the hereunder presented examples illustrate the method according to this invention . in those examples , a cgr8 murine embryonic stem cell lineage ( mountford et al ., proc . natl . acad . sci . usa 91 , 4303 - 4307 ( 1994 )), at the blastocyste stage , has been cultivated in an undifferentiated condition in the presence of lif ( leukemia inhibiting factor ) ( 103 units / ml ) on previously gelatinized culture dishes ( 0 . 1 % in pbs ) under humid atmosphere at 37 ° c . and under 5 % co 2 in an incubator . the culture medium consists in gmem / bhk21 ( glasgow &# 39 ; s modified eagles &# 39 ; s medium — gibco brl ) supplemented with 10 % foetal calf serum ( fcs ; hyclone ), 0 . 23 % sodium bicarbonate , 1 % non essential amino acids , 2 mm glutamine , 1 mm sodium pyruvate and 0 . 1 mm β - mercaptoethanol . these embryonic stem cells in an undifferentiated condition are then seeded , in the absence of lif , either on an amorphous substrate ( glass or plastic material , negative control ), or on gelatine , either on coated laminas of extracellular matrix from various cell types ( see hereunder ). cells from epithelial origin have thereby been tested ( 804g lineage ; rac - 11p lineage ; scc25 lineage ; mcf - 10 a lineage ; khsv lineage ; nbt ii lineage ; hacat lineage ) as well as fibroblastic cells ( j2 lineage ; nih - 3t3 lineage ); human dermis fibroblasts in a primary culture ). the epithelial lineages produce , amongst others , high laminin - 5 amounts , and form in vitro numerous anchorage structures of the hemidesmosome type ( langhofer m . et al ., j . cell science 105 , 753 - 764 ( 1993 )). the embryonic stem cells have also been seeded on coated laminas only with major components of the basal laminas ( purified laminin - 5 ; collagen of the purified type iv ; collagen of the purified type i ; purified fibronectins ). in order to collect their extracellular matrix , the above - mentioned lineages have been cultivated at confluence on the lamellas . once confluence has been reached , those cells are removed using a solution consisting in pbs containing 20 mm ammonium hydroxide or a hbss ( hanks balanced salt solution , gibco - brl ) solution containing 20 mm edta , 20 mm hepes and 1 mm egta ) so as to only keep the extracellular matrix intact on the lamellas . the thus “ coated ” lamellas have been stored at 4 ° c . the presence of keratinocytes has been evaluated through immunofluorescent approach with mouse monoclonal antibodies raised against the 14 ( k14 ; sigma ) and 5 ( dr . b . lane , dundee university , uk ) cytokeratins . the 14 and 5 cytokeratins are part of intermediary filaments specific to the basal keratinocytes . the embryonic stem cells were cultivated on sterile glass lamellas with or without extracellular matrix being deposited . after being washed with pbs , the cells were fixed using cold methanol for 10 minutes at − 20 ° c . the cells were then washed with pbs before being incubated for one hour with the anti - k14 primary antibody ( 1 / 100 th diluted in a pbs buffer containing 3 mg / ml bsa , in a humid chamber ) or with the anti - k5 primary antibody ( ⅕ th diluted in the same buffer , in a humid chamber ). after being again washed with pbs for 5 minutes , the lamellas were incubated for one hour in the dark with a relevant secondary antibody coupled to a marker . after a final rinsing operation , the lamellas were incubated with a nuclear marker ( hoechst or propidium iodide ), l / 1000 th diluted for 10 minutes away from light , and then mounted on laminas after being washed with distilled water . all operations were performed at room temperature . the laminas were observed under a “ zeiss axiophot ” microscope . the obtained results showed that , on an amorphous support , despite a spontaneous and anarchic differentiation , the cells did not differentiate into keratinocytes , even after a 15 day culture without lif . the results on gelatine showed that only a tiny portion of cgr8 embryonic stem cells were differentiated into keratinocytes : a keratinocyte sporadic presence could be observed after 8 days of culture and such a proportion is increased at the 15 th day , the observed keratinocytes staying predominantly isolated , without forming clusters . the results obtained on coated laminas with a total or partial fraction of the extracellular matrix of the cellular types were found to be drastically different from the results obtained on an amorphous support . results obtained on the extracellular matrix originating from the various cellular types being used a keratinocyte differentiation has been obtained on the various extracellular matrices being investigated . however , the induction efficiency variations between those various matrices require to distribute them into two distinct categories : ( a ) matrices with a high induction capacity ; ( b ) matrices with an average induction capacity ( also see