Peripheral Nucleoli Research Articles

Thecamoeba astrologa n. sp. – A new species of the genus Thecamoeba (Amoebozoa, Discosea, Thecamoebida) with an unusually polymorphic nuclear structure

Thecamoeba astrologa n. sp. was isolated from a plant litter sample. This species has a complex locomotive morphology, to a certain extent intermediate between the striate and the rugose morphotype. The shape of the actively moving cell is similar to that of striate thecamoebians, such as T. quadrilineata. However, in a slow movement, they can be easily confused with rugose species, like T. similis. Thecamoeba astrologa normally has peripheral asterisk-like nucleoli, which are unique among known thecamoebids. However, the structure of the nucleus is unusually variable during the growth of the culture and in some cases may become almost a vesicular one. The phylogenetic analysis based on the 18S rRNA gene sequence shows that this species belongs to the clade of thecamoebids, consisting of species possessing vesicular nuclei. This finding indicates that the structure of the nucleus derived from a single or few observations (as it usually happens in faunistic or ecological studies) may not be a reliable character of a thecamoebid amoeba. In some species nuclei may be highly polymorphic and dominating nuclear structure may depend on the age of the culture. Nuclei with constantly or temporarily peripheral nucleoli are now known in both major phylogenetic branches of the genus Thecamoeba.

PLASMABLASTIC LYMPHOMA OF THE ORAL CAVITY: A CASE SERIES

This study described the clinical and histopathologic data of 6 cases diagnosed as oral plasmablastic lymphoma (PBL) retrieved from the files of 2 Brazilian oral pathology services from 2000 to 2018. The mean age at presentation was 41.5 years, with a male predominance of 2:1. Involved sites were the palate, maxillary mucosa, and mandibular mucosa. All cases presented as a combination of ill-defined swellings and ulcers, with pain or bleeding. Histopathologic features revealed the proliferation of large cells, with round to ovoid eccentric nuclei, and peripheral nucleoli, as well as plasmablastic cells with prominent nucleoli. The cases were positive for CD138 and plasma cell, negative for CD20, and presented with a high proliferative index (>90%). All cases were positive for Epstein-Barr virus-encoded small ribonucleic acids (EBER). Treatment included chemotherapy associated with highly active antiretroviral therapy (HAART). Radiotherapy was performed in 1 case and HAART only was applied in 1 case. Currently, 4 patients remain alive.

Adult orbital precursor B‐lymphoblastic lymphoma with involvement of the extraocular muscles

PurposeTo report a very rare case of a precursor‐B Lymphoblastic lymphoma (pre‐B‐LBL) with periocular muscle involvement in an adult. The clinical, histopathological, and genetic findings will be reported.MethodsCase report, Immunohistochemistry, FISH, and ArrayCGH.ResultsA 56‐year‐old male experienced sudden onset of left sided ptosis and diplopia. Pain was not prominent. The patient had complete remission of an abdominal pre‐B‐LBL. On examination, proptosis and a left sided third cranial nerve palsy was found. After a few days, mydriasis without rapid afferent pupillary defect developed. MRI showed thickening of the medial and inferior rectus muscles. A biopsy of the inferior rectus muscle was obtained for histopathological examination. Histologically, an infiltrate of immature lymphocytic tumour cells was seen within the orbital skeletal muscle tissue. The tumour cells were small and presented with fine blastic nuclear chromatin along with small peripheral nucleoli. The tumour cells stained positively for CD10, CD79a, PAX5, and BCL‐2. The Ki‐67 proliferation index was 90%. These findings were consistent with a TdT‐negative pre‐B‐LBL relapse. FISH was negative for BCR/ABL, MLL‐rearrangement, and ETV‐6‐break. ArrayCGH was performed. A bone marrow biopsy confirmed systemic involvement. The patient was immediately started on NOPHO High Risk chemotherapy, followed by orbital radiation and awaits allogeneic bone marrow transplantation. The patient has no light perception in his left eye.ConclusionsThis is the first reported case of a very rare rapid growing orbital pre‐B‐LBL in an adult.

