Extended Data Fig. 6: The EC stream forms between 22 and 27 GW.

a–d, Anatomically matched coronal sections of the medial temporal lobe at the rostral tip of the uncus of the hippocampus across ages. At 18 GW (a), Ki-67⁺SOX2⁺ cells line the walls of the temporal lobe ventricle (same section level shown in (ii) in Extended Data Fig. 8c), and the medial wall facing the EC and hippocampus has fewer of these cells and no anatomical features separating the hippocampus and EC. At this age, there is a slight tissue protrusion of the hippocampus in the lateral direction towards the ventricle, a feature that becomes rapidly more prominent in the next weeks. At 22 GW (b), the hippocampus and surrounding tissue have all grown larger and a seam has begun to form between the hippocampus and EC. At this age, there is no heightened accumulation of Ki-67⁺ SOX2⁺ or DCX⁺ cells within this region. At 27 GW (c), this region has formed a higher concentration of Ki-67⁺SOX2⁺ cells as well as DCX⁺ cell clusters indicating that the EC stream has formed by 27 GW. At 38 GW (d), the density of both the Ki-67⁺ SOX2⁺ cells and the DCX⁺ cell clusters is increased, with many of both populations present within the EC stream extending from the medioventral tLV to the EC. e, Map of the location of Ki-67⁺SOX2^– (yellow) and Ki-67⁺SOX2⁺ (magenta) cells surrounding the tLV at 38 GW. Most Ki-67⁺ cells at this age are SOX2⁺, and are found in the paralaminar nucleus of the amygdala (PL) (1), the lateral V-SVZ (2), and in the EC stream (3). Scale bars: 1 mm (a–e all left panel overviews), 100 µm (a–d right panel overviews, e 1–3), 20 µm (a right insets). Abbreviations: BLA basolateral amygdala; EC: entorhinal cortex; HP: hippocampus; oSVZ: outer subventricular zone; PL: paralaminar nucleus; tLV: temporal lobe lateral ventricle; V-SVZ: ventricular–subventricular zone.