## 4 Discussion  
In summary, longitudinal OCT of postnatal retinal development has been demonstrated in developing mouse eyes from P14 to P56. Quantitative OCT analysis revealed distinct outer and inner retinal layer changes, corresponding to eye development. An HRL between the NFL and the IPL was observed in developing eyes and gradually disappeared with aging. The fine structure development of inner retina layers was analyzed via frame registration and super averaging. Three possible factors could contribute to the retinal thickness changes after eye opening. First, the eyeball diameter increases [24], causing a general thinning effect, affecting all the retinal layers. Second, cell death during retina development [25] also causes a thinning effect, affecting a few layers. Third, the retinal structure modification [26] causes a thinning or a thickening effect, affecting a few layers. The thickness change of an individual retinal layer can be the combination of all these three effects. As the eyeball expansion affects all retinal layers, the other two factors will be discussed in detail below. The NFL thickness decreased from P14 to P21 and became stable after P21 (Figure 2(E1)). The change could be explained by the combination of eyeball expansion and retinal structure modification. The RGC population remains stable during development after eye opening; therefore, the total number of axons is stable [25,27]. However, right after eye opening, until 3 weeks postnatal, there is a maximum rate of nerve fiber growth and myelination [28]. It has been reported that, during this period, the number of myelinated nerve fibers as well as the nerve fiber diameter increased [29], which is considered to be dependent on environmental light stimulation [30]. The nerve fiber modification caused NFL thickness to increase. The IPL thickness remained stable (Figure 2(E1)) from P14 to P56. This observation could be explained by the combination of eyeball expansion, cell death, and retinal structure modification. The IPL is composed of the dendrite and synapse connections between RGCs, bipolar cells, and amacrine cells. After eye opening, the population of bipolar cells keeps decreasing until P19, and the population of amacrine cells also slightly decreases [25]. The apoptosis of these cells causes the IPL thickness to decrease. However, during postnatal development, the dendritic field size of RGCs keeps increasing until 3 weeks after birth [5]. The density of both ribbons and conventional synapses in the IPL continuously increases after eye opening and reaches the peak level by around the age of P21 [7]. This dendritic refinement causes the IPL to thicken. The INL thickness decreased after eye opening (Figure 2(E1)) until P28. The change could be explained by eyeball expansion and cell death. The INL is composed of nuclei of bipolar cells, horizontal cells, and amacrine cells. The layer thickness is only related to the  
Figure 3: Comparative 3-D OCT volumetric analysis of P17 (**A**) and P56 (**B**) mouse retinas. (**A1,B1**) Representative B-scans. The purple arrows in A1 indicate individual RGCs. (**A2,B2**) The enlarged view of the purple dash square in A1 and B1. The orange arrows in B2 indicate the stratification of IPL. ((**A3–A5**) and (**B3–B5**)) are the enface images of NFL, GCL, and IPL, which correspond to the location of the blue, green, and orange arrows in A1 and B1. The red arrows in A4 and B4 indicate the individual RGCs. population of the cells. After eye opening, the population of the horizontal cells remains stable, the population of amacrine cells shows a slight decrease, and the population of bipolar cells shows a decrease until P19 [25]. The apoptosis of bipolar cells and amacrine cells causes INL thinning. Similar to the INL, the ONL thickness also decreases after eye opening (Figure 2(E2)) until P28. The change could be explained by eyeball expansion and cell death. The ONL is composed of the nuclei of retinal photoreceptors.