Ultrastructural changes in micropylar cells and formation of the micropyle during oogenesis in the medaka Oryzias latipes

Ultrastructural changes in differentiating micropylar cells and in the micropyle occur during oogenesis in the medaka, Oryzias latipes. A micropylar cell is not detectable in previtellogenic ovarian follicles. In the early vitellogenic phase, the micropylar cell becomes differentiated from neighboring granulosa cells by its electrondense cytoplasm. The micropylar cell in this phase characteristically displays an increase in rough endoplasmic reticulum, Golgi complexes, and tonofilaments around the nucleus. By the late vitellogenic phase, the enlarged micropylar cell extends a broad cytoplasmic process to the oocyte surface. A conspicuous feature of the process is a large bundle of microtubules oriented perpendicular to the oocyte surface. The inner surface of the micropylar canal has a spiral structure and is covered with the outermost layer of the chorion. In the postvitellogenic phase, the main cell body possesses many tonofilaments, mitochondria, Golgi complexes, and rough endoplasmic reticulum, and the winding cytoplasmic process contains a twisted large bundle of microtubules with a bundle of tonofilaments as its core. The spiral structure of the micropylar vestibule and the micropylar canal reflects the twisting associated with elongation of fibrous bundles of the micropylar cell anchored on the chorion at the animal pole of the oocyte.     

Previtellogenic development and vitellogenin synthesis in the fat body of a mosquito: an ultrastructural and immunocytochemical study

We describe two phases, previtellogenic and vitellogenic, in the activity of the trophocytes in the fat body of the mosquito Aedes aegypti. The previtellogenic phase, leading to trophocyte competence to synthesize vitellogenin (Vg), occurred during the first 3 days after eclosion. This phase was characterized by enlargement and activation of the nucleoli, proliferation of ribosomes and rough endoplasmic reticulum (RER), development of Golgi complexes, and extensive invaginations of the plasma membrane. During the vitellogenic phase, initiated by a blood meal, Vg was first detected, by immunofluorescence, 1 hr after feeding. The intensity of the immunoreaction increased for the next 24 hr, was declining at 30 hr, and had disappeared by 48 hr. Vg synthesis was characterized ultrastructurally by the enlargement of the RER and the formation of dense secretion granules in Golgi complexes. These secretion granules were two to three times larger at the peak of Vg synthesis than at the beginning. The granules discharged their contents by exocytosis. Two electron microscopical immunocytochemical methods, immunoferritin and peroxidase-antiperoxidase, confirmed this pathway of Vg processing. For the first 12 hr after feeding. Vg synthetic organelles proliferated and the active nucleoli were multilobed; thereafter, while Vg synthesis continued, the nucleoli began to regress into compact bodies. Termination of Vg synthesis was marked by autophagical degradation of Vg synthetic and processing organelles.     

东方扁虾卵子发生的超微结构

根据卵细胞的形态、内部结构特征及卵母细胞与滤泡细胞之间的关系,东方扁虾的卵子发生可划分为卵原细胞、卵黄发生前卵母细胞、卵黄发生卵母细胞和成熟卵母细胞等四个时期。卵原细胞胞质稀少,胞器以滑面内质网为主。卵黄发生前卵母细胞核明显膨大,特称为生发泡;在靠近核外膜的胞质中可观察到核仁外排物。卵黄发生卵母细胞逐渐为滤泡细胞所包围;卵黄合成旺盛,胞质中因而形成并积累了越来越多的卵黄粒。东方扁虾卵母细胞的卵黄发生是二源的。游离型核糖体率先参与内源性卵黄合成形成无膜卵黄粒。粗面内质网是内源性卵黄形成的主要胞器。滑面内质网、线粒体和溶酶体以多种方式活跃地参与卵黄粒形成。卵周隙内的外源性物质有两个来源:滤泡细胞的合成产物和血淋巴携带、转运的卵黄蛋白前体物。这些外源性物质主要通过质膜的微吞饮作用和微绒毛的吸收作用这两种方式进入卵母细胞,进而形成外源性卵黄。内源性和外源性的卵黄物质共同参与成熟卵母细胞中富含髓样小体的卵黄粒的形成。卵壳的形成和微绒毛的回缩被认为是东方扁虾卵母细胞成熟的形态学标志。       

Ultrastructural studies of differentiation in the oocyte of the polyclad turbellarian,Prostheceraeus floridanus

