Ultrastructural observations on oogenesis in Drosophila

The ultrastructure of the follicle cells and oocyte periplasm is described during the stages of oogenesis immediately prior to, during, and immediately subsequent to, vitellogenesis. A number of features have not been described previously in Drosophila. Some yolk appears prior to pinocytosis of blood proteins. However, most of the protein yolk forms while the periplasm is filled with micropinocytotic invaginations and tubules derived from the oolemma. These tubules retain the internal layer of material characteristic of coated vesicles and are found to fuse with yolk spheres. No accumulation of electron-dense material in the endoplasmic reticulum or Golgi of the oocyte is found. Both trypan blue and ferritin are accumulated by the oocyte. The follicle cells have an elaborate endoplasmic reticulum during the period of maximum yolk accumulation. Adjacent cells are joined at their base by a zonula adhaerens, forming a band around the cells, and by plaques of gap junctions. Gap junctions are also present between nurse cells and follicle cells. During chorion formation, septate junctions also appear between follicle cells, adjacent to the zonula adhaerens.     

Yolk formation in Isohypsibius (Eutardigrada)

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

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 observations on the oogenesis of Triops cancriformis (Crustacea,Notostraca)

Summary Each ovarian follicle of Triops cancriformis is four-celled; these cells (one oocyte and three nurse cells) are interconnected by cytoplasmic bridges. In the course of differentiation, the nurse cells are early recognizable; they increase in size more than the oocyte and their nuclei contain many nucleoli. For the first time in Arthropoda, yolk globules are reported to be present in nurse cell cytoplasm; these globules arise from the smooth endoplasmic reticulum. The functional significance of the intercellular bridges and the trophic role of the nurse cells are discussed.The authors are grateful to Dr. Bruno Sabelli for his support and to Mr. Francesco Monte for his technical assistance     

Ultrastructural studies on the formation of yolk granules in the statoblast of a fresh-water bryozoan,Pectinatella gelatinosa

The formation of protein-carbohydrate yolk in the statoblast of a fresh-water bryozoan, Pectinatella gelatinosa, was studied by electron microscopy. Two types (I and II) of yolk cells were distinguished. The type I yolk cells are mononucleate and comprise a large majority of the yolk cells. The type II yolk cells are small in number; they become multinucleate by fusion of cells at an early stage of vitellogenesis. In both types of yolk cells, electron-dense granules (dense bodies) are formed in Golgi or condensing vacuoles, which are then called yolk granules. For the formation of yolk granules, the following processes are considered: 1. Yolk protein is synthesized in the rough-surfaced endoplasmic reticulum (RER) of the yolk cells. 2. The synthesized protein condenses in the cisternal space of the RER and is packaged into small oval swellings, which are then released from the RER as small vesicles (Golgi vesicles, 300-600 A in diameter). 3. The small vesicles fuse with one another to form condensing vacuoles, or with pre-existing growing yolk granules. 4. In the matrix of the condensing vacuoles or growing yolk granules, electron-dense fibers are fabricated and then arranged in a paracrystalline pattern to form the dense body. 5. After the dense body reaches its full size, excess membrane is removed and eventually the yolk granules come to mature. Toward the end of vitellogenesis of the yolk cells, the cytoplasmic organelles are ingested by autophagosomes derived from multivesicular bodies and disappear.     

Anatomy of the ovaries of the starfish Asterias rubens (Echinodermata)

Summary The ovaries of the starfish Asterias rubens were studied histologically and ultrastructurally. The reproductive system in female specimens consists of ten separate ovaries, two in each ray. Each ovary is made up of a rachis with lateral primary and secondary folds: the acini maiores and acini minores. The ovarian wall is composed of an outer and an inner part, separated by the genital coelomic sinus. The ovarian lumen contains oocytes in various phases of oogenesis, follicle cells, nurse cells, phagocytosing cells and steroid-synthesizing cells.Oogenesis is divided into four phases: (i) multiplication phase of oogonia, (ii) initial growth phase of oocytes I, (iii) growth phase proper of oocytes I, and (iv) post-growth phase of oocytes I. The granular endoplasmic reticulum and the Golgi complex of the oocytes appear to be involved in yolk formation, while the haemal system, haemal fluid and nurse cells may also be important for vitellogenesis. The haemal system is discussed as most likely being involved in synchronizing the development of the ovaries during the annual reproductive cycle and in inducing, stimulating and regulating the function of the ovaries.Steroid-synthesizing cells are present during vitellogenesis; a correlation between the presence of these cells and vitellogenesis is discussed.     

