Histological characterization of the maturation stages of the ovarian follicles of the goldfish Carassius auratus (Linnaeus, 1758).

The goldfish is a model organism showing great potential for research, particularly in comparative endocrinology concerning the neuroendocrine signalling and regulation of vertebrate reproduction. Furthermore, this teleost is increasingly stressed as a relevant alternative to more common fish model organisms, namely zebrafish. However, quality descriptions and illustrations of the complete goldfish gonadal histology are surprisingly scarce, but needed, to support research using this fish. Therefore, the main aim of this work is to describe in detail and adequately illustrate the goldfish oogenesis, from oogonia to late maturation, by applying routine stains (haematoxylin-eosin) and special procedures (periodic acid-Schiff and Goldner's trichrome). We hypothesized that the combined strategies would enable not only to observe the most general features but also to perceive some poorly described details of oocytes better. We describe the details of the following maturation stages: oogonia proliferation, chromatin-nucleolus, primary growth (one nucleolus step, multiple nucleoli step, perinucleolar step, cortical alveoli step) and secondary growth (early secondary growth step, late secondary growth step). Additionally, we report aspects of early and late follicular atresia. The study allowed comparisons with other species and showed that the Goldner's trichrome has the best discriminative power and should be the preferred stain, despite more time-consuming.