Regional differences in the pattern of vitellogenesis in the painted frog Discoglossus pictus

During oocyte growth in the frog Discoglossus pictus two patterns of vitellogenesis are described. The first one consists of the transformation of multivesicular bodies into yolk platelets; the second is the result of a typical endocytotic process, as described in other species. The peculiarity in Discoglossus vitellogenesis consists of a regional difference of these features of vitellogenesis in vitellogenic oocytes: the multivesicular bodies transforming into yolk platelets are found only in the germinative area—the central portion of the animal half—whereas deep crypts with numerous endocytotic pits are found only in the vegetal half. The probable meaning of this regional difference in vitellogenic oocytes is discussed.     

A histochemical study of the distribution of lectin binding sites in the developing oocytes of the lancelet Branchiostoma belcheri

The distribution of carbohydrate moieties in lancelet (Branchiostoma belcheri) oocytes has been studied at different stages of development, using a peroxidase-labeled lectin incubation technique, the PAS-reaction and Alcian Blue staining. Binding sites of 5 lectins, indicating the presence of different sugar moieties (Wheat germ agglutinin (WGA) for N-acetylglucosamine, Concanavalin A (Con A) for glucose/mannose, Helix pomatia agglutinin (HPA) for N-acetyl-D-galactosamine, Ricinus communis agglutinin (RCA-I) for galactose and Ulex europaeus agglutinin (UEA-I) for fucose), were identified and were shown to undergo considerable variation during oocyte development. In the previtellogenic stage, HPA, RCA-I and UEA-I were not identified on the oocyte surface, but WGA and Con A gave strongly positive reactions at this site. In the cytoplasm, 4 lectins (Con A, HPA, RCA-I and UEA-I) gave a weak or moderate reaction, and Con A was also observed in the perinuclear region. In vitellogenic oocytes, these 4 lectins were found to also bind to the nuclear envelope, karyoplasm and nucleolus, and, with the exception of Con A, could also be found in the nuclei of more mature stages. The cytoplasmic yolk granules and Golgi vesicles of the vitellogenic oocyte, were moderately positive for Con A, HPA, RCA-I and UEA-I, but HPA, RCA-I and UEA-I were only weakly bound at the oocyte surface. In mature oocytes, all 5 lectins bound moderately or strongly to yolk granules and cell surface. HPA, RCA-I and UEA-I bound moderately or strongly to various nuclear compartments. Thus, carbohydrate content varied with the development and maturation of the oocytes, and the PAS results were in agreement with the lectin-binding results. Charged carbohydrate residues were observed in the egg envelope and Golgi bodies.These results suggest that the appearence of Con A-, HPA-, RCA-I- and UEA-I-binding glycoconjugates in the nuclei of developing oocytes show a varying pattern indicating different phases of nuclear activity which correlate with different carbohydrate synthetic activities of the oocyte.     

Do fast increasing food conditions promote the midsummer decline of Daphnia galeata?

Ovaries from mature giant red shrimp Aristaeomorpha foliacea were investigated histochemically and ultrastructurally. Four growing stages of the oocytes were distinguished: premeiosis stage, previtellogenetic stage, early vitellogenic stage and late vitellogenic stage. In addition, occasional resorptive oocytes were found. Oogonia and premeiotic oocytes were found in germinative zones. Previtellogenic and vitellogenic oocytes were localized in maturative zones. As vitellogenesis proceeded, oocytes showed a progressive development in the number of lipid droplets as well as in the extension of RER, constituted of dilated cisternae, uniformely scattered throughout the cytoplasm. The RER produced yolk granules and a lampbrush-like substance. The latter was released under the oolemma and constituted a characteristic cortical zone. The oolemma did not develop microvilli or micropinocytotic vesicles to incorporate yolk precursors. Thus, the protein yolk appeared to be of endogenous origin. Few somatic cells were found around the oocytes, but they never gave place to a continuous epithelial layer around oocytes, thus it is not possible to speak of ovarian follicle. The cytoplasm of these mesodermal-oocyte associated cells (MOAC) was characterized by a typical steroidogenic apparatus. Few resorptive immature oocytes were found inside late vitellogenic oocytes. Since the ovaries were packed with late vitellogenic oocytes and the few immature oocytes were hardly detectable, oocyte maturation occurred in a synchronous way.     

