Patent Publication Number: US-2017355944-A1

Title: Culture flask

Description:
TECHNICAL FIELD 
     The present invention relates to a culture flask used for cell culture, tissue culture and the like. 
     BACKGROUND ART 
     Culture flasks are widely used for cultivation of cells or tissues, and to implement cultivation, a culture medium is put in a flask. A culture flask typically includes a vessel for containing cells or medium, and a neck through which cells or culture medium are putting in and taking out from the vessel (see e.g. Patent Document 1). The vessel has a bottom wall, front and rear walls standing from the periphery of the bottom wall and spaced apart in the front-back direction, a pair of side walls connected to the front and rear walls and spaced apart from each other in the lateral direction, and a ceiling wall facing the bottom wall. The vessel is generally in the form of a hollow box. The neck is attached to the front wall and has a cylindrical shape with an open end. The neck is inclined upward as extending toward the end. A cap is attachable to the neck. 
     As shown in Patent Document 1, the bottom wall may be in the form of an elongated rectangle, and a bottom inclined wall (inclined surface  36 ) inclined downward as extending toward the bottom wall is provided between the front wall and the bottom wall. The bottom inclined wall extends downwards from a position adjacent to the lower end of the neck attached to the front wall to reach the bottom wall. To put culture medium into the vessel, the culture medium (liquid) is injected from a pipette that is inserted through the neck to the vicinity of the bottom inclined wall. Since the bottom inclined wall is relatively high near the neck, the supplied medium does not easily flow back toward the neck when the bottom wall is put on the lower side. The paired side walls have portions tapered to the front wall. With this shape of the vessel, every corner above the bottom wall can be reached by a pipette for sucking the culture medium or by a scraper for peeling the cell. 
     The ceiling wall is positioned higher than the upper end of the neck attached to the front wall. A plurality of culture flasks can be stacked together by letting the ceiling wall of one culture flask support the bottom wall of another culture flask. 
     When adherent cells are cultured, the cells will proliferate adhering to the culture surface (bottom wall) of a culture flask covered with a certain amount of culture medium. Conventionally, cultured cells are usually transported in a frozen state. In this case, first, the cultured cells need to be put into a special container for cryopreservation and frozen in the container, and then transported. The cultured cells are then thawed and transferred back to a culture container in which the necessary amount of culture medium has been injected. This method may have disadvantages that transferring cells between containers involves the risk of contamination and cell loss and that a lot of time and techniques for freezing/thawing are required to make the cell usable. Recent advances in constant-temperature transportation technology have made it possible to transport cells without freezing the cells while keeping the temperature suitable for culture. By transporting cells while maintaining the culture state, the cells can be immediately used for experiments, research, etc. at the destination. However, in the case of transportation maintaining the culture state, in order to prevent the cells adhered to the bottom wall from peeling off due to possible waving of the culture medium, the inside of the culture flask needs to be entirely filled with the culture medium for suppressing the movement of the culture medium. Thus, such culture-state-maintaining transportation requires a relatively large amount of culture medium to be used. 
     Depending on the type of cultured cells, the price per unit amount of culture medium is high. In transportation maintaining the culture state, the use of such an expensive culture medium in large amounts leads to an increase in the cost. 
     PRIOR ART DOCUMENT 
     Patent Document 
     Patent Document 1: JP Patent No. 2683732 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     The present invention has been proposed under the above-noted circumstances, and an object of the invention is to provide a culture flask suitable for reducing the cost of culture-state-maintaining transportation. 
     Means to Solve the Problem 
     According to the present invention, there is provided a culture flask that may be made up of: a vessel including a bottom wall, a standing wall standing from a periphery of the bottom wall, and a ceiling wall positioned above the bottom wall and covering a region surrounded by the standing wall as viewed in a vertical direction; and a cylindrical neck attached to a predetermined portion of the standing wall and including an open end. The ceiling wall includes a low region that is offset in the vertical direction toward the bottom wall from an upper end of the neck attached to the standing wall. 
     In a preferred embodiment, the proportion of the low region to the entirety of the ceiling wall is 60% or more in terms of a projected area in the vertical direction. 
