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.     

Egg envelopes of Baetis rhodani and Cloeon dipterum (Ephemeroptera, Baetidae): a comparative analysis between an oviparous and an ovoviviparous species

A comparative ultrastructural investigation on the eggshell (vitelline and chorionic envelopes) has been carried out in the nymphs of two mayfly species encompassed into the Baetidae, namely Baetis rhodani and Cloeon dipterum. During oocyte differentiation in the meroistic telotrophic ovarioles, gametes are connected to discrete nurse cells by trophic cords. In B. rhodani, each ovariole contains several eggs arranged in sequence, whereas in C. dipterum each contains a single egg. Follicle cells are competent for vitelline and chorionic envelope synthesis. Baetis rhodani is an oviparous species and the chorion is fairly thick, formed by an alveolate endochorion and a fibrillar exochorion delimited by a honey‐comb roof. Cloeon dipterum stands out among Ephemeroptera for being ovoviviparous. In B. rhodani, ovulation starts in the older nymphs with dark wing‐pads, whereas in C. dipterum choriogenesis ends in the imaginal stage. Here the chorion is very thin and laid eggs hatch almost immediately, allowing the larvulae to move out. The maturation of a single egg per ovariole is synchronized with the achievement of the adult stage. The acquisition of ovoviviparity has led to remarkable changes in the ovariole organization along with a simplification of the eggshell structure.     

The role of the follicle cells during oogenesis in the chiton Sypharochiton septentriones (Ashby) (Polyplacaphora,Mollusca)

Summary Histochemical studies and electron microscopic investigations on the role of the follicle cells during oogenesis in the chiton Sypharochiton septentriones showed that the main role of the follicle cells was the deposition of a spiny chorion around each oocyte. The chorion was composed of three layers; an inner, acid mucopolysaccharide layer, which was a primary egg membrane secreted by Golgi bodies in the cortical cytoplasm of the oocyte, an intermediate layer of protein and an outer layer of lipid. The intermediate and outer layers were secreted by the follicle cells and were thus secondary egg membranes.     

A study of the chorion and the follicle cells in relation to the sperm-egg interaction in the ascidian,Ciona intestinalis

We present ultrastructural observations on the processes underlying interaction between the spermatozoon and the egg envelopes in Ciona intestinalis. The morphological and cytochemical basis of sperm binding to the chorion and the subsequent steps such as the acrosome reaction and penetration of the spermatozoon through the chorion are described. Some speculations are also presented on the role of the follicle cells in fertilization.     

Eggshell fine structure of Bradysia aprica (Winnertz) (Diptera : Sciaridae)

The eggshell fine structure of the dark-winged fungus-gnat Bradysia aprica (Winnertz) (Diptera : Sciaridae) was investigated by scanning and transmission electron microscopy. At the anterior pole of the ovoid egg is a single micropyle, centrally located in a well-defined micropylar area. The latter is covered by many long drumstick-like chorionic processes that are longer and more numerous than those of the rest of the egg surface. Cross-sections of the eggshell show 3 concentric envelopes: the vitelline envelope, wax layer and chorion. The chorion consists of 3 components with different morphological features: the inner, intermediate and outer chorion. The latter 2 layers, involved in the organization of the drumstick-like processes, have homogeneous features, whereas the former is crystalline and resembles the innermost chorionic layer of other Diptera.     

Studies on the formation of germ cells in a compound ascidian Botryllus,primigenus oka

Gametogenesis of a compound ascidian Botryllus primigenus was studied histologically. On either side of the zooid (stage 9), in the gonadal space between the epidermis and the atrial epithelium, either a single testis or a complex of an egg follicle and a testis can be formed. The egg follicle consists of a single ovum (occasionally two ova) and its accessory cells and is connected with the atrial epithelium by the follicle stalk. The egg follicle is always accompanied by the brood pouch, a diverticulum of the atrial cavity. The testis is equipped with a vestigial spermiduct and is attached to the atrial epithelium. Buds of stage 8 comprise, besides the developing testes and, egg follicles, loose aggregations of hemoblasts and oocytes of early developmental stages, which are already accompanied by primary follicular cells. Both the oocytes and the primary follicular cells seem to arise from the hemoblasts. The young oocytes are isolated in the gonadal space of the buds nnd are transferred to buds of the succeeding generations until they finally mature. In the bud of stage 3, a compact mass of cells appears, attaching to tbe inner vesicle on either side of the body. It is derived from the hemoblasts lodged there in the preceding generation and presumably also from the circulating hemoblasts. When the cell mass receives a large oocyte derived from the preceding generation, part of the cell mass differentiates into egg envelopes, forming an egg follicle, and a follicle stalk and the remainder into a testis. When the cell mass receives no oocyte, it differentiate as a whole into a testis. In the egg follicle thus formed the outer and inner follicular cells increase in number by mitotic division. Subsequently, initial test cells are derived from the inner follicle by migration across the developing chorion; then they increas2 in number by mitosis. In the testis, meiosis and spermiogenesis take place.     

