Yolk nucleus – The complex assemblage of cytoskeleton and ER is a site of lipid droplet formation in spider oocytes

Oocytes (future egg cells) of various animal groups often contain complex organelle assemblages (Balbiani bodies, yolk nuclei). The molecular composition and function of Balbiani bodies, such as those found in the oocytes of Xenopus laevis, have been recently recognized. In contrast, the functional significance of more complex and highly ordered yolk nuclei has not been elucidated to date. In this report we describe the structure, cytochemical content and evolution of the yolk nucleus in the oocytes of a common spider, Clubiona sp. We show that the yolk nucleus is a spherical, rather compact and persistent cytoplasmic accumulation of several different organelles. It consists predominantly of a highly elaborate cytoskeletal scaffold of condensed filamentous actin and a dense meshwork of intermediate-sized filaments. The yolk nucleus also comprises cisterns of endoplasmic reticulum, mitochondria, lipid droplets and other organelles. Nascent lipid droplets are regularly found in the cortical regions of the yolk nucleus in association with the endoplasmic reticulum. Single lipid droplets become surrounded by filamentous cages formed by intermediate filaments. Coexistence of the forming lipid droplets with the endoplasmic reticulum in the cortical zone of the yolk nucleus and their later investment by intermediate-sized filamentous cages suggest that the yolk nucleus is the birthplace of lipid droplets.     

An ultrastructural investigation of the early stages of oocyte differentiation in Actinia fragacea (Cnidaria; Anthozoa)

Individuals from a population of the intertidal sea anemone Actinia fragacea (Tugwell) were collected at approximately monthly intervals over an 18 month period. Samples of gonad were removed from each anemone and examined by light and electron microscopy. During late spring and early summer, large numbers of small cells were seen in the endoderm of the female gonads, lying close to the mesoglea. For convenience, these cells were classified into three types. Type I cells are 6–9 μm in diameter, with relatively very large nuclei, which may contain synaptinemal complexes, and scant cytoplasm containing few organelles. Type II cells are larger, reaching 15 μ m in diameter, with more abundant cytoplasm containing more organelles and inclusions. The nucleus is more dense, but may also contain synaptinemal complexes. Type III cells are less common. They are similar in size to Type II cells, but their nuclei contain irregular dense chromatin masses, and the nuclear envelope is incomplete or absent. The possible significance of the various cell types is discussed. It is suggested that Type I cells are oocytes at a very early stage of differentiation and that Type II cells are rather later oocytes. The status of the Type III cells is uncertain.     

Cytomorphology of female and male gonads in Antarctic toothfish <Emphasis Type="Italic">Dissosctichus mawsoni</Emphasis> (Nototheniidae) from the Ross Sea in the summer period

Results of histological analysis of gonads of female and male Antarctic toothfish Dissostichus mawsoni caught in the Ross Sea of the Pacific sector in the period of Antarctic summer (December–February) 2004–2005 are presented. Morphological indices and index of gonad maturity are described and ecological criteria of assessment of maturity stages of ovaries, cytological indices of oocytes, and the type of toothfish oogenesis are determined. It was established that in the period of fishing Antarctic toothfish, females and males with gonads at maturity stage III dominate. Ovaries contain two groups of oocytes of the period of trophoplasmatic growth and large oocytes of the nearest spawning season that comprise the smallest proportion of total sex cells. In the testes of two male toothfish, the primary fusion of renal and generative tissues was revealed. It is suggested that the termination of gonad maturity of toothfish takes place from March to April, and spawning takes place from June to August.     

The gonads and sexual cycle of the polystyelid ascidian Distomus variolosus gaertner

Distomus variolosus from Roscoff, France, comprises two sorts, differing in their branchial and gonadal patterns. Their sexual cycle has been followed histologically and micro-anatomically. Gonads begin as clumps of lymphocytes that persist along germinal tracts. Cavitation of the clump, growth of the gonoduct, maturation of the gametes, and elaboration of accessory structures are described. Oocytes develop in a linear series in each ovary; only one or two reach maturity in each gonad. The released egg and subsequent tadpole may be held to the ovary by a “leash” formed of the partially everted outer follicle of the egg. Post-mature gonads deteriorate. Testes disrupt altogether; ovaries may persist as moribund loci of remnant germinal tissue. The sharp right-left hermaphroditism of the zooid appears to combine with a subtler anterior-posterior gradient of sexual determination.     

