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.