Synthesis and function of the fibrous layers covering the eggs of Siphlonurus lacustris (Ephemeroptera, Siphlonuridae)

Ultrastructural analysis (transmission and electron scanning microscopy) of the eggs of the mayfly Siphlonurus lacustris (Eaton) showed that they are wrapped in a thick coat composed of a network of tightly entwined filaments. Groups of twisted filaments form slightly uplifted buttons that are scattered on the coat surface. After experimentally induced egg deposition, egg–water interaction promotes marked cohesion of the eggs and their firm adhesion to the substrate. Egg masses include numerous gametes; the covering of those located close to the substrate greatly extends to anchor the whole mass. Eggs removed from the coat reveal a slightly punctuated smooth chorion and tagenoform micropyles (three to five). The coat increases egg size by about 20%. The lack of female reproductive accessory glands in Ephemeroptera transfers the synthesis of the adhesive coats to the follicle cells, which are typically competent for insect egg shell deposition (vitelline envelope and chorionic layers). This covering results from electron‐dense granules that give rise to filaments progressively organized to form superimposed layers variously orientated around the egg. In addition to egg adhesion to the substrate, a trophic function and protection from shear stress are postulated for this covering.     

In situ study of chorion gene amplification in ovarian follicle cells ofDrosophila nasuta

The temporal and spatial pattern of replication of chorion gene clusters in follicle cells during oogenesis inDrosophila melanogaster andDrosophila nasuta was examined by [^3H thymidine autoradiography and byin situ hybridization with chorion gene probes. When pulse labelled with [³H] thymidine, the follicle cells from stage 10–12 ovarian follicles of bothDrosophila melanogaster and,Drosophila nasuta often showed intense labelling at only one or two sites per nucleus.In situ hybridization of chorion gene probes derived fromDrosophila melanogaster with follicle cell nuclei ofDrosophila melanogaster andDrosophila nasuta revealed these discrete [³H] thymidine labelled sites to correspond to the two amplifying chorion gene clusters. It appears, therefore, that in spite of evolutionary divergence, the organization and programme of selective amplification of chorion genes in ovarian follicle cells have remained generally similar in these two species. The endoreplicated and amplified copies of each chorion gene cluster remain closely associated but the two clusters occupy separate sites in follicle cell nucleus.     

Formation and ultrastructure of the micropylar apparatus in Bombyx mori ovarian follicles

The formation of the micropylar apparatus during oogenesis in the silkworm, Bombyx mori, has been studied using light and transmission electron microscopy. The micropylar apparatus is formed by three types of cells: the micropylar channel-forming cells (MCFCs), the micropylar orifice-forming cells (MOFCs), and the micropylar rosette-forming cells (MRFCs). During the formation of the vitelline membrane and the chorion, each of the MCFCs extends a cytoplasmic projection serving as the mold of a micropylar-channel into the egg envelopes. The detachment and collapse of the projections takes place at the end of choriogenesis. The micropylar channels possess a common external orifice on the chorion and several internal orifices within the vitelline membrane. The MOFCs interact closely with the MCFCs and contribute to the formation of the external micropylar orifice. A petal-like rosette surrounding the orifice is imprinted on the outer chorionic surface by the MRFCs which enclose a group of the MCFCs and MOFCs.     

