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Fig. 6. Hypothetical model of growth of the allantoic bud and regenerates into
BMP4-positive mesoderm of the exocoelomic cavity. Schematic diagram
hypothesizes how growth of the allantois and allantoic regenerates might occur
by a `finger-in-a-glove' mechanism (suggested by Downs)
(Downs, 1998). (A) Normal
(non-hypothetical) neural plate/No bud (0B) stage, approximately 7.25 dpc.
Thick lines (both black and red) are BMP4-positive with black color the
mesodermal component of the yolk sac and amnion, and the red color the future
site of the allantoic bud. The localization profile of BMP4 at this stage is
from Lawson et al. (Lawson et al.,
1999). (B) In this hypothetical scenario, the normal allantois
grows into the BMP4-positive corner like a finger-in-a-glove, the result of
which is an allantoic bud whose outer surface contains BMP4-positive cells.
(C) The allantois has been microsurgically removed and, in the subsequent
hypothetical scenario, BMP4-positive mesoderm from the yolk sac and amnion
crawls onto the exposed primitive streak, thereby re-establishing the BMP4
expression pattern of the normal 0B stage in A, and subsequently that of B in
the regenerates. Abbreviations as in Fig.
1.
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