     In a preferred embodiment, the low region includes a first portion disposed at a height position in the vertical direction, where the height position is spaced from a lower end of the standing wall by not greater than one half a dimension of the standing wall in the vertical direction. 
     In a preferred embodiment, the proportion of the first portion to the entirety of the ceiling wall is 50% or more in terms of a projected area in the vertical direction. 
     In a preferred embodiment, the standing wall includes a front wall and a rear wall spaced apart from each other in a front-back direction, and a pair of side walls connected to the front wall and the rear wall and spaced apart from each other in a lateral direction. The neck is connected to the front wall. 
     In a preferred embodiment, the ceiling wall includes: a main ceiling wall portion substantially facing the bottom wall; and a ceiling-side inclined wall portion inclined downward as extending toward the rear wall. 
     In a preferred embodiment, the ceiling-side inclined wall portion includes a portion flanked by the main ceiling wall portion in the lateral direction. 
     In a preferred embodiment, the main ceiling wall portion includes an elevated portion positioned relatively higher than other portions and having a dimension in the lateral direction that increases as going toward the rear wall. 
     In a preferred embodiment, the bottom wall includes a flat main bottom wall portion and an inclined bottom wall portion that is connected to a height position adjacent to a lower end of the neck attached to the front wall and also connected to the main bottom wall portion, where the inclined bottom wall portion is inclined downward as extending from the front wall toward the main bottom wall portion. 
     In a preferred embodiment, the culture flask may further include a rib projecting downward from the standing wall, and the rib is engageable with an engagement portion formed by an upper end of the standing wall. 
     In a preferred embodiment, the periphery of the bottom wall is entirely welded to a lower edge of the standing wall. 
     In a preferred embodiment, the vessel and the neck are formed of a transparent plastic material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view showing an example of a culture flask according to the present invention. 
         FIG. 2  is a plan view of the culture flask shown in  FIG. 1 . 
         FIG. 3  is a sectional view taken along line III-III in  FIG. 2 . 
         FIG. 4  is a sectional view taken along line IV-IV in  FIG. 2 . 
         FIG. 5  is a perspective view of the culture flask of  FIG. 1  as viewed from a bottom side. 
         FIG. 6  is a perspective view of an upper member before welding as viewed from the bottom side. 
         FIG. 7  illustrates how the invention works. 
         FIG. 8  illustrates how the invention works. 
         FIG. 9  illustrates how the invention works. 
         FIG. 10  is a perspective view showing another example of a culture flask according to the present invention. 
         FIG. 11  is a sectional view taken along line XI-XI in  FIG. 10 . 
         FIG. 12  is a perspective view showing another example of a culture flask according to the present invention. 
         FIG. 13  is a sectional view taken along line XIII-XIII in  FIG. 12 . 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     Preferred embodiments of the present embodiment are described below with reference to the drawings. 
       FIGS. 1-5  show an example of culture flask according to the present invention. The culture flask A 1  of the present embodiment includes a vessel  1 , a neck  2 , and a cap  3  for attachment to the neck  2 . In Figs .  2 - 5 , illustration of the cap  3  is omitted. 
     In the present embodiment, the vessel  1  has a bottom wall  11 , a front, a rear and a pair of side walls  12 ,  13  and  14  each standing from the periphery of the bottom wall  11 , and a ceiling wall  15 . The vessel  1  has an internal space surrounded by these walls. 
     The front wall  12  and the rear wall  13  are spaced apart from each other in the front-back direction (direction x in  FIG. 2 ) and face each other. The front wall  12  is positioned at the front end of the vessel  1 , and the rear wall  13  is positioned at the rear end of the vessel  1 . The dimension of the front wall  12  in the lateral direction (direction y that is perpendicular to the direction x) is smaller than that of the rear wall  13 , and this dimension of the front wall  12  is about 0.4 to 0.5 times that of the rear wall  13 , for example. The front wall  12  has an opening at its center, and the neck  2  is connected to the front wall  12  correspondingly in position to the center. 