Ultra-structure of testes of rats born to dams treated with hydroxy-progesterone hexanoate

ObjectiveTo investigate the effects of prenatal exposure of male rats to hydroxy-progesterone hexanoate (HPH) on ultra-structures of testis. MethodsTen male rat puppies born to dams treated with 10 mg/kg body weight of HPH on the 1st, 7th and 14th day after conception were allowed to grow for 90 d; sacrificed, their testes dissected and prepared for electron microscope examination. ResultsResults showed that the testes of all rats born to dams treated with HPH had degenerated germ cells that lost their attachment to surrounding Sertoli cells (SCs) and the basement membrane. The germinal cells were abnormal with cytoplasmic cavitations. SCs had small invaginated nucleus with peripheral nucleolus, folded cell membrane and large lipid droplets. The cytoplasm of SCs had small cavities and the mitochondriae appeared distended and had lost their cristae. The interstitial spaces were wider with few Leydig cells (LCs) that had elongated nucleus with indented nuclear membrane and three prominent nucleoli with dense scattered chromatin. LCs cytoplasm contained abundant lipid droplets, numerous mitochondriae and cytoplasmic vacuoles. The seminiferous tubules germinal epithelium suffered severe degree of degeneration and there were no stages of spermatogenesis. ConclusionPrenatal exposure of male rats to HPH induces ultra-structural changes in the seminiferous tubules, SCs and LCs as well as it hampers spermatogenesis. Thus the future fecundity of males born to females treated with HPH during pregnancy will inevitably be affected.

Discrepancy between Species Borders at Morphological and Molecular Levels in the Genus Cochliopodium (Amoebozoa, Himatismenida), with the Description of Cochliopodium plurinucleolum n. sp.

Amoebae of the genus Cochliopodium are characterized by a tectum that is a layer of scales covering the dorsal surface of the cell. A combination of scale structure, morphological features and, nowadays, molecular information allows species discrimination. Here we describe a soil species Cochliopodium plurinucleolum n. sp. that besides strong genetic divergence from all currently described species of Cochliopodium differs morphologically by the presence of several peripheral nucleoli in the nucleus. Further, we unambiguously show that the Golgi attachment associated with a dictyosome in Cochliopodium is a cytoplasmic microtubule organizing center (MTOC). Last, we provide detailed morphological and molecular information on the sister clade of C. plurinucleolum, containing C. minus, C. minutoidum, C. pentatrifurcatum and C. megatetrastylus. These species share nearly identical sequences of both, small subunit ribosomal RNA and partial Cox1 genes, and nearly identical structure of the scales. Scales of C. pentatrifurcatum differ, however, strongly from scales of the others while sequences of C. pentatrifurcatum and C. minus are nearly identical. These discrepancies urge for future sampling efforts to disentangle species characteristics within Cochliopdium and to investigate morphological and molecular patterns that allow reliable species differentiation.

Morphological and molecular markers are correlated with maturation-competence of human oocytes