Cytochemistry of late ovarian chambers of Drosophila melanogaster

Summary Late ovarian chambers of Drosophila melanogaster have been examined by ultrastructural cytochemistry in an attempt to characterize some of the transformations which precede the completion of oogenesis. From stage 11 onward peroxidase activity is present in the endoplasmic reticulum of both nurse cells and oocyte, as well as in the egg-covering precursors of the columnar follicle cells. Catalase activity is restricted to the very last stages of oogenesis (stage 13–14) and appears to be located in membrane-bound organelles of the ooplasm which are continuous with the endoplasmic reticulum. Because of the presence of catalase as well as by their structural appearance, these organelles are to be identified as microperoxisomes. Catalase activity becomes cytochemically detectable in the ooplasm somehow in coincidence with the formation of glycogen. Furthermore, glycogen is first formed in intimate association with alpha-1 yolk platelets. On the basis of these findings it is suggested that glycogen synthesis occurs by a process of gluconeogenesis.     

Cells producing insulin-like androgenic gland hormone of the giant freshwater prawn, Macrobrachium rosenbergii, proliferate following bilateral eyestalk-ablation

We found that the androgenic gland (AG) of Macrobrachium rosenbergii possesses three cell types. Type I cells are small polygonal shaped-cells (13.4 μm in diameter), stain strongly with hematoxylin-eosin (H&;E), have abundant multilayered rough endoplasmic reticulum (rER), and nuclei containing mostly heterochromatin. Type II cells are slightly larger (18.6 μm in diameter), stain lightly with H&;E, have rER with dilated cisternae, and nuclei containing mostly euchromatin. Type III cells (previously undescribed) are similar in size and shape to type I cells, but the cytoplasm is unstained and they have a high amount of smooth endoplasmic reticulum (sER) and mitochondria with tubular cristae. Bilateral eyestalk-ablation resulted in AG hypertrophy with a proliferation and predominance of type I cells as determined by bromodeoxyuridine (BrdU) assays. Expression of insulin-like androgenic gland hormone (Mr-IAG), determined by immunohistochemistry, was weak in type I cells, strong in type II cells of both the intact and eyestalk-ablated, and negative in type III cells. It was also detected in spermatogonia, nurse cells, and epithelium lining of the spermatic duct. The function of Mr-IAG in these tissues is yet to be elucidated but the distribution implies a strong role in male reproduction.     

Development of the oocyte and its accessory cells of the polychaete,Diopatra cuprea (Bosc)

The oocyte-nurse cell complex of the polychaetous annelid, Diopatra cuprea, has been explored by various methods of light microscopy and by the technique of electron microscopy. Early in its development the complex appears as a string of cells floating within the coelomic cavity. As this string of cells develops, the volume of one cell (approximately the middle one) increases greatly; while that of the remaining cells, referred to as nurse cells, increase slightly. Due to this differential growth, the two opposing strands of nurse cells are displaced to one side of the oocyte. Nurse cells are joined to one another by cytoplasmic bridges. Cytoplasmic bridges also exist between the strands of nurse cells and the oocyte. The presence of numerous ribosomes within the bridges between the oocyte and nurse cells encourages us to suggest that this organelle may be transferred to the oocyte via this route. The transported ribosomes may be used by the maturing oocyte, or they may be stored by the egg to be utilized during embryogenesis. Moreover, we believe that the nurse cells are not involved in the production of the protein-carbohydrate yolk bodies for we think that these are elaborated by the endoplasmic reticulum in collaboration with certain Golgi complexes of the oocyte.     