     As shown in  FIGS. 3-5 , the bottom wall  11  has a main bottom wall portion  111  that is flat, and an inclined bottom wall portion  112  that is inclined with respect to the main bottom wall portion  111 . The main bottom wall portion  111  extends forward from the rear end of the vessel  1  and is generally in the form of an elongated rectangle. The inclined bottom wall portion  112  is connected to the front wall  12  at a position adjacent to the lower end of the neck  2 , while also connected to the main bottom wall portion  111 , and is inclined to be displaced downward as extending from the front wall  12  toward the main bottom wall portion  111 . 
     As shown in  FIGS. 1-4 , the paired side walls  14  are spaced apart from each other in the lateral direction (direction y that is perpendicular to the direction x in  FIG. 2 ) , and each of the side walls has opposite ends connected to the front wall  12  and the rear wall  13 , respectively. In the present embodiment, each of the side walls  14  is made up of a main side wall portion  141  connected to the rear wall  13  and a boundary side wall portion  142  connected to the front wall  12 . The main side wall portion  141  extends along the periphery of the main bottom wall portion  111 . The paired left and right main side wall portions  141  face each other. On the other hand, the paired left and right boundary side wall portions  142  spread apart from each other as extending backward from the front wall  12 . The front wall  12 , the rear wall  13  and the paired side walls  14  form a standing wall of the present invention. 
     As shown in  FIGS. 1-4 , the ceiling wall  15  is positioned above the bottom wall  11  and covers the entire region surrounded by the front wall  12 , the rear wall  13  and the paired side walls  14  as viewed in the vertical direction (direction z that is perpendicular to both of the direction x and the direction y). In the present embodiment, the ceiling wall  15  has a main ceiling wall portion  151  generally facing the main bottom wall portion  111  (bottom wall  11 ), and a ceiling-side inclined wall portion  152 . In the present embodiment, as better shown in  FIG. 3 , compared to the upper end P 1  of the connected portion of the front wall  12  to the neck, the ceiling wall  15  (the main ceiling wall portion  151  and the ceiling-side inclined wall portion  152 ) is provided at lower positions (i.e., closer to the bottom wall  11  in the vertical direction). 
     In the present embodiment, the main ceiling wall portion  151  includes an elevated portion  153  positioned relatively higher than other parts. The elevated portion  153  is provided over most of the length of the main ceiling wall portion  151  in the front-back direction. The dimension of the elevated portion  153  in the lateral direction varies depending on the position in the front-back direction, and the dimension in the lateral direction increases as going toward the rear wall  13  (deeper side). 
     Further, in the present embodiment, the remaining part of the main ceiling wall portion  151  other than the elevated portion  153  faces the main bottom wall portion  111  and is flat. On the other hand, the elevated portion  153  is slightly inclined to be displaced downward as extending toward the rear wall  13  (deeper side). 
     The ceiling-side inclined wall portion  152  is connected to the front wall  12  and the main ceiling wall portion  151  (the elevated portion  153 ) and inclined to be displaced downward as extending toward the rear wall  13  (deeper side). In the present embodiment, the ceiling-side inclined wall portion  152  is provided by a local part of a cylinder. As shown in  FIG. 3 , the inclination angle of the ceiling-side inclined wall portion  152  with respect to a horizontal surface is larger than the inclination angle of the elevated portion  153  with respect to a horizontal surface. In the present embodiment, as shown in  FIG. 2 , the ceiling-side inclined wall portion  152  is flanked by the main ceiling wall portion  151  in the lateral direction (direction y). The main ceiling wall portion  151  extends to the main wall  12  while flanking the ceiling-side inclined wall portion  152 . As shown in  FIG. 2 , the dimension of the ceiling-side inclined wall portion  152  in the lateral direction (direction y) is generally equal to or smaller than the dimension of the neck  2  in the lateral direction . In the present embodiment, the ceiling-side inclined wall portion  152  is generally in the form of a local part of a cylinder, and the dimension of the ceiling-side inclined wall portion  152  in the lateral direction reduces as going toward the rear wall  13  in the front-back direction (direction x). 
     Regarding the ceiling wall  15 , a region thereof that is positioned lower than the upper end P 1  of the above-noted connected portion may be referred to as a “low region” of the present invention. For instance, the proportion of the low region to the entirety of the ceiling wall  15  is at least 60%, preferably at least 80%, and more preferably at least 90%, in terms of the projected area in the vertical direction. In the present embodiment, the entirety of the ceiling wall  15  (the main ceiling wall portion  151  and the ceiling-side inclined wall portion  152 ) is positioned lower than the upper end P 1  of the connected portion, and thus the proportion of the low region to the entire ceiling wall  15  is 100%. 
