Fig. 1. The optical fractionator sampling scheme. The cerebral hemispheres were divided medially (A), and the left or right hemisphere was selected systematically randomly before dehydration and infiltration in paraffin (B). The hemispheres were sectioned exhaustively into 40 µm-thick coronal sections (C), from which a predetermined fraction, the section sampling fraction (ssf), was sampled systematically randomly (D) and subsequently stained with Giemsa. Optical disectors were positioned systematically randomly on each of the sampled sections at regular predetermined x,y-positions (E,F). The area of the counting frame of the disector to the area associated with each x,y-step represents the area sampling fraction (asf). Counts were performed in all optical disectors hitting the structure of interest (E,F). Cellular nuclei were counted by moving the counting frame through a continuous stack of thin optical planes inside the section (G). The height of the disector (h_dis) relative to the height of the section (t) represents the height sampling fraction (hsf). The disector is protected by upper and lower guard zones.