Biomedical Engineering Reference
In-Depth Information
Figure 8.5.
Scanning electron micrographs of the node (a and b) and monocilia
(arrowhead in c) of a mouse embryo. A, P, R, L indicate the anterior, posterior,
right, left side, respectively. Figure is reprinted from [60]. Copyright (2002), with
permission from Macmillan Publishers Ltd.
The beating pattern has also been characterized quantitatively. Monocilia have
a typical length of
L
5
μ
m (for mice) and they beat with typical frequencies
ω
10Hz [56].
The flow over the nodal surface induced by the beating of all monocilia is height-
dependent. With the help of exogenously introduced latex beads following the nodal
flow the velocity can be measured [13]. Okada et al. [56] find at a height of
5
μ
m
a typical fluid velocity
v
f
4
μ
m
/
s to the left (earlier measurements of the same
group yielded larger velocities
v
f
50
μ
m
/
s [54]). The counter-flow to the right
(which is necessary due to conservation of mass) takes place at a height
−
20
20
μ
m,
with typically
v
f
2
μ
m
/
s.
From a theoretical point of view, the rotational movement of the cilia is repre-
sented by an axial vector. This chiral structure aligned with respect to the A-P and
A-V axes then generates the L-R laterality as proposed by Brown and Wolpert [53].
The direction of the flow is solely determined by the tilting of the axis of rotation.
This mechanism of establishing L-R asymmetry is also conserved in rabbits and fish.
The analysis of the nodal flow in mice mutants confirms the role monocilia play
in L-R symmetry breaking.
iv
is a mouse mutant that results in randomization of
L-R determination [63]. The mutant
inv
shows a complete inversion of the L-R body
axis [64]. The experimental analysis of the properties of the monocilia of
inv
mutants
gives further support that posteriorly tilted, clockwise rotation of cilia produces the