     The above-noted low region includes an “intendedly low” portion disposed at a predetermined height in the vertical direction. For example, an intendedly low portion may have a height that is equal to or lower than one half the vertical dimension of the front wall  12 , the rear wall  13  or the paired side walls  14 , with respect to the lower end of the front wall  12 , the rear wall  13  or the paired side walls  14 . In the present embodiment, the entirety of the main ceiling wall portion  151  corresponds to the intendedly low portion. The intendedly low portion may be referred to as a first portion of the present invention. For instance, the proportion of the intendedly low portion to the entirety of the ceiling wall  15  is at least 50% and preferably at least 80%, in terms of the projected area in the vertical direction. In the present embodiment, the proportion of the intendedly low portion (the main ceiling wall portion  151 ) to the entirety of the ceiling wall  15  is about 95%. 
     As understood from  FIGS. 2-4 , the front wall  12 , the rear wall  13  and the paired side walls  14  extend in series along the periphery of the bottom wall  11  as viewed in the vertical direction, forming a closed loop. In the present embodiment, the front wall  12 , the rear wall  13  and the paired side walls  14  are substantially equal in height, and the upper edges of the front wall  12 , the rear wall  13  and the paired side walls  14  form an engagement portion  17 , which is located at a predetermined distance from the main bottom wall portion  111  (the flat portion of the bottom wall  11 ). The engagement portion  17  is formed with an indented portion  171  on its inner side. 
     In the present embodiment, a rib  18  is provided so as to project downward from the front wall  12 , the rear wall  13  and the paired side walls  14 . As understood from  FIGS. 3-5 , the rib  18  extends along the periphery of the bottom wall  11  as viewed in the vertical direction, forming a closed loop. The rib  18  extends a predetermined distance downward from the main bottom wall portion  111  (flat portion of the bottom wall  11 ). 
     As described in detail below, the rib  18  is configured to fit into the indented portion  171  of the engagement portion  17  in stacking a plurality of culture flasks A 1  as shown in  FIG. 9 . In this way, the engagement portion  17  and the rib  18  are engageable with each other. 
     The neck  2  is a portion used for putting in and taking out cells or culture medium into and from the vessel  1 . The neck  2  is cylindrical and has an opening at its end. The neck  2  is inclined upward as extending toward the end (i.e., away from the front wall  12 ). The neck  2  is formed with a male thread  21  on its outer surface. 
     The cap  3  has a cylindrical inner surface formed with a female thread (not shown) to be screwed on the male thread  21  of the neck  2  for closing the end opening of the neck  2 . Two types of caps  3 , air-permeable and sealable, may be provided and selectively used according to the situation. For example, during cell culture, the air-permeable cap  3  maybe used, though the sealable cap  3  can also be used in a loosened state. At the time of transportation, the sealable cap  3  is used in a tightened state. 
     Both of the vessel  1  and the neck  2  are formed of a transparent plastic material. Examples of usable transparent plastic material include, without limitation, polystyrene and methyl pentene which are commonly used as medical plastics, as well as other materials, such as cycloolefin polymers and cycloolefin copolymers, which may preferably be transparent. 
     In the present embodiment, the vessel  1  and the neck  2  are obtained as a single article formed by welding two members divided on upper and lower sides. In the present embodiment, the entire periphery of the bottom wall  11  is welded to the respective lower edges of the front wall  12 , the rear wall  13  and the paired side walls  14 .  FIG. 6  is a perspective view of the upper-side members as viewed from the bottom side. As understood from  FIGS. 3, 4 and 6 , the entire weld surface w is within a single flat plane. Though not illustrated, small projections arranged along the entire weld surface w are formed on the upper-side members or the lower-side member before welding, and these projections are melted during the welding process. 
     When the culture flask A 1  is used for culturing adherent cells, the culture surface (the upper surface of the main bottom wall portion  111 ) to which cells adhere is subjected to a treatment for providing hydrophilicity by corona discharge or plasma discharge, for example. 
