ABSTRACT
Figure 33. The area of the gray matter (dark gray) and white matter (light gray) at the cervical enlargement from the beginning of the second trimester (GW14), birth (NB-newborn), to 16 weeks after birth. From GW26 on, data of the myelin stained section area is used. The area graphs are stacked so that the scale on the Y axis indicates the total area of the spinal cord. These areas are measured in fi xed tissue, so the actual values in the living spinal cord will be higher by an unknown amount. The areas of the ependyma and spinal canal are so small that they do not register above the baseline. Since neurogenesis fi nishes in the fi rst trimester (see Figure 32), the gradual increase in the area of the gray matter throughout this period is due to the growth and differentiation of the neurons themselves. The white matter increases more rapidly, especially between GW19 and GW26 (compare Plates 40 and 47). There is gradual growth from GW 26 to the perinatal period (between GW37 and postnatal week 4), then more rapid growth between postnatal weeks 4 and 16 (compare Plates 95 and 106). The fi rst growth spurt is probably due to the accumulation of axons in all of the major fi ber tracts, including both parts of the corticospinal tract. The second growth spurt is probably due to the completion of myelination throughout the cervical enlargement at the 4th postnatal month. Only the outermost crescent of the corticospinal tract is still myelinating at this time (see Plate 106). From GW26 on, the area of the white matter exceeds the area of the gray matter.
