The site of commencement of maturation in mammalian retina: observations in the cat.

The outer plexiform layer (OPL) of the retina has been studied in the developing cat, from E (embryonic day) 30. Prior to E51 the layer could not be detected, the retina comprising two cell layers, an inner layer which becomes the ganglion cell layer of the adult, and an outer 'neuroblast' layer. The OPL was first detected at E51 as a narrow gap separating the neuroblast layer into inner and outer parts, which will form the inner and outer nuclear layers of the adult. At E51 the OPL was present only over a small region at the area centralis. After E51 the OPL thickens and spreads, extending over the entire retina by P (postnatal day) 10. During development, the area over which the OPL is formed is horizontally elongated, resembling the visual streak specialization of the adult retina. This pattern of development seems distinct from the pattern of cell birth in non-mammals, in which the earliest-born cells are found at the optic disc, and later-born cells are found more peripherally, with a dorso-ventral asymmetry in their distribution. In the cat the maturation of the OPL begins simultaneously with the maturation of the ganglion cell layer reported previously. It is possible that the two processes are controlled by a single mechanism.