Accumulation of yolk in a caecilian (Gegeneophis ramaswamii) oocyte: A light and transmission electron microscopic study

There is a paucity of information on the female reproductive biology of the caecilian amphibians when compared with the other vertebrate groups. Hence, the accumulation of nutrient reserves in the form of yolk and formation of yolk platelets were studied in Gegeneophis ramaswamii, adopting light microscopic histological and transmission electron microscopy analysis. Previtellogenic as well as vitellogenic follicles were observed in appropriate preparations. On the basis of the source and the routes of entry, we identified four types of yolk precursor materials, precursors 1 to 4. The earliest material appearing in the oocyte consists of abundant lipid vesicles during the previtellogenic phase, i.e., much before the follicular epithelium is fully established. This is a contribution from the oocyte mitochondria, which we identified as yolk precursor material 1, and it is autosynthetic. Once the follicle cell‐oocyte interface is fully established, there is an accumulation of the principal component of the heterosynthetic yolk by sequestration from the blood through the intercellular spaces between follicle cells in a pinocytic process. This we identified as yolk precursor material 2. There was also an indication of a lipidic yolk material synthesis in the follicle cells sequestered from maternal blood through the follicle cells in an endocytic process in which the macrovilli of follicle cells and the microvilli of the oocyte play a role. This we identified as yolk precursor material 3. Contribution to the yolk of peptidic, glycosidic, and/or lipidic material synthesized in the vitellogenic oocyte was also indicated. This we identified as yolk precursor material 4. The sequential development of intercellular associations and indications of synthesis/sequestration of the yolk have been traced. Thus, we report the mechanistic details of synthesis/sequestration of the yolk materials in a caecilian. J. Morphol., 2008. © 2008 Wiley‐Liss, Inc.     

Stages in follicle cell/oocyte interface during vitellogenesis in caecilians <Emphasis Type="Italic">Ichthyophis tricolor</Emphasis> and <Emphasis Type="Italic">Gegeneophis ramaswamii</Emphasis>: a transmission electron-microscopic study

We describe the ultrastructural organization of the vitellogenic follicle stages in two caecilian species. Monthly samples of slices of ovary of Ichthyophis tricolor and Gegeneophis ramaswamii from the Western Ghats of India were subjected to transmission electron-microscopic analysis, with special attention to the follicle cell/oocyte interface. In order to maintain uniformity of the stages among the amphibians, all the stages in the caecilian follicles were assigned to stages I–VI, the vitellogenic and post-vitellogenic follicles being assigned to stages III–VI. Stage III commences with the appearance of precursors of vitelline envelope material in the perivitelline space. Stages IV and V have been assigned appropriate substages. During the transition of stage III to stage VI oocytes, a sequential change occurs in the manifestations of follicle cells, perivitelline space, vitelline envelope and oocyte cortex. The vitelline envelope becomes a tough coat through the tunnels of which the macrovilli pass to interdigitate between the microvilli. The oocyte surface forms pinocytic vesicles that develop into coated pits and, later, coated vesicles. Contributions of the oocyte cortex to the vitelline envelope and of the follicle cells to yolk material via synthesis within them are indicated. The follicle cell/oocyte interface of vitellogenic follicles of these two caecilians resembles that in anurans and urodeles, with certain features being unique to caecilians. Thus, this paper throws light on the possible relationships of caecilians to anurans and urodeles with special reference to ovarian follicles. This research was supported by funds from the Kerala State Council for Science, Technology and Environment (KSCSTE), through the SARD facility, and by the FIST scheme of Department of Science and Technology, Government of India, New Delhi, to the Department of Zoology, University of Kerala, Thiruvananthapuram, and to the Department of Animal Science, Bharathidasan University, Thiruchirapalli (SR/FST/LSI-233/2002).     