Does the position of the germinal vesicle (GV) in human oocytes correlate with molecular and morphological parameters as well as with maturation-competence? The position of GV in human oocytes correlates with density of microtubule (MT) filaments, concentration of Fyn, nucleolus localization and the ability of the oocytes to complete maturation following GV breakdown (GVBD). Our knowledge is confined to oocytes of young mice where maturation-competence is correlated with a central GV and regulated by MTs and the presence of a chromatin ring. Fyn kinase is localized at the spindle and cortex of mouse oocytes and plays a role in both maturation and MT stabilization. Spatial localization of the GV and nucleolus (central or peripheral), the presence of a chromatin ring, the localization of Fyn, MT density and oocyte maturation were assessed in 153 human oocytes, 335 oocytes from young mice (2-month-old) and 146 oocytes from old mice (12-month-old). GV human oocytes were donated by consenting female patients (n = 57), 21-45-year-old undergoing IVF/ICSI. As a control, GV mouse oocytes were collected from female mice after injection of pregnant mares' serum gonadotrophin. Human and mouse GV oocytes allocated for immunocytochemistry were fixed on day of retrieval, stained with specific antibodies and imaged using a confocal laser-scanning microscope. Human and mouse oocytes allocated for maturation were incubated for 48 and 24 h, respectively. GVBD and extrusion of the first polar body (PBI) were assessed using differential interference contrast optics. GV location was peripheral and independent of age in 69.9% of the human oocytes, but GV location did vary with age in mice oocytes; it was central in 89.9% of the oocytes retrieved from young-mice and peripheral in 52.1% of the oocytes retrieved from old mice (P < 0.05). A central GV, whether in human or mouse oocytes, was highly correlated with a central nucleolus, absence of Fyn at the GV and a dense MT network (P < 0.05), whereas a peripheral GV correlated with peripheral nucleolus, presence of Fyn at the GV and a flimsy MT network. After 48 h in culture, no degeneration was observed in human central-GV oocytes, however, 12/95 (12.6%) of the peripheral-GV oocytes degenerated (P < 0.05). No correlation was observed between GV position and presence of a chromatin ring. The percentage of human oocytes that extruded the PBI after completing GVBD was significantly higher (73.7%) in central than in peripheral-GV oocytes (45.8%; P < 0.05). In mice oocytes, central location of the GV correlated with maturation competence in young (P < 0.05) but not old mice. The fact that the human GV oocytes used in this study were exposed to gonadotrophic stimulation but failed to mature in vivo might be a sign of their low quality and this should be considered when drawing conclusions from the data. Furthermore, our observation that only peripheral-GV human oocytes were degraded may indicate that they are of a lower quality than central-GV human oocytes. We suggest that the central location of GV within the oocytes, which is associated with an absence of Fyn at the GV and the presence of thick filamentous MTs in the ooplasm, may serve as a predictor of successful maturation and provide new insights for the use of IVM.

Comparative Morphology and Genealogical Delimitation of Cryptic Species of Sympatric Isolates of Sphaeroforma (Ichthyosporea, Opisthokonta)

Of the ancient clades of unicellular relatives of the multicellular animals, ichthyosporea are among the easiest to collect, cultivate, and analyze at the population level. Once identified, species can be correlated with their animal hosts and geographical ranges. However, the spherical stages common to many ichthyosporea provide little basis for morphological species identification. This study of the genus Sphaeroforma is the first to apply patterns of genetic discontinuity to delimit species among any of the unicellular 'holozoa.' Sequences of three loci from 148 sympatric isolates, along with type cultures, provided concordant support for new species "Sphaeroforma nootkatensis" and "Sphaeroforma gastrica," and for formally describing 'Pseudoperkinsus tapetis,' as "Sphaeroforma tapetis". We document light and electron microscopic characters that distinguish the genus but not its species. "S. tapetis" sometimes had brief amoeboid or plasmodial motile stages and endospore release through pores. Unlike closely related Creolimax, Sphaeroforma lacked a central vacuole but had multiple peripheral nucleoli. Like distantly related eccrinales, Sphaeroforma cell walls had pores and a calyx. Analyses of allele frequencies in "S. tapetis" indicated geographical differentiation but no host specificity. Accurate molecular identification of species will increase the feasibility and reliability of further studies of Sphaeroforma in its natural habit.

Phylogenetic and Morphological Diversity of Novel Soil Cercomonad Species with a Description of Two New Genera (Nucleocercomonas and Metabolomonas)

Cercomonads are important components of microbial food webs in soils and aquatic sediments. Here, we investigated the general morphology, behaviour, life cycle and 18S rDNA phylogeny of cercomonad cultures from a German grassland soil habitat. We describe ten new species including two new genera from 23 strains. Three Cercomonas, two Eocercomonas and three Paracercomonas species are described. Based on large phylogenetic distance and distinct morphology, we erect two novel clade B genera near the root of the cercomonad tree. Nucleocercomonas nov. gen. bears a number of characters unusual for cercomonads: Its anterior flagellum is extremely long, it mostly does not glide, and in its most frequent life stage the cell body does not attach to the substratum, but produces unattached pseudopodia. Furthermore, it has a unique nucleus with a peripheral nucleolus that attaches to the nuclear envelope opposite the basal body connection. Metabolomonas nov. gen. is extremely metabolic. It is characterized by a very high beating frequency of the anterior flagellum, fast gliding, rapid changes in shape and strong cytoplasmic streams. A new genus Brevimastigomonas is erected for the previously described species Paracercomonas anaerobica. The general morphology of cercomonad species often does not correspond with their phylogenetic position: closely related species may have a very different morphology.