The vitelline envelope,chorion, and micropyle of Fundulus heteroclitus eggs

The architecture and transformation of the vitelline envelope of the developing oocyte into the chorion of the mature egg of Fundulus heteroclitus have been examined by scanning and transmission electron microscopy. The mature vitelline envelope is structurally complex and consists of about nine strata. The envelope is penetrated by pore canals that contain microvilli arising from the oocyte and macrovilli from follicle cells. During the envelope's transformation into the chorion, the pore canals are lost and the envelope becomes more fibrous and compact and its stratified nature less apparent. The micropyle, of pore, through which the sperm gains access to the enclosed egg is located at the bottom of a small funnel-shaped depression in the envelope. Internally, the micropyle opens on the apex of a cone-like elevation of the chorion. During the development of the envelope, structured chorionic fibrils, the components of which are presumed to be synthesized by the follicle cells, become attached to its surface. These chorionic fibrils are though to aid in the attachment of the egg to the substratum and perhaps to help prevent water loss during low tides when the egg may be exposed.     

Ultrastructure de l'œuf de Triatoma infestans Klug

Summary The thick rigid chorion of the egg of Triatoma secreted by the follicle cells shows two porous layers: an aerial layer in the exochorion, an alveolar one in the endochorion. The anterior part of the eggshell is closed up by an operculum which is heaved up by the hatching larva. The operculum has no alveolar layer. The air enters through the numerous holes of the shell surface into the aerial layer and through the micropyles into the alveolar layer. The egg has no respiratory plastron.The follicle cells produce also a vitelline envelope whose structure shows a rapid condensation at fertilization time. During its development the embryo secretes two layers: serosal and embryonic cuticle.At high humidities, at low temperatures the egg is able to increase its weight during the early stages of embryogenesis, and this increase stops when the serosal cuticle is secreted. In a dry atmosphere the egg loses water but can develop if the temperature is higher than 20°C.The little permeability of the egg is related to the structure of its envelopes. The chorion and the vitelline envelope prevent the water from getting out of the egg. The serosal cuticle seems to be opposed to the penetration of the water into the egg. The role of the embryonic cuticle is probably limited in the transit of water.

Nous remercions Messieurs les Professeurs Maillet et Folliot qui ont mis le microscope R.C.A. à notre disposition, Madame Allo et Mademoiselle Le Gac, technicienne au microscope à balayage J.S.M. S1, pour leur collaboration technique.     

Studies on fertilization in the ascidians : II. Lectin binding to the gametes of ciona intestinalis

In the present work we have compared the binding of fluorescein-conjugated lectins (concanavalin A (ConA), wheat germ agglutinin (WGA), fucose binding protein (FBP) and soybean agglutinin (SBA)) to the sperm surface and to the egg and its envelopes of Ciona intestinalis. Only WGA is bound to the follicle cells: yet this lectin has no binding sites on the sperm surface. Both ConA and FBP are bound by the chorion, the oolemma and the sperm surface. However, while ConA reacts only with the sperm head, FBP is bound both to the head and to the flagellum. Experiments on the effect of ConA and FBP on the fertilization reaction have been carried out. The role of the lectin-binding sites that are shared by the surfaces of both gametes is discussed in connection with the nature of the sperm-binding sites.     

The role of the follicular epithelium in growing eggs of a dipteran insect during late oogenesis and cleavage

The gall midge Heteropeza pygmaea can reproduce by means of paedogenesis (i.e., larval parthenogenesis). In that process, follicles are produced that develop while floating in the hemocoele of the mother larva. A chorion is not formed at the end of oogenesis, and the growing embryos remain enveloped by the follicular epithelium. To investigate possible adaptations of the follicular epithelium to this unusual egg development, its ultrastructure has been studied during late oogenesis and cleavage. Earlier investigations had shown that the follicle cells are provided with a specifically arranged microtubular frame, which may be responsible for the anisometric growth of the egg. The present work shows that the follicle cells are always joined by desmosomes and septate junctions. During development, the septate junctions increase their surface and change their orientation to become parallel to the longitudinal egg axis, thus increasing the resistance of the follicle cells to being torn apart by growth tensions. The total surface of the follicular epithelium increases during development. Well-developed nucleoli in the nuclei and numerous ribosomes in the cytoplasm of follicle cells indicate a high level of synthetic activity. This activity may be required to support the increase in the membrane surface and the establishment of the microtubular frame. Lipid droplets, glycogen, and different inclusions in the follicle cells may represent nutrient and energy reserves. Structures indicating a quantitative significant transfer of nutrients from the follicle cells to the egg were not found.     

