Ultrastructural evidence for both autosynthetic and heterosynthetic yolk formation in the oocytes of an annelid (Phragmatopoma lapidosa: Polychaeta).

The ovaries of the reef-building polychaete Phragmatopoma lapidosa are attached to the genital blood vessels on the caudal surface of the intersegmental septa of the abdominal segments. Oogenesis is not synchronized and vitellogenesis occurs before the oocytes are released from the ovary into the coelomic cavity. A portion of each developing oocyte rests on the basal lamina of the genital blood vessel while the remaining surface of the oocyte is covered by follicle cells. Two morphologically distinct types of yolk are formed during vitellogenesis: Type I, which may be formed autosynthetically by the conjoined efforts of the rough ER and Golgi systems; and Type II, which is presumably formed heterosynthetically from endocytosis of yolk precursors from the genital blood vessel. Heterosynthetic production of yolk in an annelid has not been reported previously.     

Evolution of the concepts of vitellogenesis in Polychaete Annelids

Summary

In polychaete annelids, two types of vitellogenesis have been described: a heterosynthetic mechanism (in a number of different of worms) and an autosynthetic process (other including Nereis). Recent biochemical results suggest that the heterosynthetic mechanism is more general than previously thought. In Nereis, the vitellogenin (the precursor) is synthesized in coelomocytes and after transfer through coelomic fluid incorporated into oocytes. In germinal cells, a conversion process, involving proteolytic cleavages of vitellogenin, produces mature vitellins which are accumulated in yolk granules.

The neurohormones identified so far are not essential for vitellogenin synthesis. It is possible that these neurohormones regutate enzymatic activities in the oocytes.     

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.     

Primordial germ cells and early stages of oogenesis in Ophryotrocha puerilis (Polychaeta,Dorvillediae)

Summary Each setigerous segment of the protandric polychaete Ophryotrocha puerilis contains two primordial germ cells. A ventral furrow in the gut wall together with the peritoneal lining of the gut forms a genital blood vessel. The gonocytes are located within the peritoneum of this genital blood vessel. At sexual maturity the gonocytes undergo a proliferation cycle, the first division of which gives rise to a cell which is extruded into a forming outpocketing of the coelomic lining. The stem cell remains within the peritoneum. Inside the forming gonad the detached cell goes through a series of four mitotic divisions. The resulting 16 cells are interconnected by cytoplasmic bridges. These bridges are arranged in a very regular pattern which allows the mitotic cycles to be followed. While remaining still within the gonad the 16 cells begin to synthesize yolk and to take up exogenous yolk precursors. At this stage a differentiation into oocytes and nurse cells becomes visible. The oocytes deposit yolk platelets of the definitive size whereas the polyploid nurse cells produce only small yolk bodies that are passed to the adjacent oocytes. In a later stage the cell bridges between adjacent nurse cells are cut and pairs of one oocyte and one nurse cell are released to the coelomic cavity during breakdown of the gonadal sac. Oocyte-nurse cell-complexes then freely float in the coelomic fluid. The proliferation of gonadal cells is well synchronized within one segment. In anterior segments, however, gonadal proliferation usually begins earlier than in posterior segments but smaller oocytes in posterior segments catch up within a few days. Finally a batch of oocytes is produced in which all the oocytes are of the same size (120 m). The origin of the primordial germ cells remains unknown.     

Ultrastructure of the ovary and oogenesis in the polychaete Capitella jonesi (Hartman, 1959)

Ultrastructural features of the ovary and oogenesis in the polychaete Capitella jonesi (Hartman, '59) have been described. The ovaries are paired, sac-like follicles suspended by mesenteries in the ventral coelom throughout the midbody region of the mature worm. Oogenesis is unsynchronized and occurs entirely within the ovary, where developing gametogenic stages are segregated spatially within a germinal and a growth zone. Multiplication of oogonia and differentiation of oocytes into the late stages of vitellogenesis occur in the germinal region of the ovary, whereas late-stage vitellogenic oocytes and mature eggs are located in a growth zone. Follicle cells envelop the oocytes in the germinal zone of the ovary and undergo hypertrophy and ultrastructural changes that correlate with the onset of vitellogenesis. These changes include the development of extensive arrays of rough ER and numerous Golgi complexes, formation of microvilli along the surface of the ovary, and the initiation of extensive endocytotic activity. Oocytes undergo similar, concomitant changes such as the differentiation of surface microvilli, the formation of abundant endocytotic pits and vesicles along the oolemma, and the appearance of numerous Golgi complexes, cisternae of rough ER, and yolk bodies. Yolk synthesis appears to occur by both autosynthetic and heterosynthetic processes involving the conjoined efforts of the Golgi complex and rough ER of the oocyte and the probable addition of extraovarian (heterosynthetic) yolk precursors. Evidence is presented that implicates the follicle cells in the synthesis of yolk precursors for transport to the oocytes. At ovulation, mature oocytes are released from the overy after the overlying follicle cells apparently withdraw. Bundles of microfilaments within the follicle cells may play a role in this withdrawal process.     