Effects of starvation on reproduction of the predacious mite <Emphasis Type="Italic">Neoseiulus californicus</Emphasis> (Acari: Phytoseiidae)

Effects of starvation on gravid females of Neoseiulus californicus were investigated at 20°C and 85% RH. When females that had been reared with abundant prey were swapped, just after laying their first egg, to conditions without any prey and water, they laid 1.8 eggs and survived for 4.3 days. In the body of well-fed females, an egg with eggshell and/or two oocytes were observed in the ventral and dorsal regions, respectively. The larger oocyte had two roundish nuclei and abundant yolk granules, and was enveloped with a vitelline membrane. These two nuclei were not fused but were just close to each other. The smaller oocyte had a nucleus, but had not yet formed yolk granules and vitelline membrane. Females after 12 h starvation had an egg in the ventral region and an oocyte in the dorsal region of the body. After more than 24 h starvation females maintained an oocyte in the dorsal region of the body, but had no egg in the ventral region. The oocyte was filled with abundant yolk granules and contained two irregular nuclei when females were starved for 24 h, but when starved for more than 36 h it contained one irregular nucleus. These findings suggest that (1) gravid females maintained an oocyte in the dorsal region after laying two eggs during starvation, (2) the oocyte was not absorbed during starvation, (3) the oocyte advanced vitellogenesis and the fusion of two nuclei, and (4) the vitellogenic oocyte was not enveloped with an eggshell and had not started embryogenesis.     

Yolk sphere formation is initiated in oocytes before development of patency in follicles of the moth,Plodia interpunctella

We describe a provitellogenic stage, a previously unrecognized stage of follicle development in moths, and show that oocytes begin yolk sphere formation prior to the development of patency by the follicular epithelium. The vitellogenic activities of follicles from pharate adult femalePlodia interpunctella (Hübner) were determined by visualizing the subunits of vitellin (YP1 and YP3) and the follicular epithelium yolk protein (YP2 and YP4) using monospecific antisera to each subunit to immunolabel whole-mounted ovaries or ultrathin sections. At 92 h after pupation, yolk spheres that contained only YP2 began to proliferate in the oocytes. The inter-follicular epithelial cell spaces were closed at 92 h making vitellogenin inaccessible to the oocyte, and consequently, the vitellin subunits were not observed in the yolk spheres. YP2 uptake most likely occurred across the brush border from the follicular epithelial cells to the oocyte at this time. At 105 h, the inter-follicular epithelial cell spaces appeared closed yet trace amounts of labeling for vitellin were observed in the spaces and also in the yolk spheres along with YP2. Equivalent labeling for all four YPs in yolk spheres was finally observed at 112 h after pupation when the follicular epithelium had become patent. These data indicate that the provitellogenic stage is an extended transition period between the previtellogenic and vitellogenic stages that lasts for approximately 13 h, and it is marked at the beginning by YP2 yolk sphere formation in the oocyte and at the end by patency in the follicular epithelium.     

The cytology of the vitellogenic stages of oogenesis in Drosophila melanogaster. II. Ultrastructural investigations on the origin of protein yolk spheres

The cytology of the vitellogenic stages in the development of the oocyte of Drosophila melanogaster is described following an electron microscopic study of sections of plastic-embedded ovaries and single egg chambers. One of the first morphological manifestations of yolk deposition is an infolding of the plasma membrane of the oocyte and the abscission of membranous tubules and vesicles. The protein (alpha) yolk spheres originate along the oocyte periphery from membranous sacs to which are attached membranous tubules. It is assumed that the majority of the protein within the alpha sphere is synthesized by neighboring tubular, rough surfaced endoplasmic reticulum. The other organelles in the ooplasm are described, and their origin and possible roles in vitellogenesis are examined. The relative importance of intra- and extra-ovarian synthesis of yolk protein in different insect species is discussed.     

