Intercellular bridges between oocytes in the chicken ovary

Summary Fine structural analysis of the functional (left) ovary of the newly-hatched chick reveals the presence of true intercellular bridges between developing oocytes in the early stages of the meiotic prophase. These structures are characterized by: 1) cytoplasmic continuity between the participating oocytes, 2) a dense, fibrillar material beneath the lateral limiting membrane and 3) numerous cellular organelles within their confines. In addition, microtubular elements, parallel to the long axis of the bridge, are routinely observed. This latter finding suggests that intercellular bridges originate through incomplete cytokinesis of mitotically active oogonia and that the dense material beneath the limiting membrane may represent the cortical microfilaments associated with the contractile ring. Functionally, these structures may serve as channels for transfer of nutrients and organelles between oocytes although the possibility that certain oocytes function as nurse cells, in the sense that these cells exist in invertebrate ovaries, seems unlikely. In addition, intercellular bridges may be responsible for both restriction of oogonial mitoses and meiotic synchrony.Partial support for this study was provided by grant DE-00241 from the National Institute for Dental Research administered by Dr. Melvin Hess. We gratefully acknowledge his support of the initial aspects of this study. We are pleased to express our appreciation to Mrs. Cindy G. Wilcox, Mr. Lloyd Thibodeau and Mr. Don Driscoll for their expert technical assistance.     

Intercellular bridges in ovaries of the newborn gerbil

This study describes intercellular bridges in the ovaries of neonatal gerbils. Electron microscopy has revealed the presence of true intercellular bridges, connecting oogonia or oocytes, in ovaries of newborn gerbils. The cytoplasm of the intercellular channels is similar to that of the connected cells, with mitochondria, smooth and rough endoplasmic reticulum, and free ribosomes present. Lysosomes are also occasionally present in the intercellular bridges and they may be involved in early waves of oocyte atresia. An electron-dense substance, 350-500 A thick, is located immediately beneath the unit membrane of the intercellular bridges. Accumulation of electron-dense material increases the thickness of the walls of the intercellular bridges, supporting and maintaining the patency of the channels. It is suggested that the intercellular channels probably allow the interchange of nutrients, organelles, and possibly regulatory materials as well.     

Intercellular connections between germ cells in the developing human ovary

Summary An electron microscopic examination of human fetal ovaries reveals the presence of intercellular bridges between developing germ cells. The bridges are characterized by a band of electron-dense material beneath the lateral limiting membrane, and cell organelles are seen within the confines of these connections. Their general morphology is similar to that described in ovaries of other species. The possible functional significance of these connections is discussed.This work was supported by grants from the Edward G. Schlieder Educational Foundation, New Orleans, Louisiana State University and HD-03288 from the National Institute of Child Health and Human Development.The authors would like to thank Miss Cathy Chase for her technical assistance.     

Ovarian nests in cultured females of the Siberian sturgeon Acipenser baerii (Chondrostei,Acipenseriformes)

Ovaries of Acipenser baerii are of an alimentary type and probably are meroistic. They contain ovarian nests, individual follicles, inner germinal ovarian epithelium, and fat tissue. Nests comprise cystoblasts, germline cysts, numerous early previtellogenic oocytes, and somatic cells. Cysts are composed of cystocytes, which are connected by intercellular bridges and are in the pachytene stage of the first meiotic prophase. They contain bivalents, finely granular, medium electron dense material, and nucleoli in the nucleoplasm. Many cystocytes degenerate. Oocytes differ in size and structure. Most oocytes are in the pachytene and early diplotene stages and are referred to as the PACH oocytes. Oocytes in more advanced diplotene stage are referred to as the DIP oocytes. Nuclei in the PACH oocytes contain bivalents and irregularly shaped accumulation of DNA (DNA‐body), most probably corresponding to the rDNA‐body. The DNA‐body is composed of loose, fine granular material, and comprises multiple nucleoli. At peripheries, it is fragmented into blocks that remain in contact with the inner nuclear membrane. In the ooplasm, there is the rough endoplasmic reticulum, Golgi complexes, free ribosomes, complexes of mitochondria with cement, fine fibrillar material containing granules, and lipid droplets. The organelles and material of nuclear origin form a distinct accumulation (a granular ooplasm) in the vicinity of the nucleus. Some of the PACH oocytes are surrounded by flat somatic cells. There are lampbrush chromosomes and multiple nucleoli present (early diplotene stage) in the nucleoplasm. These PACH oocytes and neighboring somatic cells have initiated the formation of ovarian follicles. The remaining PACH oocytes transform to the DIP oocytes. The DIP oocytes contain lampbrush chromosomes and a DNA‐body is absent in nuclei. Multiple nucleoli are numerous in the nucleoplasm and granular ooplasm is present at the vegetal region of the oocyte.     

