Presence of a substance crossreacting with cortical alveolar material in “yolk vesicles” of growing oocytes ofOryzias latipes

Structural modeling and gene expression analysis of phosvitinless vitellogenin (vgc) in the Indian freshwater murrel, Channa punctatus (Bloch, 1793)

Vitellogenin (Vg) is a female-specific egg-yolk precursor protein, synthesized in the liver of fish in response to estrogens. In the present study, complete gene of phosvitinless vitellogenin (vgc) was sequenced, its 3D structure was predicted and validated by web-based softwares. The complete nucleotide sequence of vgc was 4126 bp which encodes for 1272 amino acids and showed the presence of three conserved domains viz. LPD_N, DUF1943 and DUF1944. The retrieved amino acid sequence of VgC protein was subjected to in silico analysis for understanding the structural and functional properties of protein. mRNA levels of multiple vg genes have also been quantified during annual reproductive cycle employing qPCR. A correlation has been observed between seasonal changes in gonadosomatic index with estradiol levels and hepatic expression of three types of vg genes (vga, vgb, vgc) during ovarian cycle of murrel. During preparatory phase, when photoperiod and temperature are low; low titre of E2 in blood induces expression of vgc gene. A rapid increase in the levels of E2 favours induction of vgb and vga genes in liver of murrel during early pre-spawning phase when photoperiod is long and temperature is high in nature. These results suggest that among three vitellogenin proteins, VgC is synthesized earlier than VgA and VgB during oogenesis.

Ultrastructural aspects of oogenesis and oocyte growth in fish and amphibians

Oogenesis, the early events of primary oocyte growth (meiotic arrest, synapsis, ribosomal gene duplication), and folliculogenesis can be seen to particular advantage in the germinal ridge of the syngnathan ovary. After budding off the germinal ridge (a compartment of the luminal epithelium), nascent follicles then enter into a linear array of developing follicles within which temporal and stage-specific events can be correlated with spatial distribution. Prominent features of the later phase of primary oocyte growth include intense transcriptional activity and the formation and subsequent dispersal of the Balbiani vitelline body (mitochondrial cloud) concomitant with an increase in cytoplasmic organelles and volume. Further oocyte growth is characterized by a period of cortical alveolus (in teleosts) or cortical granule (in anurans) formation, in which Golgi elements play a predominant role, and finally vitellogenesis. The latter process, which is responsible for the preponderance of oocyte growth, includes the hepatic synthesis and secretion of vitellogenin (VTG), the uptake of VTG from the bloodstream into the oocyte by receptor-mediated endocytosis, and the transport of VTG via endosomes and multivesicular bodies to forming yolk platelets. In the process, VTG is proteolytically cleaved into the yolk proteins, which assume either a monoclinic (in cyclostomes) or orthorhombic (in teleosts and amphibians) crystalline array. Other structures associated with the growing oocyte are also briefly discussed, including nuage, the vitelline envelope, intercellular junctions between the oocyte and overlying follicle cells, pigment, intramitochondrial crystals in ranidae, and annulate lamellae.

A glycoprotein from the liver constitutes the inner layer of the egg envelope (zona pellucida interna) of the fish, Oryzias latipes

A glycoprotein from the liver, which shares epitopes with chorion (egg envelope or zona pellucida) glycoproteins, is present only in the spawning female fish, Oryzias latipes, under natural conditions. This spawning female-specific (SF) substance is distinct from vitellogenin but closely resembles a major glycoprotein component, ZI-3, of the inner layer (zona radiata interna) of the ovarian egg envelope with respect to some biochemical and immunochemical characteristics. Here we report that the [125I]SF substance, injected into the abdominal cavity of the spawning female fish, was rapidly transported by the blood circulation into the ovary and incorporated into the inner layer of egg envelope of the growing oocytes. The result strongly suggests that the SF substance from the liver is a precursor substance of the major component, ZI-3, of the inner layer of egg envelope in the fish.