a–c, Human brain of an individual at 14 gestational weeks. a, Schematic of the dorsal (dHP) and ventral (vHP) hippocampus in a coronal section. Precursor cells labelled with nestin, SOX2 and vimentin are organized in ribbons between the dNE and GCL. Ki-67+ cells expressing SOX1 and vimentin or SOX2 and BLBP are present in the GCL and hilus (inset 1), along the wall of the lateral ventricle (LV) (inset 2) and between the GCL and the dNE (inset 3). The dNE is located at the edge of the ammonic neuroepithelium (aNE) closest to the fimbria. A similar organization is present in the vHP where nestin+SOX2+vimentin+ cells connect the dNE to the developing GCL. Ki-67+SOX1+vimentin+ cells are present in a strip along the ventricular wall and fill the region between the dNE and the GCL. b, Left, hemisphere at 14 gestational weeks, Nissl-stained horizontal sections. Right, Ki-67+ cells expressing SOX2 (arrows). c, 3D reconstruction of the dHP showing the field of Ki-67+ and SOX2+ cells between the dNE and GCL. d–h, Human brain at 22 gestational weeks, coronal (d) and horizontal (e) sections. The hilus and GCL contain Ki-67+SOX2+ cells (d, e (insets)) as well as nestin+SOX2+vimentin+ cells (f). These populations are asymmetrically distributed; sparse in the medial (proximal) GCL and hilus (top insets in e, f) but abundant in the lateral (distal) GCL and hilus (bottom insets in e, f). g, DCX+TUJ1+ cells and NeuN+ cells in the DG at 22 gestational weeks. NeuN+ GCL neurons in the distal GCL (arrow). h, A toluidine-blue-stained semi-thin section (top) and TEM micrographs showing the ultrastructural characteristics of DCX immunogold-labelled cells (pseudocoloured, bottom) at 22 gestational weeks. Insets of the semi-thin section show the proximal (1) and distal (2) ends of the GCL. Most DCX+ cells in the hilus and the proximal GCL have little cytoplasm, few organelles and a small, irregular nucleus (i, ii); some DCX+ cells in the hilus have an elongated, fusiform morphology (i). Some DCX+ cells in the GCL have mature neuronal characteristics such as a round nucleus, more cytoplasm, ribosomes, rough endoplasmic reticulum and mitochondria (iii); this cell type was more common in the distal GCL. At this stage, the round and more mature neuronal morphologies were observed in the distal, but not in the proximal, blade. Scale bars, 200 μm (a–h (left images)), 2 μm (a–h (insets)) and 2 μm (h (TEM)).