Plate 1 is a survey of sections from the cervicallevels of the spinal cord in the youngest 11 specimens.
All sections are shown at the same scale. The boxes
enclosing each section list: the gestational age in weeks
(GW); the crown rump length (CR) in millimeters (mm);
the specimen number preceded by M (Minot Collection)
or C (Carnegie Collection); the slide number and section
number from the set of slides containing all the sections of
that specimen; and the total area of the section in square
millimeters (mm^2 ). Since all Minot specimens have con-
secutive section numbers, no slide number is given. Full-
page normal contrast photographs of each specimen are in
Plates 2A-12A. Low contrast photographs with superim-
posed labels and outlines of structural details are in Plates
2B-12B. All A-B plates are on facing pages to allow the
user to glance back and forth at the unlabeled structure
on the left and the labeled one on the right. Plates 2-12
also feature low magnifi cation views of the spinal cord sur-
rounded by other parts of the embryo.
Gross changes in morphology. It is during earlystages of embryonic development (GW4.0-GW8.5) that the
most dramatic structural changes take place in the spinal
cord. On GW4.0 (M714) the most prominent component
is the neuroepithelium, the germinal matrix of proliferating
stem cells (NEP cells), that surround a slit-shaped spinal
canal. The roof and fl oor plates are cell bridges that close
off the spinal canal in the dorsal midline and ventral mid-
line, respectively. Only a few cells are in the differentiat-
ing fi eld of future gray matter, and the entire spinal cord is
surrounded by a thin layer of primordial white matter. By
GW8.5 (C609), the gray matter is the most prominent com-
ponent of the spinal cord; the ventral horn and the interme-
diate gray already have complete populations of neurons,
and nearly all neuronal populations are complete in the
dorsal horn and the central autonomic area. Most major
fi ber tracts are present in the white matter. The spinal canal
remains in a central location (central canal). A remnant of
neuroepithelium lines the dorsal part of the central canal,
and an ependymal/glial matrix lines the intermediate and
ventral part. Roof and fl oor plates are still visible in the
midline.
The sequence of neurogenesis between various com-ponents of the gray matter. Between GW4.0 and GW4.75,
the ventral part of the neuroepithelium is the largest com-
ponent. The fi rst neurons outside the ventral neuroepithe-
lium at GW4 are differentiating motoneurons in the ventral
horn. Between GW4.75 and GW5.5, ventral horn moto-
neurons increase as the ventral neuroepithelium recedes.
By GW5.5, the center of the ventral neuroepithelium is
thinner than its dorsal or ventral borders (see Plate 6B).
The thinned area may mark the exhaustion of the NEP
(stem) cell population that produces motoneurons. Other
parts of the ventral neuroepithelium are presumably pro-
ducing ventral horn interneurons. By GW6.8, the ventral
neuroepithelium is becoming an ependymal/glial prolifera-
tive matrix that no longer produces neurons. The interme-
diate neuroepithelium expands rapidly between GW5.25
and GW5.5 prior to growth of the intermediate gray.
The intermediate neuroepithelium gradually declines by
GW7.25 as the intermediate gray expands. By GW8.5,
the intermediate neuroepithelium becomes an ependymal/
glial matrix. Between GW5.5 and GW6.5, the dorsal neu-
roepithelium dramatically increases its size, remains large
on GW7.25, and is much smaller on GW8.5. The most
rapid growth of the dorsal horn occurs between GW7.25
and GW8.5, coinciding with the shrinkage of the dorsal
neuroepithelium (see Chapter 5, Section 5.3, Altman and
Bayer, 2001). The dorsal neuroepithelium is character-
ized by “undulations” on its outer edges. These may be
groups of premigratory postmitotic neurons, the postu-
lated “sojourn zones” that have been well documented with(^3) H–thymidine autoradiography in the spinal cord of rats on
embryonic day 15 (see Chapter 3, Figure 3-10 in Altman
and Bayer, 2001). These undulations are still present on
GW8.5 (Plate 12) indicating that a few neurons are still
being generated (or have just been generated) by the dorsal
neuroepithelium.
The sequence of appearance of various components
of the white matter. The oval bundle of His, fi rst defi nite
at GW5.25 (Plate 5), is the bifurcation zone of ingrowing
axons of the dorsal root ganglion cells, the earliest com-
ponent of the dorsal funiculus. From that lateral position,
the dorsal funiculus grows medially to approach the dorsal
midline by GW7.25 (Plate 11). By GW8.5, the dorsal
funiculus grows downward along the midline, occupying
the space vacated by the retreating roof plate and the reced-
ing dorsal neuroepithelium (Plate 12). The ventral funic-
ulus also appears by GW5.25 in the wedge-like space
between the fl oor plate, the ventral neuroepithelium, and
the ventral horn. By GW5.5, axons of the early generated
commissural neurons are crossing the midline in the ven-
tral commissure beneath the fl oor plate. The lateral part
of the white matter is still small at GW5.5 in C8998 but
is larger at GW5.5 in M2161, indicating the beginning
growth of the lateral funiculus. The dorsal, ventral, and
lateral funiculi continue to expand throughout the rest of
the fi rst trimester (see Chapter 5, Section 5.6, Altman and
Bayer, 2001).
II. The First Trimester