Ultrastructural composition of nonhistone chromosomal skeleton at different stages of mitosis in situ

In interphase cells of the SPEV culture treated with Triton X-100, 2 M NaCl, and DNAse, in the presence of 2 mM CuCl2, we clearly revealed a stabilized nuclear protein material (NPM) composed of a peripheral lamina, residual nucleolus, and internal fibrillar network. This network is formed by thin fibrils 10–20 nm in diameter, which are also revealed in the nonhistone matrix of mitotic chromosomes at all stages of mitosis. In mitotic chromosomes, NPM is represented as a network of the 10–20-nm-thick fibrils without any features of the central-axial structures. Beginning from the middle prophase, it is possible to see approached sister chromatids in contact with each other in certain sites, similar to centromeres. At these sites, the thickness of fibrils increases up to 40–50 nm, whereas the fibrils themselves are disposed more tightly; this structure can be seen in the chromosome until telophase. At the end of telophase, the decondensation of chromosomes and formation of two new nuclei whose NPM is analogous to NPM of usual interphase nucleus are observed. Thus, the NPM elements can perform the role of a skeleton in both the interphase nucleus and mitotic chromosomes.

Gonadal Sex Differentiation in Cultured Juvenile Flounder, Paralichthys orbignyanus (Valenciennes, 1839)

Gonadal Sex Differentiation in Cultured Juvenile Flounder, <i>Paralichthys orbignyanus</i> (Valenciennes, 1839)

Burkitt lymphoma of the oral cavity: an atypical presentation

Burkitt lymphoma (BL) is an aggressive form of non-Hodgkin B-cell lymphoma with 3 variants: endemic, sporadic, and immunodeficiency-associated types. The sporadic form, most commonly involving the abdomen and ileocecal region, presents as an abdominal mass, rarely presenting in the orofacial region. A 36-year-old Indian female presented to the Hospital of the University of Pennsylvania for evaluation of a persistent intraoral swelling ulceration of the lower right mandibular alveolar ridge with minimal bony invasion. Progressive systemic symptoms of fatigue, weakness, and fever developed without resolution following treatment for a presumed odontogenic infection in the 4 weeks before presentation. An incisional biopsy revealed a diffuse proliferation of intermediate- to large-sized lymphocytes with multiple small peripheral nucleoli, scant cytoplasm, and nuclear pleomorphism. Nearly all cells displayed Ki67 expression. A final diagnosis of BL was rendered following confirmation of a cMYC translocation by fluorescence in situ hybridization. This article presents a case of the sporadic form of BL with atypical presentation clinically and morphologically, primarily involving the oral soft tissue.

Number and Nuclear Localisation of Nucleoli in Mammalian Spermatocytes

In seven mammalian species, including man, the position and number of nucleoli in pachytene spermatocyte nuclei were studied from electron microscope (EM) nuclear sections or bivalent microspreads. The number and position of the nucleolar organiser regions (NORs) in mitotic and meiotic chromosomes were also analysed, using silver staining techniques and in situ hybridisation protocols. The general organisation of pachytene spermatocyte nucleoli was almost the same, with only minor morphological differences between species. The terminal NORs of Thylamys elegans (Didelphoidea, Marsupialia), Dromiciops gliroides (Microbiotheridae, Marsupialia), Phyllotys osgoodi (Rodentia, Muridae) and man, always gave rise to peripheral nucleoli in the spermatocyte nucleus. In turn, the intercalated NORs from Octodon degus, Ctenomys opimus (Rodentia, Octodontidae) and Chinchilla lanigera (Rodentia, Cavidae), gave rise to central nucleoli. In species with a single nucleolar bivalent, just one nucleolus is formed, while in those with multiple nucleolar bivalents a variable number of nucleoli are formed by association of different nucleolar bivalents or NORs that occupy the same nuclear peripheral space (Phyllotis and man). It can be concluded that the position of each nucleolus within the spermatocyte nucleus is mainly dependent upon: (1) the position of the NOR in the nucleolar bivalent synaptonemal complex (SC), (2) the nuclear pathway of the nucleolar bivalent SC, being both telomeric ends attached to the nuclear envelope, and (3) the association between nucleolar bivalents by means of their NOR-nucleolar domains that occupy the same nuclear space. Thus, the distribution of nucleoli within the nuclear space of spermatocytes is non-random and it is consistent with the existence of a species-specific meiotic nuclear architecture.