Yolk formation in an annelid (Ophryotrocha puerilis, polychaeta)

The polychaete Ophryotrocha does not show a distinct breeding season. Egg masses are produced throughout the year (continuous breeder sensu Olive and Clark, 1978). A female specimen may contain up to three different generations of oocytes with oocyte growth and maturation in each batch being well synchronized. Oogenesis takes about 18 days from proliferation of the oogonia to mature eggs. In each segment pairs of sister cells interconnected by cytoplasmic bridges are located in outpocketings of the ventral mesentery which form the gonad wall. Presumptive oocytes and nurse cells are not easily distinguished at that time. Vitellogenesis is initiated while both oocytes and nurse cells are still in the ovary. Mitochondria, multivesicular bodies (transformed mitochondria ?), dense bodies, preformed yolk bodies of smaller size and lipid droplets are probably passed through the cytoplasmic bridge from the nurse cell to the oocyte. Yolk formation includes different mechanisms and materials of different origin. Autosynthetic yolk formation predominates during the first intraovarial growth phase. After detachment of oocyte-nurse cell-complexes from the gonad pinocytotic activity of nurse cells and particularly oocytes, increases considerably. The existence of coated vesicles suggests that external sources of yolk precursors contribute to yolk formation. Prior to oocyte maturation the remnants of the nurse cell are incorporated by oocytes.     

Gonad development and evidence of protogyny in the red-throat emperor on the Great Barrier Reef

The gonad development in the red-throat emperor Lethrinus miniatus is described and the first detailed evidence for protogyny in this species provided. The identification of transitional individuals, bimodal sex-specific size-frequency distributions and female biased sex ratios suggest that L. miniatus is most likely a protogynous hermaphrodite. Transitional L. miniatus gonads were characterized by the concurrent degeneration of all oocytes and the proliferation of spermatocysts near the edge of the lamellae, an increase in blood vessels along strands of stromal tissue within the lamellae and the formation of multiple sperm sinuses. The sites of oocyte degeneration and proliferation of spermatocysts were spatially segregated. An increase in blood vessels along strands of stromal tissue within the lamellae of transitional phase gonads is likely to assist in the breakdown of oocytes and the proliferation of spermatocysts. Most mature resting females containing spermatocysts occurred within the transitional size-frequency distribution, suggesting that the presence of spermatocysts in these females may be an early sign of sex change. Oocytes within female gonads were interrupted by filamentous strands of stromal tissue within the lamellae. The testis contained a remanent ovarian lumen but no residual oocytes. Three characteristics of transitional L. miniatus gonads were found to be unusual and described for few other species of coral reef fishes. These included the absence of oocytes within testes, increased numbers of blood vessels, and the presence of strands of stromal tissue within the lamellae.     

Determination of the nucleoside triphosphate contents of eggs and oocytes of Xenopus laevis

The ribonucleotide and deoxyribonucleotide contents of eggs and oocytes of Xenopus laevis were measured. Eggs contained most deoxyribonucleotide in the form of triphosphates. dCTP, dTTP, dATP and dGTP were present in similar amounts. The egg contained sufficient deoxynucleotide triphosphate to make approximately 2500 nuclei. Oocytes contained less pyrimidine deoxyribonucleoside triphosphates than did eggs, and purine deoxyribonucleoside triphosphates were not detected. These differences may be correlated with the ability of eggs to induce nuclear DNA synthesis, a property not shown by oocytes. Both oocytes and eggs seem to contain non-phosphorylated, alpha-unsubstituted aldehydes, which may be deoxyribose derivatives. Eggs and oocytes contain similar amounts of ribonucleoside triphosphates. The low rate of RNA synthesis found in eggs, but not in oocytes, is therefore not caused by simple precursor control.     

