Synaptogenesis in the photoreceptor terminal of the mouse retina

TLDR

A combined light and electron microscopic study examined synaptogenesis in the photoreceptor terminal of visually competent mice, with implications for the timing of electroretinogram appearance during development. Photoreceptor synaptogenesis in the mouse retina proceeds through a postnatal sequence that culminates in a mature triad configuration by day 14, with horizontal cell processes contacting the terminal before bipolar dendrites.

Abstract

Abstract Observations are presented of a combined light and electron microscopic study of synaptogenesis in the photoreceptor terminal of visuallycompetent mice. Our study suggests that photoreceptor synaptogenesis in these retina progresses through a sequence of postnatal changes which leads to the ordered arrangement of processes within the adult photoreceptor terminal. On the 5th postnatal day, neuronal processes from synaptic contacts with a photoreceptor terminal, usually in the vicinity of a synaptic ribbon. Following the formation of synaptic contacts, the processes invaginate into the photoreceptor terminal, maintaining their position, lateral to a synaptic ribbon. Beginning on about the 10th day, a central process enters the terminal to yield a triad configuration which is a characteristic of a mature photoreceptor terminal. The formation of triads is essentially complete by the 14th postnatal day. Our observations suggest that horizontal cell processes form synaptic contacts with the photoreceptor terminal before bipolar cell dendrites, implying that the horizontal cells mature in advance of the bipolar cells. This possibility is discussed in terms of the appearance of the electroretinogram (ERG) in the developing mouse retina.