Ultrastructural studies of the morphological variations of the egg surface and envelopes of the African catfish Clarias gariepinus (Burchell, 1822) before and after fertilisation with a discussion of fertilisation mechanism (original) (raw)
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Structure and composition of the fish egg chorion
Journal of Ultrastructure and Molecular Structure Research, 1988
The mature egg of fishes is surrounded by an acellular coat conventionally called the chorion. In Carassius auratus three main layers can be recognized at the electron microscopic level, and we have termed them chl, ch2, and ch3 starting from the outer side to the inner side close to the egg plasma membrane. Isolated chorion from mature ovarian and ovulated eggs can be obtained by homogenization and further purification techniques. We analyzed these preparations on sodium dodecyl sulfate-palyacrylamide gel electrophoresis, and at least 20 polypeptide chains were reproducibly observed. The solubilization conditions were tested by means of various agents under reducing and nonreducing conditions. The present report focuses on the temporal and spatial pathway of chorion formation during oocyte growth, and we report the solubilization and partial characterization of protein and glycoprotein components of purified chorions.
Kutum (Rutilus frisii kutum, Kamensky 1901) is an economically important Cyprinid species endemic to the Caspian Sea. This species is anadromous, meaning it enters rivers for spawning. In this study, the oocyte development in the developing oocyte and fertilized egg of kutum was investigated with emphasis on the zona radiata (ZR) structure by light and scanning electron microscopy. Histological features of developmental stages of oocytes were described in detail using light microscopy. The results showed that ZR was not observed in the previtellogenic phase. The ZR was observed as a simple structure between the follicular layer and oocyte membrane during the cortical alveolus stage. The study of the ZR ultrastructure showed some pore canals and microvilli at the vitellogenic stage expanded structurally as the vitellogenesis process proceeded. The ZR maximum thickness (14.9 ± 1.6 μm) and maximum length of extended microvilli (10.8 ± 1.4 μm) was achieved at the end of the vitellogenic stage. During ovarian maturation, reduction in ZR thickness and microvilli length was found to be gradual. The transformation of ZR structure, i.e., clogging of the pore canals and retraction of the microvilli occurred functionally during fertilization and later on the ZR renamed as chorion (diameter 9.6 ± 0.4 μm). The chorion on the surface of fertilized eggs possessed microvilli (8.0 ± 1.1 μm in length) and some slender processes. In general, considering the changes of ZR thickness and microvilli length, it seems that they play an important role in easier transfer of yolk materials into the oocyte. In addition, after fertilization, the chorion (transformed ZR) helps adhesion of eggs to the bottom with regard to the environment of spawning.
Bolletino di zoologia, 1994
The fine structure of the chorion and micropyle of mature unfertilized eggs of the sea bass Dicentrarchus labrax was examined by scanning and transmission electron microscopy. The chorion consists of three layers: a thin electrondense outer layer, a paracrystalline middle layer and a thick inner layer consisting of 12 electrondense filamentous lamellae alternating with interlamellar material of lower electrondensity. The two more external layers are crossed by pore canals opening on the egg surface. The funnel-type micropyle is located at the animal pole of the egg. The walls of the micropylar canal have circular, rib-like thickenings reflecting the stratified inner layer of the chorion. The results are discussed and compared with those in other Teleostea.
Journal of Experimental Zoology, 1977
The vitelline envelope (VE) that surrounds an egg released from the ovary into the coelom of Xenopus laevis differs markedly, in structure and penetrability, from the VE surrounding an oviposited egg. In a coelomic egg, the filaments that form the VE are arranged in distinct fascicles or bundles. The exterior surface of the VE is irregular in contour and is permeated by channels. In an oviposited egg, the filaments are evenly dispersed and lack a fasciculated arrangement; the exterior surface is smooth and no channels are present. The fascicular arrangement of fibrils in the coelomic VE is maintained only a t neutral pH, and is not visibly altered by the cortical reaction. VEs from coelomic eggs retain their fasciculated morphology after isolation from the egg. In an in vitro test system, sperm penetrated VEs isolated from oviposited eggs, but failed to penetrate VEs isolated from coelomic eggs. The structural transformation of the VE from the coelomic type to the oviposited type occurs in the first 1-cm segment of the oviduct, prior to addition of jelly to the egg. Neither intact jelly, solubilized jelly, nor jelly extracts were capable of altering the structural organization of coelomic VEs, suggesting that the structural transformation of the VE is effected by some oviducal factor other than jelly.
