Follicle cell development is partly independent of germ-line cell differentiation in Drosophila oogenesis

Summary The developmental potential of the cells of the somatic follicular epithelium (follicle cells) was studied in mutants in which the differentiation of the germ-line cells is blocked at different stages of oogenesis. In two mutants, sn ^36a and kelch, nurse cell regression does not occur, yet the follicle cells around the small oocyte continue their normal developmental program and produce an egg shell with micropylar cone and often deformed operculum and respiratory appendages. Neither the influx of nurse cell cytoplasm into the oocyte nor the few follicle cells covering the nurse cells are apparently required for the formation of the egg shell. In the tumor mutant benign gonial cell neoplasm (bgcn) the follicle cells can also differentiate to some extent although the germ-line cells remain morphologically undifferentiated. Vitelline membrane material was synthesized by the follicle cells in some bgcn chambers and in rare cases a columnar epithelium, which resembled morphologically that of wild-type stage-9 follicles, formed around the follicle's posterior end. The normal polarity of the follicular epithelium that is characteristic for mid-vitellogenic stages may, therefore, be established in the absence of morphologically differentiating germ-line cells. However, the tumorous germ-line cells do not constitute a homogeneous cell population since in about 30% of the analyzed follicles a cell cluster at or near the posterior pole can be identified by virtue of its high number of concanavalin A binding sites. This molecular marker reveals an anteroposterior polarity of the tumorous chambers. In follicles mutant for both bgcn and the polarity gene dicephalic the cluster of concanavalin A-stained germ-line cells shifts to more anterior positions in the follicle.     

Observations on the polarity of mutant Drosophila follicles lacking the oocyte

Summary Homozygous females of the mutantsegalitarian andBicaudal-D ^R26produce follicles in which the oocyte is replaced by an additional nurse cell. Normal morphological markers for polarity can be identified in mutant follicles but the normal spatial organization of these markers is disturbed. For example, nurse-cell nuclei of different ploidy classes are present but, contrary to wild-type follicles, the nuclei show no anteroposterior ploidy gradient. The two cells with four intercellular bridges, one of which should have developed into the oocyte rather than a nurse cell, are located at the posterior pole only in young follicles (up to about stage 5), whereas during later stages they are more often found at lateral or intermediate positions. This disturbed polarity correlates with a variable aberrant pattern of extracellular ionic currents. Moreover, in the mutant follicles patches of columnar follicular epithelium differentiate locally although this type of epithelium forms normally only around the oocyte. The follicle cells at both follicle poles possess anterior quality since they migrate from both poles towards the centre of the follicle, as do the border cells restricted to the anterior pole in wild-type follicles. Our analysis indicates that in the mutants the follicular polarity is normal at first but becomes disturbed during stages 5 to 6. The secondary breakdown of polarity is likely to follow on from the absence of the oocyte.     

Structural organization of ovarian follicle cells in the cotton bug (Dysdercus intermedius) and the honeybee (Apis mellifera)

Summary The somatic epithelia of Dysdercus and Apis follicles were analyzed by electron microscopy, and the patterns of F-actin and microtubules were studied by fluorescence microscopy. The epithelia in both species differ considerably in shape and in the organization of the cytoskeleton. During previtellogenic stages, the epithelium consists of columnar-shaped cells with small (Dysdercus) or no (Apis) lateral intercellular spaces. During vitellogenesis, the follicle cells round up; the intercellular spaces increase in size in Dysdercus follicles, whereas in Apis follicles they remain small. Along the basal surface of the follicle cells, there are conspicuous parallel bundles of microfilaments perpendicular to the anteroposterior axis of the follicles. In the honeybee, these microfilament bundles are present in long filopodia, most of which are embedded in thickenings of the basement membrane and extend over the surfaces of neighbouring cells. In the cotton bug, the basal surface of the follicle cells is thrown into parallel folds. The microfilament bundles are located just underneath the cell membrane where the folds contact the basement membrane. In the polar regions of the Dysdercus follicle, the epithelial cells become flat and adhere to each other without forming intercellular spaces. The basement membrane is particularly thick in the polar areas; this has also been observed in Apis follicles around the intercellular bridge connecting oocyte and nurse cells.     

The microfilament pattern in the somatic follicle cells of mid-vitellogenic ovarian follicles of Drosophila

The microfilament pattern in the somatic follicle cells of mid-vitellogenic stage 9 to 11 follicles of Drosophila was analyzed by staining F-actin with fluorescence-labeled phalloidin. During the analyzed stages of oogenesis, the follicular epithelium differentiates morphologically and functionally. These changes are also reflected at the organization of the microfilaments. At stage 10, they show no preferred orientation in the very thin follicle cells covering the nurse cells. In contrast, the microfilaments in the basal part of the columnar follicle cells covering the oocyte become organized in parallel bundles oriented perpendicular to the long axis of the follicle. During stages 10B/11 this organization is maintained at the nurse cell/oocyte border but becomes more sloppy towards the posterior pole of the follicle. The basal part of the follicle cells containing the microfilament bundles adheres so tightly to the basement membrane that this acellular layer cannot be separated mechanically from the epithelium. Indirect evidence from inhibition studies with cytochalasins and the effects of collagenase or pronase E added to the culture medium suggest that the microfilament bundles may promote increased adhesiveness of the follicle cells to the basement membrane. The possible functional implications of the microfilaments and their orientation are discussed.     

