Drosophila vitelline membrane assembly: a critical role for an evolutionarily conserved cysteine in the "VM domain" of sV23

The vitelline membrane (VM), the oocyte proximal layer of the Drosophila eggshell, contains four major proteins (VMPs) that possess a highly conserved "VM domain" which includes three precisely spaced, evolutionarily conserved, cysteines (CX⁷CX⁸C). Focusing on sV23, this study showed that the three cysteines are not functionally equivalent. While substitution mutations at the first (C123S) or third (C140S) cysteines were tolerated, females with a substitution at the second position (C131S) were sterile. Fractionation studies showed that sV23 incorporates into a large disulfide linked network well after its secretion ceases, suggesting that post-depositional mechanisms are in place to restrict disulfide bond formation until late oogenesis, when the oocyte no longer experiences large volume increases. Affinity chromatography utilizing histidine tagged sV23 alleles revealed small sV23 disulfide linked complexes during the early stages of eggshell formation that included other VMPs, namely sV17 and Vml. The early presence but late loss of these associations in an sV23 double cysteine mutant suggests that reorganization of disulfide bonds may underlie the regulated growth of disulfide linked networks in the vitelline membrane. Found within the context of a putative thioredoxin active site (CXXS) C131, the critical cysteine in sV23, may play an important enzymatic role in isomerizing intermolecular disulfide bonds during eggshell assembly.

[Eggs fertilizability in domestic hens Gallus gallus domesticus as a function of the yolk surface area]

The increase of egg mass and reliable decrease of egg fertilizability were observed when the mass surface area of yolk (egg) increased. The results obtained suggest that, in addition to the number of viable spermatozoa penetrating across the perivitelline membrane within 15-20 min after ovulation, the probability of fertilization depends on the area of egg surface, which approximately corresponds to the area of perivitelline membrane. Apparently, the ratio of receptors' numbers and spermatozoa, which contact with them on the germ disc surface, to their number on the rest part of perivitelline membrane decreases with the increase of yolk size. The decreased egg fertilizability concomitant with the increased area of perivitelline membrane suggests that the egg size is one of the factors of fertility of the female gametes as concerns both variability of the egg composition and age.

Developing follicles of the spotted ray Torpedo marmorata express different glycoside residues in relation to granulosa differentiation and vitelline envelope formation

Lectins constitute a class of proteins/glycoproteins that specifically bind to terminal glycoside residues. The present investigation aimed to identify lectin-binding sites in developing follicles of Torpedo marmorata. Using eleven lectins (WGA, GSI-A4, GSI-B4, PSA, UEA-I, PNA, MPA, Con-A, DBA, LCA, BPA, SBA), we demonstrated that the biochemical nature and the distribution of carbohydrate residues significantly change during oogenesis in the granulosa cells and the vitelline envelope. In fact, a progressive appearance of surface glycoproteins bearing terminated ss-GlcNAc O-linked side chains was observed in the granulosa during the differentiation of pyriform-like cells from the small ones via intermediate cells simultaneously with a significant reduction of the D-Gal chains present in their nucleus. Glycoproteins bearing ss-GlcNAc O-linked side chains were first evident on the surface of small cells in contact with the oocyte, then on the intermediate ones, and finally on pyriform-like cells. The distribution pattern of such glycoproteins over the differentiated granulosa cells remained unchanged during the subsequent stages of the oocyte growth so granulosa cells preserved the same sugar distribution pattern. Furthermore, a progressive loss of D-Gal residues was evident in the nucleus of granulosa cells. In fact, staining for D-Gal was intense in the nucleus of small follicle cells and progressively reduced till disappearing in differentiated pyriform-like cells. Conversely, the small follicle cells located under the basal lamina were devoid of ss-GlcNAc residues, and the nuclear content in D-Gal remained unchanged. This finding strongly suggests that surface glycoproteins containing ss-GlcNAc residues, and the nuclear content in D-Gal might be related to the differentiation of pyriform-like cells. The present investigation also demonstrates that the content of the sugar residues of the vitelline envelope (VE) changes during oocyte growth, suggesting that pyriform-like cells may contribute to its formation.

