7.9% in a 24-week-old fetus. This drops to 2.53% in a 28-week-old fetus, being the only numerical value that differs in 24- and 28-week-old fetuses (Table 2). There are two distinct populations of labeled cells in the IGL, one with smaller cell nuclei and one with larger cell nuclei (Fig. 2C and D). Labeled cells with large cell nuclei locate mainly below the Purkinje cells (Fig. 2C and D) but a few are found at the border between the IGL and WM. At this age the border between those two layers is hard to distinguish, therefore, it is difficult to measure the exact width of the IGL. Large numbers of small Ki-67 immunopositive cells occurred inside the WM. In addition to cell proliferation in the secondary germinal layer (EGL) and in the other cerebellar cortical layers, an intensive cell proliferation occurs in the primary germinal layer of the ventricular zone (Fig. 2B). The lamina dissecans disappears between the 28th and 32nd weeks, as was observed by Rakic and Sidman [20]. As a consequence, the cerebellar cortex of a 32- week-old fetus consists of four layers Fig. 2. Photomicrographs of Ki-67 labeled neurons in the cerebellum of 24- (B, C) and 28-week-old (A, D) fetuses. (A) In the cerebellum of the 28-week-old fetus most of the cells in the EGL (e) are immunoreactive for Ki-67. In the IGL (i) the labeled cells locate close to the layer of Purkinje cells (p). Only a few labeled cells (arrows) are in the deep part of the IGL. The square indicates the magnified area shown in D. (B) Ventricular (vz) and subventricular zones of the fourth ventricle still contain large number of immunoreactive cells at this age. (C) In the EGL (e) of the cerebellum of a 24-week-old fetus, the outer and inner zones are clearly separated (dotted line). The IGL (i) contains two populations of labeled cells, one with large, ovoid cell nuclei (arrows) and another with small, round cell nuclei (open arrows). Curved arrows mark the most lightly stained cells that are accepted as specifically labeled in this study. (D) Higher magnification of the area indicated by the square in A. Labeled cells mainly locate in the outer zone of the EGL (above the dotted line). The IGL (i), below the lamina dissecans (ld), contains a heterogonous population of Ki-67 labeled cells displaying large (arrow) or small cell nuclei (open arrows). Scale bars=90 mm for A; 50 mm for B; 30 mm for C and D. Abbreviations used in the figure are e, external granular layer; i, internal granular layer; ld, lamina dissecans; m, molecular layer; p, Purkinje cell layer; vz, ventricular zone.

7.9% in a 24-week-old fetus. This drops to 2.53% in a 28-week-old fetus, being the only numerical value that differs in 24- and 28-week-old fetuses (Table 2). There are two distinct populations of labeled cells in the IGL, one with smaller cell nuclei and one with larger cell nuclei (Fig. 2C and D). Labeled cells with large cell nuclei locate mainly below the Purkinje cells (Fig. 2C and D) but a few are found at the border between the IGL and WM. At this age the border between those two layers is hard to distinguish, therefore, it is difficult to measure the exact width of the IGL. Large numbers of small Ki-67 immunopositive cells occurred inside the WM. In addition to cell proliferation in the secondary germinal layer (EGL) and in the other cerebellar cortical layers, an intensive cell proliferation occurs in the primary germinal layer of the ventricular zone (Fig. 2B).

The lamina dissecans disappears between the 28th and 32nd weeks, as was observed by Rakic and Sidman [20]. As a consequence, the cerebellar cortex of a 32- week-old fetus consists of four layers

Fig. 2. Photomicrographs of Ki-67 labeled neurons in the cerebellum of 24- (B, C) and 28-week-old (A, D) fetuses. (A) In the cerebellum of the 28-week-old fetus most of the cells in the EGL (e) are immunoreactive for Ki-67. In the IGL (i) the labeled cells locate close to the layer of Purkinje cells (p). Only a few labeled cells (arrows) are in the deep part of the IGL. The square indicates the magnified area shown in D. (B) Ventricular (vz) and subventricular zones of the fourth ventricle still contain large number of immunoreactive cells at this age. (C) In the EGL (e) of the cerebellum of a 24-week-old fetus, the outer and inner zones are clearly separated (dotted line). The IGL (i) contains two populations of labeled cells, one with large, ovoid cell nuclei (arrows) and another with small, round cell nuclei (open arrows). Curved arrows mark the most lightly stained cells that are accepted as specifically labeled in this study. (D) Higher magnification of the area indicated by the square in A. Labeled cells mainly locate in the outer zone of the EGL (above the dotted line). The IGL (i), below the lamina dissecans (ld), contains a heterogonous population of Ki-67 labeled cells displaying large (arrow) or small cell nuclei (open arrows). Scale bars=90 mm for A; 50 mm for B; 30 mm for C and D. Abbreviations used in the figure are e, external granular layer; i, internal granular layer; ld, lamina dissecans; m, molecular layer; p, Purkinje cell layer; vz, ventricular zone.