Axon development in mouse cerebellum: embryonic axon forms and expression of synapsin I

A fundamental question in central nervous system development is the timing of synaptogenesis in relation to invasion of targets by afferent axons. A related question is how growth cones transform into synaptic terminals. These two aspects of axon maturation were examined in developing mouse cerebellum, by labeling single axons with horseradish peroxidase, to study their form and cytology, and by immunocytochemical staining of a synaptic vesicle antigen, synapsin I, a phosphoprotein found on synaptic vesicles in all mature CNS synapses. From embryonic day 16 to postnatal day 3, horseradish peroxidase-labeled afferent axons extend well into the cerebellum and have simple forms. At embryonic day 16, axon growing tips are synapsin I-negative. Synapsin I is first expressed at embryonic day 17, and by embryonic day 18, fibers are stained throughout the cerebellum. Synapsin I expression coincides with a general increase in synaptic specializations, although growing tips continue to have the cytology of growth cones. During the period that axons have primitive shapes, synapsin I is distributed throughout the terminal arbor, corresponding to the presence of small vesicles along neurite lengths, even at non-synaptic sites. After postnatal day 3, when synaptic terminals develop into stereotypic shapes and engage in characteristic synaptic relations, synapsin I is restricted to boutons. Thus, the synapse-specific protein synapsin I is expressed in fetal mouse brain, long before nerve endings have the structure and connections of adult brain. In cerebellar axons, the expression of this protein follows axon arrival, coincides with the appearance of elementary synapses, and accompanies the transformation of growing tips into stereotypic synaptic boutons. The time course of expression of synapsin I, a phosphoprotein that may be involved in synaptic efficacy, suggests that transmitter release may influence early axon-target cell interactions.