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.     

Reproductive biology of the polychaete Kefersteinia cirrata Keferstein (Hesionidae). I. Ovary structure and oogenesis

The ultrastructure of the ovary and the developing oocytes of the polychaete Kefersteinia cirrata have been described. The paired ovaries occur in all segments from the 11th to the posterior. Each consists of several finger-like lobes around an axial genital blood vessel. Oogenesis is well synchronised, young oocytes start to develop in September and vitellogenesis begins in January and is completed by May.

The young oocytes are embedded among the peritoneal cells of the blood vessel wall which have accumulations of glycogen and other storage products. Each oocyte becomes associated with a follicle cell with abundant rough endoplasmic reticulum. Yolk synthesis involves the accumulation of electron dense granules along the cisternae of the abundant rough endoplasmic reticulum. Active Golgi complexes are present and are involved in the production of cortical alveoli. The oocyte has branched microvilli, which contact the follicle cells or blood sinuses between the follicle cells and peritoneal cells. In post-spawning individuals the lysosome system of the follicle cells is hypertrophied and the cells play a role in oocyte breakdown and resorption.     

Patterns of protein synthesis and accumulation during oogenesis in the polychaete,Pseudopotamilla occelata Moore

Summary

Proteins synthesized and accumulated during oogenesis or Pseudopotamilla occelata were analyzed by two-dimensional Polyacrylamide gel electrophoresis and fluorography. Significant changes in the patterns of synthesis and accumulation occur during oogenesis, paticularly between previtellogenic and vitellogenic stages, although many of the proteins are represented throughout the process. The changes may be generally dependent upon the variation in the gene expression affecting autosynthesis of proteins. However, appearance of proteins, which occur only at the vitellogenic stages but apparently not synthesized within the oocytes, indicates that heterosynthetic proteins are stage-specifically transported into oocytes.     

Structure of the ovotestis and evidence for heterosynthetic incorporation of yolk precursors in the oocytes of the nudibranch mollusc,Spurilla neapolitana

The ovotestis of Spurilla neapolitana consists of a series of spherical lobes, each of which is composed of radially arranged, sac-like acini or follicles. The male and female portions of each acinus are separated by ovarian follicle cells and testicular accessory cells. A thick basal lamina serves as a barrier between adjacent acini. The surface of each ovotestis lobe is covered by several layers of myoepithelial cells resting on a connective tissue layer. Developing oocytes are intimately associated with follicle cells except in the last stages of vitellogenesis. Follicle cells are characterized by the presence of extensive arrays of rough endoplasmic reticulum (RER) and Golgi complexes and may play a role in vitellogenesis. An ultrastructural analysis of vitellogenesis suggests that oocytes utilize both auto- and heterosynthetic mechanisms of yolk formation. Autosynthetsis is suggested by the activity of the Golgi complex and RER, while heterosynthesis is indicated by high levels of endocytotic activity by the oocyte. Follicle cell development and high endocytotic activity in the oocytes may be a reproductive adaptation to accelerate yolk synthesis, resulting in more rapid egg production.     

A cytologic analysis of the bag cell control of egg laying in Aplysia

A fine structural analysis of the ovotestis in Aplysia was undertaken in order to analyze the site of action of the bag cell hormone. Five stages of oocyte development are described. Of particular interest is the fact that the yolk seems to be synthesized primarily by the granular endoplasmic reticulum. In addition, small muscle cells whose long, thin processes surround the follicle of the ovotestis have been pointed out. This paper suggests that bag cell extract has a direct action on these small muscle cells causing them to contract and thus expel oocytes from the ovotestis. The evidence for this suggestion is that (1) these muscle cells are the most obvious effector cells in the ovotestis, (2) there are no signs of neural innervation of these muscles, (3) the time course for the liberation of the oocytes is so short that any other method of oocyte release is unlikely, (4) there is no cytologic evidence for any other expulsion process except muscular contraction, and (5) the ripe oocytes are attached to other cells of the wall of the ovotestis only by very small, simple junctions, thus making them the most likely cells to be expelled by muscular contraction.     

