Morphology of the egg shell and the developing embryo of the Red Palm Weevil,Rhynchophorus ferrugineus (Oliver)

The harvested eggs of Rhynchophorus ferrugineus are ovo-cylindrical shaped, averaged 1.09 mm in length and 0.43 mm in width, with ratio of L\W 4.42. The chorionic layer of electron dense material is seen covering the exochorion structure of the eggs. The egg main body chorion exhibits a polygonal pattern and architecture surface of the egg is supported by a system of irregular interconnecting grooves. The micropylar apparatus of the eggs of the Red Palm Weevil, R. ferrugineus is described in the present study for the first time. Two micropylar openings are found closed to the center of the posterior wide pole of the egg. Each micropylar opening presents a single small orifice and its surrounding chorion is porous and densely set with tiny projections allowing the spermatozoa to penetrate the egg. Respiratory aeropyles are distributed on the borders of reticulations in the area chorionic surface of egg capsule. The hatching region is detected on the anterior part at the opposite side of the egg. Changes in the appearance and shape of R. ferrugineus eggs as well as the incidence of embryonic development are observed.     

Surface ultrastructure of the egg chorion in the dragonfly,Ictinogomphus rapax (Rambur) (Odonata : Gomphidae)

Scanning electron microscopic examination of the egg chorion in Ictinogomphus rapax (Rambur) (Odonata : Gomphidae) shows hexagonal reticulation throughout the surface. The anterior pole of the egg bears a small rounded micropylar stalk with a group of 6 orifices arranged radially around a central boss, while the posterior pole consists of a sessile, truncated cone formed of 50–60 long, coiled filaments. The functional, taxonomic, and phylogenetic significance of various microstructures of the eggshell are discussed.     

Ultrastructure and role of the aeropyle and shell of the egg of Galleria mellanella

The structure of the eggshells and particularly the morphogenesis and the fine structure of an aeropyle were studied in the wax moth Galleria mellonella.Galleria has eggs whose chorion shows an inner trabecular layer and the walls of aeropyles are coated by threads. During oviposition air enters the trabecular layer in a sufficient quantity to allow the development of the caterpillar if the egg is immersed in boiled water or paraffin oil. This layer is in communication with the atmosphere by means of 150 to 200 aeropyles.     

Formation and structure of egg envelopes in Russian sturgeon Acipenser gueldenstaedtii (Acipenseriformes: Acipenseridae)

The covering of the eggs in Russian sturgeon Acipenser gueldenstaedtii consists of three envelopes (the vitelline envelope, chorion and extrachorion) and is equipped with multiple micropyles. The most proximal to the oocyte is the vitelline envelope that consists of four layers of filamentous and trabecular material. The structural components of this envelope are synthesized by the oocyte (primary envelope). The chorion encloses the vitelline envelope. The extrachorion covers the external surface of the egg. Examination of the arrangement of layers that comprise the egg envelopes together with the ultrastructure of follicular cells revealed that the chorion and extrachorion are secondary envelopes. They are secreted by follicular cells and are built of homogeneous material. During formation of egg envelopes, the follicular cells gradually diversify into three morphologically different populations: 1) cells covering the animal oocyte region (cuboid), (2) main body cells (cylindrical) and (3) micropylar cells. The apical surfaces of follicular cells from the first two populations form processes that remain connected with the oocyte plasma membrane by means of gap junctions. Micropylar cells are located at the animal region of the oocyte. Their apical parts bear projections that form a barrier to the deposition of materials for egg envelopes, resulting in the formation of the micropylar canal.     

Blastodermic cuticles of a springtail,Tomocerus ishibashii Yosii (Collembola : Tomoceridae)

The formation and structure of the blastodermic cuticles of a springtail, Tomocerus ishibashii Yosii (Collembola : Tomoceridae) are described together with the change of egg membrane. The blastodermic cuticles of the Collembola are 2-layered, and formed in the early stages of the embryonic development, preceding the differentiation of germ band. The first blastodermic cuticle is thicker (about 0.8-1.5 μm in thickness) and its surface is provided with complex structures, whereas the second one is thinner (about 0.2-0.4 μm in thickness) and smooth. About 3 days after oviposition, the chorion (about 2 μm in thickness) splits into 2 and the first blastodermic cuticle, provided with many projections and 4 large spines appear on the surface of the egg. Three types of projections are distinguished: button-, cone- and seta-like structures. The halves of the ruptured chorion are attached to the first blastodermic cuticle on both sides below the spines, and no projections are found in the regions concealed by the ruptured chorion. The projections of the first blastodermic cuticle are formed by cellular protrusions of the blastoderm. The conspicuous large spines on the first blastodermic cuticle are formed by the evaginations of the blastoderm. Tendrils of the primary dorsal organ run between the first and second blastodermic cuticles.     

