Biomedical Engineering Reference
In-Depth Information
Fig. 24.4
Boundary formation in the brain tube of the chicken embryo. (
A
) The primary brain
vesicles (forebrain, midbrain, and hindbrain) are separated by the permanent fore-midbrain (
FM
)
and mid-hindbrain (
MH
) boundaries. Rhombomeres (
RH
) are transient, sequential bulges in the
early hindbrain. (
B
,
C
) Axisymmetric finite element models of vesicle (
B
) and rhombomere (
C
)
morphogenesis. Contraction occurs at the apical (inner) wall. (
B
) The mid-hindbrain boundary
contracts in the circumferential direction, but the apical side of the wall contracts isotropically
elsewhere to create vesicles. (
C
) Longitudinal contraction between the more passive rhombomere
boundaries causes local bulges (rhombomeres) to form
24.3.3 Rhombomere Formation
As the primary brain vesicles form, a series of smaller, periodic bulges arise in the
hindbrain. These rhombomeres (Fig.
24.4
A), have received considerable attention
since the early 1990s as regions of cell lineage restriction and differential gene ex-
pression (reviewed in Kiecker and Lumsden,
2005
). With the spotlight on these
structures as local signaling centers, interest in the morphogenetic mechanisms of
rhombomere formation has receded. Still, some useful mechanistic details can be
garnered from the earlier literature.
In rhombomeres of chicken embryos, cell proliferation rates and apical F-actin
concentrations are higher in interboundary regions than in the boundaries (Guthrie
et al.,
1991
). In addition, the amount of extracellular space between neighboring
cells tends to increase in the boundaries during development (Heyman et al.,
1993
).
These results led to early speculation that a bowing or buckling mechanism, due to
constrained cell proliferation, drives rhombomere formation.
Alternatively, apical contraction between boundaries could play a role in rhom-
bomere formation. For example, the model in Fig.
24.4
C shows that longitudinal
contraction along the inner wall between boundaries causes these regions to bend
outward, producing a shape consistent with experimental observations.
Interestingly, rhombomeres are transient structures during brain development (as
opposed to the primary vesicle boundaries which persist through maturity) (Kiecker
