CLONAL HETEROGENEITY IN THE EARLY EMBRYONIC RODENT CORTICAL GERMINAL ZONE AND THE SEPARATION OF SUBVENTRICULAR FROM VENTRICULAR ZONE LINEAGES

The cell cycle kinetics of individual clones of cells in the embryonic cortex were studied to determine the amount of heterogeneity among co rtical progenitors, This kinetic heterogeneity may be the first sign o f heterogeneous differentiation in the proliferating cortical germinal zone, Retroviral lineage tracing of clonal progeny in the embryonic d ay 16 (E16) rat cortex (48 hr after the retroviral infection and [H-3] thymidine labeling of proliferating cells) permitted a description of the formation of the preplate in the medial cortex and the formation o f the subventricular zone (SVZ) in the lateral cortex, Forty percent o f the retrovirally tagged clones were double-labeled with a pulse of [ H-3]thymidine, corresponding to the 40% of ventricular zone cells in S -phase at the time of [H-3]thymidine injection, Most of clones had cel l numbers with powers of 2 (2, 4, and 8 cells), suggesting synchronous modes of division, Nevertheless, 15% of the retrovirally tagged clone s that were double-labeled with [H-3]thymidine at the time of [H-3]thy midine injection showed asynchronous mode of division, Clonally relate d cells in the ventricular zone showed considerable variability in cel l cycle times: 56% of the clones (8-cell clones) were composed of fast er cycling cells with cell cycle times of 12 hr, and 25% of the clones (4-cell clones) represented slower cycling cells with cell cycle time s of 16 hr. The clones migrating outside the ventricular zone differed in size and spatial distribution in the lateral versus medial cortex, In the lateral cortex, half the migrating clones were large prolifera ting 8-cell clones with all their members contained within the forming SVZ, In the medial cortex, the majority of the migrating clones were 2-cell and 4-cell clones, Given that the medial cortex matures later t han the lateral neocortex and that no SVZ has formed in the rat medial cortex by E16, we suggest that the majority of cells that leave the m edial cortical VZ by E16 are cells destined to form the neuronal popul ations of the preplate, The early embryonic cortical ventricular zone includes a mosaic of specialized progenitor cells. (C) 1997 Wiley-Liss , Inc.