AN INVESTIGATION INTO EARLY PLACENTAL ONTOGENY - ALLANTOIC ATTACHMENTTO THE CHORION IS SELECTIVE AND DEVELOPMENTALLY-REGULATED

Culture of postimplantation conceptuses was used in conjunction with m icrosurgery to investigate the timing, the mechanism and the developme ntal regulation of chorioallantoic fusion in the mouse. The timing of fusion was determined in both freshly recovered conceptuses and in tho se that had been cultured from as early as the mid-streak stage. Attac hment of the allantois to the chorion was found to have occurred in mo st conceptuses by the 6-somite stage, irrespective of whether they had been cultured. In investigating the mechanism of fusion, we wished to determine whether it depended on directed growth of the allantoic bud or on its differential adhesion to the chorion. Microsurgery was used to transplant allantoic tissue into the exocoelomic cavity of concept uses from which the resident allantois had been removed. In synchronou s grafting experiments, transplanted allantoises typically attached to the chorion despite loss of their connection with the hindgut region of the fetus. Hence selective attachment of the allantois to the chori on clearly cannot depend simply on its directed growth. While the tran splanted allantoic tissue attached to the chorion selectively, it did not attach to it precociously, despite being favourably positioned to do so. These findings argue that the initial attachment of the allanto is to the chorion depends on a selective adhesive mechanism that is de velopmentally regulated. Further grafting experiments in which donor c onceptuses were either more or less advanced than hosts revealed that attachment of the allantois to the chorion depends primarily on the st age of the allantois rather than on the stage of the chorion. Collecti vely, these findings support the hypothesis that the initial stage of chorioallantoic fusion depends on selective adhesion between regionall y differentiated mesodermal surfaces which is governed principally by the stage of development of the allantois.