the present invention arose from a desire by the present inventors to improve on existing methods for the genetic modification of an animal &# 39 ; s germ cells and for producing transgenic animals . the pre - existing art methods rely on direct injection of dna into zygotes produced in vitro or in vivo , or by the production of chimeric embryos using embryonal stem cells incorporated into a recipient blastocyst . following this , such treated embryos are transferred to the primed uterus or oviduct . the available methods are extremely slow and costly , rely on several invasive steps , and only produce transgenic progeny sporadically and unpredictably . in their search for a less costly , faster , and more efficient approach for producing transgenics , the present inventors devised the present method which relies on the in vivo or ex vivo ( in vitro ) transfection of male animal germ cells with a nucleic acid segment encoding a desired trait . the present method relies on at least one of the following strategies . a first method delivers the nucleic acid segment using known gene delivery systems in situ to the gonad of the animal ( in vivo transfection ), allows the transfected germ cells to differentiate in their own milieu , and then selects for animals exhibiting the nucleic acid &# 39 ; s integration into its germ cells ( transgenic animals ). the thus selected animals may be mated , or their sperm utilized for insemination or in vitro fertilization to produce transgenic progeny . the selection may take place after biopsy of one or both gonads , or after examination of the animal &# 39 ; s ejaculate amplified by the polymerase chain reaction to confirm the incorporation of the desired nucleic acid sequence . in order to simplify the confirmation of the actual incorporation of the desired nucleic acid , the initial transfection may include a co - transfected reporter gene , such as a gene encoding for green fluorescent protein , which fluoresces under suitable wave - lengths of ultra - violet light . alternatively , male germ cells may be isolated from a donor animal and transfected , or genetically altered in vitro to impart the desired trait . following this genetic manipulation , germ cells which exhibit any evidence that the dna has been modified in the desired manner are selected , and transferred to the testis of a suitable recipient animal . further selection may be attempted after biopsy of one or both gonads , or after examination of the animal &# 39 ; s ejaculate amplified by the polymerase chain reaction to confirm whether the desired nucleic acid sequence was actually incorporated . as described above , the initial transfection may have included a co - transfected reporter gene , such as a gene encoding the green fluorescent protein . before transfer of the germ cells , the recipient testis are generally treated in one , or a combination , of a number of ways to inactivate or destroy endogenous germ cells , including by gamma irradiation , by chemical treatment , by means of infectious agents such as viruses , or by autoimmune depletion or by combinations thereof . this treatment facilitates the colonization of the recipient testis by the altered donor cells . animals that were shown to carry suitably modified sperm cells then may be either allowed to mate naturally , or alternatively their spermatozoa are used for insemination or in vitro fertilization . the thus obtained transgenic progeny may be bred , whether by natural mating or artificial insemination , to obtain further transgenic progeny . the method of this invention has a lesser number of invasive procedures than other available methods , and a high rate of success in producing incorporation into the progeny &# 39 ; s genome of the nucleic acid sequence encoding a desired trait . primordial germ cells are thought to arise from the embryonic ectoderm , and are first seen in the epithelium of the endodermal yolk sac at the e8 stage . from there they migrate through the hindgut endoderm to the genital ridges . the primitive spermatogonial stem cells , known as a 0 / as , differentiate into type b spermatogonia the latter further differentiate to form primary spermatocytes , and enter a prolonged meiotic prophase during which homologous chromosomes pair and recombine . several morphological stages of meiosis are distinguishable : preleptotene , leptotene , zygotene , pachytene , secondary spermatocytes , and the haploid spermatids . the latter undergo further morphological changes during spermatogenesis , including the reshaping of their nucleus , the formation of acrosome , and assembly of the tail . the final changes in the spermatozoon take place in the genital tract of the female , prior to fertilization . the uptake of the nucleic acid segment administered by the present in vivo method to the gonads will reach germ cells that are at one or more of these stages , and be taken up by those that are at a more receptive stage . in the ex vivo ( in vitro ) method of genetic modification , generally only diploid spermatogonia are used for nucleic acid modification . the cells may be modified in vivo using gene therapy techniques , or in vitro using a number of different transfection strategies . the inventors are , thus , providing in this patent a novel and unobvious method for ; isolation of male germ cells , and for the in vivo and ex vivo ( in vitro ) transfection of allogeneic as well as xenogeneic dna into an animal &# 39 ; s germ cells . this comprises the administration to an animal of a composition comprising a gene delivery system and at least one nucleic acid segment , in amounts and under conditions effective to modify the animal &# 39 ; s germ cells , and allowing the nucleic acid segment to enter , and be released into , the germ cells , and to integrate into their genome . the in vivo introduction of the gene delivery