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.     

Oogenesis in the Polychaete Dipolydora commensalis

Oocytes of the polychaete Dipolydora commensalis develop in the gonad, in close contact with the wall of the genital blood vessel, up to the late stages of vitellogenesis. At the blood vessel wall, between the neighboring vitellogenic oocytes, and sometimes on the apical surface of the oocytes, there are flattened follicular cells. However, no continuous, well-expressed gonad envelope is found. Oogenesis is asynchronous. Gametes at all developmental stages, from oogonia to late vitellogenic oocytes, occur in the gonad. Dividing oogonia vary from 6 to 10 m in diameter. RNA, proteins, glycogen, and lipids accumulate in the oocytes during vitellogenesis. The breakdown of the oocyte germ vesicle occurs in the gonad. Before spawning, gametes accumulate in the coelom and reach 80–90 m in diameter, at which point a new generation appears in the gonad.     

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.     

The Presence of Intramitochondrial Yolk-crystals in Oocytes of Hypophysectomized Bullfrog Tadpoles*

Ovaries of hypophysectomized Rana catesbeiana tadpoles. weighing I to 14 g, were prepared for electron microscopic study. The oocytes are at the growth phase, ranging from 50 to 190 μm in diameter. The observation on these oocytes has revealed the presence of intramitochondrial yolk-crystals but not cytoplasmic yolk platelets. The crystalline structure, situated within the intracristal space, consists of a hexagonal array of dense particles about 50 Å in diameter and 72 Å in periodicity. Our data agree with those reported in oocytes of intact ranid species. According to literatures, crystals of intramitochondrial yolk and of cytoplasmic yolk platelets show similar ultrasturctures. The precursor of cytoplasmic yolk platelets in adult Xenopus oocytes is known to be synthesized in the estrogen-stimulated liver and incorporated via circulation into oocytes by gonadotropin-dependent micropinocytosis. The present finding suggests that the intramitochondrial yolk could be formed within oocytes, independently of the pituitary control.     

Oogenesis and oocytes

The morphological features of polychaete ovarian morphology and oogenesis are reviewed. Some basic information on ovarian structure and/or oogenesis is known for slightly more than half of recognized polychaete families although comprehensive studies of oogenesis have been conducted on 0.1 of described species. Relative to other major metazoan groups, ovarian morphology is highly variable in the Polychaeta. While some species appear to lack a defined ovary, most have paired organs that are segmentally repeated to varying degrees depending on the family. Ovaries vary widely in their location but are most frequently associated with the coelomic peritoneum, parapodial connective tissue, or elements of the circulatory system. The structural complexity of the ovary is correlated with the type of oogenesis expressed by the species. In some polychaetes, extraovarian oogenesis occurs in which previtellogenic oocytes are released into the coelom from a simple ovary where differentiation occurs in a solitary fashion or in association with nurse cells or follicle cells. In other species, intraovarian oogenesis occurs in which oocytes undergo vitellogenesis within the ovary, often in association with follicle cells that may provide nutrition. Vitellogenesis probably includes both autosynthetic and heterosynthetic processes; autosynthesis involves the manufacture of yolk bodies via the proteosynthetic organelles of the oocyte whereas heterosynthesis involves the extraovarian production of female-specific yolk proteins that are incorporated into the oocyte through a receptor-mediated process of endocytosis. Variation in the speed of egg production varies widely and appears to be correlated with the vitellogenic mechanism employed. Mature ova display a wide range of egg envelope morphologies that often show some intrafamilial similarities.     