Respiratory morphology of the Abedus herberti Hidalgo egg chorion (Hemiptera: Belostomatidae)

Although giant water bugs (Hemiptera: Belostomatidae) are large, aquatic insects known for their obligate paternal egg brooding behaviors, little research has focused on the structure of their eggs. The respiratory requirements of developing embryos likely created selection for brooding, so a thorough understanding of the respiratory morphology of belostomatid eggs could help explain how brooding behaviors facilitate embryonic gas exchange. This study used scanning electron microscopy to document the respiratory microstructure of the eggs of Abedus herberti, a back brooding giant water bug. The exochorion is similar to that of other belostomatids in texture and organization except that the respiratory region is confined to the uppermost quarter of the egg. This is the area exposed to the atmosphere by encumbered males. A plastron network made up of densely packed vertical projections demarcates the boundary between the respiratory and nonrespiratory regions of the chorion. The internal chorion is composed of alternate air‐filled and denser layers that likely facilitate the movement of oxygen from the aeropyles at the top of the eggs to the developing embryonic tissues. J. Morphol., 2011. © 2011 Wiley‐Liss, Inc.     

Changes in chorion proteins induced by the exudate released from the egg cortex at the time of fertilization in the teleost, Oryzias latipes

The chorion of unfertilized medaka Oryzias latipes eggs consists of two major proteins (77–73 and 49 kDa) and a minor 150 kDa protein. Upon fertilization, these major chorion proteins are polymerized to insoluble high molecular weight proteins via the temporary formation of several new proteins (132, 114, 62 and 61 kDa). Increasing chorion toughness is closely related to the formation of high molecular weight proteins and the increasing insolubility of the chorion proteins. The changes in chorion proteins and hardening could be induced in vitro in isolated chorions by an egg exudate, which includes cortical alveolar contents. The effects of temperature and pH on the egg exudate-induced changes in chorion proteins were examined in the present study. The major proteins could be digested by proteolytic enzymes. The 49 kDa protein was PAS-positive. Analysis with polyclonal antibodies against the major proteins demonstrated that the temporarily formed 62 and 61 kDa proteins were derived from the 77–73 kDa protein and that higher molecular weight proteins, newly formed in the process of chorion hardening, contained the same epitopes as did the 77–73 and 49 kDa proteins. The results suggest that the changes in chorion proteins of the medaka egg at the time of fertilization can be induced by an enzyme(s) released from the egg cortex into the perivitelline space.     

Micro analysis of the egg shell of Adela metallica (Poda) (Lepidoptera : Adelidae) by energy-filtering transmission electron microscopy (EFTEM)

The egg shell of the incurvarioid moth Adela metallica (Lepidoptera : Adelidae) was studied by conventional (cTEM) and energy-filtering transmission electron microscopy (EFTEM). The shell of the laid egg consists of 3 envelopes. The vitelline envelope is 0.1–0.2μm thick and homogeneous, thus exhibiting the non-exoporian character state. The single-layered chorion, which is covered by a fibrogranular mucous layer, is 0.5–0.9μm thick and homogeneous, thus exhibiting the non-ditrysian character state. The chorion is highly electron-lucent. Neither cTEM nor EFTEM revealed any sub-structural details. However, electron spectroscopic imaging (ESI) and electron energy-loss spectroscopy (EELS), revealing the elemental composition of the egg shell, indicate that the chorion and vitelline envelope are proteinaceous and hence, similar to the egg shells of other lepidopteran species. The presence of high sulphur signals associated with the vitelline envelope and the thin basal lamella of the chorion indicates that these components may be stabilized via sulphur-bridges.     