Isolation and identification of germ cells from fetal bovine ovaries

Gonadal cell suspensions were made from bovine fetuses of 35–55-, 56–80-, and 80–130-day age groups corresponding to the periods predominated by primordial germ cells (PGCs), oogonia, and meiotic cells, respectively. Germ cells identified on morphological criteria prior to their isolation from suspensions were compared histochemically and morphologically with cells in cryosections, impression smears, and semithin sections of similar gonads. Oocytes were distinguished by their chromosomal configurations in cell spreads. In suspensions from 35–55-day fetuses, cells considered to be PGCs stood out by their size, large nucleus, intracytoplasmic vesicles, and occasional blebbing. The somatic cells were smaller and contained little cytoplasm and few vesicles. In bovine gonads, in contrast to murine gonads, alkaline phosphatase (AP) activity was not specific enough to identify germ cells once they had entered the gonad. In ovaries from the 56–80-day age group, cells similar to PGCs, but slightly larger and with more cytoplasmic vesicles, were identified as oogonia. The cytoplasmic vesicles stained positively for lipid. In ovaries of 80–130-day fetuses, oogonia, oocytes, degenerating germ cells, and multinucleate germ cells were recognized. Degenerating germ cells exhibited a variety of morphological characteristics and were consistently positive for acid-phosphatase activity. Binucleate germ cells appeared around day 85 of gestation, while multinucleate germ cells were seen from day 95. It was concluded that bovine mitotic germ cells can be isolated from gonadal cell suspensions and that the best time to recover them is between 50 and 70 days of gestation. © 1994 Wiley-Liss, Inc.     

Development and early differentiation of gonad in the european eel (Anguilla anguilla [L.], Anguilliformes,Teleostei): A cytological and ultrastructural study

The structure of the gonad of the European eel (Anguilla anguilla [L.]), an “undifferentiated” gonochoristic teleost, was investigated by transmission electron microscopy from 6–8 cm elvers to 22 cm yellow eels with juvenile hermaphroditic gonads. The pear-shaped gonads of 6–8 cm elvers assume, in 12–15 cm eels, a lamellar shape and enlarge by migration of germ cells, which we refer to as primary primordial germ cells. In the gonads of ∼ 16 cm eels, the primary primordial germ cells multiply, giving rise to clusters of germ cells that have ultrastructural characteristics of the primary primordial germ cells but show giant mitochondria, enlarged Golgi complexes, and round bodies not limited by membranes. We refer to these as secondary primordial germ cells. In 16–18 cm eels, syncytial clones of oogonia interconnected by cytoplasmic bridges are also observed. In 18–22-cm-long eels, the gonads contain primordial germ cells, oogonial clones, early oocyte cysts, single oocytes in early growth stages, and primary spermatogonia. Such germ cells are present in the same cross section where they are either intermingled or are in areas of predominantly female germ cells close to areas with predominantly male germ cells. These gonads are juvenile hermaphroditic and should be considered ambisexual because in larger eels they differentiate either into an ovary or into a testis. Somatic cells always envelop the germ cells following their migration into the gonad. These somatic cells first show similar ultrastructural features and then differentiate either into early Sertoli cells investing spermatogonia, or into early follicular (granulosa) cells investing the early previtellogenic oocytes. In eels ∼ 14 cm long, primitive steroid-producing cells also migrate into the gonad. In the ambisexual gonad they differentiate either into immature Leydig cells in the male areas, or into early special cells of the theca in the female areas. Nerve fibers are joined to the steroid-producing cells. Gonad development and differentiation are also associated with structural changes of the connective tissue characterized by the progressive appearance and deposition of collagen fibrils first in the mesogonadium, then in the gonad vascular region, and then in the germinal region. The collagen-rich areas are massive in the male areas and reduced in the female ones. J. Morphol. 231:195–216, 1997. © 1997 Wiley-Liss, Inc.     

Post-embryonic development of the two-spotted spider mite Tetranychus urticae with special reference to the differentiation of the female reproductive system