Sexual and developmental differences in the protein pattern of haemolymph and body tissues of Streptocephalus dichotomus Baird (Crustacea: Anostraca)

The protein pattern of haemolymph and body tissues of the freshwater fairy shrimp Streptocephalus dichotomus has been investigated in both sexes, using polyacrylamide disc gel electrophoresis. The electropherograms of four developmental stages show variations in number and intensities of protein fractions. In Stage III, two female-specific proteins of glycolipoprotein nature appear. This stage corresponds to maturity: females begin to possess mature oocytes in the ovary. These two vitellogenic proteins are well represented in the female haemolymph, ovary and freshly laid eggs, but are absent in the male haemolymph. A heterosynthetic mode of yolk formation is thus evident in this anostracan. The two sex-limited proteins are only faintly represented in shelled eggs, suggesting an early utilization of these compounds in embryogenesis.     

The ultrastructure of the ovarian follicle of medaka,Oryzias latipes

Summary The follicular cells in the oocytes of Oryzias latipes were studied by electron microscopy in order to clarify the fine structure, and the role of the cells during yolk formation and ovulation. The smallest follicles were observed during the early phase of peri-nucleolus stage of the oocyte. The cells have flattened nuclei, and perikarya with undeveloped organelles. But when the oocytes attain diameter of about 250 (yolk vesicle stage), both types of endoplasmic reticula are present. Moreover, the microvilli of the plasma membrane of oocyte as well as the follicles protrude into the pore canals of the zona radiata. In the oocytes of yolk stage the rough-surfaced endoplasmic-reticulum is typically developed and observed around the nuclei. Other organelles (lysosomes, mitochondria and Golgi) increase in number. The relation between the changes of cytoarchitecture in the follicles and yolk formation is discussed.At 17.00 p.m. on the day preceding ovulation the microvilli withdraw somewhat. Ribosomes are attached to the vesicular and cisternal endoplasmic reticula. When the oocytes attain complete maturation (24.00 p.m. at near ovulation), striking changes of the follicles are observed. The microvilli are almost withdrawn. In the degenerating follicles the lamellar structure is formed, and lipids are deposited at the center. At this time the contents of lysosomes have mostly disappeared.     

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.     

An ultrastructural study of oogenesis in the polychaeteNephtys hombergi Savigny

The polychaeteNephtys hombergi has an annual cycle of reproduction. Ovaries were fixed for electron microscopy during the gametogenic phase from September to March, and during the breeding and post-breeding periol. Oogenesis takes place entirely within the ovary, the integrity of which is maintained by a network of simple follicle cells. Previtellogenic oocytes have close contacts with the peri-vasal cells which surround the genital blood capillaries. These contacts are lost as the oocytes enter vitellogenesis. The vitellogenic oocytes have a cytology typical of oocytes which are thought to undergo autosynthetic production of protein yolk. Biochemical studies would be required to establish whether heterosynthesis of yolk also occurs. As the oocytes proceed through vitellogenesis, cortical material is laid down near the periphery of the oocyte and a microvillous surface is developed. When the microvillous surface is complete the oocytes, by then hormone independent, are ovulated from the ovary and are ready to be spawned.     

Oogenesis in a placental viviparous onychophoran

This first ultrastruetural study of oogenesis in a placental viviparous onychophoran describes oocyte differentiation, cell interactions and reveals various unusual cellular features. The viviparous onychophoran Plicatoperipatus jamaicensis has paired ovaries medially located, attached to the dorsal body wall by muscular terminal filaments. The rest of the female reproductive tract consists of paired spermathecae oviduct/uteri (hereafter referred to as uterus). Bulbous spermathecae are joined to the oviducts by ducts. Also continuous with the oviduct lumen are two tubular structures whose lumina open to the hemolymph. The uteri contain a progression of developmental stages from implantation through stalked morulae, blastocysts, larvae and juveniles about to be born.Growing oocytes are characterized by large germinal vesicles showing synaptonemal complexes. Oocytes are surrounded by flattened follicle cells that possess extensive bundles of thick and thin filaments. Mature oocytes contain little or no yolk, but are unique among organisms in accumulating a large central reservoir of stored glycogen. The lack of yolk reflects the placental viviparous nature of the reproductive process. The glycogen reservoir provides a rapidly accessible energy source for early developmental stages.Particularly prominent also are unusually extensive and highly elaborate Golgi complexes in the cortical and peri-nuclear ooplasm. While extensive Golgi complexes have been described in oocytes of a variety of species, the particularly exaggerated size and amount of Golgi in these onychophorans suggests they may provide excellent material for the study of Golgi function. The features of the oocyte and placental viviparity show this is an ideal model to investigate the nature of the placental reproductive process analogous to mammals in an invertebrate and its implications to oogenesis.