Scientist: Research Projects: Research Articles
Craniofacial Development in the Mouse
A major research focus of Dr. Gregg Semenza’s laboratory of
the Center for Craniofacial Development and Disorders at Johns Hopkins
University is to determine the role of Msx-2 in both normal and
abnormal craniofacial development in the mouse. The mouse provides
an excellent animal model system to do this work, in that like the
human, the mouse is a vertebrate. As such, mouse and human embryonic
development share very similar morphogenetic processes, in fact
during critical early stages of embryogenesis the two species are
nearly indistinguishable morphologically. Mice generally have between
8 and 10 offspring per litter, and their gestation period is approximately
three weeks. Such characteristics make the mouse a very practical
and useful vertebrate model system, and data collected using this
animal model can be in many cases, readily extrapolated to the human.
The muscle-segment homeobox gene, MSX-2 is a homeodomain transcription
factor that has been implicated in craniofacial morphogenesis on
the basis of its expression during development in a number animal
models. In mouse embryos, Msx-2 is expressed during critical stages
of neural tube, neural crest and craniofacial development, suggesting
that it plays an important role in the early development of the
face and brain. While the expression pattern of Msx-2 expression
is intriguing, little is known about the function of this gene in
vertebrate embryonic development.
Dr. Semenza’s laboratory has generated transgenic mice which
carry a 34-kb DNA fragment encompassing the entire human MSX-2 gene
as well as critical regulatory regions that determine the temporal
and spatial pattern of gene expression during development. In the
transgenic offspring a number of craniofacial malformations were
observed including facial clefts, exencephaly, clefting of the secondary
palate, and mandibular hypoplasia (Fig 1). Interestingly, all of
the transgene-induced malformations involved neural crest derivatives
and were characterized by a deficiency of tissue. Similar malformations
are seen in the offspring of mothers exposed to various teratogens,
including ethanol.
We have previously shown that when ethanol is administered acutely
on gestational day 8, a high incidence of exencephaly and cleft
lip are produced (Fig 2). When we looked at embryos within 12 hours
of initial insult we found excessive cell death in two distinct
cell populations, the neurosomatic junction of the mid- and hindbrain,
and the anterior neural ridge. The neurosomatic junction provides
neural crest cells which are essential to maintain the critical
mass necessary to approximate the neural folds. Any condition which
leads to the reduction of this important cell population, such as
ethanol exposure, would likely prevent the completion of neurulation,
and subsequent neural tube defects would be produced. The anterior
nasal ridge, on the other hand, provides cells which contribute
to the midline of the nose and upper lip, and by the same logic
as above, reduction of this progenitor cell population would produce
midline facial deficiencies, similar to those observed in the ethanol-treated
fetuses. These data indicate that the cell death patterns found
in embryos just after ethanol administration appear to be pathogenically
correlated with the subsequent malformations.
Given the similarity in fetal phenotypes between our transgenic
and ethanol-treated fetuses, it is one our goals to determine whether
the two animal models have a common pathogenesis in regard to cleft
lip and exencephaly. In addition, we are utilizing mice that have
been genetically altered to either overexpress (transgenic) or underexpress
(knockout) Msx-2 in order to determine whether alterations in Msx-2
expression play a critical role in the pathogenesis of teratogen-induced
birth defects and to more precisely define the role of MSX-2 in
normal and abnormal craniofacial development. With this information
at hand, it ultimately may be possible to identify at-risk pregnancies
and design intervention strategies aimed at the prevention or treatment
of craniofacial malformations.
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Figure 1: Comparison of non-transgenic (a) and transgenic
(c) newborn mouse pups. A high incidence of cleft lip (c)
and exencephaly (e, left) were present in the transgenic offspring,
as compared to non-transgenic littermates (e, right).
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Figure 2: Gestational day 14 control (a) and ethanol-treated
embryos. A spectrum of facial clefts are produced when ethanol
is administered early on gestational day 8, including unilateral
clefts (b), midline clefts confined to the upper lip (c),
and midline clefts which affect the upper lip and nose (d).
A high incidence of exencephaly was also present, and in some
animals was found in conjunction with the facial clefts.
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Author: Lori Kotch, Ph.D.
Date: May 13, 1999
Last Updated:
6/27/02
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