Summary

The spider mite, Tetranychus urticae passes through four active stages in its life cycle: larva, protonymph, deutonymph, adult. The duration at 23°C and 40–60% relative humidity is 10–12 days. Each of the movable stages is followed by a resting phase (= chrysalis) where moulting processes are prepared. The general internal anatomy of the developmental stages does not differ from that of the adults. The gonadial rudiment of the female larva is an undifferentiated cell mass situated between the nervous system and the anus. In the nymphochrysalis, development of oocytes starts as indicated by synaptonemal complexes occuring in cells which are interconnected via cytoplasmic bridges. Cells exhibiting extranuclear material appear during the protonymphal stage. The presumptive uterus and vagina are formed during the deutochrysalis stage and consist of an undifferentiated tube, the distal portion of which is lined by a fine cuticle. The ovary of the deutonymph contains different cell types. Oogonia and growing oocytes are found in the cranial germ region. Oocytes, nurse cells and cells with large lobed nuclei can be observed in the caudal previtellogenic region. Oocytes protrude through the ovarian surface and invade ovarian pouches covered only by the basement membrane whereby each is connected to a tri-nucleate nurse cell via a cytoplasmic bridge. Oocytes increase in size but do not form yolk droplets. The uterus and vagina are differentiated during the late deutonymphal stage and copulation may take place as indicated by the presence of sperm in the lumen of the seminal receptacle.

The results are compared to previously published information on the female reproductive system of T. urticae and discussed with reference to co-operation of ovarian cell types and their origin.     

Reproduction and sexual development in a freshwater gastrotrich

Spermatogenesis and spermiogenesis in Lepidodermella squammata are confined to the postparthenogenic phase of the life cycle and coincide with developmental changes in the bilateral female gonads. Male stages are bilateral but asynchronous, in the lateral abdomen anterior to the female gonads. Maximum observed sperm production is two packets per side, or 64 sperm. Sperm formation occurs more rapidly at 27 degrees C than at 20 degrees C (p less than 0.001), requiring as little as 1 day. Two spermatogonial mitotic divisions produce a clone of four primary spermatocytes connected by bridges (stage 1). Centrioles are absent. Development occurs within a cyst. Meiotic divisions produce 16 spermatids (stage 2), each containing a dense, elongate, tapered nucleus. Cytoplasmic membranes enclose one end of the nuclear rod, excluding all other organelles. Completion of this process results in stage 3, a packet of 16 sperm associated with one dense sphere, a modified 'residual body' containing cytoplasmic debris. The residual body then disappears, leaving the sperm packet of stage 4. Each mature sperm is a dense nuclear rod with surrounding membranes, lacking acrosome, mitochondrion, centrioles, and flagellum. Function of sperm has not been demonstrated. The spermatozoa are of a reduced type not previously described.     

An Ultrastructural study of oocyte growth within the endoderm and entry into the mesoglea in Actinia fragacea (Cnidaria, Anthozoa)

Sea anemone gametes arise in the endoderm but migrate into the mesoglea at an early stage. In order to observe this process, large individuals of Actinia fragacea were collected from the same intertidal location at regular intervals over a 2-year period, and their gonads were examined by light and electron microscopy. The cellular origin of the oocytes is unclear, but the smallest recognizable oocytes are rounded cells, 6-8 microns in diameter, with relatively large nuclei which may contain synaptinemal complexes. Their cytoplasm contains numerous ribosomes, a flagellar basal-body-rootlet complex, and distinctive dense structures also present in male germ cells but not found in anemone nongerminal cells. During the endodermal phase of growth, the density of the oocyte nucleus increases, a single nucleolus becomes prominent, and mitochondria and glycogen accumulate in the cytoplasm. Most oocytes, but not all, only begin major vitellogenesis after entry into the mesoglea. Most oocytes enter the mesoglea vitellogenesis after entry into the mesoglea. Most oocytes enter the mesoglea before they attain a diameter of 25 microns. The oocytes migrate toward and enter the mesoglea by a process resembling amoeboid movement. During entry, the oocytes are constricted into a characteristic "hourglass" shape and become covered by a basal lamina continuous with that of the gonad epithelium. The last part of the oocyte to enter the mesoglea forms an intimate relationship with the surrounding endodermal cells, which is maintained after entry is complete, and is thought to be important in the establishment of the trophonema.     