mixture to the germ cells may be accomplished by direct delivery into the animal &# 39 ; s testis ( es ), where it is distributed to male germ cells at various stages of development . the in vivo method utilizes novel technology , such as injecting the gene delivery mixture either into the vasa efferentia , directly into the seminiferous tubules , or into the rete testis using , for example , a micropipette . to ensure a steady infusion of the gene delivery mixture , under pressures which will not damage the delicate tubule system in the testis , the injection may be made through the micropipette with the aid of a picopump delivering a precise measured volume under controlled amounts of pressure . the micropipette may be made of a suitable material , such as metal or glass , and is usually made from glass tubing which has been drawn to a fine bore at its working tip , e . g . using a pipette puller . the tip may be angulated in a convenient manner to facilitate its entry into the testicular tubule system . the micropipette may be also provided with a beveled working end to allow a better and less damaging penetration of the fine tubules at the injection site . this bevel may be produced by means of a specially manufactured grinding apparatus . the diameter of the tip of the pipette for the in vivo method of injection may be about 15 to 45 microns , although other sizes may be utilized as needed , depending on the animal &# 39 ; s size . the tip of the pipette may be introduced into the rete testis or the tubule system of the testicle , with the aid of a binocular microscope with coaxial illumination . with care taken not to damage the wall of the tubule opposite the injection point , and keeping trauma to a minimum . on average , a magnification of about × 25 to × 80 is suitable , and bench mounted micromanipulators are not severally required as the procedure may be carried out by a skilled artisan without additional aids . a small amount of a suitable , non - toxic dye , may be added to the gene delivery fluid to confirm delivery and dissemination to the tubules of the testis . it may include a dilute solution of a suitable , non - toxic dye , which may be visualized and tracked under the microscope . in this manner , the gene delivery mixture is brought into intimate contact with the germ cells . the gene delivery mixture typically comprises the modified nucleic acid encoding the desired trait , together with a suitable promoter sequence , and optionally agents which increase the uptake of the nucleic acid sequence , such as liposomes , retroviral vectors , adenoviral vectors , adenovirus enhanced gene delivery systems , or combinations thereof . a reporter construct such as the gene encoding for green fluorescent protein may further be added to the gene delivery mixture . targeting molecules such as c - kit ligand may be added to the gene delivery mixture to enhance the transfer of the male germ cell . for the ex vivo ( in vitro ) method of genetic alteration , the introduction of the modified germ cells into the recipient testis may be accomplished by direct injection using a suitable micropipette . support cells , such as leydig or sertoli cells that provide hormonal stimulus to spermatogonial differentiation , may be transferred to a recipient testis along with the modified germ cells . these transferred support cells may be unmodified , or , alternatively , may themselves have been transfected , together with - or separately from the germ cells . these transferred support cells may be autologous or heterologous to either the donor or recipient testis . a preferred concentration of cells in the transfer fluid may easily be established by simple experimentation , but will likely be within the range of about 1 × 10 5 × 10 × 10 5 cells per 10 μl of fluid . this micropipette may be introduced into the vasa efferentia , the rete testis or the seminiferous tubules , optionally with the aid of a picopump to control pressure and / or volume , or this delivery may be done manually . the micropipette employed is in most respects similar to that used for the in vivo injection , except that its tip diameter generally will be about 70 microns . the microsurgical method of introduction is similar in all respects to that used for the in vivo method described above . a suitable dyestuff may also be incorporated into the carrier fluid for easy identification of satisfactory delivery of the transfected germ cells . once in contact with germ cells , whether they are in situ in the animal or vitro , the gene delivery mixture facilitates the uptake and transport of the xenogeneic genetic material into the appropriate cell location for integration into the genome and expression . a number of known gene delivery methods may be used for the uptake of nucleic acid sequences into the cell . “ gene delivery ( or tansfection ) mixture ”, in the context of this patent , means selected genetic material together with an appropriate vector mixed , for example , with an effective amount of lipid transfection agent . the amount of each component of the mixture is chosen so that the transfection of a specific species of germ cell is optimized . such optimization requires no more than routine experimentation . the ratio of dna to lipid is broad , preferably about 1 : 1 , although other proportions may also be utilized depending on the type of lipid agent and the dna utilized . this proportion is not crucial . “ transfecting agent ”, as utilized herein , means a composition of matter added to the genetic material for enhancing the uptake of exogenous dna segment ( s ) into a eukaryotic cell , preferably a mammalian cell , and more preferably a mammalian germ cell . the enhancement is measured relative to the uptake in the absence of the transfecting agent . examples of transfecting agents include adenovirus - transferrin - polylysine - dna complexes . these complexes generally augment the uptake of dna into the cell and reduce its breakdown during its passage through the cytoplasm to the nucleus of the cell . these complexes may be targeted to the male germ cells using specific ligands which are recognized by receptors on the cell surface of the germ cell , such as the c - kit ligand or modifications thereof . “ virus ”, as used herein , means any virus , or transfecting fragment thereof , which may facilitate the delivery of the genetic material into male germ cells . examples of viruses which are suitable for use herein are adenoviruses , adeno - associated viruses , retroviruses such as human immune - deficiency virus , lentiviruses , such as moloney murine leukemia virus and the retrovirus vector derived from moloney virus called vesicular - stomatitis - virus - glycoprotein ( vsv - g )- moloney murine leukemia virus , mumps virus , and transfecting fragments of any of these viruses , and other viral dna segments that facilitate the uptake of the desired dna segment by , and release into , the cytoplasm of germ cells and mixtures thereof . the mumps virus is particularly suited because of its affinity for immature sperm cells including spermatogonia . all of the above viruses may require modification to render them non - pathogenic or less antigenic . other known vector systems , however , may also be utilized within the confines of the invention . “ genetic material ”, as used herein , means dna sequences capable of imparting novel genetic modification ( s ), or biologically functional characteristic ( s ) to the recipient animal . the novel genetic modification ( s ) or characteristic ( s ) may be encoded by one or more genes or gene segments , or may be caused by removal or mutation of one or more genes , and may additionally contain regulatory sequences . the transfected genetic material is preferably functional , that is it expresses a desired trait by means of a product or by suppressing the production of another . examples of other mechanisms by which a gene &# 39 ; s function may be expressed are genomic imprinting , i . e . inactivation of one of a pair of genes ( alleles ) during very early embryonic development , or inactivation of genetic material by mutation or deletion of gene sequences , or by expression of a dominant negative gene product , among others . in addition , novel genetic modification ( s ) may be artificially induced mutations or variations , or natural allelic mutations or variations of a gene ( s ). mutations or variations may be induced artificially by a number of techniques , all of which are well known in the art , including chemical treatment , gamma irradiation treatment , ultraviolet radiation treatment , ultraviolet radiation , and the like . chemicals useful for the induction of mutations or variations include carcinogens such as ethidium bromide and others known in the art . dna segments of specific sequences may also be constructed to thereby incorporate any desired mutation or variation or to disrupt a gene or to alter genomic dna . those skilled in the art will readily appreciate that the genetic material is inheritable and is , therefore , present in almost every cell of future generations of the progeny , including the germ cells . among novel characteristics are the expression of a previously unexpressed trait . augmentation or reduction of an expressed trait , over expression or under expression of a trait , ectopic expression , that is expression of a trait in tissues where it normally would not be expressed , or the attenuation or elimination of a previously expressed trait . other novel characteristics include the qualitative change of an expressed trait , for example , to palliate or alleviate , or otherwise prevent expression of an inheritable disorder with a multigenic basis . the method of the invention is suitable for application to a variety of vertebrate animals , all of which are capable of producing gametes , i . e . sperm or ova . thus , in accordance with the invention novel genetic modification ( s ) and / or characteristic ( s ) may be imparted to animals , including mammals , such as humans , non - human primates , for example simians , marmosets , domestic agricultural animals such as sheep , cows , pigs , horses , particularly race horses , marine mammals , feral animals , rodents such as mice and rats , and the like . other animals include fowl such as chickens , turkeys , ducks , ostriches , geese , rare and ornamental birds , and the like . of particular interest are endangered species of wild animal , such rhinoceros , tigers , cheetahs , certain species of condor , and the like . broadly speaking , a “ transgenic ” animal is one that has had foreign dna permanently introduced into its cells . the foreign gene ( s ) which ( have ) been introduced into the animal &# 39 ; s cells is ( are ) called a “ transgene ( s )”. the present invention is applicable to the production of transgenic animals containing xenogeneic , i . e ., exogenous , transgenic genetic material , or material from a different species , including biologically functional genetic material , in its native , undisturbed form in which it is present in the animal &# 39 ; s germ cells . in other instances , the genetic material is “ allogeneic ” genetic material , obtained from different strains of the same species , for example , from animals having a “ normal ” form of a gene , or a desirable allele thereof also the gene may be a hybrid construct consisting of promoter dna sequences and dna coding sequences linked together . these sequences may be obtained from different species or dna sequences from the same species that are not normally juxtaposed . the dna construct may also contain dna sequences from prokaryotic organisms , such as bacteria , or viruses . in one preferred embodiment , the transfected