Follicle formation and oocyte growth in the silver butter catfish, Schilbe intermedius Rüppell, 1832 (Siluriformes, Schilbeidae)

Abstract

This study describes folliculogenesis in the silver butter catfish Schilbe intermedius using light miscroscopy, and reports on nuclear transformations during oocyte development. The ovary is asynchronic, and as such most phases of oocyte development are found. Oocyte growth follows the general pattern observed in teleost fish. In most fish examined, left ovaries were smaller than right ones. Oocyte growth was classified into follicle, cortical alveolus; vitellogenic, and maturing phases of growth; oogonial proliferation was not detected. Prominent features in developing oocytes were the increase in size and synthesis of nucleoli in follicle and cortical alveolar stages, and the presence of yolk granules and lipid droplets in vitellogenic oocytes. A high percentage of vitellogenic and maturing oocytes, as well as the presences of atretic oocytes, were indicative of advanced maturation phases of development. During oocyte growth, shape of the nucleus changed dramatically and the nucleolus exhibited a series of transformations from two to three at the beginning of the growth period to multiple during vitellogenic stages. During folliculogenesis, pre-follicle cells developed between the oocytes, surrounding each oocyte and were completed when the oocyte and new follicle cells were completely surrounded by a basement membrane. These results suggest that ovarian follicle formation is a conserved process in S. intermedius and provide valuable foundation for further studies.