Functional differentiation in mouse mammary gland epithelium is attained through DNA synthesis, inconsequent of mitosis

摘要

Virginmousemammaryglandinexplantculturewilldifferentiateandsynthesizecaseinand?±-lactalbuminwheninsulin,hydrocortisone,andprolactin(IFPRL)arepresentintheculturemedium.ExplantswhoseDNAsynthesishasbeenblockeddifferentiatecytologically,mobilizelipid,synthesizeRNA,andincorporate3H-aminoacidsintoproteinstothesameextentasunblockedtissue.Nevertheless,caseinsynthesisasmeasuredbyimmunoprecipitationwithcasein-specificantiserumremainsatthezero-timelevelinblockedexplantswhileunblockedexplantsproducecaseinatfive-toeightfoldgreaterlevels.ElectrophoreticanalysisofimmunoprecipitatedradioactiveproteinsshowedthattheIFPRL-treatedvirgintissuemadeallfoursizeclassesofmousecasein.Immunoperoxidasestudiesofexplantsrevealedthatthenumberofmammaryepithelialcellspositiveforcaseinwas2a€“8%inblockedand24a€“31%inunblocked,ingoodagreementwiththeradioimmunoprecipitationresults.ImmunoelectronmicroscopydemonstratedtheaccumulationofcaseinwithinthecisternaeofthegranularendoplasmicreticulumandinGolgivacuolesintheunblockedepithelialcells.Similaraccumulationdidnotoccurinblockedculturesdespitethesecretoryappearanceofthecells.Autoradiographicanalysisofblockedandunblockedexplants,incubatedinthepresenceofIFPRLand[3H]thymidinefor72hr,showedthat53a€“57%oftheepithelialcellssynthesizedDNAinunblockedexplants,whereasonly2%incorporatedthelabelinthepresenceofcytosinearabinoside.WhenexplantswereincubatedwithIFPRLandvariousconcentrationsofcolchicine,only5a€“6%oftheepithelialcellswerefoundtoentermitosis.Sincecellduplicationcannotaccountfortheseveralfoldincreaseincasein-producingcellsintheunblockedexplants,theresultssuggestthattherequirementforDNAsynthesisinthissystemmayinvolveeitherpolyploidcellsortheaugmentationofspecificsequencesnecessaryforthefacilitationofterminaldifferentiation.SimilarrequirementsforDNAsynthesiswerenotobservedinmammaryexplantsfrompregnantandprimiparous(butnonpregnant)mice.