The rôle of juvenile hormone in housefly ovarian follicle morphogenesis

Oögenesis in the housefly, Musca domestica, was divided into a series of 10 stages where stage 1 was the germarium, stage 4 was the beginning of yolk deposition, stage 7 was characterized by maximal nurse cell development, stage 9 by the degeneration of the nurse cells and chorion formation, and stage 10 was the mature egg. It required 69 hr from eclosion at 27°C to develop mature eggs. This represented an oöcyte volume increase of 3700-fold, a seventeenfold increase in follicle length, and a sevenfold increase in weight. The application of 2 μg of isopropyl (E,E)-11-methoxy-3,7,11-trimethyldodeca-2,4-dienoate (ZR-515) to allatectomized (-CA) flies stimulated egg development, which progressed at the same rate as the controls. The -CA flies did not develop eggs past stage 4, which represented a cessation of development at a volume of 1·4 per cent that of a mature egg and an ovarian dry weight of 11 per cent that of a mature ovary. The follicle cells from -CA flies did not differentiate into the squamous condition over the nurse chamber, did not become columnar over the oöcyte, did not produce the chorion or vitelline membrane, and did not decrease in number as they did on the stage 10 follicles. Endomitosis in the nurse cell nuclei of -CA flies stopped development at 290 c, but maximum development of 2400 c occurred in stage 7 follicles from controls, and then the nurse cells began to disintegrate.     

Morphology and formation of the eggshell in the tarnished plant bug, Lygus lineloaris (Palisot de Beauvois) (Hemiptera: Miridae)

Scanning and transmission electron microscopy were used to study the morphology and formation of the eggshell in the tarnished plant bug, Lygus lineolaris. Eggs are bean-shaped, with an operculum at the anterior end surrounded by a row of 36-40 respiratory horns. Three micropylar openings are on the operculum, and are sealed in oviposited eggs. The chorion consists of the chorion proper and the innermost chorionic layer. An air layer composed of colonnades is present in the chorion. The innermost chorionic layer is homogeneous and electron lucent. The follicle cells secrete electron dense materials that later coalesced into the reticulated vitelline membrane. This is followed by the deposition of the innermost chorionic layer by the follicle cells. After the primordial innermost chorionic layer is formed, follicle cells at the anterior pole of the oocyte secrete the scaffold for the colonnades in the air layer. Later, the primordial scaffold matrix is redistributed and localized at the lateral and posterior end of the oocyte where it becomes secondarily modified. At the end of choriogenesis, follicle cells at the anterior pole secrete the operculum and respiratory horns.     

Formation and structure of egg capsules in scale insects (Hemiptera, Coccinea) I. Ortheziidae

The paired ovaries of the investigated species are composed of 20-30 ovarioles of a telotrophic-meroistic type. Each ovariole is subdivided into an apical tropharium (=trophic chamber) and a vitellarium that contains a single developing oocyte. This oocyte is surrounded by a mono-layered follicular epithelium that is responsible for synthesis of precursors of egg envelopes. In Orthezia, synthesis and secretion of precursors of egg envelopes (=choriogenesis) and accumulation of reserve substances in the oocyte cytoplasm (=vitellogenesis) start at the same time. The egg capsule is composed of two envelopes: an internal, thick vitelline envelope and an external, very thin chorion. The egg surface is covered with numerous, irregularly arranged waxy filaments of spiral shape. Eggs are devoid of the micropylar, aeropylar and hydropylar openings.     

Aberrant oogenesis in the patterning mutant dicephalic of Drosophila melanogaster: time-lapse recordings and volumetry in vitro

Summary Drosophila females homozygous for the mutation dicephalic occasionally produce ovarian follicles with a nurse-cell cluster on each oocyte pole (dic follicles). Most dic follicles contain 15 nurse cells as in the normal follicle, but the total nurse-cell volume is larger in dic follicles; this is in keeping with the increase in DNA content recently described. However, the relative increase in oocyte volume during nurse-cell regression (from stage 10B onward) is not significantly larger in dic than in normal follicles. Time-lapse recordings in vitro show that, as a rule, both nurse cell clusters in a dic follicle export cytoplasm to the oocyte but nurse-cell regression remains incomplete at both poles and the persisting remnants of the nurse cells cause anomalies in chorion shape. The kinematics of cytoplasmic transfer are less aberrant at that oocyte pole which harbours the germinal vesicle. Possible links are discussed between these anomalies of oogenesis and the double-anterior embryonic patterns observed in the majority of developing dic eggs.     

