Loss of yolk platelets and yolk glycoproteins during larval development of the sea urchin embryo

The fate of the yolk platelets and their constituent yolk glycoproteins was studied in Strongylocentrotus purpuratus eggs and embryos cultured through the larval stage. Previous studies have shown that the yolk glycoproteins undergo limited proteolysis during early embryonic development. We present evidence that the yolk glycoproteins stored in the yolk platelets exist as large, disulfide-linked complexes that are maintained even after limited proteolysis have occurred. We provide additional evidence that acidification of the yolk platelet may activate a latent thiol protease in the yolk platelet that is capable of correctly processing the major yolk glycoprotein into the smaller yolk glycoproteins. Because we previously showed that these yolk glycoproteins are not catabolized during early embryonic development, it was of interest to study their fate during larval development. Using a specific polyclonal antibody to a yolk glycoprotein, we found that both yolk glycoproteins and the yolk platelets disappeared in feeding, Day 7, larval stage embryos, but that starvation did not significantly affect the levels of the yolk glycoproteins. We also found that the yolk glycoproteins reappeared in 30-day-old premetamorphosis larvae.     

Subcellular organization of the yolk syncytial-endoderm complex in the preimplantation yolk sac of the shark,Rhizoprionodon terraenovae

Summary The structure of the yolk syncytial-endoderm complex of the preimplantation yolk sac of the shark is examined by light- and transmission electron microscopy. The yolk syncytium is bounded by a membrane that is anchored to the plasmalemma of adjacent endoderm cells by desmosomes. Enlarged nuclei, rough endoplasmic reticulum, Golgi complexes, mitochondria, and other cellular organelles populate the syncytium. Microtubules and filamentous elements are also observed free in the syncytium. Yolk is present as pleomorphic droplets, the profiles of which are generally spherical but may be vesicular, especially at the periphery of large yolk droplets. Occasionally, large yolk droplets have a paracrystalline configuration. Small yolk droplets are modulated through the Golgi complex of the yolk syncytium, and it is suggested that acid hydrolases are added there. Small yolk droplets released from the maturing face of the Golgi complex are sequestered in membrane-limited packets. The membrane of the packets fuses with the membrane enveloping the yolk syncytium and the yolk droplets are released into the yolk syncytialendoderm interspace. Subsequently, the yolk droplets are endocytosed by the endoderm. Yolk droplets disperse and fuse to form the large irregular yolk inclusions of the endoderm. Yolk metabolites are transported out of the endoderm through the yolk sac endothelium. The yolk sac endoderm thus mediates the transfer of metabolites from the yolk mass to the extraembryonic circulation.     

凡纳滨对虾卵母细胞卵黄发生的超微结构

利用电镜研究凡纳滨对虾卵母细胞卵黄发生的全过程。结果表明 :凡纳滨对虾卵黄的发生是双源性的。卵黄发生早、中期是内源性卵黄大量合成的阶段 ,卵黄发生中、后期则以外源性卵黄的合成为主。内源性卵黄主要由内质网、线粒体、核糖体、溶酶体、高尔基器等多种胞器活跃参与形成。其中数量众多的囊泡状粗面内质网是形成内源性卵黄粒的最主要的细胞器 ;部分线粒体参与卵黄粒的合成并自身最终演变为卵黄粒 ;丰富的游离核糖体合成了大量致密的蛋白质颗粒并在卵质中直接聚集融合成无膜的卵黄粒 ;溶酶体通过吞噬、消化内含物来形成卵黄粒和脂滴 ,且方式多样 ;高尔基器不直接参与形成卵黄粒。外源性卵黄主要通过卵质膜的微吞饮活动从卵周隙或卵泡细胞中摄取外源物质来形成     

