SIDMAN 293heavily labeled cells already had entirely left the germinal zone near the
ventricular surface. Most of these were destined never to divide again.
That is, they would differentiate and retain their full complement of
radioactive DNA for the life of the mouse, while other cells remained
in the germinal zone and returned to synthesis activity, diluting their
radioactivity in half with each subsequent division. The cells that had
ceased dividing migrated outward in patterns that had been only dimly
guessed at before, to make a cerebral cortex,^16 a cerebellar cortex,^17 a
retina,^18 and so on.
This, then, was the beginning of our precise and semi-quantitative
understanding of the genesis of form in the mammalian brain. The work
underscored the fundamental new idea, now accepted as commonplace,
that cell migration is a major event in neurogenesis. These studies also
led to the concept that a large repertory of new cell interactions, made
possible by the migration patterns, plays a dominant role in formation
of the incredibly complex nervous system. Understanding the molecular
genetic control of these migrations and interactions occupies world-wide
attention today as the central challenge in basic and clinical developmen
tal neuroscience. It all began so simply at the NIH.
Notes
- I would like to express my thanks to Drs. Ingrid G. Farreras and Victoria
A. Harden, who organized the symposium and publication, for the invi
tation to participate. - Today the National Institute of Neurological Disorders and Stroke (NINDS).
- James D. Watson & Francis H. C. Crick, “A Structure for Deoxyribonucleic
Acid,” Nature 171 (1953), 737-8. - Lloyd Guth, “Functional Recovery Following Vagosympathetic Anasto
mosis in the Cat,” American Journal of Physiology 185 (1956): 205-8. - Sanford L. Palay, “The Morphology of Synapses in the Central Nervous
System,” Experimental Cell Research 14, Suppl. 5 (1958): 275-93. - David H. Hubel, “Cortical Unit Responses to Visual Stimuli in Nonanesthe
tized Cats,” American Journal of Ophthalmology 46 (1958), 110-21. - William F. Windle, J. L. Littrell, J. O. Smart, and J. Joralemon, “Regeneration
in the Cord of Spinal Monkeys,” Neurology 6 (1956), 420-8. - W. L. Hughes, V. P. Bond, G. Brecher, E. P. Cronkite, R. B. Painter, H.
Quastler, and F. G. Sherman, “Cellular Proliferation in the Mouse as
Revealed by Autoradiography With Tritiated Thymidine,” Proceedings of
the National Academy of Sciences 44 (1958), 476-83.