An updated version of histological and ultrastructural studies of oogenesis in the siberian sturgeon acipenser baerii

Abstract

Oogenesis of Siberian sturgeon Acipenser baerii is studied on farm fish using light and electron microscopy. We have identified five stages correlated with physiological state of the ovarian follicle constituted by the oocyte surrounded by its cellular (theca and granulosa) and a-cellular (zona radiata) layers: Stages I and II before vitellogenesis, Stages III and IV during vitellogenesis, and Stage V during maturational processes. Following the oogonial stage, Stage I presents an elevated nucleoplasmic index, and the nucleus contains only one nucleolus. In this early stage, lipid globules were identified. Stage II is characterized by nucleoli multiplication and their migration toward the nuclear periphery. During this stage, the number of lipid globules increases in the oocyte cytoplasm. At the end of this stage, cortical alveoli get synthesized. The beginning of Stage III, called sub-Stage IIIa, corresponds to the first features of vitellogenin incorporation simultaneously with the elaboration of the zona radiata externa. During sub-Stage IIIb the zona radiata interna 1 is built. Stage IV is characterized by the first apparition of pigment granules and the elaboration of the zona radiata interna 2. Yolk accumulation increases and the oocyte volume grows considerably. Stage V corresponds to maturation with the beginning of nucleus migration toward the oocyte membrane. The zona radiata is completely synthesized, and a "jelly coat" is deposited on the outer surface of the zona radiata externa by synthesis of granulosa cells. Yolk accumulation ends at this stage. In conclusion, ultrastructural data allow accurate determination of oogenesis stages.