Differentiation of parietal endoderm cells of the guinea pig yolk sac, with particular reference to the development of endoplasmic reticulum☆☆☆

摘要

Cytodifferentiationinparietalendodermcellswasstudiedbylightandelectronmicroscopyinyolksacsofguineapigsbetween22daysofgestationandterm(67a€“70days).Theparietalyolksacendodermformsanepithelialcoveringofthemarginofthechorioallantoicplacentaldisc.Earlyingestationtheepitheliumissimple,butduringthemiddleofpregnancytheendodermproliferatesrapidlytobecomeathickpseudostratifiedlayer.Lateingestationtheepitheliumisagainreducedinthickness.Earlyinthecourseofdifferentiationtheendodermcellcytoplasmcontainslargenumbersoffreepolyribosomesandscantelementsofendoplasmicreticulum(ER).By28a€“30days,granularERbecomeshighlydeveloped,consistingofdilatedfenestratedcisternae.By34a€“38days,however,agranularER,intheformofanastomosingtubules,becomesthedominantcytoplasmicfeatureofthesecells.Inaddition,whorlsorlamellaeofagranularmembranesarepresent.From40daystotermthereislittlechangeinthedistributionofER.ThegranularERappearstobeinvolvedinthesynthesisofReichert'smembrane.ThefunctionalsignificanceofthewelldevelopedagranularERisnotknown,butthepossibilitiesthatitmaybeinvolvedinsteroidorcarbohydratemetabolismarediscussed.