front matter 1

A

C

B

CR 19.1 mm, C8965

CR 22.0 mm, C8553

CR 36.0 mm, M2050

DD

CR 56.0 mm, Y380-62

1 mm1 mm

1 mm1 mm

1 mm1 mm

1 mm1 mm

0.91 mm

1.01mm

0.49 mm

0.16 mm

Roof plate

(short double arch)

Roof plate

(elongated double arch)

Roof plate

(elongated vertical spike)

Roof plate

(contracting vertical spike)

Floor plate

Dorsal canal

Dorsal canal

Ventral canal

Ventral canal

Central canal

Central canal

Dorsal canal

Ventral canal

Central canal

Central canal

Gray matter

Figure 8. A continuation of the series in
the following specimens: CR 19.1 mm
(A), CR 22 mm (B), CR 36 mm
(C), and CR 56 mm (D). Note the
scale differences between specimens.
The sulcus limitans (dashed line) gets
closer and closer to the fl oor plate from
A to D, refl ecting the regression of the
ventral and intermediate neuroepithe-
lia. On the other hand, the dorsal neu-
roepithelium is expanding between A
and B, indicating that it is still pro-
ducing neurons. Note that the distance
between the roof and fl oor plates (ver-
tical bars with measurements in milli-
meters) reaches its maximum between
A and B as the dorsal neuroepithelium
stretches in the dorsoventral plane.
The dorsal neuroepithelium declines
between B and C as neurogenesis
wanes, and the distance between the
roof and fl oor plates gets shorter.
The dorsal neuroepithelium disappears
between C and D, marking the end of
neurogenesis in the cervical spinal cord.
By this time, the distance between
the roof and fl oor plates shrinks to a
smaller value than the youngest speci-
men (A, Figure 7). In A, B, and C,
the roof plate is positioned at the same
level as the most dorsal part of the
gray matter, then it sinks downward
in D, and the gray matter begins to
arch dorsally, as neurons settle and
start to differentiate in the dorsal horn.
During that downward movement, the
roof plate changes its surface features.
In all panels of Figure 7, the roof plate
surface is a single arch spanning the
midline. In A of this series, the surface
is a shallow double arch with a cleft in
the midline. The roof plate still has a
double arch in the dorsal midline in B,
but there is a noticable lengthening in
the dorsoventral plane. In C and D, the
double arch is replaced by a sharp ver-
tical spike in the midline that starts out
elongated (C) and then moves down-
ward (D) as the dorsal canal completely
disappears to leave only the central
canal. Concurrent with these changes
in the roof plate, the dorsal funiculus
fi lls with axons in its most medial parts
and marks the beginning of the fascic-
ulus gracilis and fasciculus cuneatus in
the cervical spinal cord. It is postulated
that the roof plate provides some guid-
ance or structural support for axonal
recruitment in these fasciculi (Altman
and Bayer, 2001).

Neuroepithelium, Roof and Floor Plates – GW7.0 to GW14 FIGURE 8