A comparative histochemical study of fish (Channa maruleus) and amphibian (Bufo stomaticus) oogenesis

Summary Comparative histochemical studies on the fish (Channa maruleus) and amphibian (Bufo stomaticus) oogenesis demonstrate a great similarity in the growth and differentiation of their egg follicle. The ooplasm, germinal vesicle and egg-membranes show distinct morphological and cytochemical changes during previtellogenesis and vitellogenesis.During previtellogenesis the various components of the follicle are engaged in the synthesis of protoplasm as shown by the proliferation of yolk nucleus substance, mitochondria and some lipid bodies in the ooplasm and of nucleoli in the germinal vesicle. The substance of the yolk nucleus consisting of proteins, lipoproteins and RNA first appears adjacent to the nuclear membrane. Numerous mitochondria of lipoprotein composition, and some lipid bodies consisting of unsaturated phospholipids lie in association with the yolk nucleus which forms substratum for the former. The lipid bodies, present inside the germinal vesicle, follicular epithelium, and adjacent to the plasma membrane in association with some pinocytotic vacuoles, have been considered to play a significant role in the active transport of some substances from the environment into the ooplasm and from the latter into the germinal vesicle. The follicular epithelium itself is very poorly developed, negating its appreciable role in the contribution of specific substances into the oocyte, which seem to be contributed by the germinal vesicle showing a considerable development of nuclear sap, basophilic granules and nucleoli consisting of RNA and proteins; many large nucleoli bodily pass into the cytoplasm during the previtellogenesis of Channa, where their substance is gradually dissolved. The intense, diffuse, basophilic substance of the cytoplasm is believed due to free ribosomes described in many previous ultrastructural studies.During vitellogenesis, the various deutoplasmic inclusions, namely carbohydrate yolk, proteid yolk and fatty yolk, are deposited in the ooplasm. The carbohydrate yolk bodies rich in carbohydrates originate in association with the plasma membrane and correspond to vesicles and cortical granules of previous studies. The proteid yolk consisting of proteins and some lipoproteins, and fatty yolk containing first phospholipids and some triglycerides and then triglycerides only are deposited under the influence of yolk nucleus substance, mitochondria and cytoplasm. The mitochondria and yolk nucleus substance foreshadow in some way the pattern of these two deutoplasmic inclusions and persist at the animal pole of mature egg while the other inclusions of previtellogenesis disappear from view. The pigment granules, which also show a gradient from the animal to vegetal pole in Bufo, are also formed in association with yolk nucleus substance and mitochondria. Some glycogen also appears in both the species. The nuclear membrane becomes irregular due to the formation of lobes. The lipid bodies of the germinal vesicle come to lie outside the nuclear membrane, suggesting active transport of some substances into the ooplasm; many nucleoli bodily pass into the ooplasm of Bufo, where they are gradually absorbed. The amount of basophilic granules is considerably increased in the germinal vesicle during vitellogenesis. Various egg-membranes such as outer epithelium, thin theca, single-layered follicular epithelium, zona pellucida or vitelline membrane surround the vitellogenic oocytes. The zona pellucida formed between the oocyte and follicle cells consists of a carbohydrate-protein complex. The follicle cells show lipid droplets, mitochondria and basophilic substance in their cytoplasm. The various changes that occur in the components of the follicle during vitellogenesis seem to be initiated by gonadrotrophins formed under the influence of specific environmental conditions.The author wishes to express sincere appreciation and gratitude to Dr. Gilbert S. Greenwald, who has made the completion of this investigation possible.Ph. D. Population Council Post-doctoral Fellow.     

