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Fig. 1. P/P divisions predominate during cytogenesis in the pre-cortical plate (CP)
stage. (A) Toluidine blue-stained cerebral wall of a B6C3 mouse at E13
depicting the region in question. The present study focuses on cytogenesis in
the ventricular zone (VZ) and subventricular zone (SVZ) just before and during
the emergence of the CP (pink). Cerebral walls at E14 are also composed of
sections in a pre-CP stage, mainly in the dorsal region. Smart
(Smart, 1973) suggested that a
transition from the proliferation-dominant ‘stage 1’ to
‘stage 2’ which balances proliferation and differentiation (see
text for detail) occurs in the region that we targeted for the analysis of
division patterns. IZ, intermediate zone; PP, preplate. (B) Frozen section of
an E14 dorsal region, doubly stained with anti-Ki67 and NeuroTrace (for
‘fluorescent Nissl stain’). We counted Ki67-positive and
Ki67-negative (outlined) cells in the VZ+SVZ at E13, E14 and E15 to obtain
percentage Ki67-positive/Nissl-positive (see
Table 1). (C) Slice culture
showing a DiI-labeled progenitor that divided at the ventricular surface to
give rise to two daughter cells both of which were positive for Ki67
(C', merged image from photographs using a 40x objective;
C'', observation using a 100x objective with optical slice of 1.2
µm). (D) Some forms of P/P division give rise to two daughter cells that
divide at the ventricular surface
(PS-div
PS-div+PS-div). Daughter cell
‘a’ inherited the parent cell’s basal process, whereas the
cell ‘b’ extended a new process (arrowed) to the pial surface
(asterisk). The trajectory of the interkinetic nuclear movement often differed
between such paired surface-dividing (S-div) cells, with quicker and greater
ascent by the inheritor of the basal process. (E) Another type of P/P division
produces a daughter cell that divides at the surface and another that does
not. One of the mitotic daughters born at the surface divided away from the
surface (cell ‘b’), whereas its sister cell (‘b’)
divided at the surface [entrance into M phase was confirmed by immunostaining
with anti-phosphohistoneH3 (pH3), E']
(PS-divPS-div+PNS-div). Note that the
cell ‘b’ did not inherit the parent’s radial process
(arrowhead). The thickness of the cerebral wall when the first mitosis had (or
would have) occurred was measured (approximately 150 µm in D; approximately
210 µm in E) to examine the relationship between the frequency of
occurrence of each P/P-division pattern and developmental stage
(Table 2). Scale bars: 100
µm in A; 10 µm in B,C,C',C',E'.
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