Changes in microtubule polarity orientation during the development of hippocampal neurons in culture
Microtubules in the dendrites of cultured hippocampal neurons are of nonuniform polarity orientation. About half of the microtubules have their plus ends oriented distal to the cell body, and the other half have their minus ends distal; in contrast, microtubules in the axon are of uniform polarity orientation, all having their plus ends distal (Bans, P. W., J. S. Deitch, M. M. Black, and G. A. Banker. 1988. Proc. Natl. Acad. Sci. USA. 85:8335-8339). Here we describe the developmental changes that give rise to the distinct microtubule patterns of axons and dendrites. Cultured hippocarnpal neurons initially ex- tend several short processes, any one of which can ap- parently become the axon (Dotti, C. G., and G. A. Banker. 1987. Nature (Lond.). 330:477--479). A few days after the axon has begun its rapid growth, the re- maining processes differentiate into dendrites (Dotti, C. G., C. A. Sullivan, and G. A. Banker. 1988. J. Neurosci. 8:1454-1468). The polarity orientation of the microtubules in all of the initial processes is uni- form, with plus ends distal to the cell body, even though most of these processes will become dendrites. This uniform microtubule polarity orientation is main- tained in the axon at all stages of its growth. The polarity orientation of the microtubules in the other processes remains uniform until they begin to grow and acquire the morphological characteristics of den- drites. It is during this period that microtubules with minus ends distal to the cell body first appear in these processes. The proportion of minus end-distal microtubules gradually increases until, by 7 d in cul- ture, about equal numbers of dendritic microtubules are oriented in each direction. Thus, the establishment of regional differences in microtubule polarity orienta- tion occurs after the initial polarization of the neuron and is temporally correlated with the differentiation of the dendrites.