Histological and Transcriptomic Insights into the Ovary Development of Hemibarbus labeo Injected with Spawn-Inducing Hormones.

Naturally, the ovaries of many farmed fish can only develop to stage IV (mainly including stage IV oocytes, known as full-grown postvitellogenic oocytes). Therefore, spawn-inducing hormone injections are used to promote ovary development and oocyte maturation, facilitating reproduction in the aquaculture industry. The study of spawn-inducing hormones and their underlying neuroendocrine mechanisms has been a recent focus in fish reproductive biology. However, the intra-ovarian regulatory mechanisms of ovary development and oocyte maturation after hormone injection require further investigation. In this study, we explored the histological and transcriptomic map of the ovary of Hemibarbus labeo after hormone injection to reveal changes in the ovary. The gonad index significantly increased after hormone injection for 5.5 h, after which no significant change was observed. Histological analysis showed that the nuclei had moved to one side of the oocytes at 5.5 h after hormone injection. Moreover, the volume of the oocytes increased and their yolk membranes thickened. Oocytes then underwent their first meiotic division at 5.5-11 h after hormone injection. Subsequently, the follicular membrane was ruptured, and ovulation was completed at 11-16.5 h after hormone injection. In addition, we identified 3189 differentially expressed genes (DEGs) on comparing the transcriptomes at different time points after hormone injection. These DEGs were significantly enriched in the GO terms of nervous system process, molecular transducer activity, and extracellular region, and the KEGG pathways of TNF signaling and cytokine-cytokine receptor interaction; these may play important roles in ovary development and oocyte maturation. Within these pathways, genes such as apoe, creb3, jun, junb, il11, and il8 may play important roles in steroid hormone synthesis and ovulation. Conclusively, our results show detailed sequential dynamics of oocyte development and provide new insights into the intra-ovarian regulatory mechanisms of ovarian development and oocyte maturation in H. labeo. These findings may be important for research on improving egg quality and reproduction in aquaculture.