Early pancreas organogenesis: Morphogenesis, tissue interactions, and mass effects☆

[摘要] Themidgutregionandadjacenttissuesofmouseembryosacquiretheabilitytoformexocrinepancreastissueinvitroataboutthe8-somitestage.By10somitesaregionofthegutitselfcanbeidentifiedastheprecursorofthepancreas.Theendodermofthisareaapparentlybecomescommittedordeterminedbyabout15somites,sinceatthattimeitcanformexocrineaciniwhencombinedwithmousesalivarymesoderm.Thesealterationsoccuratstagesbeforecondensedpancreaticmesodermispresentorbeforeoutgrowthofthepancreasdiverticulumoccurs.Thebeginningofpancreaticmorphogenesisismarkedbyappearanceofabroad-basedupswellingfromthedorsalgut.Thebaseofthisevaginationgraduallyconstrictstogiverisetothepancreaticduct.These,andresultsonthedistributionofmesodermalandendodermalcellsduringearlyorganogenesis,arediscussedinrelationtotheconceptthataprimarycommitmentordeterminationphaseisfollowedbyasecondarysupportphaseduringwhichnonspecificmesodermalinputissufficienttopermittissuegrowth.Theroleofsuchgrowth,asreflectedinepithelialmass,wasalsoinvestigated.Precociousappearanceofzymogengranulesoramylaseisnotevokedbyevena24-foldincreaseinmassofprimitivemousepancreaticepithelium.Ontheotherhand,fractionationofgrowingepithelia,withresultantdecreaseintissuemass,leadstopostponementofovertdifferentiation.Oncecellshavereachedaspecificchronologicalage,theydifferentiateifleftinanundisturbedtissuemassforabout2–3days.Undersuchcircumstances,differentiationcanoccurinextraordinarilysmalltissuemasses.Massperseisnotthecriticalfactorleadingtocessationofmitosisandcommencementofovercytodifferentiation.Operationally,thecellsmustbesufficientlymatureandmustbeundisturbed,inthesenseofnotbeingstimulatedtocontinuedividing,ifdifferentiationistooccur.

[学科分类] 生物科学（综合）