Oogenesis in the leech Glossiphonia heteroclita (Annelida, Hirudinea, Glossiphonidae). I. Ovary structure and previtellogenic growth of oocytes

Glossiphonia heteroclita has paired ovaries whose shape and dimensions change as oogenesis proceeds: during early previtellogenesis they are small and club-shaped, whereas during vitellogenesis they broaden and elongate considerably. During early oogenesis (previtellogenesis), each ovary is composed of an outer envelope (ovisac) that surrounds the ovary cavity and is filled with hemocoelomic fluid, in which a single and very convoluted ovary cord is bathed. The ovary cord consists of germline cells, including nurse cells and young oocytes surrounded by a layer of elongated follicle cells. Additionally, follicle cells with long cytoplasmic projections occur inside the ovary cord, where they separate germ cells from each other. The ovary cord contains thousands of nurse cells. Each nurse cell has one intercellular bridge, connecting it to a central anucleate cytoplasmic mass, the cytophore (rachis); it in turn is connected by one intercellular bridge with each growing oocyte. Numerous mitochondria, RER cisternae, ribosomes, and Golgi complexes are transported from the nurse cells, via the intercellular bridge and cytophore, to the growing oocytes. Oogenesis in G. heteroclita is synchronous with all oocytes in the ovary in the same stage of oogenesis. The youngest observed oocytes are slightly larger than nurse cells, and usually occupy the periphery of the ovary cord. As previtellogenesis proceeds, the oocytes gather a vast amount of cell organelles and become more voluminous. As a result, in late previtellogenesis the oocytes gradually protrude into the ovary cavity. Simultaneously with oocyte growth, the follicle cells differentiate into two subpopulations. The morphology of the follicle cells surrounding the nurse cells and penetrating the ovary cord does not change, whereas those enveloping the growing oocytes become more voluminous. Their plasma membrane invaginates deeply, forming numerous broad vesicles that eventually seem to form channels or conducts through which the hemocoelomic fluid can easily access the growing oocytes.     

Oocyte Polarization Is Coupled to the Chromosomal Bouquet,a Conserved Polarized Nuclear Configuration in Meiosis

The source of symmetry breaking in vertebrate oocytes is unknown. Animal—vegetal oocyte polarity is established by the Balbiani body (Bb), a conserved structure found in all animals examined that contains an aggregate of specific mRNAs, proteins, and organelles. The Bb specifies the oocyte vegetal pole, which is key to forming the embryonic body axes as well as the germline in most vertebrates. How Bb formation is regulated and how its asymmetric position is established are unknown. Using quantitative image analysis, we trace oocyte symmetry breaking in zebrafish to a nuclear asymmetry at the onset of meiosis called the chromosomal bouquet. The bouquet is a universal feature of meiosis where all telomeres cluster to one pole on the nuclear envelope, facilitating chromosomal pairing and meiotic recombination. We show that Bb precursor components first localize with the centrosome to the cytoplasm adjacent to the telomere cluster of the bouquet. They then aggregate around the centrosome in a specialized nuclear cleft that we identified, assembling the early Bb. We show that the bouquet nuclear events and the cytoplasmic Bb precursor localization are mechanistically coordinated by microtubules. Thus the animal—vegetal axis of the oocyte is aligned to the nuclear axis of the bouquet. We further show that the symmetry breaking events lay upstream to the only known regulator of Bb formation, the Bucky ball protein. Our findings link two universal features of oogenesis, the Bb and the chromosomal bouquet, to oocyte polarization. We propose that a meiotic—vegetal center couples meiosis and oocyte patterning. Our findings reveal a novel mode of cellular polarization in meiotic cells whereby cellular and nuclear polarity are aligned. We further reveal that in zygotene nests, intercellular cytoplasmic bridges remain between oocytes and that the position of the cytoplasmic bridge coincides with the location of the centrosome meiotic—vegetal organizing center. These results suggest that centrosome positioning is set by the last mitotic oogonial division plane. Thus, oocytes are polarized in two steps: first, mitotic divisions preset the centrosome with no obvious polarization yet, then the meiotic—vegetal center forms at zygotene bouquet stages, when symmetry is, in effect, broken.     