Oocyte growth in the sheepshead minnow: Uptake of exogenous proteins by vitellogenic oocytes

The structure of the vitellogenic follicle of the sheepshead minnow, Cyprinodon variegatus, is described. Follicles enlarge primarily by protein yolk accumulation (vitellogenesis) and subsequently increase in size by hydration. This study uses the electron-dense tracer, horseradish peroxidase, and a larger heterologous protein,Xenopus laevis [³H]vitellogenin, to follow the fate of exogenous proteins from the maternal circulation to yolk spheres of the growing oocyte. Materials appear to leave the perifollicular capillaries via an interendothelial route, traverse the theca and the patent intercellular channels of the follicular epithelium and the pore canals of the vitelline envelope. At the oocyte surface they are incorporated via micropinocytosis and translocated to growing yolk spheres in the peripheral ooplasm. In contrast to other studies on oocyte growth in teleosts which suggest that yolk is an autosynthetic product, this study substantiates the importance of heterosynthetic processes during oocyte growth in C. Variegatus.     

Morphology of female reproductive system of Mediterranean flatheaded peachborer,Capnodis tenebrionis (Linnaeus, 1761) (Coleoptera: Buprestidae)

Capnodis tenebrionis causes damage in many species of Rosaceae. The present study investigates on the morphology of the female reproductive system of C. tenebrionis. The female reproductive system of C. tenebrionis has a pair of ovaries, lateral oviducts, a common oviduct, spermatheca, and bursa copulatrix. Each ovary in C. tenebrionis consists of approximately 24 telotrophic meroistic type ovarioles. The ovarioles of C. tenebrionis have four regions (terminal filament, tropharium, vitellarium, and pedicel). Tropharium have trophocytes, young oocytes, and prefollicular cells. Vitellarium consists of previtellogenic, vitellogenic, and choriogenic oocytes. Previtellogenic oocyte is surrounded by cylindrical epithelial cells. Its ooplasm is homogeneous and basophilic. In vitellogenic oocyte, there are intercellular spaces between monolayered follicle cells. Its ooplasm has yolk granules and lipid droplets. Choriogenic oocyte are surrounded by chorion and single-layered cylindrical cells. There are yolk granules and lipid droplets in its ooplasm which is asidophilic. In C. tenebrionis female, spermatheca and bursa copulatrix wall is surrounded by thin cuticular intima, monolayer epithelial, glandular cells, and muscle layer. Spermatheca lumen contains a large number of spermatozoa. Bursa copulatrix lumen is filled with secretory material. This study may be useful in terms of the morphology of mature female reproductive organs of Buprestidae and other coleopteran species.     

Structural modulation of the surface and cytoplasm of oocytes during vitellogenesis in the lobster,Homarus americanus. An electron microscope-protein tracer study

The present investigation describes the ultrastructural changes which occur at the surface and in the cytoplasm of developing oocytes of the lobster, Homarus americanus, during vitellogenesis. The immature oocytes showed no surface specializations of the oolemma and no pinocytotic activity was observed. Horseradish peroxidase (HRP) tracer studies showed penetration of the tracer into the perivitelline space, but no uptake by the oocytes. The surfaces of oocytes examined during vitellogenesis, when yolk protein accumulation was maximal, exhibited numerous microvilli that projected into the perivitelline space, often appearing to be embedded in the follicular cell mass. In addition, the plasma membrane of vitellogenic oocytes contained many pinocytotic pits frequently situated at the bases of microvilli. The perivitelline space was engorged with electrondense material which appeared similar to that contained in pinocytotic structures of the oocytes. Vitellogenic oocytes incubated in HRP showed uptake of tracer reaction product by the coated pits and vesicles of the oolemma. Aggregation and subsequent fusion of these vesicles into large multivesicular bodies of ingested material were also observed in vitellogenic oocytes. Animals artificially induced to undergo vitellogenesis exhibited modulations of oocyte ultrastructure similar to those of normal vitellogenesis, notably, pinocytotic incorporation of extra-oocytic material and hypertrophy of oocyte surface microvilli. This study supports the hypothesis for a dual source of yolk protein in the American lobster.     