germ cells of the transgenic animal have the non - endogenous ( exogenous ) genetic material integrated into their chromosomes . this is what is referred to as a “ stable transfection ”. this is applicable to all vertebrate animals , including humans . those skilled in the art will readily appreciate that any desired traits generated as a result of changes to the genetic material of any transgenic animal produced by this invention are inheritable . although the genetic material was originally inserted solely into the germ cells of a parent animal , it will ultimately be present in the germ cells of future progeny and subsequent generations thereof . the genetic material is also present in the differentiated cells , i . e . somatic cells , of the progeny . this invention also encompasses progeny resulting from breeding of the present transgenic animals . the transgenic animals bred with other transgenic or non - transgenic animals of the same species will produce some transgenic progeny , which should be fertile . this invention , thus , provides animal line ( s ) which result from breeding of the transgenic animal ( s ) provided herein , as well as from breeding their fertile progeny . “ breeding ”, in the context of this patent , means the union of male and female gametes so that fertilization occurs . such a union may be brought about by natural mating , i . e . copulation , or by in vitro or in vivo artificial means . artificial means include , but are not limited to , artificial insemination , in vitro fertilization , cloning and embryo transfer , intracytoplasmic spermatozoal microinjection , cloning and embryo splitting , and the like . however , others may also be employed . the transfection of mature male germ cells may be also attained utilizing the present technology upon isolation of the cells from a vertebrate , as is known in the art , and exemplified in example 10 . the thus isolated cells may then be transfected ex vivo ( in vitro ), or cryopreserved as is known in the art and exemplified in example 11 . the actual transsection of the isolated testicular cells may be accomplished , for example , by isolation of a vertebrate &# 39 ; s testes , decapsulation and teasing apart and mincing of the seminiferous tubules . the separated cells may then be incubated in an enzyme mixture comprising enzymes known for gently breaking up the tissue matrix and releasing undamaged cells such as , for example , pancreatic trypsin , collagenase type i , pancreatic dnase type i , as well as bovine serum albumin and a modified dmem medium . the cells may be incubated in the enzyme mixture for a period of about 5 min to about 30 min , more preferably about 15 to about 20 min , at a temperature of about 33 ° c . to about 37 ° c ., more preferably about 36 to 37 ° c . after washing the cells free of the enzyme mixture , they may be placed in an incubation medium such as dmem , and the like , and plated on a culture dish . any of a number of commercially available transfection mixtures may be admixed with the polynucleotide encoding a desire trait or product for transfection of the cells . the transfection mixture may then be admixed with the cells and allowed to interact for a period of about 2 hrs to about 16 hrs , preferably about 3 to 4 hrs , at a temperature of about 33 ° c . to about 37 ° c ., preferably about 36 ° c . to 37 ° c ., and more preferably in a constant and / or controlled atmosphere . after this period , the cells are preferably placed at a lower temperature of about 33 ° c . to about 34 ° c ., preferably about 30 - 35 ° c for a period of about 4 hrs to about 20 hrs , preferably about 16 to 18 hrs . other conditions which do not deviate radically from the ones described may also be utilized as an artisan would know . the present method is applicable to the field of gene therapy , since it permits the introduction of genetic material encoding and regulating specific genetic traits . thus , in the human , for example , by treating parents it is possible to correct many single gene disorders which otherwise might affect their children . it is similarly possible to alter the expression of fully inheritable disorders or those disorders having at least a partially inherited basis , which are caused by interaction of more than one gene , or those which are more prevalent because of the contribution of multiple genes . this technology may also be applied in a similar way to correct disorders in animals other than human primates . in some instances , it may be necessary to introduce one or more “ gene ( s )” into the germ cells of the animal to attain a desired therapeutic effect , as in the case where multiple genes are involved in the expression or suppression of a defined trait . in the human , examples of multigenic disorders include diabetes mellitus caused by deficient production of , or response to , insulin , inflammatory bowel disease , certain forms of atheromatus cardiovascular disease and hypertension , schizophrenia and some forms of chronic depressive disorders , among others . in some cases , one gene may encode an expressible product , whereas another gene encodes a regulatory function , as is known in the art . other examples are those where homologous recombinant methods are applied to repair point mutations or deletions in the genome , inactivation of a gene causing pathogenesis or disease , or insertion of a gene that is expressed in a dominant negative manner , or alterations of regulating elements such as gene promoters , enhancers , the untranslated tail region of a gene , or regulation of expansion of repeated sequences of dna which cause such diseases as huntingdon &# 39 ; s chorea , fragile - x syndrome and the like . a specific reproductive application of the present method is to the treatment of animals , particularly humans , with disorders of spermatogenesis . defective