Ultrastructural diversity in the egg chorion of Hawaiian Drosophila and Scaptomyza: ecological and phylogenetic considerations

Formation of the egg shell (chorion) inDrosophila and Scaptomyza (Diptera : Drosophilidae) is a complex developmental process involving coordinated synthesis and secretion of multiple proteins by the monolayer of follicle cells surrounding the egg. Using scanning electron microscopy, the ultrastructure of the chorion in 37 endemic Hawaiian drosophilids, representing the genera Drosophila and Scaptomyza, were analyzed and compared with 7 representative species of continental Drosophila. The detailed structure of the chorion was described for 8 chorionic regions: the respiratory filaments, follicle imprints, operculum, micropyle, dorsal ridge, ventral rim, posterior pole, and the chorion cross-section. The morphology of each region is similar among related species, but strikingly different among groups. The main functions of the chorion are to protect the developing embryo from the vicissitudes of the environment and to provide channels for gas exchange during embryogenesis. Adaptation to the diverse ovipositional substrates used by Drosophila in general, and the Hawaiian species in particular, has resulted in extraordinary diversity in the various chorionic structures. The respiratory filaments differ in number and have evolved to different lengths and degrees of porosity. Furthermore, other regions also involved in respiratory exchange (the operculum, follicle imprints, the pole region, and the dorsal ridge) have diverged in parallel to the ecological divergence. The thickness and complexity of the outer endochorion are dramatically different in various groups, providing varying degrees of mechanical strength to the eggshell, which promotes embryonic survival in the diverse microenvironments. These varied chorionic structures have been found to provide useful morphological characters for phylogenetic analyses of the drosophilids.     

Buoyancy variations in eggs of Atlantic cod (Gadus morhua L.) in relation to chorion thickness and egg size: theory and observations

Egg specific gravity is a function of egg volume, chorion volume, perivitelline space (PVS) and the specific gravity of chorion and ovoplasm. Experimental studies on cod ( Gadus morhua L.), a marine multiple batch spawner, showed that approx. forty per cent of the chorion volume is incorporated between batches and that the chorionic material has a mean specific gravity of 1·20 (range: 1·14–1·35). The chorion volume and specific gravity are low at the end of the spawning period in extremely fecund fish. Specific gravity of ovoplasm was observed to be 1·017 in marine eggs and calculated to be about 1·008 in brackish water eggs. Inherent variation in PVS is found to have little influence on egg specific gravity. With the other variables constant, egg specific gravity is inversely related to egg volume supported by studies on single batches of eggs. Measurements on every batch of eggs reveal that the production of large eggs (>1·3 mm) gives low density eggs initially and high density eggs towards the end of spawning, while the production of small eggs (<1·3mm) gives moderate specific gravity. These observations are discussed in relation to vertical distribution of the eggs and reproductive success of recruit and repeat spawners. There are indications that repeat spawners produce larger eggs.     

The use of egg chorion glycoprotein of Epinephelus malabaricus for egg identification

An immuno‐probe against a glycoprotein in the egg chorion was developed for egg identification. The 97 kD glycoprotein in the chorion of unfertilized eggs of Epinephelus malabaricus was isolated and separated by SDS‐PAGE as an antigen to induce antibody from rabbit. The reactivity of the antibody as the immuno‐probe to E. malabaricus eggs was significantly positive, and was specific in that it did not react with the eggs of other fish species. The immuno‐probe should be useful in identifying the eggs of E. malabaricus among mixed egg populations.     

Eggshell fine structure of three lepidopteran pests: Cydia pomonella (L.) (Tortricidae), Heliothis virescens (Fabr.), and Spodoptera littoralis (Boisd.) (Noctuidae)

The eggshells of 3 moths, Cydia pomonella (Tortricidae), Heliothis virescens, and Spodoptera littoralis (Noctuidae) were investigated by scanning (SEM) and transmission (TEM) electron microscopy. The surface of the noctuid eggs shows structural elements (micropylar rosette, ribs, cross-ribs, and aeropyles) and regional differentiation, all typical of Lepidoptera. The egg of C. pomonella shows a different regional morphology due to its watch-glass shape and its position, lying on the flank. The micropylar structures are on the lower egg face in contact with the substrate. For S. littoralis, the surface structure (sculpturing) of the egg is not species-specific, being indistinguishable from that of S. frugiperda (Salkeld, 1984).In all 3 moths, the eggshell fine structure is basically identical, as revealed by TEM. Both the vitelline envelope and the chorion consist of several distinct layers. The vitelline envelope, bi-layered and several μm thick, undergoes a marked structural change when embryogenesis begins. At the same time, Golgi vesicles bearing dense particles, appear in the periplasm of the egg cell in fertilized eggs of H. virescens and S. littoralis. The chorion of all 3 species consists of a basal layer (C-1), a cavity layer (C-2) supported by trabecles and opening to the exterior via aeropylar canals, and a lamellar layer (C-3), which probably consists of helicoidally arranged stacks of fibrils. In H. virescens and S. littoralis, an additional epicuticle-like layer (C-4) is present. Available data from the literature are summarized and a basic scheme of the radial eggshell fine structure of ditrysian Lepidoptera is proposed.     