Ultrastructure of the ovarian follicles in the placentotrophic Andean lizard of the genus Mabuya (Squamata: Scincidae)

We studied the ultrastructural organization of the ovarian follicles in a placentotrophic Andean lizard of the genus Mabuya. The oocyte of the primary follicle is surrounded by a single layer of follicle cells. During the previtellogenic stages, these cells become stratified and differentiated in three cell types: small, intermediate, and large globoid, non pyriform cells. Fluid‐filled spaces arise among follicular cells in late previtellogenic follicles and provide evidence of cell lysis. In vitellogenic follicles, the follicular cells constitute a monolayered granulosa with large lacunar spaces; the content of their cytoplasm is released to the perivitelline space where the zona pellucida is formed. The oolemma of younger oocytes presents incipient short projections; as the oocyte grows, these projections become organized in a microvillar surface. During vitellogenesis, cannaliculi develop from the base of the microvilli and internalize materials by endocytosis. In the juxtanuclear ooplasm of early previtellogenic follicles, the Balbiani's vitelline body is found as an aggregate of organelles and lipid droplets; this complex of organelles disperses in the ooplasm during oocyte growth. In late previtellogenesis, membranous organelles are especially abundant in the peripheral ooplasm, whereas abundant vesicles and granular material occur in the medullar ooplasm. The ooplasm of vitellogenic follicles shows a peripheral band constituted by abundant membranous organelles and numerous vesicular bodies, some of them with a small lipoprotein core. No organized yolk platelets, like in lecithotrophic reptiles, were observed. Toward the medullary ooplasm, electron‐lucent vesicles become larger in size containing remains of cytoplasmic material in dissolution. The results of this study demonstrate structural similarities between the follicles of this species and other Squamata; however, the ooplasm of the mature oocyte of Mabuya is morphologically similar to the ooplasm of mature oocytes of marsupials, suggesting an interesting evolutionary convergence related to the evolution of placentotrophy and of microlecithal eggs. J. Morphol., 2010. © 2010 Wiley‐Liss, Inc.     

Uptake of the yolk protein, lipovitellin, by developing crustacean oocytes

A variety of cytochemical techniques were used to demonstrate how crustacean lipovitellin accumulates within the egg. It was found that a protein serologically identical to the lipovitellin of yolk spheres was present in the hemolymph of vitellogenic crustaceans, but was absent from the hemolymph of males and immature females.In the three crustacean species studied (Uca pugilator, Cambarus clarkii, and Libinia emarginata), pinocytosis of fluorescein-conjugated lipovitellin and trypan blue occurred only during those periods when oocytes were accumulating yolk.It may be concluded from the present studies that yolk spheres develop in crustacean eggs primarily through micropinocytotic uptake of lipovitellin from the hemolymph, although other oocyte proteins appear to be made in the oocyte.     

A Possible Extra-Ovarian Site for Synthesis of Lipovitellin During Vitellogenesis in Artemia sp. (Crustacea; Anostraca)

Summary

Lipovitellin samples, extracted from yolk platelets of cysts, were applied to SDS-PAGE. A female specific antiserum was raised against the high molecular weight apoprotein lipovitellin alpha-1 (LV-α1) of the lipovitellin complex. This anti-LV-α1 was used in the peroxidase-anti-peroxidase staining method with frontal paraffin sections (of 4μm) of whole embedded Artemia. Females were studied during a complete vitellogenic cycle. The presence of exogenous yolk precursors in the fat storage cells of the thoracopods of female Artemia was demonstrated. The amount of the female specific yolk polypeptides and the number of positively stained cells changes during the vitellogenic cycle. In vitro experiments with ³⁵S-radiolabelled methionine show the synthesis of lipovitellin-like substances in the fat storage cells of vitellogenic females.     

