Adult neuronal regeneration in the telencephalon of the killifish Aphaniops hormuzensis

Evaluation of scale development and its regeneration potency in the desert killifish under laboratory conditions

Scale development and its regeneration potency were evaluated in a desert killifish Aphaniops hormuzensis (family Aphaniidae) in laboratory conditions by using light and scanning electron microscopy. Scale development in A. hormuzensis took 156 days at room temperature. Four specific regions of scale formation were detected. The first scale development began 13 days post-hatching (dph) (total length [TL] = 8.5 mm) at the caudal peduncle region and is extended anteriorly 26 dph (TL = 13.6 mm) at the area below the dorsal fin. Scales began forming independently in the head region at 33 dph (TL = 21.7 mm), and in the abdominal region, began at 41 dph (TL = 25.8 mm). Additional points of scale origin were detected on the sides of the operculum or behind and below the eyes. Scale regeneration in the caudal peduncle started 6 days after removal (dar). In 16 dar, the microstructural features appeared and the growth circles, a wide and oblong focus (focus length = 0.6 ± 0.05 µm), and lepidonts were also formed. In 36 dar, the scale shape was gradually changed from circular to a polygon, and radii were distinguishable in the anterior field. The pattern of scale formation could be useful in enhancing the understanding of systematics and phylogeny, functional morphology, and habitat use. It could also be useful in helping to define the Larval/juvenile transition period.

Differential Regenerative Capacity of the Optic Tectum of Adult Medaka and Zebrafish

Zebrafish have superior regenerative capacity in the central nervous system (CNS) compared to mammals. In contrast, medaka were shown to have low regenerative capacity in the adult heart and larval retina, despite the well-documented high tissue regenerative ability of teleosts. Nevertheless, medaka and zebrafish share similar brain structures and biological features to those of mammals. Hence, this study aimed to compare the neural stem cell (NSC) responses and regenerative capacity in the optic tectum of adult medaka and zebrafish after stab wound injury. Limited neuronal differentiation was observed in the injured medaka, though the proliferation of radial glia (RG) was induced in response to tectum injury. Moreover, the expression of the pro-regenerative transcriptional factors ascl1a and oct4 was not enhanced in the injured medaka, unlike in zebrafish, whereas expression of sox2 and stat3 was upregulated in both fish models. Of note, glial scar-like structures composed of GFAP+ radial fibers were observed in the injured area of medaka at 14 days post injury (dpi). Altogether, these findings suggest that the adult medaka brain has low regenerative capacity with limited neuronal generation and scar formation. Hence, medaka represent an attractive model for investigating and evaluating critical factors for brain regeneration.