Cytoplasmic reorganization during the resumption of meiosis in cultured preovulatory rat oocytes

Changes in organelle topography and microtubule configuration have been studied during the resumption and progression of meiosis in cultured preovulatory rat oocytes. Germinal vesicle breakdown (GVBD) was reversibly inhibited by dibutyryl cAMP (DcAMP) or nocodazole, a microtubule-disrupting agent. The microtubule stabilizing agent taxol did not inhibit GVBD, but did impair further maturation. The migration of acidic organelles and chromatin in living oocytes was analyzed using the vital stains acridine orange and Hoechst 33258, respectively. Germinal vesicle stage oocytes undergo perinuclear aggregation of acidic organelles during GVBD and these organelles subsequently disperse into the cell cortex as the first meiotic spindle migrates to the oocyte periphery. DcAMP and nocodazole block the perinuclear aggregation of acidic organelles, whereas, in taxol-treated oocytes, organelle aggregation and GVBD occur but the dispersion of acidic organelles was arrested. Dose-response studies on the effects of nocodazole showed that GVBD was generally retarded and that a 50% inhibition of GVBD was achieved at concentrations in excess of 1.0 microM. Concentrations of taxol at 10 microM or above effectively inhibited both chromatin condensation and meiotic spindle formation. Indirect immunofluorescence microscopy with anti-tubulin antibodies revealed dissolution of microtubules with 1.0 microM nocodazole. Taxol had little effect on microtubule organization in germinal vesicle or chromatin condensation stage oocytes; however, when oocytes that had formed first meiotic spindles were treated with taxol, numerous microtubule asters appeared which were preferentially associated with the oocyte cortex. The changes in organelle topography, microtubule configuration, and drug sensitivity are discussed with respect to the regulation of cytoplasmic reorganization during the meiotic maturation of rat preovulatory oocytes.     

Early oogenesis in the short-tailed fruit bat Carollia perspicillata: transient germ cell cysts and noncanonical intercellular bridges

The ovaries of early embryos (40 days post coitum/p.c.) of the bat Carollia perspicillata contain numerous germ-line cysts, which are composed of 10 to 12 sister germ cells (cystocytes). Variability in the number of cystocytes within the cyst and between the cysts (defying the Giardina rule) indicates that the mitotic divisions of the cystoblast are asynchronous in this bat species. Serial section analysis showed that the cystocytes are interconnected via intercellular bridges that are atypical, strongly elongated, short-lived, and rich in microtubule bundles and microfilaments. During slightly later stages of embryonic development (44-46 days p.c.), somatic cells penetrate the cyst, and their cytoplasmic projections separate individual oocytes. Separated oocytes surrounded by a single layer of somatic cells constitute the primordial ovarian follicles. The oocytes of C. perspicillata are similar to mouse oocytes and are asymmetric. In both species, this asymmetry is clearly recognizable in the localization of the Golgi complexes. The presence of germ-line cysts and intercellular bridges (although noncanonical) in the fetal ovaries of C. perspicillata suggest that the formation of germ-line cysts is an evolutionarily conserved phase in the development of the female gametes in a substantial part of the animal kingdom.     

Stable intercellular bridges in development: the cytoskeleton lining the tunnel

A wide variety of intercellular junctions that are involved with cell adhesion or signal transduction have been described in recent years. A widespread but less well-characterized type of intercellular junction is the stable intercellular bridge. Several organisms use stable intercellular bridges as cytoplasmic connections, probably to allow rapid transfer of information and organelles between cells. Here, the authors take a detailed look at the assembly of intercellular bridges called ring canals in the Drosophila germline and discuss how examination of mutants that disrupt Drosophila ovarian ring canal assembly indicates that these bridges are required for intercellular transport of cytoplasm.     