Toward a dynamic model of deposition and utilization of yolk steroids

The discovery by Schwabl that maternal steroid hormones aretransferred to the egg yolk and have effects on the phenotypeof offspring revealed a new pathway for non-genetic maternaleffects. The initial model relied on passive transfer. The thinkingwas that steroids passively entered the lipophillic yolk duringyolk deposition and then were deposited in the yolk until theywere passively delivered to the embryo as the yolk was used.Subsequent studies revealed that the system is much more dynamicthan that. Here, we explore questions about how dynamic thesystem really is and look at questions like: Is transfer ofmaternal steroids to the yolk passive or is it actively regulated?At what stages of the maternal reproductive cycle are steroidstransferred? During reproduction, how dynamic are the levelsof yolk steroids? Especially in the case of potentially deleterioussteroids (e.g., androgens in female offspring; glucocorticoids),once deposited can they come out of the yolk over time? Canthey be metabolized by the yolk or by the embryo? During incubation,how much do steroid levels in the yolk change? Can steroidsdiffuse from the yolk to the embryo prior to yolk utilization?Does the embryo contribute to yolk steroid levels as it develops?We believe that comprehensive answers to questions like thesewill eventually allow us to generate a much more accurate andcomplete model of the transfer and utilization of yolk steroidsand that this model will be much more dynamic and active thanthe one initially proposed.     

Ultrastructure and cytochemistry study of the yolk syncytial layer in the alevin of trout (Salmo fario trutta L.) after hatching

After hatching, the yolk syncytial layer of Salmo fario trutta may be subdivided into two zones, namely, the vitellolysis zone (containing numerous yolk platelets), and the cytoplasmic zone (where yolk platelets are rare). In the vitellolysis zone, two stages in the utilization of the yolk are observed: 1) The first stage, comprises the formation of yolk platelets from coalescent yolk by spherical cutting out and basal scission. This process seems to be achieved by the invagination of fibrillar elements into the coalescent yolk to form individual yolk platelets surrounded by a limiting membrane. 2) The second stage essentially consists of the extrusion or budding of yolk matter from a yolk platelet. Again, where the yolk matter leaves a platelet, fibrillar elements are evident and show an alkaline phosphatase activity. The platelets of the vitellolysis zone have a homogeneous content and variable diameter; they never acquire a heterogeneous and polymorphic aspect which could be interpreted as an intermediate stage in their degradation.     

Distribution of yolk polypeptides in the follicle cells during the differentiation of the follicular epithelium in Sarcophaga bullata egg follicles

In S. bullata, the ovaries contribute to the synthesis of yolk polypeptides. A specific antiserum for yolk polypeptides was used to visualize the presence of yolk polypeptides in the follicle cells during their differentiation. After vitellogenesis has started, all follicle cells contain yolk polypeptides. The squamous follicle cells covering the nurse cells and the border cells lose yolk polypeptides before mid-vitellogenesis, whereas the follicle cells over the oocyte contain yolk polypeptides until after late vitellogenesis. All follicle cells are immunonegative afterwards. In vitro translation of poly(A)+ RNA demonstrated that the presence of yolk polypeptide mRNA correlates well with follicle cell immunopositivity for yolk polypeptides. This suggests that the follicle cells synthesize the ovarian yolk polypeptides. Differences in cellular and nuclear morphology, total and poly(A)+ RNA synthesis and the rate of yolk polypeptide synthesis were shown to be correlated with the presence or absence of yolk polypeptides in the differentiating follicular epithelium. The possible relationship between these different aspects of follicle cell differentiation, follicle cell polyploidy and the extracellular current pattern around follicles are discussed.     

Yolk sac development in lizards (Lacertilia: Scincidae): New perspectives on the egg of amniotes