     The advantages of the culture flask A 1  having the above structure are described below. 
     In the culture flask A 1 , the ceiling wall  15  of the vessel  1  has a low region located lower than the upper end P 1  of the neck  2  connected to the front wall  12 . This arrangement substantially reduces the volume of the inner space of the vessel  1 , which is defined between the bottom wall  1  and the ceiling wall  15 . When adherent cells such as iPS cells are transported while being in a culture state, it is necessary to prevent the peeling of cells adhering to the bottom wall due to waving of the culture medium, and to this end the inside of the culture flask A 1  needs to be filled entirely with the culture medium to suppress the movement of the culture medium. According to the present embodiment, the ceiling wall  15  is made relatively low, whereby the amount of expensive culture medium to be used can be reduced for culture-state-maintaining transportation. In addition, the low region of the ceiling wall  15  accounts for a high proportion (100% in the present embodiment) of the size of the entire ceiling wall  15  as viewed in plan, and the intendedly low region of the ceiling wall  15  also accounts for a predetermined high proportion (about 95% in the present embodiment). According to such a structure, the volume of the inner space of the vessel  1  can be greatly reduced as compared with the conventional structure, and a particularly large effect can be expected for culture-state-maintaining transportation with respect to the reduction of the amount of the medium used. Culture-state-maintaining transportation is a very important technology in the field of drug development and regenerative medicine using iPS cells, because it allows iPS cells to be used immediately for experiments, research, etc. at the destination due to the culture state of iPS cells being maintained during transportation. Using culture flasks A 1  for such transportation can achieve considerable cost reduction. 
     The present embodiment includes the ceiling-side inclined wall portion  152 , which is connected to the front wall  12  and the main ceiling wall portion  151  and inclined to be displaced downward as extending toward the rear wall  13 . According to this arrangement, in scraping the adhesive cells from the bottom wall  11  (main bottom wall portion  111 ), it is possible to scrape off the cells over an area covering the deeper side (the rear wall  13  side) as well as the front side (the front wall  12  side) on the main bottom wall portion  111  by using the scraper S (indicated by dashed lines in  FIG. 7 ). 
     In the present embodiment, the ceiling-side inclined wall portion  152  is flanked by the main ceiling wall portion  151  in the lateral direction (direction x). With such a structure, the proportion of the main ceiling wall portion  151  in the ceiling wall  15  may preferably be as high as possible. It may also be favorable to reduce the volume of the inner space of the container portion  1 . 
     In the present embodiment, the ceiling wall  15  (main ceiling wall portion  151 ) has an elevated portion  153  which is positioned relatively higher than other portions and of which dimension in the lateral direction (direction y in the figure) increases as going toward the rear wall  13 . According to this arrangement, when scraping the adhesive cells from the bottom wall  11  (main bottom wall portion  111 ), the scraper S can reach corners on a deeper side of the main bottom wall portion  111 , as shown in  FIGS. 7 and 8 . Likewise, a pipette for sucking culture medium can reach corners on a deeper side of the main bottom wall portion  111 , so that culture medium can be sucked completely. 
     In the present embodiment, the elevated portion  153  is inclined downward as extending toward the rear wall  13  (deeper side). Accordingly, the remaining portions of the ceiling wall  15  other than the elevated portion  153  are located at much lower heights. Hence, the volume of the inner space of the vessel  1  can be made advantageously small, which is favorable for reducing the cost for culture-state-maintaining transportation. 
     The vessel  1  and the neck  2  are provided as a single article formed by welding two upper and lower members. The entire periphery of the bottom wall  11  is welded to the weld surface w or the lower edges of the front wall  12 , rear wall  13  and paired side walls  14 . According to this structure in which the lower edge of each wall and the periphery of the bottom wall  11  are joined along the weld surface w (single flat plane), it is possible to apply even pressure to the entire weld surface w during welding, which contributes to enabling proper sealing of the vessel  1  by welding. 
