Development of the eye in grass carp (Ctenopharyngodon idella): Ocular histomorphology, corneal surface ultrastructure, and retinal calretinin and tyrosine hydroxylase immunoreactivity.

Eye posthatching development was examined in grass carp (Ctenopharyngodon idella) from 1 day posthatching (1 dph) to 365 dph. Eye histology and morphometry, retinal expression of calretinin (CR) and tyrosine hydroxylase (TH), and corneal ultrastructure were studied using light and scanning electron microscopy. Basic eye structure was detectable at 1 dph, though important features, including retina and iris pigmentation as well as photoreceptor differentiation, were delayed till 4 dph. Differentiated photoreceptors seemed to emerge at 4 dph with evident increases in their count and shape diversity at 20 dph. The corneal surface epithelium appeared smooth at 20 dph. However, it featured corrugations and micropits at 60 dph, excessive desquamations at 90 dph, and distinct intercellular boundaries at 365 dph. Iris differentiation was evident at 20 dph. The annular ligament was seen at the iridocorneal junction at 20 dph with noticeable intracellular coarse secretory granules in yearling fish. The choroidal rete appeared massively filled by blood at 90 dph. Morphometrically, eye diameter and corneal thickness increased markedly between 20 dph and 365 dph. While the iris and central parts of the retina showed a rapid increase in thickness between 4 dph and 20 dph. Immunohistochemically, CR- and TH-immunoreactive cells were detected in the retina from 1 dph onwards, with CR localized to the inner nuclear and ganglion cell layers and TH to the inner nuclear layer. Numbers of immunoreactive cells per mm2 peaked at 90 dph for CR and 20 dph for TH, suggesting a differential timeline for their activities. CR immunoreactive cells were equally distributed between central and peripheral parts of the retina. While TH immunoreactive cells revealed a significantly higher count at the peripheral parts of the retina between 20 dph and 365 dph. This study characterizes early posthatching ocular development in C. idella and provides a reference for future studies of the cyprinid eye.