Fine structure of the egg shells of Habrophlebia fusca (curtis) and h. consiglioi Biancheri (Ephemeroptera : Leptophlebiidae)

The eggs of 2 mayflies, Habrophlebia fusca and H. consiglioi (Ephemeroptera : Leptophlebiidae) were observed with scanning and transmission electron microscopes. The external surface of the eggs in both species had longitudinally oriented costae. The chorion of H. fusca had different structures in its costal and intercostal zones. Three distinct layers could be recognized: an inner layer close to the vitelline coat, consisting of electron-dense lamellae perpendicular to the egg surface; an intermediate layer, consisting of loosely structured fibrillar material; and an outer highly electron-dense layer, consisting of 2 separate laminae, divided by an electron-transparent line. In the egg of H. fusca, the costal area of the chorion shows a columnar structure. The columns merge distally to create wide chambers. This organization has been observed with the SEM in H. consiglioi as well. The chambers are interconnected and communicate with the exterior through openings along the costal edges. Masses of mucus-like substance are present both in the chambers and outside the chorion; they show fibrillar material and electron-dense bodies with a paracrystalline structure.     

Functional anatomy of the egg pore in Callosobruchus maculatus: a trade-off between gas-exchange and protective functions?

Abstract. The chorion of the egg in Callosobruchus maculatus (Coleoptera: Bruchidae) provides protection but also has to allow exchange of respiratory gases. A single opening in the chorion (the egg pore) allows diffusion of gases, and young larvae die if the egg pore is blocked. Comparison of respiration rates using a Gilson respirometer showed an increase in oxygen uptake from the first to the seventh day after the egg was laid; the increase was from 6.8 to 11.2 pl/egg/day in a Brazil strain and from 8.6 to 23.5 pl/egg/day in a Yemen strain. The Yemen strain had double the metabolic rate of Brazil-strain insects in the larvae, which are exposed to seed toxins, but not in the non-feeding adults. It is suggested that the higher metabolic rate in the Yemen larvae is related to their ability to develop in seeds that are toxic to the Brazil strain. The egg pore also differed in size and shape between the two strains. The egg pore was cylindrical in the Brazil strain, but shorter and funnel-shaped with an increased external diameter in the Yemen strain. Calculation showed that the different shape and size of the Yemen egg pore would allow a 2–3-fold increase in gas conductance compared with the Brazil strain, and this increase was consistent with the enhanced oxygen requirement of the Yemen larvae, which was a little over twice that of the Brazil larvae at 7 days. The anatomy of egg pores in Cmaculatus thus seems to represent a trade-off between two functions, allowing adequate gas exchange while maintaining the protective function of the chorion.     

Pressure-volume relationship of the Fundulus egg in sea water and in sucrose

Upon activation, an internal hydrostatic pressure develops within the Fundulus egg, and compresses the egg proper to a reduced volume. When the perivitelline pressure is abolished by a highly hypertonic sucrose solution, the egg volume increases. As sucrose penetrates the chorion, the volume again decreases. The relation between P and V in these conditions is inverse, and approximates a rectangular hyperbola. The limiting factor causing most of the deviation is shown to be the incompressible fraction. It is concluded that the volume of the egg proper is controlled by the perivitelline pressure, and that the effect of hypertonic sucrose solution is exerted by lowering the pressure and thereby increasing membrane permeability non-specifically. It is also shown that some permanent alterations occur within the plasma membrane during activation that reduce the permeance, and thereby, increase the incompressible fraction.     

Ultrastructural studies of the formation of the egg capsule in the hermaphroditic species, Isohypsibius granulifer granulifer Thulin, 1928 (Eutardigrada: Hypsibiidae)

The egg capsule of Isohypsibius granulifer granulifer Thulin 1928 (Eutardigrada: Hypsibiidae) is composed of two shells: the thin vitelline envelope and the multilayered chorion. The process of the formation of the egg shell begins in middle vitellogenesis. The I. g. granulifer vitelline envelope is of the primary type (secreted by the oocyte), but the chorion should be regarded as a mixed type: primary (secreted by the oocyte), and secondary (produced by the cells of gonad wall). During early choriogenesis, the parts of the chorion are produced and then connected into a permanent layer. The completely developed chorion consists of three layers: (1) the inner, medium electron dense layer; (2) the middle labyrinthine layer; (3) the outer, medium electron dense layer. After the formation of the chorion, a vitelline envelope is secreted by the oocyte.     