Histochemical studies on the yolk-nucleus in fish oogenesis

Summary A histochemical study of the oogenesis of two species of fresh water fishes, Channa maruleus and Heteropneustes fossilis, was undertaken to reveal the origin, structure, histochemical nature, and function of the so-called yolk-nucleus. The basophilic substance of the yolk-nucleus, which is situated in the juxta-nuclear cytoplasm, gradually accumulates adjacent to the nuclear membrane. It is a homogeneous, spherical mass. In Channa, some basophilic, dense bodies develop in the yolk-nucleus. Histochemical tests show that the yolk-nucleus and dense bodies are rich in RNA and proteins. Mitochondria of lipoprotein composition and lipid inclusions, composed of unsaturated phospholipids, appear in association with the yolk-nucleus. Throughout previtellogenesis, the yolk-nucleus continues to proliferate its basophilic, RNA-containing substance and other inclusions. Finally it disintegrates while lying in the peripheral cytoplasm of the larger oocytes which show the synthesis of yolk bodies. During yolk formation, lipid inclusions and mitochondria start disappearing from view but the RNA-containing substance, originated from the yolk-nucleus of previtellogenesis, continues to persist among the growing yolk bodies. The latter arise de novo from the ground cytoplasm, under the influence of the RNA-containing substance, mitochondria and lipid inclusions of previtellogenesis.This work was carried out in the Department of Zoology, University of Gorakhpur, Gorakhpur, India.Population Council Post-Doctoral Fellow.     

Cytoplasmic inclusions in the oogenesis of ophiocephalus punctatus

Summary TheGolgi apparatus can be seen in the usual juxtanuclear position in a very young oocyte. Later, the mass increases in size and resolves itself into very minuteGolgi spheres which disperse gradually in the general cytoplasm. There is some evidence of the infiltrati on of theGolgi bodies from the follicular epithelium to the egg. The granular mitochondria arise also in the juxtanuclear area, and with the increase in the size of the oocyte they get dispersed in the cytoplasm. The filamentar and beaded mitochondria appear in the older oocytes. Fatty yolk arises through the agency of theGolgi bodies by the swelling of aGolgi granule or by the deposition of new material on the concave side of one or more crescenticGolgi bodies. Albuminous yolk appears to be formed directly or indirectly by mitochondria. With 8 figures in the text.     

Cytological and histochemical observations on the cortical zone of oocytes of a teleost fish, Nandus nandus (Ham)

The cortical zone of oocyte of Nandus nandus has been studied by cytological and histochemical techniques. In an early stage of oocyte development some granular substances appear in the juxtanuclear region which during oocyte growth move to the peripheral ooplasm and forms a thick cortical granular layer. Cytochemically, this cortical granular layer consists of proteins and carbohydrates along with RNA positive material, lipids, mitochondria and Golgi bodies. This cortical granular layer, later on, converts into the cortical vacuolar layer and ultimately forms the cortical alveolar structures in the vitellogenic oocyte of Nandus nandus. The results suggest that the cortical zone plays an important role in the storage of substances transported from the follicular epithelium to the oocyte. Further, the cortical granular substances which are rich in proteins, carbohydrates and lipoidal material, also help in the formation of jelly layers of the developing oocytes.     

Oocyte-follicle cell differentiation in two species of amphineurans (Mollusca), Mopalia mucosa and Chaetopleura apiculata

Differentiating oocytes and associated follicle cells of two species of amphineurans (Mollusca) Mopalia muscosa and Chaetopleura apiculata have been studied by techniques of light and electron microscopy. In addition to the regularly occurring organelles, the ooplasm of young oocytes contains large, randomly situated, basophilic regions. These regions are not demonstrable in mature eggs. As oocytes differentiate, lipid, pigment and protein-carbohydrate yolk bodies accumulate within the ooplasm. Concomitant with the appearance of pigment and the protein carbohydrate containing yolk bodies, the saccules of the Golgi complex become filled with a dense material. Associated with the Golgi complex are cisternae of the rough endoplasmic reticulum which are filled with an electron opaque substance which is thought to be composed of protein synthesized by this organelle. That portion of the cisternae of the endoplasmic reticulum facing the Golgi complex shows evaginations. These evaginations are thought to finalize into protein containing vesicles that subsequently fuse with the Golgi complex. Thus, the Golgi complex in these oocytes might serve as a center for packaging and concentrating the protein used in the construction of the protein containing pigment or protein-carbohydrate yolk bodies. The suggestion is made that the Golgi complex may also synthesize the carbohydrate portion of the formentioned yolk bodies. In an adnuclear position in young oocytes are some acid mucopolysaccharide containing vacuolar bodies. In mature eggs, these structures are found within the peripheral ooplasm and we have referred to them as cortical granules. There is no alteration of these cortical granules during sperm activation.     

