A
B
C
CR 3.3 mm, C6144
CR 8.0 mm, M2065
CR 10.5 mm, C6517
0.25 mm0.25 mm
0.5 mm0.5 mm
1 mm1 mm
0.25 mm
0.59 mm
0.74 mm
Medial face of the
neuroepithelium
Roof plate
Floor plate
Dorsal canal
Ventral canal
Anterior cut edge
of the neuroepithelium
Gray matter
Central canal
Dorsal canal
Ventral canal
Central canal
Dorsal canal
Ventral canal
Central canal
Figure 7. The undivided neuroepithelium, roof plate, fl oor plate, and gray matter on the
left side of the spinal cord in the following specimens: CR 3.3 mm (A), CR 8 mm (B), and
CR 10.5 mm (C). Note the scale differences between specimens. The dashed lines run
through the sulcus limitans. Notice that the sulcus is closer to the roof plate in A and shifts
downward in B and C. That coincides with the expansion of the dorsal neuroepithelium,
which is located above the sulcus limitans. In these young specimens, the roof plate is the
uppermost structural feature of the spinal cord, and is always above the level of the dorsal
horn. In addition, the roof plate has a smooth domed surface with no indication of a cleft in
the midline. The vertical bars show the actual distance (in millimeters) between the roof
and fl oor plates in the approximate center section of each model. The distances continu-
ally increase from A to C indicating that the neuroepithelium has a net growth during this
period, even though many young neurons have already been generated and their stem cells
are no longer in the neuroepithelium.
FIGURE 7 Neuroepithelium, Roof and Floor Plates – GW3.5 to GW5.5