-free contact and, for example, controls the operation of the pump  24  or  24 ′. For example, when activated, the reed contact  116  directs the pump to pump additional culture liquid  30 . 
     In one possible implementation of the culturing device, the change from a submerged nutrient supply condition to basal nutrient supply condition can be accomplished. For example, the forked photoelectric barrier  98  can be adjusted downward and excess culture liquid can be manually drained or emptied from the system. A preliminary liquid level can be set to a level significantly below that desired for basal nutrient supply. The liquid level is then increased by the pump under control of the output signal of the forked photoelectric barrier  98 , which was previously set to an appropriate level for basal nutrient supply. 
     One may prefer to periodically switch between the basal and the submerged nutrient supply conditions. In one example of this approach, a second forked photoelectric barrier can be installed. One barrier can be adjusted permanently to detect appropriate liquid levels for basal nutrient supply. The other barrier can then be set to detect liquid levels for submerged nutrient supply. A second control system which corresponds to that shown in FIG. 8, can be provided for operation of the second forked photoelectric barrier. The reed contact  116  of one system can control a bidirectional pump to deliver liquid to increase the culture liquid level when desired. The reed contact  116  of the other system would then control operation of the bidirectional pump for removal of culture liquid. 
     The various mechanical components of the culturing device described above are preferably made of glass to provide visual access to the interior of the device. The glass must be of sufficient strength and rigidity to withstand the expected rigors of use. Suitable glass components should be capable of sterilization at temperatures of 120° C. without damaging the glass in order to provide a satisfactory germ-free initial state before carrying out the experiments. The various tubing and connections described above are also preferably capable of being sterilized at 120° C. without causing any problems. The tubing elements may optionally be made from a silicone tubing material. 
     As explained above, only one level sensor is preferably used for controlling the level of the culture liquid in all the various culture containers  12 . For this reason, it is necessary to arrange the various culture containers  12  precisely level with one another, so that the surfaces on which the cell cultures grow lie in a common horizontal plane. Two position strips  118  and  120  are provided to accurately position the various culture modules and to lock the culture modules in the outer housing. These strips cooperate with the front edges and/or back edges of the lower plates of the temperature-control housing  60  in order to assist in leveling the housings. 
     In FIGS. 1 a  and  2   a , the cell culture inserts  15  are shown schematically as being incorporated as part of the container  12  substrates in order to simplify the drawings. However, in practice, the cell culture inserts  15  are preferably not fixed in the culture containers  12 , but rather are preferably discrete parts that can be removed from the containers. As indicated in FIG. 4 for a culture container  12 , a commercial cell culture insert  122  can be in the shape of a cylindrical beaker with a flat bottom wall  124  and a cylindrical peripheral wall  126 . By placing the cell culture inserts  122  into the culture container  12  and securing or supporting them with the aid of the glass bridges  57 , the various bottom walls  124  can be assured to lie in a common horizontal plane. The cell culture inserts  122  may optionally be made of a porous synthetic material as can the cell culture inserts  15  in FIG. 1 a  and  2   a.    
     In the culturing device described above, one can very simply change from submerged to basal nutrient supply of the cell cultures. By changing the temperature of the introduced temperature-control liquid, one can provide additional variation of the growth conditions. A line  128  can also be provided to introduce desired gaseous substances into the outer housing  52 , the temperature controlled housings  60 , or the individual containers  12 . The cell cultures can be exposed to predetermined stressing or therapeutic conditions for desired substances. In the same way, exposure to aerosols can also be performed. 
     Analogous to treatment with gases and/or aerosols as noted previously, one can also apply or dust particulate active ingredients directly onto the cell cultures to investigate additional desired effects. If one selects particles having an appropriate density relative to the selected culture liquid so that the particles are lighter than the culture liquid, the particles can be lifted off the cell culture when the level of the culture liquid is raised (submerged nutrient supply). On the other hand, when the culture liquid is lowered (basal nutrient supply), the particles again will contact with the cell culture. Such suitable particles are very fine dusts, for example, wood dust or hydrophobic particles, which settle insufficiently or not at all in suspension. 
     Special attachments can be added to the culture modules to further equip the device for directly applying other substances to the cell culture material. For example, ozone could be directly applied to cells through a separate attachment. 
     Particles and other complex substances can also be applied to the cells at the air/liquid interface while maintaining the pulsed or periodically varied technique described herein. 
     Biosensors can also be adapted to cooperate withe the culture modules for either continuous or discontinuous detection of special cell components, such as, for example, calcium, and other cell products. Measuring probes and measuring devices can also be added for continuous or discontinuous measurement of gaseous components, such as CO 2  or NO 2 , or particulate components, such as total particulate matter or TPN, of airborne substances in the atmospheric environment exposed to the cells. 