Oocyte differentiation in the polyclad turbellarian Prostheceraeus floridanus has been examined to determine the nature of oogenesis in a primitive spiralian. The process has been divided into five stages. (1) The early oocyte: This stage is characterized by a large germinal vesicle surrounded by dense granular material associated with the nuclear pores and with mitochondria. (2) The vesicle stage: The endoplasmic reticulum is organized into sheets which often contain dense particles. Vesicles are found in clusters in the cytoplasm, some of which are revealed to be lysosomes by treatment with the Gomori acid phosphatase medium. (3) Cortical granule formation: Cortical granules are formed by the fusion of filled Golgi vasuoles which have been released from the Golgi saccules. The association between the endoplasmic reticulum and Golgi suggests that protein is synthesized in the ER and transferred to the Golgi where polysaccharides are added to form nascent cortical granules. (4) Yolk synthesis: After a large number of cortical granules are synthesized, yolk bodies appear. They originate as small membrane-bound vesicles containing flocculent material which subsequently increase in size and become more compact. Connections between the forming yolk bodies and the endoplasmic reticulum indicate that yolk synthesis occurs in the ER. (5) Mature egg: In the final stage, the cortical granules move to the periphery and yolk platelets and glycogen fill the egg. At no time is there any evidence of uptake of macromolecules at the oocyte surface. Except for occasional desmosomes between early oocytes, no membrane specialization or cell associations are seen throughout oogenesis. Each oocyte develops as an independent entity, a conclusion supported by the lack of an organized ovary.     

Previtellogenic and vitellogenic oocytes in ovarian follicles of cultured siberian sturgeon Acipenser baerii (Chondrostei,Acipenseriformes)

Previtellogenic and vitellogenic oocytes in ovarian follicles from cultured Siberian sturgeon Acipenser baerii were examined. In previtellogenic oocytes, granular and homogeneous zones in the cytoplasm (the ooplasm) are distinguished. Material of nuclear origin, rough endoplasmic reticulum, Golgi complexes, complexes of mitochondria with cement and round bodies are numerous in the granular ooplasm. In vitellogenic oocytes, the ooplasm comprises three zones: perinuclear area, endoplasm and periplasm. The endoplasm contains yolk platelets, lipid droplets, and aggregations of mitochondria and granules immersed in amorphous material. In the nucleoplasm, lampbrush chromosomes, nucleoli, and two types of nuclear bodies are present. The first type of nuclear bodies is initially composed of fibrillar threads only. Their ultrastructure subsequently changes and they contain threads and medium electron dense material. The second type of nuclear bodies is only composed of electron dense particles. All nuclear bodies impregnate with silver, stain with propidium iodide, and are DAPI‐negative. Their possible role is discussed. All oocytes are surrounded by follicular cells and a basal lamina which is covered by thecal cells. Egg envelopes are not present in previtellogenic oocytes. In vitellogenic oocytes, the plasma membrane (the oolemma) is covered by three envelopes: vitelline envelope, chorion, and extrachorion. Vitelline envelope comprises four sublayers: filamentous layer, trabecular layer 2 (t2), homogeneous layer, and trabecular layer 1 (t1). In the chorion, porous layer 1 and porous layer 2 are distinguished in most voluminous examined oocytes. Three micropylar cells that are necessary for the formation of micropyles are present between follicular cells at the animal hemisphere. J. Morphol. 278:50–61, 2017. ©© 2016 Wiley Periodicals,Inc.     

Previtellogenic oocytes of South African largemouth bass Micropterus salmoides Lacépède 1802 (Actinopterygii,Perciformes) - the Balbiani body,cortical alveoli and developing eggshell

The ovaries of the largemouth bass Micropterus salmoides, an alien and invasive species in South Africa, contain a germinal epithelium which consists of germline and somatic cells, as well as previtellogenic and late vitellogenic ovarian follicles. The ovarian follicle consists of an oocyte surrounded by follicular cells and a basal lamina; thecal cells adjacent to this lamina are covered by an extracellular matrix. In this article, we describe the Balbiani body and the polarization and ultrastructure of the cytoplasm (ooplasm) in previtellogenic oocytes. The nucleoplasm in all examined oocytes contains lampbrush chromosomes, nuclear bodies and several nucleoli near the nuclear envelope. The ultrastructure of the nucleoli is described. Numerous nuage aggregations are present in the perinuclear cytoplasm in germline cells as well as in the ooplasm. Possible roles of these aggregations are discussed. The ooplasm contains the Balbiani body, which defines the future vegetal region in early previtellogenic oocytes. It is comprised of nuage aggregations, rough endoplasmic reticulum, Golgi apparatus, mitochondria, complexes of mitochondria with nuage-like material, and lysosome-like organelles. In mid-previtellogenic oocytes, the Balbiani body surrounds the nucleus and later disperses in the ooplasm. The lysosome-like organelles fuse and transform into vesicles containing material which is highly electron dense. As a result of the fusion of the vesicles of Golgi and rough endoplasmic reticulum, the cortical alveoli arise and distribute uniformly throughout the ooplasm of late previtellogenic oocytes. During this stage, the deposition of the eggshell (zona radiata) begins. The eggshell is penetrated by canals containing microvilli and consists of the following: the internal and the external egg envelope. In the external envelope three sublayers can be distinguished.     

