the present invention involves a novel method for preserving either ( i ) cultured epithelial sheets obtained by culturing epidermal keratinocytes , corneal keratinocytes or other epithelial cell types ; ( ii ) cultured but non - confluent epithelial cells ; ( iii ) skin substitutes , equivalents or the like ( understanding skin equivalent as a epithelial sheet obtained by culture of epidermal keratinocytes onto a gel made of extracellular matrix components such as collagen or hyaluronic acid or a dermal equivalent such as de - epidermized dermis and containing mesenchymal cells or not ); or ( iv ) mesenchymal - derived cells or tissues for wound repair . cells useful for wound repair of a human patient are particularly contemplated . cells of the foregoing type may be cultured , detached or not , with substratum , and combined , with or without , a backing material . examples of substratum are well known to those skilled in the art and include gels , films and extracellular matrices such as hyaluronic acid , collagen , fibrin glue and synthetic products . backing materials , likewise , are known to those skilled in the art and include gauze , plastics , silicones , hydrogels and dextran . the tissue sample to be preserved first is incubated in a solution that may be a nutrient medium or an isotonic solution containing a cryoprotectant amount of a monosaccharide or a disaccharide . after a period of incubation , typically between about 5 minutes and 8 hours at room temperature , the excess solution then is removed from the cells and the cells are frozen , dried or freeze - dried in the presence of the cryoprotectant . a cryoprotectant amount of a monosaccharide or a disaccharide is that amount of a monosaccharide alone , a disaccharide alone or a mixture of monosaccharides and / or disaccharides which will permit the cryopreservation of epithelial or mesenchymal cells according to the methods of the invention without the need of exogenous cryopreservation agents that interfere with wound healing when applied to a wound bed . for example , dmso and pvp are used routinely in a cryopreservation but are always washed from tissue grafts prior to application of the graft to the wound bed . the present invention permits cryopreservation without such agents , thereby avoiding the need for extra , unnecessary washing steps . in the case of the preferred monosaccharide , glucose is used at concentrations between about 0 . 05m and 3 . 5m . the lower limit is about 8 - 10 times greater than the concentration of glucose in blood , extracellular fluid or typical nutrient media ( 0 . 0055 m ) and is about twice as much as is present in nutrient media with the highest levels of glucose ( 0 . 025 m ). the preferred amount of glucose is on the order of between 0 . 1 m and 1 m . the preferred amounts of other monosaccharides and disaccharides will depend upon the particular conditions employed , particular cells cultured and upon the presence and concentration of any other biocompatible protective agents such as human serum or human serum albumin . in all cases , cryoprotectant amounts exceed by at least two times those amounts of monosaccharides and disaccharides present in nutrient media and the like . the solutions of the invention preferably are free of amounts of exogenous materials that interfere with wound healing when applied to a wound bed . such exogenous materials are well known to those skilled in the art and include materials commonly used as cryoprotective agents . for example , dmso , pvp and polyhydroxy carbohydrates are often used as cryoprotective agents . it is common practice to remove such agents from the thawed tissue by washing prior to applying the tissue to a wound bed . likewise , dextran and nonhuman sources of protein including albumin are used in cryopreservation protocols and are thoroughly removed prior to applying the preserved tissue to a wound bed . on the other hand , materials such as human serum albumin , proline , spermine , myristic acid , low concentrations of zinc and many other cryopreservative agents do not interfere with wound healing . thus , the present invention in its preferred embodiments contemplates biocompatible preservation solutions that do not cause additional wash steps prior to application of the preserved tissue to a wound bed . the invention contemplates preserving the tissue in a minimum volume of solution . this is a departure from the prior art and is an advantage of the present invention . one advantage is simplified handling , whereby the tissue can be placed in bags or dishes and preserved without concern for spillage , without the need for sealed dishes and without other cumbersome protocols characteristic of the prior art . an unexpected advantage is the ability to