front matter 1

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