Ultrastructural study of oogenesis in Phoronopsis harmeri (Phoronida)

Temereva, E.N., Malakhov, V.V. and Yushin, V.V. 2011. Ultrastructural study of oogenesis in Phoronopsis harmeri (Phoronida). —Acta Zoologica (Stockholm) 92 : 241–250. The successive stages of oogenesis in Phoronopsis harmeri were examined by electron microscopy methods. During the oogenesis, each oocyte is encircled by vasoperitoneal (coelomic) cells forming a follicle. The previtellogenic oocytes are small cells which accumulate ribosomes for future synthesis; their cytoplasm contains characteristic clusters of mitochondria and osmiophilic particles resembling a germ plasm of other metazoans. The cytoplasm of the vitellogenic oocytes includes numerous mitochondria, cisternae of the rough endoplasmic reticulum, Golgi bodies and annulate lamellae. The synthesis of three types of inclusions was observed: strongly osmiophilic granules (lipid droplets) as a prevalent component, distinctly larger granules surrounded by membrane (proteinaceous yolk) and numerous large vesicles with pale flocculent content. No inclusions which could be unequivocally interpreted as the cortical granules were detected. The surface of the vitellogenic oocytes is covered by microvilli which increase in number and length during development. The oogenesis in Phoronida may be interpreted as follicular because of close association of oocytes with the vasoperitoneal tissue. However, well‐developed synthetic apparatus together with a strongly developed microvillous surface and absence of endocytosis indicate a clear case of autosynthetic vitellogenesis. Thus, in phoronids, there is a combination of simply developed follicle and autosynthesis that, apparently, is plesiomorphic character.     

Oogenesls in the terrestrial hermit crab,coenobita clypeatus (decapoda,anomura): II. Vitellogenesis

Oocytes from the land hermit crab, Coenobita clypeatus, in various stages of vitellogenesis were examined by light and electron microscopy. Early vitellogenic oocytes are characterized by accumulations of discrete vesicles of endoplasmic reticulum in the perinuclear cytoplasm. As oocytes develop, the endoplasmic reticulum becomes abundant, and numerous Golgi complexes are seen. There is a well developed Golgi-endoplasmic reticulum interaction. Within the confines of the reticulum are discrete intracisternal granules, which can be seen coalescing into electron-dense yolk bodies. Lipid accumulation is seen throughout the cytoplasm. Coincident with the burst of intra-oocytic metabolism are oolemma modifications and micropinocytosis, which provide ultrastructural evidence for extra-oocytic yolk production. The mature oocyte contains numerous yolk and lipid vesicles of varying electron density that comprise both intra- and extra-oocytic substrates.     

Differentiation of a calsequestrin-containing endoplasmic reticulum during sea urchin oogenesis

We have used light and electron microscopic immunolocalization to study the distribution of a sea urchin calsequestrin-like protein (SCS) during sea urchin oogenesis. SCS was localized exclusively in the lumen of the endoplasmic reticulum (ER) and in the nuclear envelope of oocytes of all maturation stages. Immunoelectron microscopy also revealed that SCS is not present in golgi complexes of oocytes. Double label immunofluorescent staining of frozen sections of ovary with the SCS antiserum and an antibody to the cortical granule protein hyalin indicated a dramatic morphogenesis of the SCS-containing ER (SCS-ER) coincident with oocyte maturation. This differentiation included an apparent increase in the amount and complexity of the cytoplasmic SCS-ER network, the transient appearance of stacks of SCS-ER cisternae in synthetically active vitellogenic oocytes, and the restructuring of the SCS-ER in the cortex. Immunofluorescence of isolated oocyte cortices showed a plasma membrane-associated SCS-ER which was much less dense and regular than that found surrounding the cortical granules in the mature unfertilized egg cortex. Cytoplasmic and cortical microtubule arrays are present in oocytes and may provide the basis for the SCS-ER distributional dynamics. The results of this study underscore the dynamic nature of ER and how it's organization reflects cellular functions. We suggest that the establishment during oogenesis of the dense SCS-ER tubuloreticulum provides the egg with the calcium sequestration and release apparatus that regulates calcium fluxes during egg activation and early development.     