Dynamics and cytochemistry of oogenesis in Astyanax fasciatus (Cuvier) (Teleostei, Characiformes, Characidae) from Rio Sapucaí, Minas Gerais State, Brazil

Oogenesis involves a set of transformations which are undergone by female germ cells These cells change into oogonias and then into mature oocytes. Sexually mature female fish were collected monthly, during one year, from the Sapucai River, a tributary of the Rio Grande, which is part of the Furnas Reservoir in the state of Minas Gerais. During the several stages of maturation, we observed small round oogonias with a large nucleus, a single nucleolus, and weakly stained cytoplasm with eosinophilic granules. The primary oocytes showed a large basophilic nucleus, with a developed peripheral nucleolus and a reduced cytoplasm. The previtellogenic oocytes presented voluminous cytoplasm and nucleus with several small peripheral nucleoli. The oocytes underwent vitellogenesis with the development of the zona radiata and the follicle cells. Their cytochemical reactions indicated that the two layers of the zona radiata of A. fasciatus contained proteins and polysaccharides. The initially squamous follicle cells, became cuboidal. In mature oocytes, the nucleus moved toward the periphery, next to the micropyle, and the yolk granules formed by proteins, fulfilled the cytoplasm. The clear unstained vesicles are likely to be the cortical alveoli in the perivitelline region.

Dynamics and cytochemistry of oogenesis in Leporinus striatus Kner (Teleostei, Characiformes, Anostomidae) from the Rio Sapucaí, Minas Gerais State, Brazil

Oogenesis involves a sequense of transformations which are undergone by female germ cells. These cells change into oogonias and then into mature oocytes. Sexually mature females were collected monthly, during one year, from the Rio Sapucaí, tributary of the Rio Grande, which is part of the Furnas Reservoir system in the state of Minas Gerais. The observed material showed that oogonias were small spherical cells, had a big spherical nucleus, with a single nucleolus, and weakly stained cytoplasm with eosinophilic granules (FG stained), which indicate their protein content. The primary oocytes showed a big basophilic nucleus, with a large peripheral nucleolus, and several smaller nucleoli. They show a reduced cytoplasmic content. The previtellogenic oocytes presented voluminous cytoplasm and nucleus with several small peripheral nucleoli. The oocytes underwent vitellogenesis with the development of the zona radiata and the follicle cells. The zona radiata had two layers, the outer and the inner, which showed its protein content when stained with CM and FG techniques. TB pH 2.5 and pH 4.0 staining showed that oocytes undergoing vitellogenesis presented weakly stained cytoplasm and peripheral cytoplasmic vesicles. The follicle cells that were squamous became cuboidal. In mature oocytes, the yolk granules that filled the cytoplasm became green and blue when stained with FG and CM techniques, indicating their protein content. The perivitclline region showed rosy stained vesicles (TB pH 2.5 and pH 4.0) spread among the weakly stained peripheral vesicles, which seemed to be the cortical alveoli. The zona radiata cells, CM and FG stained, still showed two layers like the oocytes from the previous stage, but thicker.