An electron microscope study on the cytoplasmic and nuclear components of rat primary oocytes

Summary This paper reports on the structure of rat primary oocytes, as observed with the electron microscope. Four main components are described in the cytoplasm: Golgi apparatus, centrioles, mitochondria and multivesicular bodies.The components of the Golgi apparatus are forming a single mass confined to a limited region of the cytoplasm and the centrioles were found located in a clear zone sited in the middle of this mass. Mitochondria are scattered at random in the cytoplasm. Multivesicular bodies are elements integrated by an enveloping membrane containing a varied number of tiny vesicles. They are generally found associated with a short number of small free vesicles. Only one two groups of this kind are found per oocyte. This contrast with what has been observed previously in full-grown rat oocytes, where the groups are numerous and constituted by many units.Two components were described for the oocyte nucleus: nucleoli and chromosomes. Nucleoli are constituted by a tangled thread whose elemental component is a fine fibrous material of high electron density.At the age studied on this paper, primary oocytes are undergoing meiotic prophase, chromosomes have at this time the same components observed by different authors in primary spermatocytes. These are two thick ribbon-like threads helically twisted around a thinner medial filament. Each tripartite group is attached by one end to the nuclear membrane. It was actually seen tripartite groups incompletely organized; the images recorded of such groups suggest that the medial filament is the first to appear in the nucleoplasm. The possible significance of these filaments in respect to the meiotic phase called chromosome pairing is discussed.     

Intercellular organelle traffic through cytoplasmic bridges in early spermatids of the rat: mechanisms of haploid gene product sharing

Stable cytoplasmic bridges (or ring canals) connecting the clone of spermatids are assumed to facilitate the sharing of haploid gene products and synchronous development of the cells. We have visualized these cytoplasmic bridges under phase-contrast optics and recorded the sharing of cytoplasmic material between the spermatids by a digital time-lapse imaging system ex vivo. A multitude of small (ca. 0.5 microm) granules were seen to move continuously over the bridges, but only 28% of those entering the bridge were actually transported into other cell. The average speed of the granules decreased significantly during the passage. Immunocytochemistry revealed that some of the shared granules contained haploid cell-specific gene product TRA54. We also demonstrate the novel function for the Golgi complex in acrosome system formation by showing that TRA54 is processed in Golgi complex and is transported into acrosome system of neighboring spermatid. In addition, we propose an intercellular transport function for the male germ cell-specific organelle chromatoid body. This mRNA containing organelle, ca. 1.8 microm in diameter, was demonstrated to go over the cytoplasmic bridge from one spermatid to another. Microtubule inhibitors prevented all organelle movements through the bridges and caused a disintegration of the chromatoid body. This is the first direct demonstration of an organelle traffic through cytoplasmic bridges in mammalian spermatogenesis. Golgi-derived haploid gene products are shared between spermatids, and an active involvement of the chromatoid body in intercellular material transport between round spermatids is proposed.     

Mouse early oocytes are transiently polar: three-dimensional and ultrastructural analysis

The oocytes of many invertebrate and non-mammalian vertebrate species are not only asymmetrical but also polar in the distribution of organelles, localized RNAs and proteins, and the oocyte polarity dictates the patterning of the future embryo. Polarily located within the oocytes of many species is the Balbiani body (Bb), which in Xenopus is known to be associated with the germinal granules responsible for the determination of germ cell fate. In contrast, in mammals, it is widely believed that the patterning of the embryo does not occur before implantation, and that oocytes are non-polar and symmetrical. Although the oocytes of many mammals, including mice and humans, contain Bbs, it remains unknown how and if the presence of Bbs relates to mouse oocyte and egg polarity. Using three-dimensional reconstruction of mouse neonatal oocytes, we showed that mouse early oocytes are both asymmetrical and transiently polar. In addition, the specifics of polarity in mouse oocytes are highly reminiscent of those in Xenopus early oocytes. Based on these findings, we conclude that the polarity of early oocytes imposed by the position of the centrioles at the cytoplasmic bridges is a fundamental and ancestral feature across the animal kingdom.     

Myelin-like structures as a possible source of the smooth endoplasmic reticulum in early amphibian oocytes

A comparative study of amphibian oocyte ultrastructural organization has shown a significant accumulation of elements of the smooth endoplasmic reticulum in the oocyte cytoplasm at the third stage of development. The analysis of oocytes of two frog species, Xenopus laevis and Rana temporaria, at the first and second stages of their development enabled us to recognize in the cytoplasm of the oocyte some myelin-like structures (MLs) made of 30-40 densely packaged membranous layers and shaped as dense bodies. MLs are also present in the adjacent follicular cells and in the intercellular space. In the oocyte cytoplasm these structures are located near the nuclear envelope and other intracellular organelles. At the third stage of oogenesis, which is characterized by a high functional activity of the cells, MLs are seen to unwrap sequentially into double-layer membranes similar to the smooth endoplasmic reticulum cisternae. Intermediate steps of this process being also observed. It is supposed that MLs may play the role of membrane stocks to be used eventually for the formation of nascent endoplasmic membranes in the amphibian oocytes.     