Formation of an egg envelope in the pycnogonid, Propallene longiceps (Pycnogonida, Callipallenidae)

Vitellogenic oocytes of all pycnogonids studied so far contain dilated elements of the endoplasmic reticulum, filled with characteristic electron-dense bodies. During vitellogenesis these bodies fuse and form larger, almost spherical granules that were traditionally interpreted as nascent yolk granules. Here, we present the results of ultrastructural investigations of previtellogenic and early vitellogenic oocytes of Propallene longiceps (Pycnogonida, Callipallenidae). We show that the intra-cisternal bodies/granules of pycnogonids are not involved in vitellogenesis but contain macromolecules that are released from the oocyte and contribute to the formation of an egg envelope. The obtained results are discussed in a phylogenetic context. We suggest that the presence of the intra-cisternal electron-dense bodies in the oocyte cytoplasm represents a plesiomorphic character of arthropods inherited from the arthropod ancestor.     

The ultrastructure of oogenesis and yolk formation in Labidocera aestiva (Copepoda: Calanoida)

Yolk formation in the oocytes of the free-living, marine copepod, Labidocera aestiva (order Calanoida) involves both autosynthetic and heterosynthetic processes. Three morphologically distinct forms of endogenous yolk are produced in the early vitellogenic stages. Type 1 yolk spheres are formed by the accumulation and fusion of dense granules within vesicular and lamellar cisternae of endoplasmic reticulum. A granular form of type 1 yolk, in which the dense granules within the cisternae of endoplasmic reticulum do not fuse, appears to be synthesized by the combined activity of endoplasmic reticulum and Golgi complexes. Type 2 yolk bodies subsequently appear in the ooplasm but their formation could not be attributed to any particular oocytic organelle. In the advanced stages of vitellogenesis, a single narrow layer of follicle cells becomes more developed and forms extensive interdigitations with the oocytes. Extra-oocytic yolk precursors appear to pass from the hemolymph into the follicle cells and subsequently into the oocytes via micropinocytosis. Pinocytotic vesicles fuse in the cortical ooplasm to form heterosynthetically derived type 3 yolk bodies.     

Ultrastructural investigations of the ovary and oogenesis in the pycnogonids Cilunculus armatus and Ammothella biunguiculata (Pycnogonida, Ammotheidae)

Abstract. Ovarian ultrastructure and oogenesis in two pycnogonid species, Cilunculus armatus and Ammothella biunguiculata , were investigated. The ovary is morphologically and functionally divided into trunk and pedal parts. The former represents the germarium and contains very young germ cells in a pachytene or postpachytene phase, whereas the latter houses developing previtellogenic and vitellogenic oocytes and represents the vitellarium. Intercellular bridges were occasionally found between young (trunk) germ cells. This indicates that in pycnogonids, as in other animal groups, at the onset of oogenesis clusters of germ cells are generated. As nurse cells are absent in the ovaries of investigated species, the clusters must secondarily split into individual oocytes. In the vitellarium, the oocytes are located outside the ovary. Each oocyte is connected to the ovarian tissue by a stalk composed of several somatic cells. The stalk cells directly associated with the oocyte are equipped with irregular projections that reach the oocyte plasma membrane. This observation suggests that the stalk cells may play a nutritive role. The ooplasm of vitellogenic oocytes comprises mitochondria, free ribosomes, stacks of annulate lamellae, active Golgi complexes, and vesicles derived from these complexes. Within the latter, numerous electron-dense bodies are present. We suggest that these bodies contribute to yolk formation.     