Oogenesis and the ovarian cycle in Salamandra salamandra infraimmaculata Mertens (Amphibia; Urodela; Salamandridae) in fringe areas of the taxon's distribution

Reproductive cycle and oogenesis were studied in specimens of Salamandra salamandra infraimmaculata Mertens that inhabit fringe areas of the taxon's distribution in the Mediterranean region. Both ovarian mass and length are correlated significantly with body mass and length. Ovarian length is also correlated with the number of oocytes. During the oogenetic cycle six stages in oocyte development were recognized. Three occur during previtellogenesis: stage 1, in which oogonia divide and form cell nests; stage 2 in which oogonia differentiate into oocytes; and stage 3, in which the oocyte cytoplasm increases in volume. In the vitellogenic phase two additional stages, 4 and 5, were recognized: stage 4, in which lipid accumulates in vacuoles in the periphery followed by the appearance of yolk platelets near the cytoplasmic margin; and stage 5, in which oocyte volume increases rapidly due to increased number of yolk platelets until it reaches its maximal size. During postvitellogenesis one stage was recognized: stage 6, in which the beginning of maturation is characterized by movement of the nucleus toward the animal pole. Oogenesis continues year-round. The first four stages were seen in all ovaries examined. The ovarian cycle is independent of season and reproductive stage apart from the number of mature, postvitellogenic oocytes that increases following gestation toward the beginning of spring (March-April). J. Morphol 231:149–160, 1997. © 1997 Wiley-Liss, Inc.     

Oogenesis of Mozambique tilapia. IV. Yolk formation

Yolk globules in developing oocytes of Tilapia mosambique are formed by two processes: 1) biosynthetical activity of oocyte organoides; 2) vitellogenin migration by micropinocytosis and its further transformation. Undoubtedly, yolk globules of endogenic and exogenic origin are fused. The primary yolk globules are spherical, and the secondary ones are lobular. The latter originate by incorporating the former. The fast growth of the late vitellogenic stage oocytes occurs as a result of active migration of primary yolk globules into the central part of the oocyte and as their association with the secondary yolk globules. In vitellogenic oocytes of T. mosambique no yolk vesicles (cortical granules), were found by any existing methods.     

Immuno-localization of ferritin polypeptides in oocytes and somatic tissue of the freshwater snails Lymnaea stagnalis L. and Planorbarius corneus L.

Summary Antibodies were raised in rabbits against the 19000 Mr and 24000 Mr polypeptides of snail ferritin from Lymnaea stagnalis L. Anti-24000 Mr polypeptide antibodies were purified by an affinity-purification step and were made monospecific for their antigen by preabsorption with the 19000 Mr antigen. These purified antibodies were then used for in situ detection of their respective antigens by the indirect immunofluorescence method. The 19000 Mr polypeptide was found widely distributed in tissues of both pulmonate snails investigated (Lymnaea stagnalis L. and Planorbarius corneus L.) with the most intense antigen-directed fluorescence in certain connective tissue cells, secretory cells of the midgut gland and Sertoli cells and epithelia of the gonadal acini. In contrast, the 24000 Mr polypeptide could be detected only in yolk platelets of vitellogenic oocytes. The results indicate that yolk and somatic cell ferritins differ in immunoreactivity and structure and, accordingly may differ in function.This investigation was supported by the Deutsche Forschungsgemeinschaft. I greatly appreciate the advice given to me by Drs. U. Mays and V. Riedel, Münster.     

An autoradiographic and cytophotometric study of oogenesis in a pulmonate snail,Planorbarius corneus