Embryos of oviparous reptiles develop on the surface of a large mass of yolk, which they metabolize to become relatively large hatchlings. Access to the yolk is provided by tissues growing outward from the embryo to cover the surface of the yolk. A key feature of yolk sac development is a dedicated blood vascular system to communicate with the embryo. The best known model for yolk sac development and function of oviparous amniotes is based on numerous studies of birds, primarily domestic chickens. In this model, the vascular yolk sac forms the perimeter of the large yolk mass and is lined by a specialized epithelium, which takes up, processes and transports yolk nutrients to the yolk sac blood vessels. Studies of lizard yolk sac development, dating to more than 100 years ago, report characteristics inconsistent with this model. We compared development of the yolk sac from oviposition to near hatching in embryonic series of three species of oviparous scincid lizards to consider congruence with the pattern described for birds. Our findings reinforce results of prior studies indicating that squamate reptiles mobilize and metabolize the large yolk reserves in their eggs through a process unknown in other amniotes. Development of the yolk sac of lizards differs from birds in four primary characteristics, migration of mesoderm, proliferation of endoderm, vascular development and cellular diversity within the yolk sac cavity. Notably, all of the yolk is incorporated into cells relatively early in development and endodermal cells within the yolk sac cavity align along blood vessels which course throughout the yolk sac cavity. The pattern of uptake of yolk by endodermal cells indicates that the mechanism of yolk metabolism differs between lizards and birds and that the evolution of a fundamental characteristic of embryonic nutrition diverged in these two lineages. Attributes of the yolk sac of squamates reveal the existence of phylogenetic diversity among amniote lineages and raise new questions concerning the evolution of the amniotic egg. J. Morphol. 278:574–591, 2017. © 2017 Wiley Periodicals, Inc.     

An Ultrastructural Investigation on Vitellophage Invasion of the Yolk Mass during and after Germ Band Formation in Embryos of the Stick Insect Carausius morosus Br.

Developing embryos of the stick insect Carausius morosus were examined ultrastructurally with a view to studying vitellophage invasion of the yolk mass during and after germ band formation. Newly laid eggs in C.morosus have a unique yolk fluid compartment surrounded by a narrow fringe of cytoplasm comprising several small yolk granules. Vitellophages originate mainly from a thin layer of stem cells, the so-called yolk cell membrane, interposed between the germ band and the yolk mass. Throughout development, a thin basal lamina separates the yolk cell membrane from the overlying embryo.
Vitellophages extend from the yolk cell membrane with long cytoplasmic processes or filopodia to invade the central yolk mass. Along their route of entrance, filopodia engulf portions of the yolk mass and sequester it into membrane-bounded granules. As this process continues, the yolk mass is gradually partitioned into a number of yolk granules inside the vitellophages.
Later in development, the yolk cell membrane is gradually replaced by the endodermal cells that emerge from the anterior and posterior embryonic rudiments. From this stage of development onwards, vitellophages remain attached to the basal lamina through long filopodia extending between the endodermal cells. Yolk confined in different vitellophagic cells appears heterogeneous both in density and texture, suggesting that yolk degradation may be spatially differentiated.     

Effects of egg yolk during the freezing step of cryopreservation on the viability of goat spermatozoa

Four experiments were conducted to investigate the effects of egg yolk during the freezing step of cryopreservation (namely, the process except for the cooling step), on the viability of goat spermatozoa. The effects of egg yolk on sperm motility and acrosome integrity during the freezing step were investigated in Experiment 1. Spermatozoa diluted with Tris-citric acid-glucose (TCG) solution containing 20% (v/v) egg yolk were cooled to 5 degrees C, washed, and then frozen in TCG with egg yolk (TCG-Y), TCG without egg yolk (TGG-NY), 0.370 M trehalose with egg yolk (TH-Y), or trehalose without egg yolk (TH-NY). All extenders contained glycerol. In frozen-thawed spermatozoa, the inclusion of egg yolk in the freezing extenders increased (P<0.05) percentages of motile sperm, progressively motile sperm, and the recovery rate (ratio of post-thaw to pre-freeze values), but decreased (P<0.05) acrosomal integrity. Moreover, extenders with trehalose had better (P<0.05) post-thaw sperm viability. In Experiment 2, the effects of egg yolk on acrosome status before and after freezing were studied. Egg yolk significantly decreased the proportion of intact acrosomes before freezing, leading to fewer (P<0.05) intact acrosomes post-thaw and lower (P<0.05) recovery rates for intact acrosomes. In Experiment 3, including sodium dodecyl sulfate (SDS) in a diluent containing egg yolk tended to preserve the acrosome compared with the egg yolk containing diluent free of SDS, however, spermatozoa had a lower (P<0.05) proportion of intact acrosomes than those in a yolk-free diluent. However, after cooling, spermatozoa were diluted with a glycerolated extender containing egg yolk. Therefore, the objective of Experiment 4 was to explore whether the egg yolk or glycerol was responsible for the reduced intact acrosome percentage. In this experiment, after cooling and washing the spermatozoa were diluted in TCG with glycerol and/or egg yolk. The combination of glycerol and egg yolk in the extender reduced (P<0.05) the proportion of intact acrosomes compared with egg yolk or glycerol alone. In conclusion, the inclusion of egg yolk significantly improved sperm motility, indicating its beneficial effects during the freezing step of cryopreservation; trehalose appeared to synergistically increase its cryoprotective effects. Furthermore, although neither glycerol nor egg yolk per se affected the proportion of intact acrosomes, the combination of the two significantly reduced the proportion of acrosome-intact spermatozoa.     