freeze - dry cultured cells and tissue using the cryoprotective agents of the invention . in particular , the present invention permits sheets of epithelial cells to be freeze - dried , stored , rehydrated ( or not ) and used in wound repair . storage can be at relatively high temperatures , permitting shipment and use in remote locations and avoiding the need for expensive , space - limited − 70 ° c . freezer space . freeze drying is facilitated by removing as much cryoprotectant solution as practicable prior to freezing ; with a minimum volume of fluid , freezing , drying and thawing proceeds quickly and with minimal functional damage to cells . the present invention thus provides a preserved epithelial wound repair tissue comprising a dry , confluent sheet of cultured epithelial cells which , after being stored in a dry state and applied to the surface of a wound in a patient , maintains structural integrity and functional characteristics sufficient to induce wound healing in a patient . minimum volumes can be achieved by aspirating excess solution and / or by gravity - draining solution from the cells . other methods will be apparent to those skilled in the art . it is desirable to have no more than 30 ml solution per graft of 50 cm 2 of tissue although even lower amounts are preferred . cryoprotective agents useful according to the invention include permeant and non - permeant agents , as well as agents which must be washed off prior to application of the preserved tissue to a wound bed ( although agents that are biocompatible are preferred ). agents known to be useful as cryoprotectants include the following : monosaccharides ( e . g . glucose , fructose , maltose , ribose , mannose , xylose ); disaccharides ( e . g . trehalose , sucrose , cellobiose , lactose ); trisaccharides ( e . g . raffinose ); sugar alcohols ( e . g . mannitol , sorbitol , myo - inositol , phosphorylated inoitols , glycerol ); polysaccharides ( e . g . hydroxyethyl starch ( hes ), dextran , phosphorylated dextran , heparine , heparan sulphate , hyaluronic acid , dermatan sulphate , chondrollin sulphate , agarose ); carboxylic acids ( e . g . pyruvate , 2 , 3 ,- diphosphoglycerate ); protein and protein mixtures ( e . g . blood , animal serum , plasma , human albumin , bovine albumin , bovine gelatin , fish gelatin ). the preferred cryopreservative solution includes a monosaccharide and a protein or a mixture of protein . the preferred monosaccharide , glucose , is added at a concentration of between 0 . 05 and 3 . 5m , and the protein or protein mixture is added at a concentration of between 0 . 1 and 40 . 0 mg / ml . most preferably , the solution consists of nutrient media containing between 0 . 1 and 1 . 0m glucose plus 1 . 0 - 5 . 0 mg per ml human serum albumin ( or 0 . 05 - 40 mg / ml serum , most preferably human ). the material to be preserved is immersed in the solution at a temperature range between 5 ° c . and 40 ° c . and preferably at room temperature . the time of incubation is between about 5 minutes and 8 hours , although longer periods are possible , but not necessarily desirable . after the incubation , the material is frozen in the minimum volume of solution remaining after total aspiration or gravity - draining leaves no more than about 30 ml of solution per 50 cm 2 of tissue and , more typically , leaves minimum volumes ( i . e . amounts so low as to be impractical to measure ). the material may be frozen , freeze - dried or dried . conventional freezing , freeze - drying and drying protocols may be employed . an advantage of the present invention is that any of the known protocols for freezing are useful according to the present invention . a simple two - step procedure has been found to be particularly suitable , wherein the cultured epithelial sheets are first maintained at a temperature of about − 20 ° c . for 5 to 12 hours , after which the cells are frozen to − 80 ° c . over the course of about 2 hours . the frozen material then may be stored or banked at such temperatures . it should be understood , however , that the material may be frozen in a one - step procedure to a temperature of − 20 ° c . and stored or banked at that temperature . cultured epithelial sheets have been successfully maintained either at − 20 ° c . or − 80 ° c . for a year . according to one aspect of the invention , the cultured epithelial sheets are freeze - dried . the resultant material represents the first cultured epithelial sheets preserved in a dry state which , after being stored for even an extended period of time , can be applied to the surface of a wound , maintaining the structural integrity and functional characteristics sufficient to induce wound healing . as mentioned above , it is even possible to apply the dried epithelial