The ultrastructure and function of follicle cells in Foucartia squamulata (Herbst) (Curculionidae)

Summary The follicle cells of Foucartia squamulata are involved in the formation of both vitelline membrane and chorion. Precursors for these egg coverings are synthesized by the rough endoplasmic reticulum and condensed within dictyosomes. The vitelline membrane and the chorion appear on the oocyte surface simultaneously, which is an unusual phenomenon for insects. The follicular epithelium has not been found to contribute to vitellogenesis in the species under study.     

Follicle cell development is partly independent of germ-line cell differentiation in Drosophila oogenesis

Summary The developmental potential of the cells of the somatic follicular epithelium (follicle cells) was studied in mutants in which the differentiation of the germ-line cells is blocked at different stages of oogenesis. In two mutants, sn ^36a and kelch, nurse cell regression does not occur, yet the follicle cells around the small oocyte continue their normal developmental program and produce an egg shell with micropylar cone and often deformed operculum and respiratory appendages. Neither the influx of nurse cell cytoplasm into the oocyte nor the few follicle cells covering the nurse cells are apparently required for the formation of the egg shell. In the tumor mutant benign gonial cell neoplasm (bgcn) the follicle cells can also differentiate to some extent although the germ-line cells remain morphologically undifferentiated. Vitelline membrane material was synthesized by the follicle cells in some bgcn chambers and in rare cases a columnar epithelium, which resembled morphologically that of wild-type stage-9 follicles, formed around the follicle's posterior end. The normal polarity of the follicular epithelium that is characteristic for mid-vitellogenic stages may, therefore, be established in the absence of morphologically differentiating germ-line cells. However, the tumorous germ-line cells do not constitute a homogeneous cell population since in about 30% of the analyzed follicles a cell cluster at or near the posterior pole can be identified by virtue of its high number of concanavalin A binding sites. This molecular marker reveals an anteroposterior polarity of the tumorous chambers. In follicles mutant for both bgcn and the polarity gene dicephalic the cluster of concanavalin A-stained germ-line cells shifts to more anterior positions in the follicle.     

Stages in follicle cell/oocyte interface during vitellogenesis in caecilians <Emphasis Type="Italic">Ichthyophis tricolor</Emphasis> and <Emphasis Type="Italic">Gegeneophis ramaswamii</Emphasis>: a transmission electron-microscopic study

We describe the ultrastructural organization of the vitellogenic follicle stages in two caecilian species. Monthly samples of slices of ovary of Ichthyophis tricolor and Gegeneophis ramaswamii from the Western Ghats of India were subjected to transmission electron-microscopic analysis, with special attention to the follicle cell/oocyte interface. In order to maintain uniformity of the stages among the amphibians, all the stages in the caecilian follicles were assigned to stages I–VI, the vitellogenic and post-vitellogenic follicles being assigned to stages III–VI. Stage III commences with the appearance of precursors of vitelline envelope material in the perivitelline space. Stages IV and V have been assigned appropriate substages. During the transition of stage III to stage VI oocytes, a sequential change occurs in the manifestations of follicle cells, perivitelline space, vitelline envelope and oocyte cortex. The vitelline envelope becomes a tough coat through the tunnels of which the macrovilli pass to interdigitate between the microvilli. The oocyte surface forms pinocytic vesicles that develop into coated pits and, later, coated vesicles. Contributions of the oocyte cortex to the vitelline envelope and of the follicle cells to yolk material via synthesis within them are indicated. The follicle cell/oocyte interface of vitellogenic follicles of these two caecilians resembles that in anurans and urodeles, with certain features being unique to caecilians. Thus, this paper throws light on the possible relationships of caecilians to anurans and urodeles with special reference to ovarian follicles. This research was supported by funds from the Kerala State Council for Science, Technology and Environment (KSCSTE), through the SARD facility, and by the FIST scheme of Department of Science and Technology, Government of India, New Delhi, to the Department of Zoology, University of Kerala, Thiruvananthapuram, and to the Department of Animal Science, Bharathidasan University, Thiruchirapalli (SR/FST/LSI-233/2002).