Ultrastructure of the pre-implantation shark yolk sac placenta

During ontogeny, the yolk sac of viviparous sharks differentiates into a yolk sac placenta which functions in gas exchange and hematrophic nutrient transport. The pre-implantation yolk sac functions in respiration and yolk absorption. In a 10.0 cm embryo, the yolk sac consists of six layers, viz. (1) somatic ectoderm; (2) somatic mesoderm; (3) extraembryonic coelom; (4) capillaries; (5) endoderm; and (6) yolk syncytium. The epithelial ectoderm is a simple cuboidal epithelium possessing the normal complement of cytoplasmic organelles. The endoplasmic cisternae are dilated and vesicular. The epithelium rests upon a basal lamina below which is a collagenous stroma that contains dense bodies of varying diameter. They have a dense marginal zone, a less dense core, and a dense center. The squamous mesoderm has many pinocytotic caveolae. The capillary endothelium is adjacent to the mesoderm and is delimited by a basal lamina. The endoderm contains yolk degradation vesicles whose contents range from pale to dense. The yolk syncytium contains many morphologically diverse yolk granules in all phases of degradation. Concentric membrane lamellae form around yolk bodies as the main yolk granules begin to be degraded. During degradation, yolk platelets exhibit a vesicular configuration.     

半滑舌鳎胚胎发育中后期卵黄囊超微结构观察

在胚胎发育中期,半滑舌鳎胚胎由胚体、卵黄囊和卵周液构成.对半滑舌鳎胚胎发育中后期的卵黄囊进行超微结构观察.结果表明,卵黄囊是由卵黄囊膜和包裹其内的卵黄物质组成.在半滑舌鳎胚胎发育过程中,卵黄囊内的卵黄物质逐渐消耗产生低分子量的卵黄磷蛋白分裂小泡.分裂小泡转移到卵黄囊内部消黄细胞中,在消黄细胞的作用下分裂小泡转化成卵黄颗粒.随后卵黄颗粒在卵黄囊内表面聚集成囊状结构并转移运输到卵黄囊膜内部,最后把卵黄物质从卵黄囊膜转移并释放到卵周液中,为胚胎发育提供营养.     

Cloning and expression of the yolk protein of the tsetse fly Glossina morsitans morsitans

Two major families of nutritional proteins exist in insects, namely the vitellogenins and the yolk proteins. While in other insects only vitellogenins are found, cyclorraphan flies only contain yolk proteins. Possible sites of yolk protein synthesis are the fat body and the follicle cells surrounding the oocyte. We report the cloning of the yolk protein of the tsetse fly Glossina morsitans morsitans, a species with adenotrophic viviparity. The tsetse fly yolk protein could be aligned with other dipteran yolk proteins and with some vertebrate lipases. In contrast to the situation in most fly species, only a single yolk protein gene was found in the tsetse fly. Northern blot analysis showed that only the ovarian follicle cells, and not the fat body represents the site of yolk protein synthesis.     