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.     

Ultrastructural features of the ventral prostate epithelial cells in the Australian plains rat, Pseudomys australis

The fine structure of epithelial cells of the small ventral prostate of Pseudomys australis males was studied. The cells were sometimes binucleated, exhibited relatively little granular endoplasmic reticulum, generally few secretory granules (probably reflecting a low proteinaceous secretory activity), but had abundant agranular endoplasmic reticulum (AER), similar to that in steroidogenic cells. Some of the mitochondria also showed tubular cristae. Furthermore, most cells had some irregular dense bodies in the cytoplasm which may have developed, by a gradual transformation process, from the membranes of AER, mitochondria and other organelles; they could be the product of degenerative changes in these organelles. These findings indicate a significant difference in the structure of these cells from those present in the ventral prostate of the hopping mouse, Notomys alexis. It is speculated that this gland secretes relatively little protein but perhaps more lipid or cholesterol-derived substance.     

Fate of sperm tail components after incorporation into the hamster egg

The changes in the cytoplasmic organelles of sperm tail in golden hamster eggs fertilized in vivo were observed by electron microscopy. Eggs were obtained from oviducts of hamsters that had been superovulated and inseminated by injection of cauda epididymal spermatozoa into the uteri. In the egg cytoplasm 10 hours after insemination, some of the mitochondria of the spermatozoon midpiece had begun to swell, and a number of multivesicular bodies were observed surrounding the midpiece. The fibrous sheath of the principal piece quickly disappeared prior to the first cleavage, whereas the axoneme and outer dense fibers were unaltered. During the two-cell stage, numerous multivesicular bodies gathered around the midpiece and fused with the mitochondria. The heterophagic vacuoles thus formed then gradually separated from the axial fibers. The outer dense fibers were disarranged and partially torn into small segments; then they seemed to dissociate into substructural granular components. The axonemal microtubules had begun to swell but were still present in the two blastomeres. It is indicated from these observations that at least the mitochondria of the tail constituents carried into the oocyte are digested into small molecular elements by the multivesicular bodies and are possibly distributed as nutrients for the blastomeres during the early stage of development.     

Further morphological and histochemical studies on the yolk nucleus and associated cell components in the developing oocyte of the Indian wall lizard

In March through April when the oocyte growth in the ovaries of the wall lizard (Hemidactylus) is very rapid, the yolk nucleus continues to persist through various stages of previtellogenesis. This persisting yolk nucleus and associated cell components have been studied with histochemical techniques. The spherical and dense yolk nucleus stains for protein, lipoprotein and RNA. It does not form any close morphological association with the other cell components such as the mitochondria, lipid bodies (L₂), spaces or canals, diffuse sudanophilic substance and dense bodies, which are arranged into three zones round the yolk nucleus proper. The mitochondria stain for lipoprotein; the L₂ bodies consist of phospholipid; the spaces do not contain any material demonstrable with histochemical techniques; and the ooplasm containing the diffuse sudanophilic substance and dense bodies shows lipoprotein, protein and RNA. Eventually, the yolk nucleus disintegrates, and its substance as well as the other cell components are distributed in the cortical ooplasm of oocytes which are ready to form the yolk bodies. Concepts of the origin, morphology, cytochemistry and function of the yolk nucleus in the oocytes of invertebrates and vertebrates, which have come about recently through the application of cytochemical and submicroscopical techniques, are discussed.     

