Anterior-Posterior Patterning 
in the Early Mammalian Embryo 
Janet Rossant, Ph.D. — International Research Scholar 
Dr. Rossant is Professor of Molecular and Medical Genetics at the University of Toronto and Senior 
Scientist at the Samuel Lunenfeld Research Institute of Mount Sinai Hospital, Toronto. She received her 
undergraduate training in zoology at Oxford University and her Ph.D. degree from Cambridge University. 
Her postdoctoral training was in Richard Gardner's laboratory at Oxford. Before joining Mount Sinai 
Hospital, she was Associate Professor of Biological Sciences at Brock University, St. Catherines. 
SHORTLY after implantation in the mother's 
uterus, the mammaHan embryo is transformed 
from an undifferentiated group of cells with no 
axis of symmetry into a trilayered structure with 
anterior-posterior (A-P) and dorsal -ventral (D-V) 
polarity and the beginnings of segmentation. One 
of the main challenges in mammalian develop- 
ment is to understand the cellular processes that 
underlie these events and how they are geneti- 
cally controlled. 
Studies in vertebrates other than the mouse 
have implicated inductive interactions between 
tissue layers as important in both determining 
new tissue types and establishing regional do- 
mains along the A-P axis. We have devised an ex- 
plant-recombination system that allows us to 
address the importance of such inductive interac- 
tions in the mouse. Using the mouse homeobox- 
containing engrailed-like (En) genes as markers 
for a specific anterior domain of the nervous sys- 
tem, we have shown that expression of these 
genes in ectoderm depends on interaction with 
underlying mesoderm. 
Isolated anterior ectoderm from premesoderm 
stages of development will not initiate En gene 
expression in vitro but will express En genes 
after aggregation with later anterior mesoderm. 
Anterior mesoderm will also induce En expres- 
sion in posterior ectoderm, which never nor- 
mally expresses En proteins. Posterior mesoderm 
is incapable of En induction when combined 
with either early ectoderm or later posterior 
ectoderm. 
These experiments and others in progress indi- 
cate that there is regionalization in the capacity 
of mesoderm to induce specific neural struc- 
tures. Furthermore, preliminary evidence sug- 
gests that the basic patterning of the forebrain, 
midbrain, and anterior hindbrain structures is 
laid down by mesoderm induction at the early 
neural plate stage and that the later interac- 
tions that lead to the localized expression of 
other anterior genes may be confined to the 
neuroectoderm . 
We are continuing these kinds of studies on 
both mesoderm and neural induction in the 
mouse, making use of the increasing number of 
early marker genes for the processes of A-P pat- 
terning. We expect these studies to define in 
more detail the temporal and spatial parameters 
of inductive interactions. The data will help in 
the search for the underlying molecular basis of 
patterning in the embryo. 
One factor thought to play a role in A-P pattern- 
ing is retinoic acid (RA). There is considerable 
circumstantial evidence to implicate RA in help- 
ing to establish the boundaries of specific A-P do- 
mains in the developing embryo. We have pro- 
vided more such evidence by showing that 
transgenic mice, carrying an RA-responsive ele- 
ment upstream of a neutral promoter-/<2cZ con- 
struct, express the bacterial lacZ gene in a spe- 
cific posterior domain of the embryo. 
The boundary of this expression domain coin- 
cides, at the early neural plate stage, with the 
anterior boundary of the Hox gene, Hox-2.9. As 
development proceeds, the boundary of expres- 
sion of the transgene recedes in concert with the 
establishment of more-posterior //ox gene bound- 
aries in the hindbrain. Hox genes are thought to 
be involved in A-P patterning, especially in defin- 
ing the identity of the segmental hindbrain rhom- 
bomeres. The coincidence of Hox gene bound- 
aries and the receding RA-responsive transgene 
boundary suggests a possible involvement of RA 
in establishing Hox gene expression domains. 
We have shown that anterior members of the 
Hox- 2 gene cluster respond within four hours to 
exogenous RA in vivo. This response involves an 
anterior shift in their expression boundaries, and 
the altered expression domain persists in later 
embryos and can be correlated with the specific 
teratogenic effects of RA in the hindbrain. Further 
experiments are planned to confirm that endoge- 
nous RA is important for Hox gene patterning. 
Our knowledge of the genetic control of pat- 
terning in the gastrulating mouse embryo is still 
very limited. If one could screen rapidly through 
large numbers of genes for their pattern of ex- 
pression at gastrulation, one could hope to iden- 
tify overlapping expression patterns that indicate 
fundamental developmental domains. In coUabo- 
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