Development of structure and function in the mammalian yolk sac: I. Developmental morphology and vitamin B12 uptake of the rat yolk sac☆

[摘要] Morphologicalandphysiologicalaspectsofdifferentiationhavebeenanalyzedinthevisceralsplanchnopleureoftheinvertedyolksacplacentaofthealbinoratbetween13daysandterm.Thevisceralyolksacisknowntobeinvolvedinthetransferofantibodiesfromthematernaltothefetalblood.Theabsorptivecellsofthevisceralendodermpossessultrastructuralfeaturescharacteristicofprotein-absorbingcells.Theappearanceofthefreesurfaceofthesesimplecolumnarcellssuggestsmotility.Itisprimarilymodifiedtoformlong,branchingmicrovillithatoftenseemtobefusing.Invaginationsofthefreesurfaceoriginatebetweenmicrovilli,andmostprobablyconnectwithanextensivecanalicularsystemofsmoothmembraneslocatedinthesuperficialcytoplasm.Theinnersurfaceofboththeinvaginationsandthecanaliculipossessesafilamentouscoatthatmayrepresentavastinternalsurfacefortheattachmentofproteins.Morphologicalandphysiologicalevidencesuggeststhatlargeabsorptivevacuolesariseasdilatationsofthecanaliculi.Thecontentofthemoredeeplylocatedvacuolesismorecondensed,andtheymaybecalledabsorptivedroplets.Thus,asuperficialsystemofsmoothmembranesisdirectlyinvolvedintheuptakeofmacromolecules,suchasantibodiesandotherproteins.Duringthisperiodofgestationvariousmorphologicalchangesoccurthatmayaffectabsorptiveactivityconsiderably.Asgestationproceeds,themicrovillibecomeshorterandlessbranched;thesmoothmembranesystemisaltered;andintercellularassociationschangeaslargedilatationsappearintheintercellularspaceoftheabsorptiveepithelium.Inaddition,thereisathickeningoftheplacentalbarrierthatintervenesbetweentheyolksac(oruterinecavity)andthevitellinecirculation.ThepresentfindingsindicatethatthevisceralyolksacpossessesahighlyspecificmechanismforaccumulatingconsiderablevitaminB12andB12-intrinsicfactorcomplexinvitro.Theparietalwalllacksthismechanism.ThecapacitytoconcentratevitaminB12andB12-intrinsicfactorbothinvivoandinvitroisgreaterat13daysthanneartermwhendeterminedonthebasisofradioactivityperunitweight.Thissuggestsadecreaseinabsorptivecapacitypercellasgestationproceeds.Thereducedabsorptiveactivityofeachcellis,however,morethancounterbalancedbyaseventyfoldincreaseinthemassofthisplacentalmembrane.Microscopicexaminationrevealedthatthemorphologyofthevisceralyolksaciswellpreservedduringa60-minuteperiodofinvitroincubation.Experimentalfindingsinvitroindicatethatalthoughthevisceralsplanchnopleurecanaccumulatealargemacromolecularcomplex,suchasvitaminB12-IF,itlacksthecapacitytoconcentratesmallmolecules,suchasl-valineandα-methylglucoside,againstaconcentrationgradient.Thisevidencesuggeststhatthisplacentalmembraneintheratmaybehighlyspecializedforthetransferofmacromolecules.

[学科分类] 生物科学（综合）