Quantitative studies of mitoses in fetal rat brain: orientations of the spindles.

In continuation of our quantitative studies on 15-day fetal rat brain development, evidence was obtained of ventricular mitoses whose spindles are perpendicular to the surface of the ventricle. In most locations the proportion (ratio) between cells with perpendicular mitoses and predominant cells with mitoses parallel to ventricle is only 0.064 (+/- 0.057): 1; however, in two specific locations of lamina terminalis this proportion is much higher: 2.006 (+/- 1.24): 1 and 0.634 (+/- 0.42): 1. Total mitotic counts in these two locations were also higher: 1.8 and 1.5 X higher than the average elsewhere in cerebral hemispheres. It is suggested that these perpendicular orientations are not accidental but serve some specific purpose: either they give rise to a different cell population, or they represent a stage preparatory for cell migration.