Formation and ultrastructural organization of gonads in Oikopleura gracilis Lohmann, 1896 (Urochordata: Appendicularia)

This study deals with the formation and ultrastructural organization of the gonads in a common species of appendicularian, Oikopleura gracilis, from Peter the Great Bay. Light microscopy observations show that the gonads develop from a transparent primordium that is located in the basolateral part of the gonad cavity; the primordium increases in size in the process of development and differentiates into the testis and ovary. The testis is covered by a single layer of ultrastructurally uniform follicular epithelium and contains a population of proliferating male gonocytes. The ovary contains two types of germ line nuclei, which are large polyploid nuclei that belong to the auxiliary cells and small meiotic nuclei of the oocytes. The two nuclei types, together with a common cytoplasm, form a syncytium of the ovary, or the coenocyst. As in the dioecious Oikopleura dioica, the coenocyst of O. gracilis produces naked oocytes that are devoid of a type III follicular membrane. The coenocyst is covered by a single-layered follicular epithelium, in which two cell types can be distinguished ultrastructurally. Thus, the synchronous maturation of sex products in O. gracilis is achieved by the formation of the germ-line syncytium in the testis and the coenocyst in the ovary, which generates a large number of simultaneously ripening oocytes that are competent for fertilization.     

Development of the reproductive system of Caenorhabditis elegans

A morphological study of the growth and the development of the reproductive system of the nematode Caenorhabditis elegans has been carried out. When the first stage larva hatches from the egg it contains four primordial gonadial cells. These cells proliferate and form the entire adult reproductive system, consisting of approximately 2500 nuclei, in 45 hr at 25°C. Several distinctive morphological featues of gonadogenesis and early embryogenesis that are recognizable in the compound microscope can be used to chart the development of the nematode. The mature gonad presents a linear developmental axis both temporally and morphologically of the formation of oocytes, fertilization, and the early stages of embryogenesis. The structure of the adult ovary indicates that the cytoplasm of each newly formed oocyte is derived from a common core of cytoplasm within the multinuclear ovary.     

The structure of symplasmic early oocytes and their enveloping sheath cells in the polychaete,Platynereis dumerilii

1. Early oocytes of Platynereis dumerilii are found in clusters floating in the coelom. The oocytes of a cluster form a syncytium which is enveloped by several sheath cells. 2. At stage 2, only a single sheath cell per cluster remains, and it penetrates the group of rounded oocytes, enveloping each one of them. At stage 4, this cell contains a reticular basket made up of bundles of filaments and is inferred to be degenerating, from the presence of vacuoles, clumps of pigment-like material, and atypical mitochondria. 3. Synaptonemal complexes are typical of the nuclei of stage 2 oocytes. Oocytes of stage 4 (early vitellogenesis) contain stacks of endoplasmic reticulum in a distinctive arrangement, with interspersed electron-dense masses. Similar masses accumulate in the cytoplasm close to the nucleus and adjacent to the nuclear pores. 4. From the present observations, a physical supporting rather than a nutritive function is attributed to the sheath cell, which ensures cohesion among the oocytes connected with each other throughout the cluster phase by cytoplasmic bridges. This finding is discussed with respect to conclusions drawn from oocyte transplantation experiments.     

Ultrastructural and cytochemical aspects of the germarium and the vitellarium in Syndesmis patagonica (Platyhelminthes,Rhabdocoela, Umagillidae)

The cytoarchitecture of the female gonad of the endosymbiont umagillid Syndesmis patagonica has been investigated using electron microscopy and cytochemical techniques. The female gonad consists of paired germaria and vitellaria located behind the pharynx in the mid‐posterior region of the body. Both the germaria and the vitellaria are enveloped by an outer extracellular lamina and an inner sheath of accessory cells which contribute to the extracellular lamina. Oocyte maturation occurs completely during the prophase of the first meiotic division. Oocyte differentiation is characterized by the appearance of chromatoid bodies and the development of endoplasmic reticulum and Golgi complexes. These organelles appear to be involved in the production of round granules, about 2–2.5 μm in diameter, with a homogeneous electron‐dense core surrounded by a granular component and a translucent halo delimited by a membrane. These egg granules migrate to the periphery of mature oocytes, are positive to the cytochemical test for polyphenol detection, are unaffected by protease and have been interpreted as eggshell granules. The mature oocytes also contain a small number of yolk granules, lipid droplets, and glycogen particles scattered throughout the ooplasm. The vitellaria are branched organs composed of vitelline follicles with vitellocytes at different stages of maturation. Developing vitellocytes contain well‐developed rough endoplasmic reticulum and small Golgi complexes involved in the production of eggshell and yolk globules. Eggshell globules are round, measure 4–5 μm in diameter, and have a mosaic‐like patterned content which contains polyphenols. The yolk globules, 2–3 μm in diameter, show a homogeneous protein content of medium electron density, devoid of polyphenols, and completely digested by protease. The mature vitellocytes also contain glycogen as further reserve material. The presence of polyphenolic eggshell granules in the oocytes and of polyphenolic eggshell globules with a mosaic‐like pattern in the vitellocytes have been considered apomorphic features of the Rhabdocoela + Prolecithophora. J. Morphol. 275:703–719, 2014. © 2014 Wiley Periodicals, Inc.     

