Purpose: Calbindin-D 28K (CaD) and parvalbumin (PV) are calcium-binding proteins thought to act as an intraneural calcium ion buffering system in the central nervous system. We previously reported that PV appears at birth in some cells in the outer portion of the nuclear layer of the retina that can differentiate into horizontal cells. CaD is also distributed in horizontal cells; however, it is not clear at which developmental stage CaD appears.
Methods: The development of horizontal cells and the distribution of CaD and PV was examined ultrastructurally and immunohistochemically in rat retinas from birth (P0) to postnatal day 50 (P50).
Results: Our ultrastructural data showed clear differentiation of horizontal cells from other retinal cells in the early postnatal days. These cells became nearly mature around P10 to P12. PV appeared in the outer nuclear layer at birth (P0) and, by P7, was observed in the ganglion cell layer, amacrine cells, and horizontal cells. CaD-immunoreactive cells, however, were not seen in any portions of the retina until P7. At P10, CaD-immunoreactivity was faintly observed in horizontal cells, and its immunoreactivity gradually increased in the horizontal cells from P10 to P15 when the eyes of the rats opened. CaD was not found in the ganglion cell layer or in amacrine cells at any period examined.
Conclusion: In the horizontal cells, PV appeared when they were ultrastructurally distinguished from other retinal cells, while CaD appeared when they already had some structure for endocytosis and synaptic transmission, suggesting that PV and CaD may be good morphological and functional markers for horizontal cells, respectively.