The Development of Neuronal Polarity: A Retrospective View

Abstract

In 1988, Carlos Dotti, Chris Sullivan, and I published a paper on the establishment of polarity by hippocampal neurons in culture, which continues to be frequently cited 30 years later (Dotti et al., 1988). By following individual neurons from the time of plating until they had formed well developed axonal and dendritic arbors, we identified the five stages of development that lead to the mature expression of neuronal polarity. We were surprised to find that, before axon formation, the cells pass through a multipolar phase, in which several, apparently identical short neurites undergo periods of extension and retraction. Then one of these neurites begins a period of prolonged growth, becoming the definitive axon; the remaining neurites subsequently become dendrites. This observation suggested that any of the initial neurites were capable of becoming axons, a hypothesis confirmed by later work. In this Progressions article, I will try to recall the circumstances that led to this work, recapture some of the challenges we faced in conducting these experiments, and consider why some of today's neuroscientists still find this paper relevant.

Figure 1.

Stages of development of hippocampal neurons in culture. The approximate times when cells enter each of the stages are indicated. Reproduced with permission from Dotti et al. (1988).

Figure 2.

Axonal specification is marked by a profound change in the pattern of neurite growth and retraction. A–F, Single frames from a time lapse recording illustrating phases of growth and retraction during Stage 2 and Stage 3 of development. Before the axon formed, different minor neurites exhibited periods of growth (g) followed by periods of retraction (r). Eventually, one process began to grow (*) and did not retract, but instead developed into the axon. The times shown refer to hours and minutes after plating. G, Graph representing changes in the length of four different neurites from this same cell before and after axon specification. Bold symbols indicate growth spurts. The x axis shows time after plating. This figure, showing data collected since our original work, offers a clearer view of these events, thanks to improved imaging equipment and the availability of Adobe Photoshop (Teresa Esch, unpublished observations).