Nervous System 



GROWTH PATTERNS 



Proliferative Sources. In the ganglia, cell 

 division is rather ubiquitous. In the CNS, on 

 the other hand, mitoses are, in post-neurula- 

 tion stages, confined to the inner surface, 

 lining central canal and brain ventricles 

 (Fig. 1J9). More peripheral layers are es- 

 sentially devoid of mitotic figures; the ceils 

 there continue to grow in size, but without 

 ensuing divisions. Whether the inner "ger- 

 minal" layer is subject to some active mito- 

 genic stimulation by its exposure to the 

 lumen, or rather division in the outer layers 

 is actively inhibited by local conditions in 

 the mantle, is uncertain. However, upon 

 injury to the early CNS (Hooker, '25), as 

 well as after unilateral ablation (cord: 

 Detwiler, '44; Holtzer, '51; not observed in 

 midbrain: Detwiler, '46b), mitotic cells may 

 appear throughout the mantle, which seems 

 to disprove an early loss of divisory faculty. 

 There is a remote possibility that the con- 

 finement of mitotic figures to the inner 

 surface might not truly express the position 

 of the germinal cells in the resting stage, 

 but that the latter might merely rise to the 

 surface during mitosis (Sauer, '35). The 

 observation that x-irradiation of embryos 

 destroys a cell layer somewhat deeper than 

 the inner lining (Hicks, '52) could be in- 

 terpreted in two ways: either these deeper 

 cells are the true germinal ones, but are im- 

 paired in their normal premitotic centripetal 

 movement, or else they are postmitotic cells 

 defying the general supposition that germinal 

 cells are the most sensitive to radiation 

 (Hicks, '53). At any rate, once neuronal 

 differentiation has become marked, none of 

 the resulting nerve cells would ever divide 

 again under ordinary circumstances. We 

 therefore can confine our consideration of 

 proliferative patterns to the irmer, "ger- 

 minal," layer, whatever its precise delinea- 

 tion may be. In this layer, mitotic density 

 and rate vary characteristically both along 

 the longitudinal and in the dorsoventral di- 

 rection, as will be described more fully 

 below. 



Growth and Shape. Daughter cells of a 

 germinal mitosis may either remain in the 

 germinal layer or move off into the deeper 

 layers. In the former event, they can continue 

 to proliferate, while in the latter event, they 

 merely add to the bulk of the mantle with- 

 out further reproduction. Since the relative 

 frequency of the two events will depend on 

 the spatial arrangement, the configuration of 



373 



early brain and cord assumes significance 

 for their over-all growth rate. According to 

 Hertwig's rule, the plane of division of a 

 radially elongate medullary cell should lie 

 parallel to the surface; hence, of the two 

 daughter cells only one would remain a 

 germinal cell. We know neither what re- 

 arrangements take place after division, nor 

 what actuates and guides the movement of 

 the proliferated cells. However, it seems that, 

 at least in the earlier stages, they glide along 

 other medullary cells stretching across the 



Fig. 139. Cross section of spinal cord (15th seg- 

 ment) of GYz-day chick embryo showing mitotic 

 activity of germinal layer, predominantly in dorsal 

 half (from Hamburger, '48). 



neural tube and brain wall, or along radial 

 fibers deposited by the medullary cells. If 

 all new cells kept on being thus displaced 

 in the radial direction, the neural tube would 

 gain only in width, but not in length. 



The factor counteracting this trend seems 

 to be the longitudinal stretch to which the 

 tube is passively subjected by the lengthwise 

 extension of the surrounding tissues, espe- 

 cially the notochord (Horstadius, '44; Kitchin, 

 '49). This would divert a certain fraction of 

 the new cells in the longitudinal, rather than 

 radial, direction, and by extending the ger- 

 minal surface area, also lead to a progressive 

 expansion of the proliferative source. While 

 still hypothetical, this view is supported by 