     The rib  18  is provided so as to project downward from the front wall  12 , the rear wall  13  and the paired side walls  14 . The rib  18  extends a predetermined distance downward from the main bottom wall portion  111  (flat portion of the bottom wall  11 ). The engagement portion  17 , constituted of the upper edges of front wall  12 , the rear wall  13  and the paired side walls  14 , has a substantially uniform height and is located at a predetermined distance from the main bottom wall portion  111  (the flat portion of the bottomwall  11 ). The engagement portion  17  and the rib  18  are engageable with each other. This allows a plurality of culture flasks A 1  to be stacked as shown in  FIG. 9 . In the stacked state, the main bottom wall portions  111  of the respective culture flasks A 1  are parallel to each other. Thus, it is possible to stack many culture flasks A 1  in a stable manner. This contributes to, for example, reduction of the space occupied by the culture flasks A 1  in an incubator during culture. 
     Although the culture flask A 1  maintains the ceiling wall  15  relatively low as described above in order to greatly reduce the amount of the culture medium for culture-state-maintaining transportation, operation of a scraper or a pipette for the flask, stacking properties and the like do not differ from those for typical flasks. Thus, the present culture flask can be used without changing the way of handling to which many researchers are accustomed. 
       FIGS. 10-13  illustrate another example of culture flask according to the present invention. In these figures, the elements that are identical or similar to those of the foregoing embodiment are designated by the same reference signs as those used for the foregoing embodiment, and the description is omitted appropriately. In  FIGS. 10-13 , illustration of the cap  3  described in the foregoing embodiment is omitted. 
     The culture flask A 2  shown in  FIGS. 10 and 11  has a weld surface w adjacent to the upper edges of the front wall  12 , rear wall  13  and paired side walls  14 , and hence, has other differences from the culture flask A 1 . 
     As understood from  FIG. 11 , the lower one of the two welded members includes the bottom wall  11 , the front wall  12 , the rear wall  13 , the paired side walls  14  and the neck  2 . The upper one of the two welded members includes the ceiling wall  15 . The ceiling wall  15  includes a main ceiling wall portion  151 , a ceiling-side inclined wall portion  152  and a supporting peripheral portion  154 . The main ceiling wall portion  151  is almost entirely flat. The supporting peripheral portion  154  stands from the periphery of the flat main ceiling wall portion  151  and supported on the upper edges of the front wall  12 , rear wall  13  and paired side walls  14 . As understood from  FIG. 11 , the entire weld surface w is within a single flat plane. The ceiling-side inclined wall portion  152  is inclined downward as extending toward the rear wall  13  (deeper side), and is flat. 
     The entirety of the main ceiling wall portion  151 , most part of the ceiling-side inclined wall portion  152  and most part of the supporting peripheral portion  154  are positioned lower than the upper end P 1  of the neck  2  connected to the front wall  12 , and referred to as a low region of the invention. In the present embodiment, the proportion of the low region to the entirety of the ceiling wall  15  is about 92% in terms of the projected area in the vertical direction. Moreover, in the present embodiment, the entirety of the main ceiling wall portion  151  is at a height position corresponding to or lower than one half the dimension of the front wall  12 , the rear wall  13  and the paired side walls  14  in the vertical direction from the lower ends of these walls, and constitutes the intendedly low portion in the vertical direction (corresponding to a first portion of the invention). In the present embodiment, the proportion of the intendedly low portion (the main ceiling wall portion  151 ) to the entirety of the ceiling wall  15  is about 80%. 
     In the culture flask A 2 , the ceiling wall  15  of the vessel  1  has a low region (the entirety of the main ceiling wall portion  151 , most part of the ceiling-side inclined wall portion  152  and most part of the supporting peripheral portion  154 ) positioned lower than the upper end P 1  of the neck  2  connected to the front wall  12 . According to the culture flask A 2  in which the ceiling wall  15  is made relatively low, the amount of culture medium used can be reduced for culture-state-maintaining transportation. Thus, by using the culture flask A 2 , the cost for culture-state-maintaining transportation is reduced. 
     The culture flask A 3  shown in  FIGS. 12 and 13  has a weld surface w at a position different from that in the culture flask A 1 , and has other differences from the culture flask A 1 . 