spermatogenesis or spermiogenesis frequently has a genetic basis , that is , one or mutations in the genome may result in failure of production of normal sperm cells . this may happen at various stages of the development of germ cells , and may result in male infertility or sterility . the present invention is applicable , for example , to the insertion or incorporation of nucleic acid sequences into a recipient &# 39 ; s genome and , thereby , establish spermatogenesis in the correction of oligozoospermia or azoospermia in the treatment of infertility . similarly , the present methods are also applicable to males whose subfertility or sterility is due to a motility disorder with a genetic basis . the present method is additionally applicable to the generation of transgenic animals expressing agents which are of therapeutic benefit for use in human and veterinary medicine or well being . examples include the production of pharmaceuticals in domestic cows &# 39 ; milk , such as factors which enhance blood clotting for patients with types of haemophilia , or hormonal agents such as insulin and other peptide hormones . the present method is further applicable to the generation of transgenic animals of a suitable anatomical and physiological phenotype for human xenograft transplantation . transgenic technology permits the generation of animals which are immune - compatible with a human recipient . appropriate organs , for example , may be removed from such animals to allow the transplantation of , for example , the heart , lung and kidney . in addition , germ cells transfected in accordance with this invention may be extracted from the transgenic animal , and stored under conditions effective for later use , as is known in the art . storage conditions include the use of cryopreservation using programmed freezing methods and / or the use of cryoprotectants , and the use of storage in substances such as liquid nitrogen . the germ cells may be obtained in the form of a male animal &# 39 ; s semen , or separated spermatozoa , or immature spermatocytes , or whole biopsies of testicular tissue containing the primitive germ cells . such storage techniques are particularly beneficial to young adult humans or children , undergoing oncological treatments for such diseases such as leukemia or hodgkin &# 39 ; s lymphoma . these treatments frequently irreversibly damage the testicle and , thus , render it unable to recommence spermatogenesis after therapy by , for example , irradiation or chemotherapy . the storage of germ cells and subsequent testicular transfer allows the restoration of fertility . in such circumstances , the transfer and manipulation of germ cells as taught in this invention are accomplished , but transfection is generally not relevant or needed . in species other than humans , the present techniques are valuable for transport of gametes as frozen germ cells . such transport will facilitate the establishment of various valued livestock or fowl , at a remote distance from the donor animal . this approach is also applicable to the preservation of endangered species across the globe . the invention will now be described in greater detail by reference to the following non - limiting examples . the pertinent portions of the contents of all references , and published patent applications cited throughout this patent necessary for enablement purposes are hereby incorporated by reference . in vivo adenovirus - enhanced transferrin - polylysine - mediated delivery of green lantern reporter gene delivery system to testicular cells the adenovirus enhanced transferrin - polylysine - mediated gene delivery system has been described and patented by curiel al . ( curiel d . t ., et al . adenovirus enhancement of transferrin - polylysine - mediated gene delivery , pnas usa 88 : 8850 - 8854 ( 1991 ). the delivery of dna depends upon endocytosis mediated by the transferrin receptor ( wagner et al ., transferrin - polycation conjugates as carriers for dna uptake into cells , pnas ( usa ) 87 : 3410 - 3414 ( 1990 ). in addition this method relies on the capacity of adenoviruses to disrupt cell vesicles , such as endosomes and release the contents entrapped therein . this system can enhance the gene delivery to mammalian cells by as much as 2 , 000 fold over other methods . the gene delivery system employed for the in vivo experiments was prepared as shown in examples below . human transferrin was conjugated to poly ( l - lysine ) using edc ( 1 - ethyl - 3 -( 3 - dimethyl aminopropyl carbodiimide hydrochloride ) ( pierce ), according to the method of gabarek and gergely ( gabarek & amp ; gergely , zero - length cross - linking procedure with the use of active esters , analyt . biochem 185 : 131 ( 1990 )). in this reaction , edc reacts with a carboxyl group of human transferrin to form an amine - reactive intermediate . the activated protein was allowed to react with the poly ( l - lysine ) moiety for 2 hrs at room temperature , and the reaction was quenched by adding hydroxylamine to a final concentration of 10 mm . the conjugate was purified by gel filtration , and stored at − 20 ° c . the green lantern - 1 vector ( life technologies , gibco brl , gaithersberg , md .) is a reporter construct used for monitoring gene transfection in mammalian cells . it consists of the gene encoding the green fluorescent protein ( gfp ) driven by the cytomegalovirus ( cmv ) immediate early promoter . downstream of the gene is a sv40 polyadenylation signal . cells transfected with green lantern - 1 fluoresce with a bright green light when illuminated with blue light . the excitation peak is 490 nm . adenovirus di312 , a replication - incompetent strain deleted in the ela region , was propagated in the ela trans - complementing cell line 293 as described by jones and shenk ( jones and shenk , pnas usa ( 1979 ) 79 : 3665 - 3669 ). a large scale preparation of the virus was made using the method of mittereder and trapnell ( mittereder et al ., “ evaluation of the concentration and bioactivity of adenovirus vectors for gene therapy ”, j . urology , 70 : 7498 - 7509 ( 1996 )). the virion concentration was determined by uv spectroscopy , 1 absorbance unit being equivalent to 10 viral particles / ml . the purified virus was stored at − 70 ° c . 