## 4 Discussion
The population of photoreceptors keeps decreasing until P24 [25], which could cause ONL thinning. The ELM-RPE thickness increased after eye opening (Figure 2(E2)) until P28. The ELM to RPE layer is composed of the photoreceptor inner segment, outer segment, and RPE layers. The change could be explained by the combination of eyeball expansion, cell death, and photoreceptor inner and outer segment elongation. The population of photoreceptors keeps decreasing until P24, which could cause layer thinning. However, the photoreceptor inner segmentation and outer segmentation both expand significantly [26], which causes this layer to thicken. To the best of our knowledge, the HRL between the NFL and the IPL in the OCT B-scans of mouse retina has not been reported before. For human OCT images, the hyporeflective layer between the NFL and the IPL is the ganglion cell layer (GCL). However, for mouse retina, most studies considered that the RGCs are buried in the NFL or the IPL. As the intensity of RGCs is much weaker than axons, dendrites, and synapses in the NFL and the IPL, they are hard to identify in the OCT image [21]. Therefore, for mouse retina segmentation, some people consider the RGCs are included in the NFL and call it the NFL-GCL complex [31; 32; 33; 34]; some people consider the RGCs are included in the IPL and call it the GCL-IPL layer [35; 36], while some people simply use NFL or GCL to denote the layer that is above the IPL [37; 38]. From the enface image (Figure 3(A3,B3)) at the location of the HRL, we could see individual RGC somas. By comparing enface images of P17 and P56 mice, we can see that the RGCs of P17 were \"cleaner\" and the RGCs of P56 are co-located with nerve fibers and neuron dendrites and synapses. Considering the IPL and nerve fiber refinements after eye opening, we speculate that this hyporeflective band between the GCL and the IPL are the RGCs. The actual mechanism of the HRL vanishing is not well understood. We have two hypotheses. First, during the refinement of the IPL, the RGCs are well surrounded by the neuronal dendrites of bipolar cells, amacrine cells, and RGCs, and thus reduce the difference of light properties in neighboring regions. The other hypothesis is that, during the refinement of the nerve fiber, the RGCs are well packed into the NFL layer, and thus reduce the difference of light properties in neighboring regions. Either way, the HRL of RGCs will vanish because of being buried in other hyperreflective tissues. An immunofluorescent study could help to test these hypotheses. This immunofluorescent study will not only help to explain the vanishing of the HRL, but also help to identify the correlation between the retinal OCT image and the anatomic correlate. The observation of the morphological changes of retinal inner layers, such as the NFL and the IPL, during retinal postnatal development, is consistent with the observation of the IPL modification after eye opening [7], RGC dendritic refinement [4; 5; 6], and nerve fiber growth [29]. The fine structure is hard to observe with OCT. However, with super averaging processing after frame registration, the nerve fibers, individual RGCs, and stratification of the IPL could be observed [19; 20; 21]. In a previous study, the whole frame registration was all executed by ImageJ. A few hours are needed to finish the frame registration of 150 OCT volumes which contain 512 A-scans times 512 B-scans in each volume. In our approach, the registration information is computed by ImageJ; however, the volume image data transformation is executed by MATLAB. Due to the good 3D matrix computation performance capability and parallel computing, the total time cost for frame registration was within 30 min for 200 OCT volumes. Each OCT volume consisted of 500 B-scans, and each B-scan included 500 A-scans. There are several similarities and differences between the observations of mouse and human retinal postnatal development by OCT [39]. Photoreceptor development can be observed both in the human and mouse retina [40].