Zonagenesis and vitellogenesis in the swordfish (Xiphias gladius L.)

The work reports che results obtained by che histological, histochemical, immnunohistochemical and ultrastructural study of swordfish ovaries collected during the pre-spawning period. In particular, we dealt with the correlation between two events of oogenesis: zonagenesis and vitellogenesis. These phenomena, in teleost fish, depend on hepatic synthesis of the molecular componenrs for both the eggshell and the oocytic energy deposits. Both hepatic processes are controlled by plasma levels of estradiol. Fragments of ovaries from 4 specimens caught in the North Ionian Sea in May were: a) fixed in Boun's solution and embedded in paraffin wax; b) fixed in 3% GTA in 0.1 M cacodilate buffer (pH 7.4). post-fixed in 1% OsO4 and embedded in Araldite. Paraffin sections were scained wirh Ematoxylin-eosin, PAS and Alcian blue (AB) pH 2.5. Certain sections were immunostained wich rabbit ariti salmon zona radiata proteins (ZRP) and rabbit anti swordfish vitellogenin (VTG) sera. Thin sections obtained by Araldite embedded samples, were stained with uranyl acetate and lead citrate. The ovaries showed the presence of follicles in different maturity stages (diameter ranging from 20 to 550 (um). Each follicle was constituted by an oocyce surrounded by a single layer of follicular cells. Zona radiata appeared as a thin PAS positive ring in follicles with diameter >l60 mm and its thickness increased up to 12 um as follicle developed. Anti ZRP serum showed immunoreactivity with follicles with diameter >220 um. The immunostaining. initially was localised on both some follicular cells and oocyte surface, but in more developed follicles (diameter >250 um) it was detected also in cortical alveoli of the peripheral ooplasm. The cortical alveoli showed also positive reaction to PAS and AB. Anti VTG seuum reacted with che ooplasm of follicles wich diameter >250 um showing an increase in staining intensity correlated with vitellogenic development. The main ultrastructural features observed were; a) follicular cells with numerous mitochondria, few ribosome, scarce endoplasmic reticulum and Golgi complex; b) zona radiata constituted by an electrondense amorphous matrix crossed by poro canals in which oocyte microvillar projections extended; e) oolemma involved in endocytosis activity; d) cortical ooplasm characterized by conspicuous endoplasmic reciculum associaced with cortical alveoli. The results obtained in the present study show that the formation of the zona radiata involves phenomena both of follicular and extra-follicular origm. In addition, this work suggests an ordered sequence of initiation of zonagenesis before initiation of vitellogenesis.