Neurogenesis in the chick ventral lateral geniculate and ectomammillary nuclei: relationship of soma size to birthdate.

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This study had 3 objectives: to determine if there are gradients of neuron production in the chick diencephalon and mesencephalon, to determine if large neurons are produced before small neurons, and to determine the period of neurogenesis in the ventral lateral geniculate nucleus (GLv) and ectomammillary nucleus (EMN). Tritiated thymidine autoradiography and a cumulative labeling analysis were used in determining neuron birthdates. In our studies the diencephalon, mesencephalon, and EMN did not exhibit clearcut gradients of neuron proliferation. The GLv possessed an inside to outside laminar gradient and a more subtle caudal to rostral gradient of neurogenesis. There was a consistent negative correlation between neuron size and embryonic age among nuclear groups of the diencephalon, in general, and within the GLv and EMN in particular. The neurons of the GLv and EMN were produced from the third to the seventh day of incubation. It was concluded that there is an irregular sequence of production of nuclear groups in the diencephalon and mesencephalon rather than a series of recognizable neurogenetic gradients. Neuron size, however, is an important factor in predicting the order of neuron production.