Intersex,a temperature-sensitive mutant of the nematode Caenorhabditis elegans

A temperature-sensitive mutation, isx-1(hc17), is reported in the nematode Caenorhabditis elegans which alters the sexual phenotypes of both genotypic sexes. At the restrictive temperature, XX animals are functionally female rather than hermaphroditic due to the absence of spermatogenesis, and XO animals develop as intersexes. These intersexes have normal male head and tail structures and exhibit some mating behavior, but possess hermaphrodite-like gonads which produce no sperm and usually contain a few oocytes. An abortive vulva is usually present and evidence is presented which suggests that the formation of the vulva by the hypodermis is induced by the underlying gonad. The direct effects of the mutation are confined to the descendants of four primordial gonad cells. Gametogenesis and gonad sheath development do not seem to be tightly coupled and are shown to differ in their responses to X-chromosome dosage. The interaction of the intersex mutation with mutant alleles of two transformer genes tra-1 and tra-2 is discussed and a model for the action of these genes in gonad development and sex determination is proposed.     

Pattern formation fails after blastoderm formation by rapid cell cycles in an artificially activated insect egg

Oocytes explanted from adult ovaries of the arrhenotokous Hymenopteron Pimpla turionellae remain in an inactive state, because development has not been initiated by mechanical deformation during natural oviposition. However, they could be induced to enter development by injecting cleavage energids into the posterior pole. After lag phases of up to 32 h, the implanted nuclei initiated a normal cleavage process, except that the polarity of its progress was reversed. In other oocytes, the injected energids congregated in a ring-shaped region at the egg surface to form a superficial nuclear front, which slowly advanced towards the anterior egg pole, thereby successively stimulating portions of the quiescent ooplasm to take part in development. Up to 41 rapid cell cycles started from that front, each of them with an anaphase wave running backwards into the region already peripherally occupied by nuclei. Thus, the blastoderm was formed extremely metachronously and by rapid obviously biphasic cell cycles, which never occur at the egg surface during normal cleavage. A germ band, however, was only formed under the following conditions: (1) that cleavage did not follow the nuclear front mode, and (2) that ooplasm from the donor's posterior pole was co-injected with the graft nuclei. We conclude that embryonic differentiation requires some of the events which had been omitted in eggs where development failed, especially the exponential increase of the cell cycle length, and the activity of some posterior factor(s) during egg activation.     

The Nutrimentary Egg Development of the Mite,Varroa jacobsoni (Acari,Arachnida), an Ectoparasite of Honey Bees

The fine structure of the female gonad of Varroa jacobsoni is described. There are two components: the ovary proper and the so-called lyrate organ. The ovary is the place where oocytes mature, embedded in a supporting tissue composed of two cell types: somacells 1 and somacells 2. The lyrate organ has a nutrimentary function and is comprised of two components: supporting cells and nutritive tissue. The supporting cells are similar to the somacells 2 in that they contain abundant microtubules. The nutritive tissue is an extensive syncytium. It is connected with the oocytes via intercellular bridges, the nutritive cords. Oocytes and nutritive tissue are thought to have derived from common stem cells. From fine structural evidence it is concluded that ribosomes are one of the most important components to be transported via the nutritive cords into the oocytes. However, an increase in number of mitochondria in the middle-stage oocytes may also be a consequence of transport of these organelles from the nutritive tissue into the oocytes. Further characteristics make plausible that the interdependences of oocytes and nutritive tissue are comparable to those found in meroistic ovarioles of insects. The somatic components do not seem to be as important as the follicle cells of insects, however. It is assumed that the evolution of a nutrimentary oogenesis speeds up embryogenesis. Thus, the differentiation of the female gonad of Varroa jacobsoni may have facilitated the species' adaptation to a development completed in a short and limited time within the shelter of the covered brood cell of the bee.