Ultrastructural characterization of the gonads of Triops cancriformis (Crustacea, Notostraca) from populations containing both females and males: no evidence for hermaphroditic reproduction

 The gonads of female and male Triops cancriformis specimens from populations of the northern part of the species distribution area were examined by conventional transmission electron microscopy in order to gain insight into the complicated reproductive mode of the species. Ovarian follicles consisted of an oocyte and three nurse cells and were surrounded by a thin layer of follicular cells. Oocytes are initially smaller than nurse cells and contained mitochondria of the cristae type as well as many free ribosomes. The prominent nucleus contained a nucleolus. The cytoplasm of oocytes was filled with yolk globules that were surrounded by membranes of the endoplasmic reticulum. Nurse cells also contained yolk globules. The follicle duct walls consisted of closely packed elongated cells covered by a lamellar basal lamina. No distinct Golgi apparatus was found in the follicle duct cells. The passage of oocytes through the duct was accompanied by a marked flattening of the follicle duct cells. Simultaneously, the oocytes were wrapped in eggshell material before entering the longitudinal oviduct. Testicular lobes were not found in any of the investigated female specimens. In male animals, the epithelial wall of the testicular tubules consisted of both germinal and vegetative cells. Maturing spermatids formed multicellular nests located in clearly delimited cysts in the intercellular space of the testicular epithelium. The lumen of the testicular tubules contained, exclusively, free mature spermatozoa. These characteristics point to the cystic type of spermiogenesis. Mature spermatozoa were non-flagellate, had a regular ovoid shape of 4–5 μm diameter, and an acrosome was not found. Only a negligible amount of spermatozoa (< 5%) showed signs of degeneration. In conclusion, the results provide evidence for a parthenogenetic and/or bisexual reproduction mode rather than a hermaphroditic one in the populations examined. Accepted: 4 August 1997     

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.     

Yolk organelles and their membranes during vitellogenesis ofXenopus oocytes

Summary The yolk platelets ofXenopus laevis have been studied by thin-section and freeze-fracture electron microscopy to characterize the boundary membrane during yolk formation. Throughout vitellogenesis, large yolk platelets are in close contact with smaller nascent yolk organelles. Two types of primordial yolk platelets (I and II) have been discriminated. After membrane fusion these precursors can be completely incorporated into the main body of existing platelets, numerous yolk crystals then merge and form one uniformly stratified core. Lipid droplets are tightly attached to the membrane at all developmental stages of yolk platelets. A direct connection of endoplasmic reticulum to the membranes of yolk platelets was not observed. On freezeetching replicas, yolk-platelet membranes present fracture faces with intramembranous particles (IMP) of various sizes and a heterogeneous distribution of approximately 200–600 IMP/μm² at the E face, and 1200–2100 IMP/μm² at the P face. Again, this presentation of the membrane exhibits neither anastomoses to the endoplasmic reticulum, nor caveolae that exclude the uptake of yolk-containing vesicles into these yolk organelles. Proteinaceous yolk platelets tend to fracture along their periphery through the superficial layers.     

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.     

Cytochemical characterization of the endomembranous system during oocyte secondary growth in Serrasalmus spilopleura (Teleostei, Characiformes, Characidae)

