front matter 1

VII. Summary and Conclusions

to the next indicate developmental changes that allow a

more accurate interpretation of real events (the fi rst 6 data

points in Figure 32 graphs). These quantitative data,

when combined with the qualitative evidence in the pho-

tographs, allow the construction of timetables of develop-

mental events among neuronal populations in the spinal

cord as shown in Table VII-1.

Figures 34 through 37 feature pie-charts showing

relative changes in the areas of varous components of the

spinal cord throughout its development. The problems

associated with fi xative, cutting planes, section thickness,

and staining types are much less relevant here. Figures

34 and 35 look at the entire range of development from

GW4.0 to the 4th postnatal week. Figures 36 and 37 look

at the changes in maturation of cervical through sacral

levels in 6 specimens from GW8.5 to the 4th postnatal

month.

Finally, Table VII-2 shows a semi-quantitative

analysis of the sequential steps in myelination in several of

the major fi ber tracts in the spinal cord at the level of the

cervical enlargement. These events can be used to make

some generalizations about myelination sequences in fi ber

tracts, not only in the spinal cord, but throughout the cen-

tral nervous system. These conclusions are explained in

the caption of Table VII-2.

This section features a quantitative summary of the

major developmental events that have been qualitatively

demonstrated in photographs of the spinal cord at imma-

ture through mature stages. In order to compare the com-

plete timetable of development, the cervical level is dealt

with most completely because that level is illustrated at all

ages. First, absolute changes in area are discussed, fol-

lowed by relative changes in area. Finally, a semi-quan-

titative approach using myelin staining density is used to

determine myelination sequences in various fi ber tracts.

Figures 32 and 33 show progressive changes in

area of various components of the spinal cord in absolute

values (mm^2 ). It is important to note that the absolute

values are based on a single specimen at each age; these

specimens were collected over a period of several years,

were preserved with different fi xatives, and were not all

cut at the same thickness or in exactly the same plane.

All of these factors contribute to “noise” in the data, and

the graphs are not smooth progressions from one age to

another. The time when most noise in the data exists is

between GW4.0 and GW6.8 in the fi rst trimester. Nine

specimens from this period are shown in Plates 2 through

10. Fortunately, six of these specimens are from the Minot

Collection, where the fi xative is constant and the cutting

plane is more regular between specimens. When all of the

specimens in the Carnegie Collection are removed from

the data, the changes in absolute areas from one age