SYNCHRONY OF CLONAL CELL-PROLIFERATION AND CONTIGUITY OF CLONALLY RELATED CELLS - PRODUCTION OF MOSAICISM IN THE VENTRICULAR ZONE OF DEVELOPING MOUSE NEOCORTEX

We have analyzed clonal cell proliferation in the ventricular zone (VZ ) of the early developing mouse neocortex with a replication-incompete nt retrovirus encoding human placental alkaline phosphatase (AP). The retrovirus was injected into the lateral ventricles on embryonic day 1 1 (Ell), i.e., at the onset of neuronogenesis., Three days postinjecti on, on E14, a total of 259 AP-labeled clones of various sizes were fou nd in 7 fetal brains. There are approximately 7 cell cycles between El l and E14(Takahashi et al., 1995a), and there is a 1-2 cell cycle dela y between retroviral injection and the production of a retrovirally la beled ''founder'' cell; thus, we estimate that the ''age'' of the clon es was about 5-6 cell cycles. Almost one-half of the clones (48.3%) id entified were pure proliferating clones containing cells only in the V Z. Another 18.5% contained both proliferating and postproliferative ce lls, and 33.2% contained only postproliferative cells. It was striking that over 90% of the clonally related proliferating cells occurred in clusters of two or more apparently contiguous cells, and about 73% of the proliferating cells occurred in clusters of three or more cells. Regardless of the number of cells in the clone, these clusters were ti ghtly packed and confined to a single level of the VZ. This clustering of proliferating cells indicates that clonally related cells maintain neighbor-neighbor relationships as they undergo interkinetic nuclear migration and progress through several cell cycles, and, as a result, the ventricular zone is a mosaic of small clusters of clonally related and synchronously cycling cells. In addition, cells in the intermedia te zone and the cortical plate were also frequently clustered, indicat ing that they became postproliferative at a similar time and that the output of the VZ is influenced by its mosaic structure.