Structure of ovaries and formation of egg envelopes in the stonefly, Leuctra autumnalis Aubert, 1948 (Plecoptera: Leuctridae). Ultrastructural studies

The investigation of ovaries and the formation of egg envelopes of the stonefly Leuctra autumnalis was carried out with light and transmission electron microscopes. The ovary of the studied species is paired and consists of several dozen panoistic ovarioles opening individually to the oviduct. The process of egg capsule formation already begins in previtellogenesis. At this time the follicular cells secrete precursors of the vitelline envelope. Analysis of the presented data suggests that the oocyte itself also takes part in the formation of the vitelline envelope during late vitellogenesis. Simultaneously, the follicular cells produce precursors of further layers of the egg capsule, i.e. two-layered chorion and extrachorion, consisting of two gelatinous layers and a flocculent one. The completely developed capsule contains channels, probably micropylar ones.

Ovarian follicle ultrastructure in the teleost Synbranchus marmoratus (Bloch, 1795), with special reference to the vitelline envelope development

Synbranchus marmoratus is a protogynous diandric teleost fish widely distributed throughout South America. The aim of this work was to study the ultrastructure of the vitelline envelope and the relationship among oocyte and their follicular cells during oogenesis. During perinucleolar stage, the oocyte and the follicular cells form microvillar processes that project into the perivitelline space. The oocyte secretes a dense and amorphous material, which appears as the first evidence of the vitelline envelope (VE) development. The VE passes from a double to a multilayered structure during oocyte growth. In mature oocytes, the VE reach a mean thickness of 11 microm, having up to 30 layers. Oocyte microvilli are thinner than the follicular ones and were seen in contact with the follicular plasmalema, however we could not find any contact between the follicular microvilli and the oolemma. Before ovulation, microvillar processes retract and the pore canals seem to collapse. An outer electron dense layer occludes the superficial pore and forms a continuous layer. No jelly or adhesive coatings were seen at least in ovulated eggs sampled from ovarian lumen. Follicular cell and oocyte cytological characteristics do not differ from those described in other teleosts species.

Observations on the mechanism of eggshell formation in the liver fluke, Fasciola hepatica

A mechanism for eggshell production in Schistosoma mansoni has been proposed (Wells & Cordingley, 1991), and suggests that the release of eggshell protein globules from the vitelline cells occurs under alkaline conditions within the ootype followed by their subsequent fusion to form the eggshell. Fusion and tanning of these components produces eggshell which autofluoresces. The present study was carried out to determine whether a similar process operates in Fasciola hepatica. A number of drug treatments were used to disrupt key steps in the maturation of vitelline cells. Treatment with the calcium ionophore lasalocid (1 x 10(-5) M) led to the premature release of eggshell globules from the vitelline cells but not their fusion. Incubation in monensin (1 x 10(-6)M), a sodium ionophore and ammonium chloride (NH4Cl) (5 x 10(-2) M), a weak base, resulted in the premature fusion of eggshell protein globules within the vitelline cells and premature tanning of the eggshell protein material. The copper-containing enzyme, phenol oxidase, is thought to be involved in the tanning process during the production of eggs. Diethyldithiocarbamate, (DDC, 1 x 10(-3) M) is a phenol oxidase inhibitor and treatment with this compound, in combination treatments with monensin and NH4Cl, prevented fusion of the vitelline cell globules and tanning of the shell protein material. The results of the study suggest that the mechanism for eggshell formation in F. hepatica is similar to that proposed for S. mansoni and may be common to other trematodes as well.