     These various possibilities of cell culture are obtained with a simple construction of the apparatus of the culturing device. The culturing devices described above are well suited for the universal use of commercial cell culture inserts. The modular construction of the device permits the culture containers or wells to be adapted or replaced to suit specific desired insert sizes without the need for significantly altering the device. 
     Performing experiments using the disclosed device is very flexible and three or more parallel experimental batches can be run per module. Although the disclosed device includes three exemplary containers per each of four module, more or fewer culture containers and more or fewer modules can be optionally provided in such a device. When using all modules of the culturing device according to FIGS. 3 to  7 , one can run 4 by 3 parallel experimental batches. This permits, for example, determining cell reactions as a function of concentration and/or time. 
     Some exemplary special cell biological problems that can be investigated and/or solved using the disclosed culturing devices include the following: 
     1. Treatment of cells with substances such as gases, aerosols, or dusts directly at the air/liquid boundary. 
     2. Cell interactions by cocultivation of different cell types such as epithelial cells and fibroblasts. 
     3. Attachment behavior of cells in a system that is supplied with culture liquid in a pulsed manner. 
     4. Differentiation of cells from cultures upon basal supply of the culture liquid (polarization). 
     5. Investigation of cilia-carrying cells with basal and/or intermittent supply with culture liquid. 
     6. Continuous analysis of cell excretion products during the experiment. 
     7. Intermittent addition of growth-stimulating and growth-inhibiting active ingredients without interruption of the culturing process. 
     8. Intermittent addition of the substances named under # 6  above during exposure of the cells to active or damaging substances. 
     A culturing device is shown in FIGS. 3 to  7  with its own temperature-control device. However, the device of FIGS. 3 to  7  can be provided without temperature-control, similar to the culturing devices shown in FIGS. 1 a  and  2   a  Such a device can simply be placed into a temperature-control cabinet. 
     Preparation of the culturing device for conducting an experiment is described below. The culturing device in the assembled state but without the cell culture inserts, supply device and level sensor, is sterilized, and particularly is autoclaved at 120° C. in an autoclave for 30 minutes. The device is autoclaved together with other necessary materials, such as pump tubings, tweezers for introducing the cell culture inserts, bottles for holding the culture medium, etc. After the autoclaving, the level sensor is mounted in a clean room workbench, then the culture medium tubing is attached to the pump and the pump speed is adjusted to the intended experimental conditions. Before introducing the cell culture inserts, the culturing device is rinsed by filling the device with the cell-type-specific culture medium and then emptying the device again, after which the used medium is discarded. Then the entire culturing device is filled with the cell-type-specific medium to below the culture containers. The prepared cell culture inserts, which are packed individually and provided by the manufacturer in a sterile form, are introduced to the device with the aid of the tweezers within the clean room workbench. Then the selected program for controlling the pump is started and the culturing device is transferred, if needed, optionally into the temperature-control cabinet. The pump program controls a cycle, which includes the steps of (1) forward-pumping culture medium and culturing time and (2) back-pumping and waiting. 
     After initialization, the pump carries out its forward pumping function automatically. For this purpose, an adequate pump speed is chosen depending on the cell type and type of culture. The program monitors the output signal of the photocells and terminates the pumping activity as soon as the desired liquid level is reached. Typically, termination of pump activity occurs only after two or several signal detections monitored by the program. This permits avoidance of the detection of erroneous signals which can be produced, for example, by the development of bubbles, and, in the case of a single signal detection, would lead to a premature and undesirable ending of the pumping activity. The subsequent culturing time is preferably selectable and adjustable. 
     After the culturing time has elapsed, the back pumping or pumping out process of the culture medium is initialized, whereupon, again, the pump speed is preferably selectable and adjustable. The waiting time required for the pumping-off process can be obtained from the selected pumping off speed and the volume of the medium. The cycle as described above and preferably controlled by the program can be repeated once, twice or any desired number of times. 
     The cell culture inserts can be removed any time in the clean room work-bench. In order to do this, the computer program is stopped. In a case where only a few inserts are removed for analysis, the program can be started again and the above program and cycles repeated. 
     When the culturing device inserts are completely removed, the entire device can be cleaned. For cleaning, the entire culturing device is rinsed several times with distilled water. The glass parts can also be cleaned with alcohol or other suitable cleaning agents without causing any problems. 
     The disclosed culturing devices were explained above in conjunction with a particular level sensor. However, it will be evident to those skilled in the art that other level sensors are also suitable and are able to recognize an instantaneous liquid level in the culture container  12  and can provide a corresponding output signal. These include glimmer-controlled sensors, sensors operating on a dielectric principle, sensors with a number of vertically spaced electrodes, other infrared sensors, and ultrasonic sensors. 