table i ). ( a ) matrices with a high induction capacity of the keratinocyte differentiation a ) extracellular matrix produced by nhf cells : the nhf ( normal human fibroblasts ) were obtained from prepuce biopsies and maintained in dmem ( dulbecco &# 39 ; s modified eagles &# 39 ; s medium — gibco brl ) supplemented with 10 % svf ( hyclone ). the cells were maintained in a humid atmosphere at 37 ° c . and under 5 % co 2 in an incubator . at confluence , the nhf cells were removed and the cgr8 embryonic stem cells were seeded on the extracellular matrix deposited by the nhf cells . on the 8th day of culture of the cgr8 embryonic stem cells on the matrix produced by the nhfs , numerous isolated keratinocytes could be identified through immunofluorescence . on the fifteenth day , more numerous keratinocytes could be observed in big clusters (“ patches ”) forming some kind of an epidermal small sheet . numerous isolated keratinocytes can also be detected . thus , the presence of a matrix secreted by the nhf cells causes a significant effect on the differentiation of the embryonic stem cells into keratinocytes . indeed , whereas no differentiation could be observed in a direct culture on an amorphous substrate even after a 15 - day culture , a large amount of keratinocytes is obtained in a direct culture on the extracellular matrix produced by the nhf cells . moreover , such a differentiation is earlier than that obtained via the embryoid bodies since as early as the 8 th day of culture in a monolayer without lif , a high proportion of keratinocytes is to be observed . b ) extracellular matrix produced by the nih - 3t3 , rac - 11p , khsv and nbt ii cellular lineages : the above - mentioned experiment ( nhf cells ) was reproduced on the matrix produced by the following cellular lineages : i ) nih - 3t3 : atcc , crl 1658 . such cells were maintained in dmem ( dulbecco &# 39 ; s modified eagles &# 39 ; s medium — gibco brl ) supplemented with 10 % svf ( hyclone ). the cells were maintained in a humid atmosphere at 37 ° c . and under 5 % co 2 in an incubator . ii ) rac - 11p : sonnenberg a . et al . ( 1996 ) j . cell science 106 : 1083 . such cells were maintained in dmem ( dulbecco &# 39 ; s modified eagles &# 39 ; s medium — gibco brl ) supplemented with 10 % svf ( hyclone ). the cells were maintained in a humid atmosphere at 37 ° c . and under 5 % co 2 in an incubator . iii ) khsv : miquel c . et al . ( 1996 ). exp . cell res . 224 : 279 - 290 . such cells were maintained in dmem ( dulbecco &# 39 ; s modified eagles &# 39 ; s medium — gibco brl ) containing 50 % ham f12 medium ( gibco brl ) and supplemented with 10 % svf ( hyclone ), 0 . 4 μg / ml hydrocortisone , 0 . 1 ng / ml cholera toxin and 10 ng / ml epidermal growth factor . the cells were maintained in a humid atmosphere at 37 ° c . and under 5 % co 2 in an incubator . iiii ) nbt ii : atcc , crl 1655 . such cells were maintained in dmem ( dulbecco &# 39 ; s modified eagles &# 39 ; s medium — gibco brl ) supplemented with 10 % svf ( hyclone ). the cells were maintained in a humid atmosphere at 37 ° c . and under 5 % co 2 in an incubator . in these four other examples , similar results are obtained to those obtained with the nhf cell matrix . indeed , the deposit of the embryonic stem cells on the coated laminas with the extracellular matrix produced by such cellular lineages leads to the appearance of k14 - positive keratinocytes as early as the 8 th day of culture in a monolayer . the qualitative and quantitative differences are shown in table 1 . ( b ) matrices with an average induction capacity of the keratinocyte differentiation a ) extracellular matrix produced by the 804g lineage : the 804g lineage , derived from epithelial cells of rat &# 39 ; s bladder ( riddelle ks et al ., j . ( 1991 ) j . cell biol . 112 , 159 - 168 ), has been previously cultivated in dmem ( dulbecco &# 39 ; s modified eagles &# 39 ; s medium — gibco brl ) supplemented with 10 % svf ( hyclone ). the cells were maintained in a humid atmosphere at 37 ° c . and under 5 % co 2 in an incubator . at confluence , the 804g cells were removed and the cgr8 embryonic stem cells were seeded on the extracellular matrix deposited by the 804g cells . on the 8th day of culture of the cgr8 embryonic stem cells on the matrix produced by the 804gs , isolated keratinocytes could be identified through immunofluorescence . on the fifteenth day , more keratinocytes could also be detected . thus , the presence of a matrix secreted by the 804g cells causes a significant effect on the differentiation of the embryonic stem cells into keratinocytes . indeed , whereas no differentiation could be observed in a direct culture on an amorphous substrate even after a 15 - day culture , a large amount of keratinocytes is obtained in a direct culture on the extracellular matrix produced by the 804g cells . moreover , such a differentiation is earlier than that obtained via the embryoid bodies since as early as the 8th day of culture in a monolayer without lif , a high proportion of keratinocytes is to be observed . the qualitative and quantitative differences are shown in table 1 . b ) extracellular matrix produced by the scc25 , mcf - 10a and hacat cellular lineages : the above - mentioned experiment ( 804g cells ) was reproduced on the matrix produced by the following cellular lineages : i ) scc25 : atcc crl 1628 . such cells were maintained in dmem ( dulbecco &# 39 ; s modified eagles &# 39 ; s medium — gibco brl ) containing 50 % ham f12 medium ( gibco brl ) and supplemented with 10 % svf ( hyclone ) and 0 . 