Km. Downs et Rl. Gardner, AN INVESTIGATION INTO EARLY PLACENTAL ONTOGENY - ALLANTOIC ATTACHMENTTO THE CHORION IS SELECTIVE AND DEVELOPMENTALLY-REGULATED, Development, 121(2), 1995, pp. 407-416
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.