TRANSIENT AXONAL SIDE BRANCHES IN THE DEVELOPING MAMMALIAN OPTIC-NERVE

Optic axons were labelled with horseradish peroxidase to establish the presence of side branches and examine their distribution and morpholo gy in the developing optic nerve of the quokka wallaby, Setonix brachy urus, the cat and rat at stages when axon numbers are at their peak. I n each species, three quarters of the axons were essentially straight and lacked side branches. The remaining axons took significantly longe r paths and bore side branches, mostly at points where axons undulated or changed direction. Side branches occurred at intervals of 28-43 mu m, had lengths of 2-3 mu m and were usually simple rather than branch ed. A minority (1%) of the axons crossed diagonally between fascicles and two thirds of these had more side branches (interval: 10-18 mu m) on the interfascicular portion than were found on the forward-directed axons. A small number of axons (0.01%) doubled back to grow retrograd ely towards the eye, these axons also bore relatively more side branch es (interval: 8-22 Gun), especially at points where the axons changed direction. Ultrastructural reconstruction showed that side branches re sembled small axonal profiles and constituted 2% of the total axon num ber. It is suggested that side branches are involved in the fine-tunin g of growth cone navigation. Most side branches are lost by adulthood, indicating their transient nature. The absence of retrogradely-direct ed axons from adults suggests that cells with such axons are removed b y naturally occurring cell death.