Oocyte development in the mouse: an ultrastructural comparison of oocytes isolated at various stages of growth and meiotic competence

An ultrastructural comparison of mouse oocytes isolated at various stages of growth and meiotic competence has been carried out. Progressive changes in the nucleoli, ribosomes, mitochondria, endoplasmic reticulum, Golgi complex, and other organelles and inclusions of the oocyte have been examined as a function of oocyte size by transmission electron microscopy. The observations presented support the idea that growth of the mammalian oocyte involves not just tremendous enlargement of the cell, but extensive alterations in its overall metabolism as reflected in the ultrastructure of the oocyte at various stages of growth.     

The cytoskeleton organizes germ nuclei with divergent fates and asynchronous cycles in a common cytoplasm during oogenesis in the chordate Oikopleura

Germline cysts are conserved structures in which cells initiating meiosis are interconnected by ring canals. In many species, the cyst phase is of limited duration, but the chordate, Oikopleura, maintains it throughout prophase I as a unique cell, the coenocyst. We show that despite sharing one common cytoplasm with meiotic and nurse nuclei evenly distributed in a 1:1 ratio, both entry into meiosis and subsequent endocycles of nurse nuclei were asynchronous. Coenocyst cytoskeletal elements played central roles as oogenesis progressed from a syncytial state of indistinguishable germ nuclei, to a final arrangement where the common cytoplasm had been equally partitioned into resolved, mature oocytes. During chromosomal bouquet formation in zygotene, nuclear pore complexes clustered and anchored meiotic nuclei to the coenocyst F-actin network opposite ring canals, polarizing oocytes early in prophase I. F-actin synthesis was required for oocyte growth but movement of cytoplasmic organelles into oocytes did not require cargo transport along colchicine-sensitive microtubules. Instead, microtubules maintained nurse nuclei on the F-actin scaffold and prevented their entry into growing oocytes. Finally, it was possible to both decouple meiotic progression from cellular mechanisms governing oocyte growth, and to advance the timing of oocyte growth in response to external cues.     

Light and electron microscope studies on ovarian follicles in the lizard Chalcides ocellatus

The development of ovarian follicles in a skink has been studied with light and electron microscopy. In early stages the previtellogenic oocyte has a follicular covering (granulosa) comprising only two cell types, small cells and pyriform cells. A complex microvillous interdigitation between follicle cells and oocyte is present from very early stages but regresses as a mature size is reached. The outer thecal layer differentiates into distinct interna and externa as growth proceeds. Occasional biovular follicles are formed. Pyriform cells establish direct continuity with the oocyte via cytoplasmic bridges which traverse the layer of microvilli interdigitating in the zona pellucida. Such bridges appear most frequently just before the onset of yolk deposition; the organelles and cytoplasmic constituents presumed to be transferred across them may stimulate this activity. As the follicles grow, the pyriform cells shrink and disappear to leave just the small cells forming the single layered granulosa. There is asynchrony in recruitment and/or early growth rates of follicle crops and uniformity of oocyte size appears only as vitellogenesis nears completion (with up to five oocytes, about 1 cm in diameter, on each side). Yolk deposition may involve transformation of golgi vesicles or pinocytotic vesicles but there is no evidence to show mitochondria as foci for deposition.     

Yolk formation in Isohypsibius (Eutardigrada)

Summary In Isohypsibius granulifer, yolk is autosynthesized. The Golgi apparatus is mainly responsible for the formation of yolk, which consists of irregular platelets with heterogeneous contents and a diameter of about 1 m. Dense globules, 300 nm in diameter, are visible among yolk platelets. These develop in the vesicles of the rough endoplasmic reticulum. The genesis of these vesicles is associated with the outer membrane of the nuclear envelope, which forms blebs intensively during previtellogenesis and early vitellogenesis. The developing oocytes are assisted by nurse cells, to which they are jointed by cytoplasmic bridges. For every oocyte, there are a number nurse cells, which are sister cells of the oocyte. In addition to rRNA, nurse cells transfer to the oocyte lipids, platelets of yolk formed in their cytoplasm, mitochondria and cortical granules.     