6 μg transferrin - polylysine complex from example 1 were mixed in 7 . 3 × 10 7 adenovirus d1312 particles prepared as in example 3 , and then mixed with 5 ug of the green lantern dna construct of example 2 , and allowed to stand at room temperature for 1 hour . about 100 ul of the mixture were drawn up into a micropipette , drawn on a pipette puller , and slightly bent on a microforge . the filled micropipette was then attached to a picopump ( eppendorf ), and the dna complexes were delivered under continuous pressure , in vivo to mice as described in example 6 . controls were run following the same procedure , but omitting the transferrin - poly - lysine - dna - viral complexes from the administered mixture . the conjugated adenovirus particle complexed with dna were tested on cho cells in vitro prior to in vivo testing . for these experiments a luciferase reporter gene was used due to the ease of quantifying luciferase activity . the expression construct consists of a reporter gene encoding luciferase , is driven by the cmv promoter ( invitrogen , carlsbad , calif . 92008 ). cho cells were grown in dulbecco &# 39 ; s modified eagle &# 39 ; s medium ( dmem ) with 10 % fetal calf serum . for gene transfer experiments cho cells were seeded into 6 cm tissue culture plates and grown to about 50 % confluency ( 5 × 10 5 cells ). prior to transfection the medium was aspirated and replaced with serum free dmem . cells were either transfected with transferrin - polylysine - dna complexes or with lipofectin dna aggregates . for the transferrin - polylysine mediated dna transfer , the dna - adenovirus complexes were added to the cells at a concentration of 0 . 05 - 3 . 2 × 10 4 adenovirus particles per cell . plates were returned to the 5 % co 2 incubator for 1 hour at 37 ° c . after 1 hour 3 ml of complete media was added to the wells and the cells were allowed to incubate for 48 hours before harvesting . the cells were removed from the plate , counted and then lysed for measurement of luciferase activity . for cells transfected by lipofectin , 1 μg of cmv - luciferase dna was incubated with 7 μl of lipofectin ( life technologies ). the dna - lipofectin aggregates were added to the cho cells and allowed to incubate at 37 ° c . at 5 % co 2 for 4 hours . three mls of complete medium was added then to the cells and they were allowed to incubate for 48 hours . the cells were harvested , counted and lysed for luciferase activity . the luciferase activity was measured by a luminometer . the results obtained are shown in table 1 . the data included in table 1 below show that the adenovirus - enhanced transferrin - polylysine gene delivery system is 1 , 808 fold more efficient than lipofection for transfection of cho cells . the gfp dna - transferrin - polylysine viral complexes , prepared as described in example 4 above , were delivered into the seminiferous tubules of three ( 3 )- week - old b6d2f1 male mice . the dna delivery by transferrin receptor - mediated endocytosis is described by schmit et al . and wagner et al . ( schmit et al ., cell 4 : 41 - 51 ( 1986 ); wagner , e ., et al . pnas ( 1990 ), ( usa ) 81 : 3410 - 3414 ( 1990 )). in addition , this delivery system relies on the capacity of adenoviruses to disrupt cell vesicles , such as endosomes and release the contents entrapped therein . the transfection efficiency of this system is almost 2 , 000 fold higher than lipofection . the male mice were anesthetized with 2 % avertin ( 100 % avertin comprises 10 g 2 , 2 , 2 - tribromoethanol ( aldrich ) and 10 ml t - amyl alcohol ( sigma ), and a small incision made in their skin and body wall , on the ventral side of the body at the level of the hind leg . the animal &# 39 ; s testis was pulled out through the opening by grasping at the testis fat pad with forceps , and the vas efferens tubules exposed and supported by a glass syringe . the gfp dna - transferrin - polylysine viral complexes were injected into a single vasa efferentia using a glass micropipette attached to a hand held glass syringe or a pressurized automatic pipettor ( eppendorf ), and trypan blue added to visualize the entry of the mixture into the seminiferous tubules . the testes were then placed back in the body cavity , the body wall was sutured , the skin closed with wound clips , and the animal allowed to recover on a warm pad . nine ( 9 ) days after delivery of the genetic material to the animals &# 39 ; testis , two of the animals were sacrificed , their testes removed , cut in half , and frozen in liquid nitrogen . the dna from one half of the tissues , and the rna from the other half of the tissues were extracted and analyzed . the presence of gfp dna in the extracts was tested 9 days after administration of the transfection mixture using the polymerase chain reaction , and gfp specific oligonucleotides . gfp dna was present in the testes of the animals that had received the dna complexes , but was absent from sham operated animals . the presence of gfp mrna was assayed in the testes of experimental animals as follows . rna was extracted from injected , and non - injected testes , and the presence of the gfp messages was detected using reverse transcriptase pcr ( rtpcr ) with gfp specific primers . the gfp message was present in the injected testes , but not in the control testes . thus , the dna detected above by pcr analysis is , in fact , episomal gfp dna , or gfp dna which has integrated into the chromosomes of the animal . the