Goldfish retinotectal system: continuing development and synaptogenesis.

The development of the retina and tectum in goldfish was studied using light and electron microscopy. Soon after hatching the retina is well differentiated in that all the layers of the adult retina are present. The tectum at this time lacks the characteristic layered structure of the adult and innervation in the stratum opticum is extremely sparse, being confined mainly to the rostral region. The retina grows rapidly and retinal layers increase in thickness. This continues into adulthood. Optic innervation of the tectum increases and in fish 19 mm in body length the adult pattern of layers seen by silver staining and by electron microscopy is recognizable. At this time the optic nerve contains large number of unmyelinated axons. The thickness of tectal layers continues to increase over the entire size range of fish studied, well into adulthood. Synaptic densities in the layer of optic termination also change. Density falls in the rostral region as the fish increase in size. In the caudal region there is an initial decrease followed by a small increase. Total numbers of synapses in the main layer of optic termination increase both rostrally and caudally over the entire range of fish studied. Optic and nonoptic fibers contribute to this. The optic nerve at this stage is almost completely myelinated. The continuing growth of both the retina and tectum, including synaptogenesis, may provide a basis for the remarkable regeneration and plasticity shown by this system.