Xenopus laevis sperm-egg adhesion is regulated by modifications in the sperm receptor and the egg vitelline envelope

The biochemical and ultrastructural changes in the envelope of the Xenopus laevis egg that occur during oviposition and fertilization have been thoroughly studied (Hedrick, J. L., and Nishihara, D. M., Methods Cell Biol. 36, 231-247, 1991; Larabell, C. A., and Chandler, D. E., J. Electron Microsc. Tech. 17, 294-318, 1991). However, the biological significance of these changes with respect to gamete interaction has been unclear. In the current study, it was found that changes in the envelope are directly responsible for regulating sperm-egg adhesion, an initial step of fertilization. As a result of these transformations, sperm bind only to unfertilized oviposited eggs, not to oocytes or coelomic eggs. In addition, they do not bind to fertilized eggs. The molecular and cellular basis of the regulation of the sperm binding process was investigated in the context of our recent findings that two structurally related envelope glycoproteins, gp69/64, serve as sperm receptors during fertilization (Tian, J.-D., Gong, H., Thomsen, G. H., and Lennarz, W. J., J. Cell Biol. 136, 1099-1108, 1997). Although the purified gp69/64 glycoproteins isolated from the oocyte or coelomic egg envelopes exhibited sperm binding activity, when these proteins are part of the intact oocyte or coelomic egg envelopes, they are not accessible to either anti-gp69/64 antibodies or to sperm. During the conversion from the coelomic to the vitelline envelope, the gp69/64 sperm receptors become exposed on the surface, an event that correlates with proteolytic cleavage of gp43 and accompanying ultrastructural alterations in the envelope. Conversely, after fertilization, when the vitelline envelope of the egg is converted to the fertilization envelope of the zygote, limited proteolytic cleavage of the sperm receptor results in loss of sperm binding activity. In addition, formation of a fertilization layer on top of the structurally altered VE adds another physical block to sperm binding. These results provide new insights into structure-function relationships between envelope components of the anuran egg, and provide further evidence supporting the key role of gp69/64 as sperm receptors during X. laevis fertilization.

Early embryonic angiogenesis in the chick area vasculosa

The rate and pattern of growth as well as vessel ultrastructure of the area vasculosa were examined in the chick. The embryos were grown in shell-less culture after 3 d in ovo and staged according to Hamburger & Hamilton (1951) and the rate of increase in the diameter of the area vasculosa was measured. This revealed an increase in the area vasculosa diameter of 0.4 +/- 0.02 mm h-1 (n = 62) for embryos between stages 15 and 20. To determine the growth pattern of the sinus terminalis (the advancing edge of the area vasculosa), a marked length of the sinus was photographed at hourly intervals over a period of 9 h. It was found that this vessel grows by new vessels forming external to the sinus in the form of parallel plexuses, one of which then replaces the original sinus as the major route of bloodflow. Ultrastructurally the capillaries of the area vasculosa were simple tubes of endothelial cells, lacking a basement membrane. The endothelial cell cytoplasm contained only a few organelles, mainly mitochondria and rough endoplasmic reticulum. These findings indicate that the chick area vasculosa capillaries bear similar structural and growth characteristics to those associated with tumour angiogenesis and suggest that they may prove to be a useful model system for studying the factors involved in pathological angiogenesis.

Histogenesis of the yolk sac in the chick

The histogenesis of the chick yolk sac was studied from incubation day 3 to posthatching day 8. During incubation the epithelial lining forms complex folds which penetrate into the yolk mass. Hemopoiesis is at a maximum between incubation days 11 and 13. The mesoderm is a thin layer during early incubation but thickens in the later stages to form a covering serosa, an intermediate connective tissue layer, and an inner vascularized layer which forms lymphatic tissue after hatching. Involution includes the processes of autolysis and phagocytosis. Protein assimilation, by pinocytosis and phagocytosis, reaches a peak at incubation day 13. Glycogen is present in peak amounts between incubation days 7 and 9. Endodermal alkaline phosphatase activity is localized in the apical cytoplasm between days 7 and 13 and in the basal cytoplasm during the latter incubation stages. Acid phosphatase activity increases throughout incubation to peak levels at day 2 posthatching. Maximum lipid absorption occurs during the latter incubation stages. Ribonucleic acid reveals only moderate staining throughout incubation. Electron microscopy shows that the endoderm is active in pinocytosis and phagocytosis. Lysosomes, rich in acid phosphatase, are numerous within the apical endoderm.