Biomedical Engineering Reference
In-Depth Information
at the three-somite stage and the thickened neural plate with a middle line, named neural keel, can be
revealed through THG modality (Figure 14.8b). However, because the rate of cell mitosis at this stage is
retarded and there are no nerve fibers existing, almost no SHG signals were observed inside the neural
keel. As the embryo developed into the five-somite stage, the first neuron inside the neural keel began
to differentiate at the five-somite stage and elongated the nerve fibers (Kimmel et al. 1995), while mean-
while, SHG signals arising from the nerve fibers appeared again in the middle of the neural keel and they
gradually extended to be linear-like (Figure 14.8c). As the development went on from the five-somite
stage to prim-15 stage (Figures 14.8d through 14.8f), SHG modality dynamically recorded the growth
of the nerve fibers and SHG signals became much stronger and more linear-like. On the other hand, the
morphological changes from neural keel to a rod-like structure called neural rod, and the formation of
the neural tube, which is a hollow structure with several lumens, can all be revealed by THG modality.
During the whole process of the brain development, interface-sensitive THG provides the 3D sketch
of different structures from a neural plate to a hollow neural tube; nerve-fiber- and mitotic-spindle-
sensitive SHG can tell us much more stories about how cells behave during the development process. In
addition to the brain, the axons elongated from the ganglia in the retina were also observed through the
SHG and THG also showed the structure of the eyes, including the retina and the lens.
By enlarging the SHG/THG images of the neural tube (Figure 14.9a), different SHG contrast sources
can be clearly recognized. With the structures of the neural tube revealed by the THG modality, in
the enlarged image (Figure 14.9b), SHG signals can be identified to arise from the mitotic spindles
(arrowhead) and the nerve fibers (arrow) according to their specific distribution and dynamic changes
of the distribution. In the neural tube, SHG signals from the mitotic spindles were found to be highly
concentrated in the region near the middle line, called ventricular zone or gray matter (Figure 14.9c).
This completely matches the fact that stem cells or precursor cells only exist in the ventricular zone and
mitosis can only occur in this zone (Kimmel et al. 1995). On the other hand, the linear-like SHG signals
FIgurE 14.9
SHG signals in the neural tube. (a) THG shows the outline of the neural tube and SHG reveals the
nerve fiber distribution in the neural tube. (b) Inset from (a) shows the SHG in the ventricular zone (VZ) near the
lumens (L). In this region, SHG arises from both mitotic spindles (arrowhead) and nerve fibers (arrow). (c) Enlarged
image corresponding to the area of inset in (b) shows clearly the mitotic spindles in the ventricular zone of the
neural tube. (d) Cell mitosis (arrow) at the bottom of the retina (R). SHG images corresponding to (a)-(c) are shown
in (a-S)-(c-S), respectively.