The endomembranous system of Serrasalmus spilopleura oocyte secondary growth was analysed using structural and ultrastructural cytochemical techniques. In vitellogenic oocytes, the endoplasmic reticulum components, the nuclear envelope intermembranous space, some Golgi dictiossomes, lysosomes, yolk granules, regions of the egg envelope and sites of the follicle cells react to acid phosphatase detection (AcPase). The cortical alveoli, some heterogeneous cytoplasmic structures, regions of the egg envelope, and sites of the follicle cells are strongly contrasted by osmium tetroxide and zinc iodide impregnation (ZIO). The endoplasmic reticulum components, some vesicles, and sites of the follicle cells also react to osmium tetroxide and potassium iodide impregnation (KI). The biosynthetic pathway of lysosomal proteins, such as acid phosphatase, required for vitellogenesis, involves the endoplasmic reticulum, Golgi complex, vesicles with inactive hydrolytic enzymes, and, finally, lysosomes. In S. spilopleura oocytes at secondary growth, the endomembranous system takes part in the production of the enzymes needed for vitellogenesis, and in the metabolism of yolk exogenous components (AcPase detection). The endomembranous system compartments also show reduction capacity (KI reaction) and are involved in the metabolism of proteins rich in SH‐groups (ZIO reaction).     

An electron microscope study on the developing oocytes of the crab Cancer pagurus L. with special reference to yolk formation (Crustacea)

Summary The developing oocytes of the crab Cancer pagurus L. were studied with the light and electron microscope.Protein yolk formation was found to take place in two different ways. Yolk precursors of type 1 accumulate within the cisternae of an extensively developed granular endoplasmic reticulum. Also further growth and transformation into the definite yolk body occur within the reticular membranes. There is no structural indication that any other cell organelle contributes to the synthesis of this type of yolk building.Protein yolk formation of type 2 involves accumulation and transformation of material within a limiting membrane of the smooth type. The enclosed material is presumably derived from micropinocytosis, enclosed cellular elements and vesicles originating from the Golgi complex.It thus appears that the cell organelles play an important role in the process of drotein yolk formation in the growing oocytes of Cancer pagurus.     

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.     

Ultrastructural studies on the vitellogenesis of Tetrodontophora bielanensis (Waga) (Collembola)

Summary Vitellogenesis in Tetrodontophora bielanensis (Waga) is of the mixed type. Part of the yolk material is produced inside the oocyte (auto-synthesis), while part is absorbed by micropinocytosis. During autosynthesis polyribosomes, rough endoplasmic reticulum and dictyosomes take part. Regardless of their origin, mature yolk spheres are constructed identically and are composed of three elements: cortex layer, matrix and crystals. Histochemical tests show that polysaccharides are present in the yolk spheres. Lipid droplets have been observed in the ooplasm; they develop without visible contact with any of the organelles. Among the reserve materials the following have been found: rough endoplasmic reticulum, dictyosomes, polyribosomes, mitochondria and a few microtubules.     

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

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

中华稻蝗卵子卵黄发生期超微结构研究

利用透射电镜研究了中华稻蝗Oxya chinesis卵子发生中卵黄发生期的超微结构.卯黄发生初期,滤泡上皮细胞胞质内出现大量粗面内质网及线粒体等细胞器,可能与为卵母细胞提供营养有关.卵黄发生期卵母细胞胞质内卵黄球逐渐增多,它也许有多种来源.观察到环形片层结构,并讨论了其可能功能.     

Mechanisms of protein yolk synthesis and deposition in crustacean oocytes

Summary Electron microscope studies on the oocytes of several crustacean species demonstrate that the protein yolk arises within vesicular and lamellar forms of the rough-surfaced endoplasmic reticulum. The vesicular form of the endoplasmic reticulum may have its origin from a blebbing process of the outer layer of the nuclear envelope. Disc-shaped granules, representing precursor elements of the yolk granules, appear within the vesicular and lamellar profiles of endoplasmic reticulum. Autoradiographic results suggest that the ribosomes attached to the endoplasmic reticulum take part in the biosynthesis of yolk proteins. Numerous disc-shaped granules accumulate within the cisternae of the endoplasmic reticulum, but eventually they undergo a transformation into a finely granular yolk granule. Thus, both the origin and growth of protein yolk granules occur within membranes constituting the endoplasmic reticulum. The results provide evidence that intra-ooplasmic synthesis of yolk protein occurs in these oocytes.This investigation was supported by research grants (HD-00699; GM-09229) and a Career Development Award (GM-11,524) from the National Institutes of Health, U.S. Public Health Service.