Rat retinas were studied during the first three postnatal weeks. Outer segments began developing during the fifth postnatal day and were in close association with pigment epithelium. Developing photoreceptor lamellae were seen to be continuous with smooth endoplasmic reticulum. The outer plexiform layer began to form on the fifth day by the lateral extension of the horizontal cell processes. On the 12th day synapses were formed in the inner plexiform layer, and it was at this time the first response was recorded on the electroretinogram.
Weidman TA, Kuwabara T. Postnatal Development of the Rat RetinaAn Electron Microscopic Study. Arch Ophthalmol. 1968;79(4):470-484. doi:10.1001/archopht.1968.03850040472015