Brefeldin A impairs porcine oocyte meiotic maturation via interruption of organelle dynamics

Brefeldin A (BFA) is a lactone antibiotic synthesized from palmitic acid by several fungi that could block anterograde transport of proteins from endoplasmic reticulum to Golgi apparatus by reversible disruption of the Golgi complex. Previous investigations have shown that BFA induces the apoptosis of cancer cells in mitosis and impairs asymmetric spindle positioning in meiosis. Here, we document that exposure to BFA in porcine oocytes compromises the meiotic maturation via disrupting both nuclear and cytoplasmic maturation. We found that BFA exposure collapsed the cytoskeleton assembly by showing the aberrant spindle organization with misaligned chromosomes and defective actin dynamics. Furthermore, the distribution of both mitochondria and cortical granules (CGs), two important indexes of cytoplasmic maturation of oocytes, was disturbed following BFA exposure. We finally validated that the localization of ovastacin, a component of CGs that is essential for the postfertilization removal of sperm-binding sites in the zona pellucida, was also perturbed in BFA-exposed oocytes, which might weaken their fertilization capacity. Collectively, these findings indicate that Golgi-mediated protein transport is indispensable for the porcine oocyte meiotic maturation.     

Seasonal ultrastructural variations in pinealocytes of the woodchuck,Marmota monax

The ultrastructure of the pinealocyte in the woodchuck, Marmota monax, was studied during the four seasons of the year. Fall cells have a fairly uniform cytoplasmic density, organelles consistent with synthetic and/or secretory activity and rather extensive pericapillary and intercellular spaces. Many winter pinealocytes are nearly devoid of ribosomes and granular endoplasmic reticulum but contain lipid droplets associated with mitochondria. Pericapillary and intercellular spaces are minimal. Spring glands have the greatest variation in cytoplasmic density with intercellular and pericapillary spaces similar to that seen in fall glands. Cells containing electron dense cytoplasm have Golgi zone associated, secretory granules, free ribosomes, short sections of granular endoplasmic reticulum and dense bodies. Cells with a more electron lucent cytoplasm are similar to the most frequently observed summer pinealocytes which have numerous Golgi zones but few associated secretory granules. Microtubules are prominent in the cytoplasm of these cells, the plasma membranes are smooth and intercellular and pericapillary spaces are minimal. A yearly rhythm or cyclic activity of the pinealocyte is suggested.     

Dirofilaria immitis: ultrastructural aspects of oocyte development and zygote formation.

Ultrastructural observations of the ovary and uterus of Dirofilaria immitis reveal some characteristics of oogonia, oocytes, and uterine sperm. Oogonia are confined to the distal portion of the ovary including a blind tip, where a morphologically distinct terminal cap cell was not observed. These cells contain a nucleus with a nucleolus, numerous dense bodies, scanty ribosomes, lipid droplets, and an occasional mitochondrion. Endoplasmic reticulum is lacking and Golgi complexes were observed only in fully grown oogonia. Primary oocytes located in the middle portion of the ovary are large, elongate, and have a complete set of organelles including many small mitochondria, fragmentary endoplasmic reticulum, ribosomes, Golgi complexes, and very few dense bodies. These cells are arranged into many rosettes about central cytoplasmic masses, the rachises, to which they maintain cytoplasmic continuity by pseudopodlike processes. The rachises contain no organelle except a few dense granules and are bound by winding membranes. Oocytes from the proximal portion differ from those of the middle portion of the ovary in their larger size, round shape, absence of many organelles, presence of small dense granules, and lacking a rachis. Dense bodies are specific to the oogonia and exhibit DNase susceptibility and a positive reaction for a mitochondrial enzyme. These findings together with their decreased number and a concomitant increase of mitochondria in the oocytes suggest a relationship between these bodies and mitochondria.Uterine sperm of D. immitis are of the amoeboid type and contain several chromatin masses without a nuclear envelope, many mitochondria, and specialized membranous organelles referred to as mesosomelike vesicles. The vesicles are probably originated from the sperm plasma membrane. Upon fertilization, the entire spermatozoon penetrates the oocyte and its contents are gradually dissolved in the ooplasm with a simultaneous appearance of large numbers of ribosomes at the site of dissolution. Ribosomes were later found in the nucleus. A pronucleus was not observed. These findings are basically in agreement with those described for Ascaris but differ in the morphologic features and number of rachises, presence of dense bodies, absence of refringent granules in the oocytes and the absence of a refringent body and presence of several chromatin masses in the sperm.