     As understood from  FIG. 13 , the lower one of the two welded members includes the bottom wall  11 , the front wall  12 , the rear wall  13 , the paired side walls  14  and the neck  2 . The upper one of the two welded members includes the ceiling wall  15 . The ceiling wall  15  includes a main ceiling wall portion  151  and a ceiling-side inclined wall portion  152 . The main ceiling wall portion  151  is almost entirely flat. 
     Each of the front wall  12 , the rear wall  13  and paired side walls  14  has a stepped portion in the middle in the height direction, and the portions above the stepped portions are offset outward. The stepped portions of the front wall  12 , the rear wall  13  and the paired main side wall portions  141  are horizontally flat. The stepped portions of the paired boundary side wall portions  142  are inclined upward as extending toward the front wall  12  and connected to the stepped portion of the front wall  12 . In the culture flask A 3 , the stepped portions of the front wall  12 , the rear wall  13  and the paired side walls  14  constitute a weld surface w. The weld surface w is not entirely within a single flat plane. The ceiling-side inclined wall portion  152  is inclined downward as extending toward the rear wall  13  (deeper side) and is flat. 
     The entirety of the main ceiling wall portion  151  and most part of the ceiling-side inclined wall portion  152  are positioned lower than the upper end P 1  of the neck  2  connected to the front wall  12 , and referred to as a low region of the present invention. In the present embodiment, the proportion of the low region to the entirety of the ceiling wall  15  is about 97% in terms of the projected area in the vertical direction. Moreover, of the low region, the entirety of the main ceiling wall portion  151  is at a height position corresponding to or lower than one half the dimension of the front wall  12 , the rear wall  13  and the paired side walls  14  in the vertical direction from the lower ends of these walls in the present embodiment, and hence constitutes the intendedly low portion in the vertical direction (corresponding to the first portion of the present invention). In the present embodiment, the proportion of the intendedly low portion (the main ceiling wall portion  151 ) to the entirety of the ceiling wall  15  is about 85%. 
     In the culture flask A 3 , the ceiling wall  15  of the vessel  1  has a low region (the entirety of the main ceiling wall portion  151  and most part of the ceiling-side inclined wall portion  152 ) positioned lower than the upper end P 1  of the neck  2  connected to the front wall  12 . According to the culture flask A 3  in which the ceiling wall  15  is made relatively low, the amount of culture medium used can be reduced for culture-state-maintaining transportation. Thus, by using the culture flask A 3 , the cost for culture-state-maintaining transportation is reduced. 
     Similarly to the culture flask A 1 , for the culture flasks A 2  and A 3  again, operation of a scraper or a pipette for the flask, stacking properties and the like do not differ from those for typical flasks, and they can be used without changing the way of handling to which many researchers are accustomed. 
     Although the embodiments of the present invention have been described above, the scope of the present invention is not limited to the above-described embodiments, and all variations within the scope of the matters described in each claim are included in the scope of the present invention. 
     In the above-described embodiment, description is made of the paired side walls  14  constituted of paired main side wall portions  141  facing each other and paired boundary side wall portions  142  that spread apart from each other as extending backward. However, the present invention is not limited to such a structure. For example, the paired side walls may be configured such that the entirety of the paired side walls uniformly spread apart from each other as extending backward. 
     In the above-described embodiments, description is made of a standing wall constituted of the front wall  12 , the rear wall  13 , and the paired side walls  14 , but the configuration of the standing wall is not limited to this. For example, the standing wall may have a generally circular shape as viewed in the vertical direction. 
     LIST OF REFERENCE CHARACTERS 
     A 1 , A 2 , A 3  Culture flask 
     P 1  Upper end of the connecting portion 
     w Weld surface 
       1  Vessel 
       11  Bottom wall 
       111  Main bottom wall portion 
       112  Inclined bottom wall portion 
       12  Front wall (standing wall) 
       13  Rear wall (standing wall) 
       14  Side wall (standing wall) 
       141  Main side wall portion 
       142  Boundary side wall portion 
       15  Ceiling wall 
       151  Main ceiling wall portion 
       152  Ceiling-side inclined wall portion 
       153  Elevated portion 
       154  Supporting peripheral portion 
       17  Engagement portion 
       171  Indented portion 
       18  Rib 
       2  Neck 
       21  Male thread 
       3  Cap