     The culturing devices as described above make a variety of different culturing and exposure conditions possible. The height or level of the culture medium inside the culture vessel can be adjusted so that it lies above the top of the cell culture insert membrane to produce a submerged nutrient supply culture. The level can also be adjusted to a level just contacting the membrane or somewhat within the membrane height in order to produce a basal nutrient supply culture from underneath. Alternately, the liquid level can be intermittently adjusted between a submerged and a basal condition as desired. 
     Changing from a submerged culture to a basal culture can easily be done by controlling the culture medium supply device. The supply device is controlled to either introduce a predetermined amount of culture medium to the culture vessel or to remove an amount of the medium from the vessel. An output signal from the level sensor corresponds to a specific set level target value to control operation of the supply device. Adjustability of the level of the culture medium in the culture container can be achieved very precisely and simply without reading of marks or similar manual monitoring of equipment. The parts of the apparatus necessary for creating different culturing conditions are simple and inexpensive. 
     A difference or variance between the submerged and basal nutrient supply can be adjusted very precisely. The various areas of a cell culture carried on a cell culture insert grow under exactly the same conditions. The culture medium located in the individual culture containers can be analyzed during the growth phase. From the composition of the culture medium, one can draw conclusions regarding the growth of cell cultures through their metabolic products. One can grow a large number of cell cultures under exactly the same conditions. The design of such a culturing device for a large number of cell cultures is relatively simple and requires only a relatively small space. 
     The disclosed devices preferably utilize only one level sensor for the entire device so that no individual cell culture container requires its own sensor. The device can use two sensors, but a second sensor is only necessary where a separate sensor is desired for detecting each of the basal and the submerged liquid levels for the system as described above. 
     The disclosed device is designed in order to easily determine the culture medium level within the culture container from the outside. Therefore, the containers can have a smooth surface and are free from inserts that would make cleaning the device difficult. The culture containers can also be sterilized simply at high temperatures. 
     The disclosed device also permits manual adjustment of the target level for the culture medium. The device is equally adaptable to provide for adjusting the target level electronically. The level sensor preferably can measure a large height range of the culture container. The level can also be predetermined by adjusting the level target value signal of the circuit and/or by adjusting the sensor position. A photoelectric sensor for level detection, such as a forked photoelectric barrier, can optionally be used, which are relatively inexpensive and readily available components. 
     The disclosed culturing device is also well suited for testing multiple cell cultures simultaneously at relatively low cost. The housings, culture containers, inserts and miscellaneous components can be fabricated as mass produced standard components (modules) to further reduce cost and complexity of the device. 
     The disclosed device also permits removal of culture medium without a residue in the same way from all modules of the culturing device. One can still grow different cell cultures in the different modules. In addition, it is also possible to investigate the progress of growth through the metabolic products for the various cultures (for the various modules). 
     The disclosed device also permits consistent control of the temperature of various cultures. Without requiring stirring, the device ensures that no temperature gradient develops within the individual constant-temperature containers. 
     In the disclosed culturing device, the connection between the individual modules of the culturing device is relatively simple between modules. Therefore, the module arrangement can be expanded to include more modules or reduced to include only one or two modules. 
     In the disclosed culturing device, the interior space of the culture container is separate and sealed from the surrounding atmosphere. The interior space of the entire housing or of individual modules or containers of the device can also be filled with a different gaseous medium to investigate the influence of the gaseous medium on the growth of the cultures. 
     The disclosed device can provide a satisfactory germ-free initial growth condition, if properly sterilized, for the cell cultures. The disclosed device can be fabricated from a material that is suitable for sterilization and that provides visual inspection of the cell cultures within the interior of the device. A glass material is used in one embodiment and can be selected so that it will not be damaged by or cause damage to the material in the culture container. 
     In the disclosed culturing device, both the introduction and the discharge of the culture medium can be done utilizing a bidirectional pump and delivery system. The system can be set up to quickly attain a desired target level value and to avoid level oscillations. 
     In the disclosed culturing device, the cultures can be exposed to stimuli wherein the growth conditions are periodically and easily altered. In addition, the culture can be exposed to substances that are not present in the liquid phase of the nutrient medium, such as gaseous substances or solids that cannot be dispersed in the liquid phase. The device also permits both submerged nutrient supply and basal nutrient supply to be alternated periodically. 
     The disclosed culture medium in the present device can be controlled via a computer program. This program can be run utilizing virtually any operating system, such as under MS-DOS operating systems. The program can be suitable for controlling all functions of the pumping system. The pumping system can be connected to the computer through a serial interface. The program can be used to detect and to process the output signal of the photocell. The progress of the experiment can be recorded manually, or preferably using the computer program. 
     Although certain apparatus constructed in accordance with the teachings of the invention have been described herein, the scope of coverage of this patent is not limited thereto. On the contrary, this patent covers all embodiments of the teachings of the invention fairly falling within the scope of the appended claims either literally or under the doctrine of equivalents.