sheet directly to the wound , allowing the body fluids to rehydrate the epithelial sheet . when freeze - drying , conventional procedures are employed except that monosaccharides and / or disaccharides are used as cryoprotectants and the cells are frozen in a minimum volume of cryoprotectant solution . thus , the epithelial sheets can be frozen by cooling , for example , to − 70 ° c . for about 20 minutes , and , subsequently , the frozen epithelia can be lyophilized in conventional equipment . the freeze - dried epithelial sheets then may be maintained in heat - sealed bags and banked or stored , for example , at 4 ° c . this is the first known example of freeze - drying of cultured epithelial cells to result in a useful wound repair product . alternatively , the cells may be dried at a range of temperatures without freezing . under the foregoing storage conditions , the in vitro generated tissue maintains a biosynthetic activity about the same as or higher than that found in tissue preserved by the methods of the prior art . structural integrity , protein synthesis and cell secretion ability is maintained similar to or higher than the corresponding features in cells preserved by methods known in the art . because the preferred preservative solutions employ cryoprotectants that are not toxic and not immunogenic , the rinsing of cultured cells , cultured epithelial sheets or skin equivalents typical of prior art methods is obviated . likewise , because a minimum of solution is used in the freezing process , thawing occurs in a minimum amount of time ( as little as in a minute ), by allowing the material to warm up in a 37 ° c . bath . the material , of course , can be directly immersed into a thawing medium . the following specific examples will illustrate the procedures of the invention , when applied to the preservation of cultured epidermal sheets . however , as described above , the same procedures are extended for preservation of cultured epithelial sheets ( obtained by culture of human or nonhuman epithelial cells from epidermis , cornea or another epithelial cell types ), skin substitutes or other cultured cell types . epidermal sheets were obtained by culturing human neonatal foreskin keratinocytes , using the procedures developed by rheinwald & amp ; green ( 1975 , supra ). the epithelial sheets were detached from culture dishes using dispase 11 ( boehringer mannheim ) at a final concentration of 2 . 5 mg / ml . after detaching from culture dishes , the epithelial sheets are washed with phosphate buffered saline ( pbs ) at room temperature , mounted in a backing material , and incubated for 10 minutes with preservation solution consisting in dulbecco - vögt modification of minimal esential medium ( dmem ) containing glucose and human serum albumin at the indicated concentrations ; if desired , preservation solution may be buffered , % nith 20 . 0 mm hepes . after incubation , the preservation solution is aspirated leaving the minimum volume of solution in the vessel containing the epithelium . afterwards , epithelia , which may be mounted on a backing material , are kept in heat - sealed bags and frozen following a two - step procedure : first , they are maintained at − 20 ° c . during 5 hours ; then cooled overnight to − 70 ° c . finally , the frozen epithelia are banked or stored at − 70 ° c . epithelial sheets were thawed in culture medium ( dulbecco - vögt modification to minimum essential medium , dmem ) or in buffered saline containing trehalose and serum albumin , and viability was determined by quantification of [ 35 s ]- methionine incorporation into protein released to culture medium . the thawed epithelia were incubated with low methionine ( 6 μg / ml ) dmem containing [ 35 s ]- methionine ( 4 . 0 μci / ml ) at 37 ° c ., and radioactivity incorporated into tca percipitable material was determined in aliquots of culture medium . table i shows the results obtained in this experiment . to normalize radioactivity incorporation values , cultured epithelial sheets used as controls were incubated with dmem containing 10 %( v / v ) animal or human serum and 10 %( v / v ) glycerol ( standard procedure ) for 10 minutes , and frozen following the above two - step procedure . after thawing , epithelial viability was determined by measuring the [ 35 s ]- methionine incorporation into released protein , and results from these control epithelia were referred as the unit . it is shown the mean value from triplicate epithelia . in other experiments , after incubation in preservation solution , epithelia were lyophilized or dried . cultured epithelial sheets are detached , mounted in a backing material and incubated in the preservation solution as above . the preservation solution is aspirated , leaving the