In utero development and maturation of the retina of a non-primate mammal: a light and electron microscopic study of the guinea pig.

A light and electron microscopic examination of retinogenesis in the fetal guinea pig has revealed an early development of synapses and photoreceptor cells. Differentiation of the neural retina begins around day 23 of gestation. By 34 days the retina reaches its maximum thickness. It differentiates an inner plexiform layer in which vesicle-containing processes and primitive synapses are evident. Synaptic ribbons are found in processes of this layer by 43-45 days of gestation. An outer plexiform layer develops within the neuroblast layer at 40 days of gestation; from its first appearance the outer plexiform layer contains synapses complete with synaptic ribbons. Receptor terminals of the alpha, paranuclear and beta type are present well before birth. Photoreceptor cells form inner segments by 40 days; the formation of outer segments is indicated by 45 days but not widespread until 49 days. The retina appears mature by day 51-57. It is clear that the primate is not unique in the early differentiation of its retinal synapses relative to the time of maturation of its photoreceptor cells. The potential functional capacities of precocious retinae, and the mechanisms of synapse development are discussed.