Ovary of Zebrafish during Spawning Season: Ultrastructure and Immunohistochemical Profiles of Sox9 and Myostatin

Simple Summary Reproduction is a pivotal physiological process in various biological organisms. As the zebrafish has been suggested as a model for the study of numerous aspects of ovarian development, this research aimed to pinpoint the cellular and microenvironmental changes of the ovary in zebrafish during the spawning season, in addition to providing information on the ovarian expression of Sox9 and myostatin. Various stages of oogenesis were described. Furthermore, immunohistochemical analysis revealed SOX9 and myostatin expression in primary and pre-vitellogenic oocytes and the theca cell layers. Moreover, a wide diversity of cells was detected in the ovarian stroma. Collectively, the findings of this study support the importance of the zebrafish ovary as a model for follicular development studies.Abstract This study sought to examine the ovarian cellular and stromal components of the zebrafish (Danio rerio) throughout the spawning season using light and electron microscopic tools. The ovaries of zebrafish showed oocytes in all stages of follicular development and degeneration (atresia). Six stages of oogenesis were demonstrated: oogonia, early oocytes, late oocytes, vacuolated follicles, the yolk globule stage (vitellogenesis), and mature follicles. The SOX9 protein was expressed in the ooplasm of the primary and previtellogenic oocytes and the theca cell layer of the mature follicles. Myostatin was expressed in the granulosa and theca cells. Many stem cells in the ovarian stroma expressed myostatin and SOX9. During the spawning season, the EM results indicated that the zona radiata increased in thickness and was crossed perpendicularly by pore canals that contained processes from both oocytes and zona granulosa. The granulosa cells contained many mitochondria, rER, sER, and vesicles. Meanwhile, the thecal layer consisted of fibroblast-like cells. Atretic follicles could be demonstrated that involved both oocytes and their follicular walls. Several types of cells were distinguished in the ovarian stroma, including mast cells, telocytes, lymphocytes, fibroblasts, endocrine cells, macrophages, adipocytes, dendritic cells, and steroidogenic (stromal) cells. The ovary of the zebrafish serves as a model to investigate follicular development.