Identification and immunolocalization of a 65 kDa drought induced protein in cultivated tomatoLycopersicon esculentum

A 65 kDa protein with a pI value of 5.2 accumulated gradually in tomato leaves during water stress. Protein levels returned to those of the control upon rehydration of the plants. Antiserum raised against the protein, purified from two dimensional electrophoresis gels, was obtained and used as a probe to localize the protein in tomato leaves by immunofluorescence and immunogold labeling. The protein was found to be mainly localized in different areas of nuclei (peripheral chromatin masses, nucleoli and nucleoplasm), chloroplasts, and some leaf cell cytoplasmic regions. Quantification of the gold labeling clearly demonstrated that the amount of the protein increased significantly in nuclei and chloroplasts of cells in drought-stressed plants. Cytological changes occurring in leaf tissues during water stress are also reported.

Nuclear fine structure at interphase and during encystment in two forms of the testacean Arcella vulgaris

Arcella vulgaris and A. vulgaris var. multinucleata have two and seven vesicular nuclei, respectively. In early interphase, the nuclei are spherical, with a main central nucleolus and several small peripheral nucleoli. The main nucleolus has mixed fibrillar and granular components and no apparent chromatin bodies in the nucleolus-organizing regions (NORs). The nuclear chromatin is dispersed except for small heterochromatin lumps. Nuclear bodies of fibrous structure occur outside the main nucleolus. The nuclear envelope shows an internal lamina and an external layer of tangential fibres. In middle interphase, the nuclei become irregular in shape and adjacent to the plasma membrane at the dorsolateral cell surface. The condensation of the chromatin increases. The nucleolus is often eccentric and its NORs show conspicuous bodies of condensed chromatin surrounded by a halo and a fibrous transcription zone. By the end of interphase, the main nucleolus becomes polymorphic and segregated into a fibrillar basal part which contains a vacuolar area with dense inclusions, and a fibro-granular cap which contains many fibrous electron dense bodies. These are likely to be the nuclear chromatin elements. In early resting cysts, the nuclei detach from the cell membrane and approach one another. The main nucleolus segregates into large, mainly peripheral fibrillar blocks inside a granular mass. The NORs consist of condensed chromatin bodies without a transcription zone around them. The nuclear chromatin is condensed. Conspicuous intranuclear annulate lamellae bearing pore complexes occur only in cysts.

Localization of NORs in the nucleolus-DNA body complex of the oocyte from the nemertine Amphiporus lactifloreus.

The nucleus of the premeiotic oocyte from the hoplonemertine Amphiporus lactifloreus contains a characteristic DNA-rich body that rapidly generates a peculiar nucleolar apparatus previously described as the nucleolus-DNA body complex. During early vitellogenesis, the nucleolar complex is centrally located in the nucleus and consists of many spherical and ribbon-shaped components-the nucleolar spherulae and lamellae-which produce hundreds of small peripheral nucleoli and several granular bodies as vitellogenesis progresses. Distributions of the nucleolar organizer regions (NORs) in various nucleolar components were studied by silver staining, and results were compared with previous findings obtained by cytochemical and autoradiographic techniques.The core of each spherula in the nucleolus-DNA body complex is strongly Ag-positive, and so, later on, is a minor part of each peripheral nucleolus. In contrast, the cortices of the nucleolar spherulae, in which slightly decondensed chromatin is distributed uniformly, and the nucleolar lamellae produced through self-peeling of the spherulae, do not react with silver. Thus, the NOR proteins identified by silver deposits correspond to numerous loci at which the most decondensed, transcriptionally active chromatin is made visible by the osmium-ammine reaction. These results are discussed in relation to the functional evolution of the initial DNA body of the oocyte from A. lactifloreus, the transcriptional abilities of its by-products, and the general process of the amplification of ribosomal genes.

Laboratory observations on spawning, brood size and egg hatchability of the Antarctic krill Euphausia superba from Prydz Bay, Antarctica

Forty-three female Euphausia superba were collected from three sites in Prydz Bay, Antarctica in January 1985. Most gravid females took 3 to 14 d to spawn after capture and some less mature females released eggs after 21 d. Most females spawned only once; overall mean brood size was 2 809 eggs and mean hatchability for eggs from the premoult females was 69.7%. For seven females spawning a second time, overall mean brood size was 597 eggs. Mean hatchability for second-spawn eggs from five premoult females was 67.1%. Possible origins of the second-spawned eggs were small numbers of matured eggs left in the ovaries after the first spawning or eggs developed from those of peripheral nucleolus and vitellogenic stages. Throughout the 8 wk of observations, there was no apparent development of eggs in females which had released all their eggs at one spawning.