4 μg / ml hydrocortisone . the cells were maintained in a humid atmosphere at 370c and under 5 % co 2 in an incubator . ii ) mcf - 10a : atcc crl 10317 . such cells were maintained in dmem ( dulbecco &# 39 ; s modified eagles &# 39 ; s medium — gibco brl ) containing 25 % ham f12 medium ( gibco brl ) and supplemented with 10 % svf ( hyclone ), 1 . 5 ng / ml triiodo - l - thyronin , 5 μg / ml insulin , 0 . 5 μg / ml hydrocortisone , 20 μg / ml adenine , 5 μg / ml apotransferrin and 2 ng / ml epidermal growth factor . the cells were maintained in a humid atmosphere at 370c and under 5 % co 2 in an incubator . iii ) hacat : boukamp p . et al . ( 1988 ). j . cell biol . 106 : 761 - 771 . such cells were maintained in dmem ( dulbecco &# 39 ; s modified eagles &# 39 ; s medium — gibco brl ) containing 50 % ham f12 medium ( gibco brl ) supplemented with 10 % svf ( hyclone ) and 1 % of a non essential amino acid solution . the cells were maintained in a humid atmosphere at 37 ° c . and under 5 % co 2 in an incubator . in these three other examples , similar results are obtained to those obtained with the 804g cell matrix . indeed , the deposit of the embryonic stem cells on the coated laminas with the extracellular matrix produced by such cellular lineages leads to the appearance of k14 - positive keratinocytes as early as the 8 th day of culture in a monolayer . the qualitative and quantitative differences are shown in table 1 . results obtained with the d3 embryonic stem cell lineage on the extracellular matrix produced by the above - mentioned lineages the d3 lineage ( doetschman et al ., j . embryol . exp . morphol . 87 , 27 - 45 ( 1985 )) was previously cultivated on a nutrient layer of murine embryonic fibroblasts . results similar to those obtained with the lineage of cgr8 embryonic stem cells have been obtained . results similar to those obtained with the matrices were observed using the conditioned medium of the same cellular lineages . the method according to the present invention has therefore the advantage , compared to the hanging drop method as applied by bagutti et al . ( op . cit .) for inducing the differentiation of mouse embryonic stem cells into keratinocytes , of allowing keratinocytes to be more efficiently and more quickly obtained . this is particularly important in cases where it is required to mass produce such keratinocytes , as for example in the case of producing artificial skin . the skin is indeed an organ consisting in three tissues from different embryological origins : the ectoderm for the epidermis , the mesoderm for the dermis and the hypodermis . the keratinocytes account for 95 % of the epidermal population and they permanently renew from a basal germinal layer following a differentiation programme ending to the formation of a stratum corneum consisting of corneocytes . the production of artificial skin is a technology aiming at treating patients suffering from thermal wounds , more particularly severely burnt people , vascular wounds such as ulcers , as well as patients suffering from pathologies associated to healing deficiencies . three main pattern types for regenerating human skin in laboratory are currently available and implementable : the equivalent dermis ( procacci et al ., j . inv . dermatol . 115 , 518 ( 2000 ); bell et al ., proc . natl . acad . sci . usa 76 , 1274 - 1278 ( 1979 ); bell et al ., j . invest . dermatol . 81 , 2s - 10s ( 1983 )), the allogenic dermo - epidermal composite skin ( burke et al ., am . surg . 194 , 413 - 428 ( 1981 )) and the allogenic culture epidermis ( hansbrough et al ., j . am . med . ass . 262 , 2125 - 2140 ). such different patterns are prepared from primary cultures of keratinocytes and / or fibroblasts . whereas such patterns are well suited for pharmaceutical studies , their use for therapeutic purposes is however considered unsatisfactory for a number of surgical teams over the world , more particularly as it is associated with graft rejection ( compron et al ., lab . invest . 