泥螺卵子发生的超微结构研究

利用透射电镜观察了泥螺卵子发生过程。结果表明 ,泥螺的卵子发生可划分为卵原细胞、卵黄发生早期、卵黄发生中期及卵黄发生后期卵母细胞 4个时期。卵原细胞核大而圆 ,胞质内分布有少量的线粒体和高尔基囊泡 ,细胞表面具微绒毛。卵黄发生早期的卵母细胞 ,胞质中各类细胞器发达 ,并出现数量较多的类朦胧子。卵黄发生中期的卵母细胞胞体迅速增大 ,核伸出伪足状突起 ,卵质中各种细胞器活动活跃 ,并参与形成卵黄粒和脂滴。此期还可观察到卵母细胞与滤泡细胞间的物质交换现象。卵黄发生后期的卵母细胞体积增至最大 ,细胞器数量减少。本文就卵黄发生前后卵母细胞内部构造的变化、意义及滤泡细胞与卵母细胞蛋白来源间的关系作了探讨     

Fine structure of the corpus Luteum of the raccoon during pregnancy

Summary Ultrastructure of the granulosa lutein cells of the raccoon from throughout pregnancy has been described. The lutein cells often from epithelial cords which are separated by the connective tissues, capillaries and lymphatics. Based on the arrangements and modifications of the cytoplasmic organelles and inclusions, three types of lutein cells have been recognized. The type I lutein cells predominantly contain tubular, agranular endoplasmic reticulum, juxtanuclear Golgi complexes, a few round to rod-shaped mitochondria, some free ribosomes, and occasional lipid droplets. Occasionally the tubular cristae of mitochondria and tubular smooth endoplasmic reticulum appear contiguous. The type II cells contain abundant lace-like and/or stacked fenestrated endoplasmic reticulum cisternae that frequently form membranous whorls, some tubular, agranular endoplasmic reticulum, mitochondria, and lipid droplets. Mitochondria are usually small, but unusual large ones also occur. The small, rod-to round-shaped mitochondria usually have tubular cristae; but the large, oval, elongate, and cup shaped mitochondria possess tubular, lamellar, plate like, and whorl-like cristae. The plasma membranes of the cells are complexly elaborated and folded, especially when apposing each other. In favorable sections, strands of fenestrated cisternae appose the folds of the plasma membranes. In general, the amount of cytoplasmic organelles and inclusions vary greatly in the cells. The type III cells predominantly contain lipid droplets and sparse cytoplasmic organelles. The type I and II cells are found throughout pregnancy, but the type III cells are observed from mid gestation to term. The cytological features of type I and II cells suggest that they probably secrete most of the steroids, whereas the type III cells primarily store lipids.This research was supported by UPSHS grant AM-11376 and NIH contract 69-2136.     

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.     

PATTERN OF OSMIUM DEPOSITION IN THE PARIETAL CELLS OF THE STOMACH

Parietal cells of the stomach of the hamster show extensive amounts of dense material in a variety of organelles after prolonged exposure to a solution of osmium tetroxide. Conspicuous amounts of reduced osmium compounds are evident within the granular endoplasmic reticulum, perinuclear cisterna, and vesicular elements of the Golgi complex. Dense material is also apparent within cristae of the mitochondria, the surface coat of the microvilli of the intracellular canaliculus, and vesicular elements of the multivesicular bodies. Multivesicular bodies, containing numerous small osmiophilic elements, are often seen surrounding and/or in close contact with mitochondria. The proximity of the multivesicular bodies to the mitochondria appears to be related to an autophagic process involving degradation of mitochondria. The distribution and intensity of the precipitates within the organelles of the parietal cells vary in different regions of the gastric glands. The findings of this study emphasize that cell structures other than the Golgi complex may contain large concentrations of reduced osmium compounds after prolonged exposure to a solution of osmium tetroxide.     