Morphological specializations of the yolk sac for yolk processing in embryonic corn snakes (Pantherophis guttatus: Colubridae)

Non‐avian reptiles commonly are assumed to be like birds in their overall patterns of development. However, colubrid corn snakes (Pantherophis guttatus ) have mechanisms of yolk cellularization and processing that are entirely different from the avian pattern. In birds, a vascular “yolk sac” surrounds and digests the liquid yolk. In contrast, in corn snakes, the yolk material is converted into vascularized cords of yolk‐filled cells. In this study, we used stereomicroscopy, histology, and scanning electron microscopy to analyze this unusual developmental pattern in corn snakes. Our observations reveal that the yolk sac cavity is invaded by endodermal cells that proliferate, absorb yolk spheres, and form aggregates of interconnected cells within the liquid yolk mass. As development proceeds, small blood vessels arise from the yolk sac omphalopleure, penetrate into the yolk mass, and become tightly encased in the endodermal cells. The entire vitellus ultimately becomes converted into a mass of vascularized, “spaghetti‐like” strands of yolk‐laden cells. The resulting arrangement allows yolk to be digested intracellularly and yolk products to be transported to the developing embryo. Indirect evidence for this pattern in other species raises the possibility that it is ancestral for squamates and quite possibly Reptilia in general.     

Abnormal behavior of the yolk centrosomes during early embryogenesis of Drosophila melanogaster

After the 10th nuclear cycle the yolk centrosomes follow an irregular pathway. Unlike the somatic centrosomes, which move to the opposite poles of the nuclei to form the bipolar spindles, the yolk centrosomes remain as pairs at one pole of the yolk nuclei or shift feebly and nucleate irregular spindles, most of which have only one main pole. The yolk centrosomes are no longer observed near the yolk nuclei, but progressively move away into the surrounding cytoplasm. Despite the irregular behavior of the centrosomes and although the yolk nuclei cease to divide, the yolk centrosome duplication cycle continues. The early development of Drosophila thus provides an excellent natural system for the study of the uncoupling of the nuclear and centrosomal cycles.     

Ultrastructural studies on the formation of yolk granules in the statoblast of a fresh-water bryozoan,Pectinatella gelatinosa

The formation of protein-carbohydrate yolk in the statoblast of a fresh-water bryozoan, Pectinatella gelatinosa, was studied by electron microscopy. Two types (I and II) of yolk cells were distinguished. The type I yolk cells are mononucleate and comprise a large majority of the yolk cells. The type II yolk cells are small in number; they become multinucleate by fusion of cells at an early stage of vitellogenesis. In both types of yolk cells, electron-dense granules (dense bodies) are formed in Golgi or condensing vacuoles, which are then called yolk granules. For the formation of yolk granules, the following processes are considered: 1. Yolk protein is synthesized in the rough-surfaced endoplasmic reticulum (RER) of the yolk cells. 2. The synthesized protein condenses in the cisternal space of the RER and is packaged into small oval swellings, which are then released from the RER as small vesicles (Golgi vesicles, 300-600 A in diameter). 3. The small vesicles fuse with one another to form condensing vacuoles, or with pre-existing growing yolk granules. 4. In the matrix of the condensing vacuoles or growing yolk granules, electron-dense fibers are fabricated and then arranged in a paracrystalline pattern to form the dense body. 5. After the dense body reaches its full size, excess membrane is removed and eventually the yolk granules come to mature. Toward the end of vitellogenesis of the yolk cells, the cytoplasmic organelles are ingested by autophagosomes derived from multivesicular bodies and disappear.     

Incorporation of 35S-sulfate into yolk platelets of Xenopus laevis embryos. A study using electron microscope autoradiography.

Inorganic 35S-sulfate was injected into Xenopus laevis embryos before first cleavage to study incorporation of the label into the yolk platelets in order to localize glycosaminoglycan synthesis. Electron microscope autoradiography of embryonic thin sections from blastulae and gastrulae revealed that the primary site of label incorporation is at the edge of the yolk platelets, and, to a lesser extent, in their interiors. Autoradiography of isolated yolk platelets, lacking unit membranes, indicated the absence of label. Thus, edge associated label comes from the yolk platelets membrane, and interior label is solubilized in the glycerol-water gradient during yolk platelets isolation. Ruthenium red staining of yolk platelet in situ shows haavy deposits of the dye on the yolk platelet membrane surface facing the cytoplasmic surface. The crystalline main body of isolated yolk platelets does not take up the dye. It appears that continuous synthesis or sulfation of glycosaminoglycan occurs primarily at the outer surface yolk platelet membranes during early development, providing a novel site for this process.     