60 , 600 - 612 ( 1989 )). the autograft of regenerated epidermis from a cutaneous biopsy on the patient , followed by a culture of the keratinocytes for about three weeks would be the best solution as it would prevent any immunological rejection risk . however , such a solution has the disadvantage of being long to implement . currently , for the surgeon , the expanded allograft of corpse skin remains the best cutaneous substitute . however , this solution is not feasible in the long term because , in addition to the problem of the lack of donors , such a solution exposes the patient to a potential viral contamination . that is why attempts have been made for providing an alternative to such methods . since the culture of human embryonic stem cells is from now on possible in a laboratory , the represented methods allow for the unlimited achievement of graftable regenerated epidermis without rejection risk , the embryonic stem cells having the double advantage of being “ immortal ” without being immortalized and of being easy to handle through transfection and homologous recombination . in order that such keratinocytes remain in the recipient patient , modifications of some loci , such as that of the genes of the major histocompatibility complex playing a part in the recognition of foreign cells by the immune system , allow to create universal immunotolerant totipotent lineages . effect of various matrices on the keratinocyte differentiation matrices d8 d15 glass − − gelatine −/+ + nhf +++ ++++ 3t3 +++ ++++ 804g ++ ++ rac11 p ++ ++++ scc25 ++ + mcf10 +++ + khsv +++ +++ nbtii + +++ hacat + + the es cells were deposited on matrices secreted by cells from various origins . the presence of newly formed keratinocytes was detected on 8 ( d8 ) and 15 days ( d15 ) through anti - keratin 14 immunomarking . such a quantification represents the results of various independent experiments performed in triplicate . it has been experimentally observed that the embryonic stem cells ( es cells ) could be induced to differentiate from one another in keratinocytes through the addition of bmp - 4 in the culture medium , whatever the substrate on which the cells are cultivated . bmp - 4 is a morphogenic protein belonging to the tgf - β superfamily . it is known that the neuroectodermal cells , during the early embryonic development , become either epidermal or neuronal , depending on the bmp - 4 local concentration , the high concentration of bmp - 4 enhancing the formation of the epidermis ( wilson p . et al . ( 1997 ) development 124 , 3177 - 3184 ; chang c . et al . ( 1997 ) development 124 , 827 - 837 ). according to the method of the invention , as soon as the lif factor has been removed , a 0 , 5 nm bmp - 4 solution diluted in 0 , 1 % pbs - bsa was added to the culture medium with which the embryonic stem cells are cultivated in a monolayer , and this treatment was repeated every 2 days . it was possible to observe , through immunomarking with a anti - cytokeratin - 14 antibody , the appearance of an important proportion of keratinocytes on the 15 th day of treatment . it was also observed that if , instead of adding to the culture medium a bmp - 4 solution , a solution was added in the same conditions containing 50 μg / ml ascorbic acid , an important proportion of keratinocytes on the 15 th day of treatment appeared . in addition , the deposition in the absence of lif of undifferentiated stem cells , cultivated on an equivalent or de - epidermized dermis , allows for the formation of a double layer of keratinocyte differentiated cells as early as the 8 th day of immersion culture . through immunofluorescence , all the cells adhering to the dermal substrate indeed proved to be positive for the cytokeratin - 14 . at the end of fourteen additional days of cell culture at the air - liquid interface according to the method disclosed by basset - seguin n . et al . ( basset - seguin n . et al . ( 1990 ) differentiation 44 , 232 - 238 ), it is possible to obtain a stratified epidermis having all the markers specific for the various murine epidermis cell layers , as well as the deposit by the laminin - 5 basal layer at the level of the basal lamina . comparatively , adding bmp - 4 in the culture medium of embryonic stem cells , already upon the deposit on the equivalent dermis , allows to obtain a double layer of keratinocyte differentiated cells as early as the 4 th day of immersion culture .
the present invention relates to a method for inducing the differentiation of mammals &# 39 ; embryonic stem cells into keratinocytes , comprising the following steps of : isolating an extracellular matrix secreted by at least one mammals &# 39 ; cell type , cultivating mammals &# 39 ; embryonic stem cells in parallel in an undifferentiated condition in an appropriate culture medium in the presence of lif , seeding the embryonic stem cells as a monolayer on said extracellular matrix , cultivating the thus seeded embryonic stem cells in the absence of lif for a period of time sufficient for their differentiation into keratinocytes , and collecting the thus obtained keratinocytes .