transfected gene was being expressed . two males , one having received an injection with the gfp transfection mixture and a control to whom only surgery was administered , were sacrificed 4 days after injection , and their testes excised , and fixed in 4 % paraformaldehyde for 18 hours at 4 ° c . the fixed testis was then placed in 30 % sucrose in pbs with 2 mm mgcl 2 for 18 hours at 4 ° c ., embedded in oct frozen on dry ice , and sectioned . when the testes of both animals were examined with a confocal microscope with fluorescent light at a wavelength of 488 nm . bright fluorescence was detected in the tubules of the gfp - injected mice , but not in the testes of the controls . many cells within the seminferous tubules of the gfp - injected mouse showed bright fluorescence , which evidences that they were expressing fluorescent green protein . gfp transfected males were mated with normal females . the females were allowed to complete gestation , and the pups to be born . the pups ( f1 offspring or progeny ) were screened for the presence of the novel genetic material ( s ). cells were isolated from the testes of three 10 - day - old mice . the testes were decapsulated and the seminiferous tubules were teased apart and minced with sterile needles . the cells were incubated in enzyme mixture for 20 minutes at 37 ° c . the enzyme mixture was made up of 10 mg bovine serum albumin ( embryo tested ), 50 mg bovine pancreatic trypsin type iii , clostridium collagenase type i , 1 mg bovine pancreatic dnase type i in 10 mls of modified htf medium ( irvine scientific , irvine , calif .). the enzymes were obtained from sigma company ( st . louis , mo . 63178 ). after digestion , the cells were washed twice by centrifugation at 500 × g with htf medium and resuspended in 250 μl htf medium . the cells were counted , and 0 . 5 × 10 6 cells were plated in a 60 mm culture dish in a total volume of 5 ml dmem ( gibco - brl , life technologies , gaithesburg , md . 20884 ). a transfection mixture was prepared by mixing 5 μg green lantern dna ( gibco - brl , life technologies , gaithesburg , md . 20884 ) with 20 μl superfect ( quagen , santa clarita , calif . 91355 ) and 150 , μl dmem . the transfection mix was added to the cells and they were allowed to incubate for 3 hours at 37 ° c ., 5 % co 2 the cells were transferred to a 33 ° c . incubator and incubated overnight . the following morning the cells were assessed for transfection efficiency by counting the number of fluorescent cells . in this experiment the transfection efficiency was 90 % ( figure not shown ). the testicular cells transfected with green lantern viewed with nomaski optics × 20 show the same cells viewed with fitc . nearly all the cells were fluorescent , which is confirmation of their successful transfection . the cells were injected into the testis via the vasa efferentia using a micropipette . 3 × 10 5 cells in a total volume of 50 μl were used for the injection . the cells were mixed with trypan blue prior to the injection . three adult mice were injected with transfected cells . the balb / cbyj recipient mice had been irradiated 6 weeks prior to the injection with 800 rads of gamma irradiation . one mouse became sick and was sacrificed 48 hours after the injection . the testes from this mouse were dissected , fixed and processed for histology . the two remaining males were bred with normal females as shown . after 4 months pups were born . litters are currently being screened for the integration of the transgene . a cell suspension was prepared from mice of different ages as described below . the mice &# 39 ; s testes were dissected , placed in phosphate buffered saline ( pbs ) decapsulated , and the seminiferous tubules were teased apart . seminiferous tubules from groups i and ii were transferred to hepes buffered culture medium ( d - mem ) ( gibco - brl , life technologies , gaithesburg , md . 20884 ) containing 1 mg / ml bovine serum albumin ( bsa ) ( sigma , st . louis , mo . 63178 ) and collagenase type i ( sigma ) for the removal of interstitial cells . after a 10 minute incubation at 33 ° c ., the tubules were lifted into fresh culture medium . this enzymatic digestion was not carried out on the testes from group i because of their fragility . the tubules from group ii and iii mice or the whole tissue from group i mice were transferred to a petri dish with culture medium and were cut into 0 . 1 - 1 mm pieces using a sterile scalpel and needle . the minced tissue was centrifuged at 500 × g for 5 minutes and the pellet was resuspended in 1 ml of enzyme mix . the enzyme mix was made up in d - dmem with hepes ( gibcobrl ) and consisted of 1 mg / ml bovine serum albumin ( bsa ) ( sigma , embryo tested ), 1 mg / ml collagenase i ( sigma ) and 5 mg / ml bovine pancreatic trypsin ( sigma ) and 0 . 1 mg / ml deoxyribonuclease i ( dn - ep , sigma ). the tubules were incubated in enzyme mix for 30 minutes at 33 ° c . after the incubation , 1 ml of medium was added to the mix and the cells were centrifuged at 500 × g for 5 min . the cells were washed twice in medium by centrifugation and resuspension . after the final wash the cell pellet was resuspended in 250 μl of culture medium and counted . ( a ) propanediol ( proh )- sucrose method testicular cells from a total of 31 mice ( age 8 - 12 weeks ) were cryopreserved using 6 different freezing and thawing protocols . in addition to freezing cell supsensions , pieces of testicular tissue were frozen ( see freezing method above ). the cell suspension was prepared as described above . the cell suspension was incubated in a buffer stock solution consisting of 80 % phosphate buffered saline ( pbs ) and 20 % human serum ( spr , helsinki , finland ) for 5 minutes . the cells were then incubated in 1 . 5m proh for 10 minutes , pelleted by centrifugation and resuspended in 1 . 5m proh with 0 . 