3H-thymidine autoradiographic analysis of telencephalic histogenesis in the chick embryo: II. Dynamics of neuronal migration, displacement, and aggregation.

The movements of cells from their sites of origin in the proliferative neuroepithelium to their final positions in the chick telencephalon were traced by autoradiographic analysis of 3H-thymidine-labeled brains. A series of chick embryos were labeled on successive days of development between days 5 and 9 and fixed for autoradiography between days 6 and 10. Isochrone maps visualizing neuronal positions on each day of development between days 6 and 10 provided direct information concerning cell migrations, displacements, and aggregations leading to compartmentalization of the telencephalic wall and the generation of the "outside-in" pattern of histogenesis characteristic of the avian telencephalon.

3H-thymidine autoradiographic analysis of telencephalic histogenesis in the chick embryo: I. Neuronal birthdates of telencephalic compartments in situ.

The birthdates of neuronal populations comprising the chick telencephalon were determined by 3H-thymidine labeling and were mapped with respect to their terminal positions in the 16-day embryo. Essentially all neurons were generated between four and nine days of embryonic development. Each telencephalic structure (based on terminology used by Karten and Hodos, '67) was characterized by a specific range of birthdates: some regions such as the core of the ectostriatum or the paleostriatum primitivum, were generated within a single day, while others, such as the hyperstriatum accessorium, required up to five days for generation of the complete population.

Self-assembly of cortical plate cells in vitro within embryonic mouse cerebral aggregates. Golgi and electron microscopic analysis.

Mouse isocortical cells were dissociated at 18 days of embryonic development and were reaggregated in vitro by rotation in gyratory incubator shaker. The internal organization of the resulting aggregates was studied by conventional light microscopy, Golgi impregnation, and electron microscopy, establishing the following pattern of reassembly: (1) the predominant cell type in the aggregates was the pyramidal neuron; (2) each of these pyramidal neurons tended to orient its apical dendrite toward the surface of the aggregate; and (3) in larger aggregates (diameter > 600 microns) there was prominent parallel alignment of pyramidal cell apical dendrites. These characteristics resulted in an in vitro reconstruction of the major features of isocortex observed in situ, including formation of a superficial, rather acellular plexiform layer.

Synaptic vesicles of inhibitory and excitatory TERMINALS IN THE CEREBELLUM.

Populations of synaptic vesicles within cerebellar terminals considered excitatory or inhibitory on the basis of physiological evidence differ with respect to size and shape. Size rather than shape appears to be the main morphological difference between these populations. Elongation of vesicles is depenident on fixation with aldehyde fixatives, and both size and elongation change with age mainly during maturation.