Fish Physiology and Biochemistry, 2020
Egg-envelope, an acellular coat, surrounds the egg and is essential for vitellogenin incorporation. It also plays a pivotal role during fertilization and provides protection to the developing embryo. In the present study, scanning electron microscopy was used to elucidate the structural details of isolated egg-envelopes from the Indian freshwater murrel, Channa punctatus. Several pores and single micropyle were observed on outer surface, whereas inner layer indicated deposition of proteinaceous material. The constituent proteins of egg-envelope were further characterized by Fourier transform infrared (FT-IR) spectroscopy, and electrophoresis and mass-spectrometry (MALDI-TOF-MS/ MS). The secondary structure of egg-envelope proteins showed the presence of antiparallel ß-pleated sheets and aromatic amino acids. These proteins resolved into two peptides (130 kDa and 68 kDa) under denaturing conditions, which exhibited glycoprotein nature. The peptide band with low molecular mass showed significant similarity with transmembrane protein, whereas peptide band with high molecular mass matched with choriogenin protein of other fishes. These results confirm that chorion is derived from precursor protein, Choriogenin, in murrel. Chemical composition of eggenvelope supports that chorion is responsible exchange material and chemical defence during embryogenesis.
Journal of Experimental Zoology, 1987
It has been recently shown that, in several genera of annelids, including Chaetopterus, fertilizing sperm attach to and fuse with egg microvilli which penetrate the vitelline envelope. This suggests that the annelid vitelline envelope may have no direct or obligatory role in normal fertilization. The present study was undertaken to investigate the involvement of the vitelline envelope in fertilization in Chuetopterus experimentally, by examining the fertilization of vitelline envelope-free eggs quantitatively and qualitatively. Brief exposure of the eggs to isotonic sucrose-EDTA removed the vitelline envelope as determined by both phase-contrast and electron microscopy, rendered the eggs more sensitive to polyspermy and substantially reduced the binding of supernumerary sperm to eggs but did not decrease fertilizability as determined by sperm dilution assay and did not make the eggs more sensitive to cross-fertilization. The events of fertilization were examined by electron microscopy and found to be very similar in vitelline envelope-free eggs to those in intact eggs. We conclude that the vitelline envelope in Chaetopterus has binding sites for sperm but that it has no obligatory role in fertilization and is primarily involved in the prevention of polyspermy.
Studies on chorion hardening inhibition and dechorionization in turbot embryos
Aquaculture, 2007
Embryo dechorionization is a common practice used in certain fish species for different purposes. It facilitates techniques like microinjection, transfection or electroporation in embryos. Dechorionization is easily achieved in some fish species but is a more complex problem in species that have very thick chorions. In this study, we address this problem in turbot embryos, where chorion removal is practically unachievable post-chorion hardening. For this purpose, different solutions that lacked ions required for the hardening of this envelope or contained inhibitors of enzymes involved in the process were used during egg fertilization. The toxicity of the solutions was assessed, and their effect on embryo cleavage and on chorion structure was studied by light and scanning electron microscopy (SEM). The results demonstrated that embryos are very sensitive to these solutions and that first cellular cleavages are affected with most of them. This study also provides the first report on turbot chorion structure, analyzed by SEM. The chorion is a very thick envelope in this species, and its total removal was not observed with the employed treatments. Nevertheless, partial dechorionization was achieved when embryos were fertilized in some of the tested solutions and later treated with pronase (3 mg/ml).