minimal volume of solution at the vessel containing the epithelium . then , epithelia are frozen by cooling at − 70 ° c ., at least for 20 minutes . afterwards , frozen epithelia are lyophilized in a lyphlock 6 freeze dry / shell freeze system ( labconco ). the freeze - dried epithelia can be kept in heat - sealed bags and banked or stored at 4 ° c . after preservation for at least 48 h , epithelia were reconstituted by immersion in dmem , and epithelial viability was assayed as described at example 1 . results are shown at table ii . in this illustrative experiment , [ 35 s ]- methionine incorporation values were also normalized , using epithelial sheets frozen after preincubation with dmem plus 10 %( v / v ) fbs and 10 %( v / v ) glycerol as references . similar results were obtained when other solutions such as phosphate buffered saline , or isotonic saline containing serum albumin and / or mono - or disaccharides , were used for reconstitution of the epithelial sheets , skin equivalents or cells . since the way to determine cell viability by cell growth parameters or specific dye staining , requires trypsinization of the cell sheet into single cells , and since this procedure of trypsinization by itself causes cell damage and decreases cell viability and cell growth ability , cultured human epidermal keratinocytes were harvested during exponential growth phase using a ( 1 : 1 ) mixture of trypsin 0 . 15 % and edta 0 . 02 %. cells were extensively washed with culture medium and pelleted . then , cells were resuspended and incubated with preservation medium . after incubation cells were frozen following the two - step procedure described above , or freeze - dried as in the example ii . the cryopreserved cell suspensions were stored at temperatures about − 20 ° c . or lower , and freeze - dried cells were stored at ambient or lower temperatures at least during 24 hours . cells were thawed or reconstituted with culture medium , and aliquots were used to determine cell viability by trypan blue dye exclusion . results from this type of experiment are shown . in similar experiments to those shown in example iii , human epidermal keratinocytes were harvested , washed , and after pelleting , cells were resuspended and incubated with preservation medium consisting of dmem plus 0 . 005m to 3 . 5m glucose , plus 0 . 1 mg / ml to 40 . 0 mg / ml serum albumin plus either 1 . 0 %. 10 %, ( v / v ) glycerol or 1 %- 10 % ( v / v ) dmso . after incubation , cells were frozen following the step procedure described above or freeze - dried as in the example ii . the cryopreserved cell suspensions were stored at temperatures about − 20 ° c . or lower , and freeze - dried cells were stored at ambient or lower temperatures at least 24 hours . cells were thawed or reconstituted with culture medium , and cultured to determine cell proliferation abilities . the results obtained from these experiments are as shown in example i . the procedures illustrated above were successfully employed for the preservation of cultured epithelial sheets or epithelial cell suspensions either by freezing , freeze - drying or drying . in all cases , the dressings , cells or tissue samples were incubated with cryopreservation solutions containing either glucose ( 0 . 005m to 3 . 5m ) and serum albumin ( 0 . 1 mg / ml to 40 . 0 mg ,/ ml ) or glucose ( 0 . 005m to 4 . 0m ) and animal or human serum ( 0 . 05 mg / ml to 40 . 0 mg / ml ), or glucose ( 0 . 005m to 3 . 5m ) plus serum albumin 0 . 1 to 40 . 0 mg / ml dmso ( 1 %- 10 %) or glycerol ( 1 %- 10 %). the disclosed methods ensure cell / tissue structural integrity and metabolic activities such as protein synthesis and release ( as examples of growth factors or extracellular matrix components ). these procedures are useful to preserve epithelial sheets obtained by culture of human or nonhuman epithelial cells from epidermis , cornea or other epithelial cell types , skin substitutes or another cultured cell types ; therefore , they should be valuable for multiple fields such as therapeutic , pharmacological and also for research . various modifications and equivalents will be apparent to those skilled in the art .
methods and products relating the preservation of cultured mammalian epithelial or mesenchymal cells are provided . the methods involve the pre - treatment of epithelial cells with a solution containing a cryoprotectant amount of monosaccharides and / or disaccharides . the treated cells then are frozen , dried or freeze - dried in a minimum volume of solution and stored for later use . the invention avoids the use of materials that must be washed away from the preserved tissue prior to application of the tissue to a wound bed . the invention also permits the storage of epithelial tissue in a dry state .