Incorporation of 35S-Sulfate into Yolk Platelets of Xenopus laevis Embryos

Inorganic ³⁵S-sulfate was injected into Xenopus laevis embryos before first cleavage to study incorporation of the label into the yolk platelets in order to localize glycosaminoglycan synthesis. Electron microscope autoradiography of embryonic thin sections from blastulae and gastrulae revealed that the primary site of label incorporation is at the edge of the yolk platelets, and, to a lesser extent, in their interiors. Autoradiography of isolated yolk platelets, lacking unit membranes, indicated the absence of label. Thus, edge associated label comes from the yolk platelets membrane, and interior label is solubilized in the glycerol-water gradient during yolk platelets isolation.
Ruthenium red staining of yolk platelet in situ shows heavy deposits of the dye on the yolk platelet membrane surface facing the cytoplasmic surface. The crystalline main body of isolated yolk platelets does not take up the dye.
It appears that continuous synthesis or sulfation of glycosaminoglycan occurs primarily at the outer surface yolk platelet membranes during early development, providing a novel site for this process.     

Development of yolk sac and chorioallantoic membranes in the Lord Howe Island skink,Oligosoma lichenigerum

Development of the yolk sac of squamate reptiles (lizards and snakes) differs from other amniote lineages in the pattern of growth of extraembryonic mesoderm, which produces a cavity, the yolk cleft, within the yolk. The structure of the yolk cleft and the accompanying isolated yolk mass influence development of the allantois and chorioallantoic membrane. The yolk cleft of viviparous species of the Eugongylus group of scincid lizards is the foundation for an elaborate yolk sac placenta; development of the yolk cleft of oviparous species has not been studied. We used light microscopy to describe the yolk sac and chorioallantoic membrane in a developmental series of an oviparous member of this species group, Oligosoma lichenigerum. Topology of the extraembryonic membranes of late stage embryos differs from viviparous species as a result of differences in development of the yolk sac. The chorioallantoic membrane encircles the egg of O. lichenigerum but is confined to the embryonic hemisphere of the egg in viviparous species. Early development of the yolk cleft is similar for both modes of parity, but in contrast to viviparous species, the yolk cleft of O. lichenigerum is transformed into a tube‐like structure, which fills with cells. The yolk cleft originates as extraembryonic mesoderm is diverted from the periphery of the egg into the yolk sac cavity. As a result, a bilaminar omphalopleure persists over the abembryonic surface of the yolk. The bilaminar omphalopleure is ultimately displaced by intrusion of allantoic mesoderm between ectodermal and endodermal layers. The resulting chorioallantoic membrane has a similar structure but different developmental history to the chorioallantoic membrane of the embryonic hemisphere of the egg. J. Morphol. 2012. © 2012 Wiley Periodicals, Inc.     

Ultrastructural studies of male-incubated ovaries of the gypsy moth,Lymantria dispar

Ovaries from Lymantria dispar females were transplanted into an environment lacking the vitellogenin ligand; i.e., the male milieu. Transmission electron micrographs comparing the terminal oocytes of male-grown ovaries and normal ovaries showed that yolk sphere diameters were reduced in the male-grown oocytes. However, there were larger numbers of these small yolk spheres per unit area of cytoplasm, indicating that the coalescence of endosomes into yolk spheres is reduced in the absence of vitellogenin. Although there are larger numbers of yolk spheres in male-grown oocytes, the smaller diameter of yolk spheres resulted in less area being taken up by yolk spheres per unit area of cytoplasm in male-grown oocytes, yielding lowered yolk production. This lowered yolk production is a result at least in part of the lowered number of coated vesicles per unit area of submembrane space and in part of the reduced interfollicular spaces seen in male-grown ovaries.     

Cytochemistry of yolk elements in the Amphioxus egg

Summary The Amphioxus egg develops compound yolk and lipid yolk, besides the cortical vacuoles described previously. The compound yolk elements consisting of carbohydrates, proteins, lipoproteins, and triglycerides originate within the ooplasmic masses that are constituted by the yolk nucleus substance and mitochondria. The lipid yolk elements, which are poorly developed, stain for phospholipids; the exact mode of their formation could not be determined. The behaviour and cytochemistry of organelles (yolk nucleus and mitochondria) have also been described.