Dynamics of tubulin structures in Xenopus laevis oogenesis

The distribution of tubulin and/or tubulin-containing structures was examined in separate classes of Xenopus laevis oocytes and in germinal vesicles isolated from them. Although a monoclonal antibody has been used, the technique applied on paraffin sections does not allow clear-cut definition of the state of tubulin present (monomeric, dimeric or polymerized form); however, the probable existence of assembled microtubules is indicated by supplementary techniques, i.e. histology and immunoperoxidase staining. Immunofluorescence reveals maximum tubulin concentration in the Balbiani body and in a ring-shaped formation around the nucleus in young oocytes. The Balbiani body disintegrates in the course of vitellogenesis, granules formed from its periphery migrate into the cytoplasm and gradually fill the entire cytoplasm as radial cords. In the ring-shaped formation around the nucleus strongly fluorescent cords and fibres are formed, particularly on the future vegetal-half-facing part of the nucleus. Reorganization of tubulin may be related to the establishment of a structure directing two-way shifts (1) of cytoplasmic organelles from the Balbiani body to the cytoplasm, and (2) of yolk proteins containing endosomes derived from the endocytically active oolemma to the yolk platelets. A distinct fluorescent fibrillar network is found inside the isolated germinal vesicles, near the nucleus membrane. Peripheral nucleoli, often present in nuclear membrane protuberances, seem to be surrounded by this material, which is oriented along the surface, and as a basket towards the inside of the nucleus. It is assumed that the structures may participate in the transport of nucleoli from the nucleus to the cytoplasm via the nuclear envelope.

Ultrastructural evidence for loss of the trophoblastic layer in the chorioallantoic placenta of Australian Bandicoots (Marsupialia: Peramelidae)

In most marsupials, placentation involves only the yolk sac; however, in the bandicoot family, Peramelidae, a functional chorioallantoic placentation develops in addition (Hill, 1895, 1897, 1900; Flynn, '22, '23). This duality is viewed as having evolutionary significance because most eutheria have both placentae. Furthermore, the bandicoot trophoblast was reported to vanish from the chorioallantoic site in late gestation (Hill, 1897; Flynn, '23); whereas, the eutherian trophoblast is identifiable throughout later pregnancy and may act as an immunological barrier between maternal and fetal genotypes (Kirby '68). Thus we have re-examined this singular chorioallantoic placenta of the bandicoot in plastic sections with light and electron microscopy. A distinctive feature of the bandicoot placentation is the transformation of the uterine simple columnar luminal epithelium into a highly vascular lining composed almost entirely of discrete syncytial masses (homokaryons). Endometrial blood vessels penetrate among the homokaryons to create a rich network of large diameter capillaries at extremely superficial locations near the maternal surface. In the chorioallantoic placenta (7 mm to 10-11 mm crown-rump embryos) the microvillous surface of the maternal homokaryons interdigitates with the microvillous border of the fetal trophoblast with desmosomal interaction. This trophoblast consists of a single layer of tall columnar undifferentiated cells rich in ribosomes-polysomes, poor in cytoplasmic membranes, and with large nuclei that have distinct clumps of heterochromatin and conspicuous nucleoli. It is thus remarkable that these undifferentiated cells disappear as a recognizable layer later in gestation (12 mm crown-rump embryos). Flynn's hypothesis that the trophoblastic cells disappear by fusing with maternal syncytia gains support from the existence of two populations of nuclei in the syncytial masses only at the chorioallantoic site. One population is comparable to that occurring in the homokaryons pf the yolk sac placenta, i.e., pale staining nuclei with little heterochromatin and small peripheral nucleoli. However, the other nuclei resemble those of the trophoblast cells. Since the trophoblastic cells before their disappearance as a layer possess properties associated with potential for further differentiation, the possibility of fusion between the maternal homokaryons and fetal trophoblastic cells to form heterokaryons composed of two genotypes merits fur