Accessory nuclei revisited: the translocation of snRNPs from the germinal vesicle to the periphery of the future embryo

Oocytes of certain insects contain peculiar organelles termed accessory nuclei (AN). These organelles originate by budding off from the envelope of the oocyte nucleus and contain 1-2 dense inclusions immersed in a translucent ground substance. We have demonstrated that in the wasp Vespula germanica each inclusion consists of two elements: a spherical body, and a hemispherical structure composed of numerous 20-30 nm particles. Immunoelectron microscopy and whole-mount in situ hybridization have shown that the inclusions contain AgNOR-staining proteins, p80-coilin, Sm proteins, and small nuclear RNAs (snRNAs). These results indicate that the inclusions and hemispherical structures are homologous to Cajal bodies and B-snurposomes of Xenopus germinal vesicles, respectively. During previtellogenesis, AN (together with their Cajal bodies) migrate to the cortical ooplasm of the oocyte where they reside at least until the onset of embryogenesis. We suggest that AN are vehicles for the transport and localization of snRNPs to the periphery of the oocyte, i.e., to the region where the blastoderm of the embryo develops and where there is a requirement for a high concentration of RNA-processing factors.     

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.     

A normal rabbit serum containing Golgi-specific autoatibodies identifies a novel 74-kDa trans-Golgi resident protein

A normal rabbit serum has been identified which contains Golgi-specific autoantibodies. In indirect immunofluorescence experiments the serum was found to stain the juxtanuclear Golgi complex in a variety of cell lines, including human skin fibroblasts, rat osteoblasts, rat myoblasts (L6), baby hamster kidney epithelial cells, and human embryonic kidney cells (293). Thus, the antigen(s) recognized by this serum seems to be well conserved and universally expressed in various mammalian cell types. Immunoelectron microscopy revealed that the epitope resides in the luminal side of the Golgi membranes, and that the antigen is concentrated in the trans-face of the Golgi stacks. In agreement with these results, brefeldin A treatment did not release the antigen from the membranes, but caused its redistribution partly into the endoplasmic reticulum but also into the juxtanuclear area, similarly as with other proteins known to be present in the trans-Golgi cisternae or trans-Golgi network. Our immunoprecipitation studies in human skin fibroblasts demonstrated that the serum recognizes specifically only a single protein with a molecular size of 74 kDa. This protein also cosedimented with a known trans-Golgi-specific marker protein, galactosyltransferase, after fractionation of subcellular organelles by Nycodenz gradient centrifugation. The widespread and polarized expression of this 74-kDa trans-Golgi resident protein suggests that it is required for the late Golgi functions in different mammalian cell types.     

The fine structure of the parathyroid gland

The fine structure of the parathyroid of the macaque is described, and is correlated with classical parathyroid cytology as seen in the light microscope. The two parenchymal cell types, the chief cells and the oxyphil cells, have been recognized in electron micrographs. The chief cells contain within their cytoplasm mitochondria, endoplasmic reticulum, and Golgi bodies similar to those found in other endocrine tissues as well as frequent PAS-positive granules. The juxtanuclear body of the light microscopists is identified with stacks of parallel lamellar elements of the endoplasmic reticulum of the ergastoplasmic or granular type. Oxyphil cells are characterized by juxtanuclear bodies and by numerous mitochondria found throughout their cytoplasm. Puzzling lamellar whorls are described in the cytoplasm of some oxyphil cells. The endothelium of parathyroid capillaries is extremely thin in some areas and contains numerous fenestrations as well as an extensive system of vesicles. The possible significance of these structures is discussed. The connective tissue elements found in the perivascular spaces of macaque parathyroid are described.     

Basic protein and ribonucleic acid in the cytoplasm of the ovarian oocyte in the golden hamster

Summary Cytochemical and autoradiographic studies indicate that granular-fibrillar cytoplasmic bodies and intermitochondrial substance in the hamster ovarian oocyte contain both basic protein and ribonucleic acid. The studies suggest that the granular-fibrillar bodies and intermitochondrial substance, although morphologically similar, are not functionally identical. It is proposed that the granular-fibrillar bodies represent stored inactive maternal messenger RNA synthesized during the lampbrush chromosome stage of oogenesis for use in the developing embryo. The function of the intermitochondrial substance is still obscure. The suggestion is put forward that it may represent the synthetic machinery for the manufacture of those mitochondrial proteins for which the mitochondrial DNA is insufficient to code.The author wishes to thank Mrs. E. M. Lloyd-Davies for her expert technical assistance in preparing the material for autoradiography.     

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.     

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

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