1m sucrose . the cell suspension was loaded into straws ( 0 . 25 μm , paillette , l &# 39 ; aigle , france ) or 1 ml cryogenic vials ( nunc cryotube ). samples were frozen in a controlled temperature freezing machine ( planer kryo , series iii , planer biomed , sunbury on thames , uk ). the samples were cooled at a rate of 2 ° c ./ min to − 8 ° c ., and seeded manually using forceps cooled in liquid nitrogen . after 10 min the samples were cooled at 0 . 3 ° c ./ min to − 30 ° c . after which they were cooled at a rate of − 50 ° c ./ min to − 150 ° c . samples were then stored in liquid nitrogen at − 196 ° c . the samples were removed from liquid nitrogen and kept at room temperature for 2 min . the samples were incubated in 1m proh + 0 . 1m sucrose for 5 min , followed by an incubation in 0 . 5m proh + 0 . 1m sucrose for 5 min and then in 0 . 1m sucrose for 10 min . the cell suspension was placed in buffer stock . the cell suspension was pipetted into a vial and the yolk buffer freezing medium ( irvine scientific , santa ana , calif .) was added drop by drop to make up approximately 50 % of the total volume . the samples were cooled in a controlled freezer at an initial cooling rate of 0 . 5 ° c ./ min to a temperature of 1 . 5 ° c . the samples were then cooled at 10 ° c ./ min until they reached a temperature of − 80 ° c . on reaching this temperature the samples were placed in liquid nitrogen for storage . samples were removed from liquid nitrogen and thawed at room temperature . the suspension was centrifuged and the pelleted cells were resuspended in pbs . cells were pipetted into a cryogenic vial containing 60 % medium 199 with earle &# 39 ; s salts ( gibco , gaithesburg , md . ), 20 % human ab serum . 20 % dmso was added to the cells drop by drop to make up 50 % of the total volume . the cells were cooled at a rate of 4 ° c ./ min to 0c . and then at 1 ° c ./ min to − 80 ° c ., then at 10 ° c ./ min to − 100 ° c . and finally at 20 ° c ./ min to − 160 ° c . the samples were then stored in liquid nitrogen . samples were removed from liquid nitrogen and thawed at room temperature . the suspension was centrifuged and the pelleted cells were resuspended in pbs . cells were pipetted into a cryogenic vial . a solution containing 45 % 5000 u / ml heparin ( tovens medicinske fabrik , ballerup , denmark ), 15 % dmso and 40 % albumin ( spr ) in pbs was added drop by drop to make up 50 % of the total volume . the freezing and thawing programme was the same as that used for the glycerol yolk buffer method . cells were pipetted into a cryogenic vial and a freezing solution containing 90 % fetal calf serum and 10 % dmso was added at room temperature to make up 90 % of the total volume . the samples were placed in a − 70 ° c . freezer ( revco scientific corp ., asheville , n . c .) for 24 hours . the samples were then stored in liquid nitrogen . the thawing procedure was that same as that used for the glycerol yolk method . the cells were pipetted into a cryogenic vial and a freezing solution containing 70 % dmem , 20 % fetal calf serum and 10 % filtered glycerol was added to the cells to make up 90 % of the total volume . the resuspension was incubated at 37 ° c . for 10 min . the samples were placed in a − 70 ° c . freezer for 24 hours after which they were stored in liquid nitrogen . the thawing procedure was the same as that described for the glycerol yolk method . the method used for freezing whole testicular tissue was the same as the method we described previously for freezing ovarian tissue ( hovatta , et al ., human reprod . 11 : 1268 - 1272 ( 1996 ). the testicles of 6 mice were decapsulated in culture medium ( d - mem ) and cut into 0 . 3 - 1 . 0 mm pieces . the tissue pieces were placed in medium containing 1 . 5m proh in pbs with 20 % serum for 10 min . at room temperature . they were transferred to cyrogenic vials and cooled at 2 ° c ./ min to − 8 ° c . the vials were seeded manually with forceps dipped in liquid nitrogen . after 10 min the cooling was continued at a rate of 0 . 3 ° c ./ min to − 30 ° c . and then at a rate of 50 ° c ./ min to − 150 ° c . when the samples reached this temperature they were transferred to liquid nitrogen . the vials were removed from the liquid nitrogen and allowed to come to room temperature for 2 min . they were then placed in a water bath at 30 ° c . until they had thawed . the tissue pieces were transferred to a petri dish containing 1 . 0m proh , 0 . 1m sucrose and 20 % serum in pbs for 5 min . they were then transferred to a solution containing 0 . 5m proh , 0 . 1m sucrose and 20 % serum in pbs for 5 min and then to a solution containing 0 . 1m sucrose with 20 % serum in pbs for 10 min . the cells were kept in culture medium . the results obtained from the above experimental procedures are summarized in table 2 below . from table 2 above , it may be seen that the testicular cells that had been frozen using the propanediol - sucrose method had the highest percentage of viable cells upon thawing than cells frozen using the other methods . the propanediol - sucrose freezing method was significantly less damaging to testicular cells than the dmso method used by avarbock et al ., 1996 for freezing testicular cells prior to transfer . the propanediol - sucrose method was also shown to be good for freezing human ovarian tissue as described by hovatta et al . ( hovatta et al ., human reprod . 11 : 1268 - 1272 ( 1996a ), the relevant part of which is incorporated herein by reference , and pieces of testicular tissue . the testicular spermatozoa from a human biopsy were frozen - thawed using the glycerol - yolk buffer method , and then used for intracytoplasmic injection of eggs ( icsi ). a successful pregnancy resulted ( hovatta , o . et al ., pregnancy resulting from intracytoplasmic injection of spermatozoa from a frozen thawed testicular biopsy , human reprod . 11 : 2472 - 2473 ( 1996b ).