Scanning electron microscopy of Drosophila embryogenesis: 1. The structure of the egg envelopes and the formation of the cellular blastoderm

As part of a series of detailed observations on embryogenesis in Drosophila, the protective coverings of the egg and surface changes in the embryo prior to gastrulation have been studied with the SEM. Four specializations of the chorion are described: the plastron, micropylar cone, operculum, and the posterior thickening. After removal of the protective coverings the surface changes during development can be observed. During the first eight synchronous nuclear divisions a dense array of thin microprojections covers the whole embryo. After the ninth division between 373 and 408 nuclei reach the surface and become located in cytoplasmic projections. From counts of the number of surface bulges during the syncytial blastema stages, it was established that 13 synchronous divisions take place producing between 5600 and 6500 surface nuclei. During formation of the cellular blastoderm, the location of the prospective cells becomes obscured by a dense pattern of microprojections from each cell. However, with the completion of the blastoderm, the surfaces of the cells become smooth and the cell outlines distinct. The usefulness of the SEM in developmental studies is discussed.     

Ultrastructural and histochemical studies of the egg capsules of Perla marginata (Panzer, 1799) and Dinocras cephalotes (Curtis, 1827) (Plecoptera : Perlidae)

Eggshells of stone flies P. marginata and D. cephalotes (Plecoptera : Perlidae), inhabiting mountain streams, were examined using scanning and transmission electron microscopes, a phase-contrast light microscope and histochemical methods to detect proteins, lipids and polysaccharides.The eggshells of the species investigated consist of a vitelline envelope, chorion and gelatinous sheet decorated on its outer surface with mushroom-like structures. An anchoring structure (attachment disc) is situated on the posterior pole of the egg. The structure and function of the attachment disc, as well as the possible taxonomic applications, are discussed. The morphology and histochemical composition of all these elements of the shell clearly demonstrate good adaptation to land and aquatic habitats; the chorion consists of 2 layers, the internal layer being finely perforated by numerous aeropyles. The external layer, with fewer, regularly placed aeropyles, protects the egg interior against dehydration in the land habitat. The gelatinous sheet seems to provide additional protection. Mushroom-like structures, situated on its surface, correspond with the positions of aeropylar openings. These and other interrelations between chorion structure and function are discussed.     

Effects of wheat germ agglutinin on tunicate egg activation and fertilization: Is there a plasma membrane sperm receptor system on Ascidia ceratodes eggs?

Little work has been carried out on the sperm recognition systems present on the egg plasma membrane. Here it is shown that wheat germ agglutinin (WGA) interferes with the sperm-interacting system on the plasma membrane of eggs of the ascidian, Ascidia ceratodes. The WGA activates the dechorionated egg, indicating that a plasma membrane sugar residue can be directly tied to egg activation. Low concentrations of this lectin do not activate the eggs, but reduce fertilizability. This observation suggests that the WGA binding site might be part of a sperm reception–activation complex in the plasma membrane. While WGA also affects sperm binding to the chorion, the mechanisms of sperm interaction at the plasma membrane and chorion show different sensitivities to lectins, sugars and enzymes.     

Ultrastructure of the natural and factitious host eggs of Trichogramma galloi Zucchi and Trichogramma pretiosum Riley (Hymenoptera: Trichogrammatidae)

The surface and structure of the chorion of eggs of Diatraea saccharalis (F.) (Lepidoptera: Crambidae), Anticarsia gemmatalis (Huebner), Heliothis virescens F., Spodoptera frugiperda (Smith) (Lepidoptera: Noctuidae), Sitotroga cerealella (Oliver) (Lepidoptera: Gelechiidae), Ephestia kuehniella Zeller and Corcyra cephalonica Stainton (Lepidoptera: Pyralidae), that are hosts of Trichogramma galloi Zucchi and Trichogramma pretiosum Riley (Hymenoptera: Trichogrammatidae) were studied on SEM and TEM. Other characteristics of these eggs, such as changes in their color during embryonic development, size and volume were also recorded. Sculpturing and texture of the surface of the chorion greatly varied among the species studied, as well as the number of layers of the chorion and their thickness. Eggs of the factitious hosts were among the smallest and their volume was very close to each other. All these characteristics would provide basic information for a better understanding of the host selection behavior and are useful for the development of a suitable artificial host egg for the in vitro rearing of these egg parasitoids.