Oogenesis in the annelid Enchytraeus albidus with special reference to the origin and cytochemistry of yolk

In the annelid Enchytraeus albidus the ovary is composed of packets containing eight synchronously developing oocytes. Each oocyte in the packet is connected, via a bridge, to a common cytoplasmic mass. Developmental synchrony of oocytes within individual packets is probably related to the ooplasmic continuity. The young previtellogenic oocyte contains many polysomes, a few cisternae of smooth and rough endoplasmic reticulum, small Golgi complexes, and mitochondria. Many of the mitochondria are dumbbell-shaped and may thus represent division stages. Vitellogenesis is marked by the appearance of peripherally located lipid yolk and small, densely staining granules scattered throughout the ooplasm. There is an increase of smooth endoplasmic reticulum, mitochondria, and enlarged Golgi elements. Small multivesicular-like bodies, the early stages of developing yolk, are derived from the Golgi complex. The mature yolk sphere is bipartite and consists of (a) a variable number of dense spheres, the core bodies, which are produced in the ooplasm by the Golgi complex and which become embedded in (b) a dense matrix. The electron opaque tracer, horseradish peroxidase is incorporated into the oocyte and deposited in the matrix suggesting that this component of the yolk sphere is obtained by micropinocytosis. Enzyme digestions and various cytochemical techniques suggest that the core bodies are rich in carbohydrate, probably as glyco- or mucoproteins, and that the matrix is rich in lipid.     

Oogenesis in Hydra. III. The growth and fusion of the oocytes

The ultrastructure of oocytes in Hydra has been studied prior to the beginning of their phagocytic activity. The formation of Golgi complexes and vacuolar system by means of the rough endoplasmic reticulum budding and the formation of numerous enzymatic granules within the Golgi complex are considered with respect to the preparation of cells for the processes of phagocytosis and intracellular digestion of structures. The oocyte situated in the center of each accumulation of syncytially connected cells, descendants of one interstitial cell, becomes the egg, whereas all others are consumed by it and transform into endocytes. The partial and complete confluence of oocytes is described. In the first case, a part of cytoplasm of the cells surrounding the growing oocyte is torn away and included in this oocyte. In the second case, the confluence of large oocytes containing endocytes in their cytoplasm takes place. The growing oocyte forms pseudopodia and cytoplasmic processes which move apart the epithelial-muscle cells, reach the mesogloea and come in contact with the gastroderm cells. The role of these contacts in the oocyte growth is discussed.     

Oogenesis in Actinoposthia beklemischevi (Platyhelminthes, Acoela): an ultrastructural and cytochemical study

The oogenesis of the acoel Actinoposthia beklemischevi can be divided into a previtellogenic and a vitellogenic stage. Maturing oocytes are surrounded by accessory cells (a.c.) that produce electrondense granules, the content of which is released into the space between the oocyte and a.c. and gives rise to a thin primary egg envelope. The a.c. may also contribute to yolk synthesis by transferring low molecular weight precursors to the oocyte. Two types of inclusion are produced in maturing oocytes. Type I inclusions are small, roundish granules produced by the Golgi complex. They have a proteinaceous non-polyphenolic content which is discharged in the intercellular space and produce a thicker secondary egg envelope. Type I inclusions represent eggshell-forming granules (EFGs). Type II inclusions are variably sized globules progressively changing their shape from round to crescent. They appear to be produced by the ER, contain glycoproteins and remain scattered throughout the cytoplasm in large oocytes. Type II inclusions represent yolk. The main features of oogenesis in Actinoposthia are: (a) EFGs have a non-polyphenolic composition; (b) the egg envelope has a double origin and is not sclerotinized; (c) yolk production appears to be autosynthetic. The present ultrastructural findings are compared with those from other Acoelomorpha and Turbellaria.     

Cytodifferentiation and vitellogenesis during oogenesis in arachnida: Cytological studies on developing oocytes of a harvestman

Light and electron microscope studies were made on harvestman oocytes during the course of their origin, differentiation, and vitellogenesis. The germ cells appear to originate from the ovarian epithelium. They subsequently migrate to the outer surface of the epithelium, where they remain attached often by means of stalk cells which suspend them in the hemocoel during oogenesis. The “Balbiani bodies,” “yolk nuclei,” or “nuage” constitute a prominent feature of young, previtellogenic oocytes, and take the form of large, but variable sizes of electron-dense cytoplasmic aggregates with small fibrogranular components. The cytoplasmic aggregates fragment and disperse, and cannot be detected in vitellogenic oocytes. The young oocytes become surrounded by a vitelline envelope that appears to represent a secretory product of the oocyte. The previtellogenic oocytes are impermeable to horseradish peroxidase under both in vivo and in vitro conditions. In addition to mitochondria, dictyosomes, and abundant ribosomes, the ooplasm of the previtellogenic oocyte acquires both vesicular and lamellar forms of the rough-surfaced endoplasmic reticulum. In many areas, a dense homogeneous product appears within the cisternae of the endoplasmic reticulum and represents nascent yolk protein synthesized by the oocyte during early stages of vitellogenesis. Later in vitellogenesis, the oocyte becomes permeable to horseradish peroxidase under both in vivo and in vitro conditions. This change is associated with a massive process of micropinocytosis which is reflected in the presence of large numbers of vesicles of variable form and structure in the cortical ooplasm. Both spherical and tubular vesicles are present, as are coated and uncoated vesicles. Stages in the fusion of the vesicles with each other and with developing yolk platelets are illustrated. In the harvester oocytes, vitellogenesis is a process that involves both autosynthetic and heterosynthetic mechanisms.     

