Developmental time course of the sympathetic postganglionic innervation of the rat eye.

The time course of the innervation of the rat iris by sympathetic postganglionic axons from the superior cervical ganglion (SCG) has been studied in postnatal rats. A retrogradely transported fluorescent dye, Fast blue (FB) was injected bilaterally into the anterior eye chamber of rats within 3 h of birth or at 7 or 14 days postnatal. Littermates were killed at 24 h intervals, both SCG removed, fixed and sectioned. A substantial number of sympathetic neurones were labelled retrogradely from the eye at birth. The number of labelled cells tripled over the first postnatal week to reach proportions equivalent to those labelled in 3-week-old rats. The degree of sympathetic fibre ingrowth to the iris was examined at 3 postnatal ages, using fluorescence histochemistry. This method showed a small proportion of catecholamine-containing fibres in the iris at birth with a more extensive fibre density over the iris by 7 days after birth. A further increase in fibre density had occurred by the end of the 3rd postnatal week even though the number of cells projecting to the iris at these two ages was similar. This difference in fibre density may arise from ramification of fibres which have reached the iris by 7 days postbirth.