Summary The spatial and temporal patterns of macromolecular syntheses in oocytes and somatic auxiliary cells of the snail Planorbarius corneus have been investigated by autoradiography and cytophotometry. Oogenesis has been divided into three stages, comprising early meiosis up to diplotene (stage I), previtellogenetic growth phase (stage II), and vitellogenesis (stage III). No DNA synthesis was found in any oocyte stage. In stage-I oocytes, only nucleoli were found labelled with ³H-uridine. Oocyte nuclei of stage II and III actively synthesize RNA in nucleoli and chromosomes. The most intense incorporation of uridine in chromatin probably occurs during the previtellogenesis — vitellogenesis transition period during which cytological findings suggest well developed lampbrush chromosomes. RNA synthesis in amphinucleoli of stage-III oocytes is restricted to basophilic nucleolar parts, whereas acidophilic parts (protein bodies) neither synthesize nor store RNA. During vitellogenesis oocytes incorporate amino acids into yolk platelet proteins. Radioactive proteins are found in yolk platelet precursors 5 h after injection of the tracer and in yolk platelets 3 h thereafter. The labelling pattern suggests that oocytes synthesize certain hitherto unidentified yolk components. No evidence for the participation of follicle cells in synthesis and transport of vitellogenic proteins has been obtained from autoradiography. Cytological findings suggest an important role for these cells in oogenesis. They are highly active in RNA and protein synthesis. Cellular differentiation is accompanied by polyploidization of the nuclei which attain a highest DNA content of 256 c. Polyploidization probably occurs in incremental steps as indicated by complete endomitotic chromosomal cycles. Autoradiographs show that, during vitellogenesis, oocytes do not incorporate significant amounts of glucose, and only certain follicle cells were labelled with glucose, probably indicating the synthesis of glycogen.     

An ultrastructural study of differentiation of pyriform cells and their contribution to oocyte growth in representative squamata.

An electron microscopic study of the differentiation of pyriform cells and their contribution to oocyte growth in three lizards (Tarentola mauritanica, Cordylus wittifer, Platysaurus intermedius) and one colubrid snake (Coluber viridiflavus) revealed that pyriform cells differentiate from small follicle cells via intermediate cells after establishing an intercellular bridge with the oocyte (see also Hubert: Bull Soc Zool Fr 102:151-158, 1977; Filosa et al: J Embryol Exp Morphol 54:5-15 1979; Klosterman: J Morphol 192:125-144, 1987). Once differentiated, pyriform cells display ultrastructural features indicative of synthetic activity, including abundant ribosomes, Golgi membranes, vacuoles, mitochondria, and lipid droplets. These cellular components extend to the apex of the cell at the level of the intercellular bridge, suggesting that constituents of pyriform cells may be transferred to the oocyte. Furthermore, we demonstrate for the first time that pyriform cells incorporate exogenous yolk. The yolk is segregated inside maturing yolk granules that form in the pyriform cell in the same manner as described for vitellogenic oocytes in non-mammalian vertebrates (see Wallace: Developmental Biology, A Comprehensive Synthesis 127-177, 1985). It is the first clear evidence that pyriform cells and the oocyte may fulfill similar vitellogenic functions. The establishment of an intercellular bridge may represent a crucial event in the development of an integrated system in which pyriform cells and oocyte cooperate.     

An ultrastructural study of oogenesis in a marine triclad

The ultrastructural features of oocyte differentiation were studied in the marine triclad Cercyra hastata. Oocytes at several stages of maturation, each surrounded by follicle cell projections, are present within each of the two ovaries. A pre-vitellogenic and a vitellogenic stage have been detected in the oogenesis of C. hastata. The pre-vitellogenic stage is mainly characterized by an increase in the nuclear and nucleolar volume and activity, and the appearance and development of cortical granule precursors which are elaborated by the Golgi complex. In early phases of the vitellogenic stage, intense delamination and blebbing of the nuclear envelope occurs which probably contributes to an increase in number of cytoplasmic membranes and to transfer of nuclear material to the cytoplasm. The rough endoplasmic reticulum is extensively developed and often assumes a ‘whorl’ array. Several areas of yolk precursor formation appear in the whorls. Numerous 2–5 μm protein yolk globules are subsequently formed which appear surrounded by a double membrane (cisternae of the smooth endoplasmic reticulum) and become randomly distributed throughout the cytoplasm of mature oocytes. The peripheral ooplasm is occupied by a monolayer of electron-dense cortical granules. Finally, the evolutionary significance of the autosynthetic mechanism of yolk production is discussed.