Cytochemical and fine structural analysis of oogenesis in the gastropod, Ilyanassa obsoleta

An analysis of differentiating oocytes of the gastropod, Ilyanassa obsoleta, has been made by techniques of light and electron microscopy. Early previtellogenic oocytes are limited by a smooth surfaced oolemma and are associated with each other by maculae adhaerentes. Previtellogenic oocytes are also distinguished by a large nucleus containing randomly dispersed aggregates of chromatin. Within the ooplasm are Golgi complexes, mitochondria and a few cisternae of the rough endoplasmic reticulum. When vitellogenesis begins, the oolemma becomes morphologically specialized by the formation of microvilli. One also notices an increase in the number of organelles and inclusions such as lipid droplets. During vitellogenesis there is a dilation of the saccules of the Golgi complexes and cisternae of the endoplasmic reticulum. Associated with the Golgi complexes are small protein-carbohydrate yolk precursors encompassed by a membrane. These increase in size by fusing with each other. The “mature” yolk body is a membrane-bounded structure with a central striated core and a granular periphery. At maturity a major portion of the ooplasmic constituents such as as mitochondria and lipid droplets occupy the animal region while the bulk of the population of yolk bodies are situated in the vegetal hemisphere. The follicle cells incompletely encompass the developing oocyte. In addition to the regularly occurring organelles, follicle cells are characterized by the presence of large quantities of rough endoplasmic reticulum and Golgi complexes whose saccules are filled with a dense substance. Associated with the Golgi saccules are secretory droplets of varied size. Amongst the differentiating oocytes and follicle cells are Leydig cells. These cells are characterized by a large vacuole containing glycogen. A possible function for the follicle and Leydig cells is discussed.     

凡纳滨对虾卵母细胞卵黄发生的超微结构

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

A light and electron microscope study on oogenesis in the freshwater pulmonate snail Biomphalaria glabrata

In the freshwater snail Biomphalaria glabrata the formation and composition of yolk granules and the role of the follicle cells were studied by histochemical and electron microscopical techniques. The rough endoplasmic reticulum and the Golgi apparatus appeared to be involved in yolk formation, which is a continuous process throughout oogenesis. From the very beginning of yolk formation two main types of yolk granules were distinguished morphologically. However, with histochemical and enzyme cytochemical methods no differences were observed between these types. The granules acquire lysosomal enzymes after oviposition, indicating that their main function is probably digestion of perivitelline fluid, which contains nutrients for the developing embryo.Yolk formation and the activity of the follicle cells were studied in successive stages of oogenesis by quantitative electron microscopy. The data strongly suggest that the follicle cells are involved in the formation of the follicular cavity and hence in the ovulation process.     

Sexual cycle and seasonal changes in the ovary of the red mullet, Mullus surmuletus, from the southern coast of Brittany

The reproductive cycle of the red mullet is described on a macroscopic scale in terms of the GSI, HSI and K , and on a microscopic scale in terms of histological changes in the ovary and changes in the oocyte size frequency distribution. On the southern coast of Brittany the red mullet breeds in May and June. During oogenesis, the previtellogenic period lasts 6 months and the secondary phase of vitellogenesis no more than 3 months. When spawning commences the process of vitellogenesis ceases and up to 20% of the vitellogenic oocytes become atretic. Prior to spawning a single batch of oocytes can be seen to be entering secondary vitellogenesis. During the immediate prespawning and spawning periods the existing vitellogenic oocytes mature but there is no recruitment from the stock of previteilogenic oocytes. This results in a gap or hiatus in the oocyte size frequency distribution between previtellogenic and vitellogenic oocytes within which there are very few resting or maturing oocytes. The red mullet appears to be a determinate spawner, in which egg loss through atresia considerably reduces the potential fecundity.