Confocal scanning laser microscopy of the follicular epithelium in ovarioles of the stick insect Carausius morosus

Ovarian follicles of the stick insect Carausius morosus were analyzed by confocal laser microscopy and immunocytochemistry with a view to studying cell polarity in the follicular epithelium. Such probes as anti-α-tubulin antibodies and Rh-phalloidin were employed to establish how the follicle cell cytoskeleton changes during ovarian development. Data show that α-tubulin prevails over the basal end, while F-actin appears more abundant along the apical end of the follicle cells. This finding was further corroborated by immunogold cytochemistry, showing that label along the basal end is primarily associated with microtubules, while that along the apical end is due to follicle cell microvilli interdigitating with the oocyte plasma membrane. A monoclonal antibody specifically raised against a vitellin polypeptide was used to investigate the role the follicular epithelium might play in relation to vitellogenin (Vg) uptake by the oocyte. Data show that under these conditions label is restricted to the intercellular channels of the follicular epithelium, thus providing further support to the notion that Vg enters the oocyte through the extracellular pathway leading from the basement lamina to the oocyte surface. By contrast, the use of a monoclonal antibody raised against a fat-body-derived protein of 85 kDa that is specifically sulfated within the follicle cells provides evidence for the existence of an alternative way of gaining access to the oocyte surface, that is by transcytosis through the follicular cell epithelium. These findings confirm our earlier observations on stick insect ovarioles whereby polarization in the follicular epithelium is primarily addressed to sustain a transcytotic vesicular traffic between opposite poles of the follicle cell of Vg toward the oocyte surface.     

Oogenesis in the honeybee Apis mellifera: cytological observations on the formation and differentiation of previtellogenic ovarian follicles

Summary Oogenesis is known to be important for embryonic pattern formation. For this reason we have studied the early differentiation of the honeybee ovariole histologically, ultrastructurally, and by staining F-actin with rhodaminyl-phalloidin. At the anterior tip of the ovariole, stem cells are lined up in a single file; they are organelle-poor but contain characteristic electrondense bodies with lysosomal properties. The presence of these bodies in cystocytes as well as prefollicle cells indicates that both cell types may be derived from the apical stem cells. During later stages of oogenesis, the follicle cells differentiate cytologically in different regions of the follicle. The organization of the intercellular bridges between cystocytes derived from a single cystoblast has been studied in detail. The polyfusomes in the intercellular bridges of cystocyte clusters stain with rhodaminyl-phalloidin and hence contain F-actin. Later, when the polyfusomes begin to desintegrate, F-actin rings form which line the rims of the intercellular bridges. Actin might be recruited from conspicuous F-actin stores which were detected in the germ-line cells. The F-actin rings are dissembled some time before the onset of vitellogenesis when the nurse chamber has grown to a length of about 200 m. At the basal side of the follicle cells (close to the basement membrane facing the haemocdele) parallel microfilament bundles encircle the ovariole. The microfilament bundles which are oriented mostly perpendicular to the long axis of the ovariole were first observed around the zone where the cystocyte divisions occur; after this phase the micro-filament bundles become organized differently in the follicle cells associated with the nurse cells and in the follicular epithelium of the oocyte. Correspondence to: H.O. Gutzeit     

Program of F-actin in the follicular epithelium during oogenesis of the german cockroach, Blattella germanica

Extensive programmed structural and functional changes of insect follicular epithelium during oogenesis provide a model to study modulation of cytoskeletal organization during morphogenesis in a non-dividing cell population. Rhodamine-phalloidin staining of whole mounted and cryosectioned oogenic follicles reveal changing F-actin filament organization from pre- to post-vitellogenic stages consistent with the presumptive dorsal-ventral orientation of the future embryo. Filaments are not abundant in pre-vitellogenic follicle cells up to day 2. Differences between dorsal and ventral follicle cells appear first on day 3. Obviously patent follicle cells are seen only on the ventral follicle surface which exhibits stronger F-actin fluorescence than the dorsal non-patent epithelium. On the presumptive ventral side of midvitellogenic follicles morphologically distinct bundles of actin filaments orient peripherally into projections connecting adjacent follicle cells and from the center of follicle cells apically into macrovillar projections extending toward the oocyte surface. The mid-vitellogenic dorsal follicle cell layer also possesses macrovillar extensions containing F-actin which reach and appear to penetrate the oolema. During chorion deposition major reorganization of actin of follicle cells takes place. After chorion deposition all F-actin filaments within a given follicle cell are arranged into large parallel bundles with semi-regular cross-striations which exclude fluorescent label. The parallel orientation of actin striated filament bundles within each follicle cell appears to be random with respect to the orientation of bundles in neighboring follicle cells over much of the mid-latitude of the follicle epithelium. At anterior and posterior follicle poles a more axial orientation of striated bundles is evident. This muscle-like tissue arrangement is appropriate for cooperation in ovulating the chorionated oocyte from the follicle into the oviduct.     

A novel pattern of follicular epithelium morphogenesis in higher dipterans

In fly ovaries, the follicular epithelium surrounding germline cells diversifies into several morphologically distinct cell subpopulations. This complex process is crucial for the formation of a regionally complex eggshell and establishment of polarity of the future embryo. Morphogenetic changes accompanying patterning of the follicular epithelium have been best characterized in the model fly, Drosophila melanogaster. Here, we analyze follicular epithelium diversification in the ovaries of Tachypeza nubila, a brachyceran fly closely related to the group Cyclorrhapha, which also includes Drosophila. We provide morphological evidence that in Tachypeza, the diversification process differs from that described in the Drosophila model system in several important respects: (i) follicle cells differentiate into five subpopulations (versus eight in Drosophila); (ii) only one of these subpopulations (i.e. border cells) is migratory (versus four in Drosophila); (iii) the main body follicle cells form a uniform epithelium with no distinct border between follicle cells covering the nurse cell compartment and the oocyte; (iv) chorionic material is deposited not only on the surface of the oocyte but also on the nurse cells; (v) there is no centripetal migration of the follicle cells; (vi) the resulting eggshell is morphologically simple with no regional specializations except for the micropylar apparatus at the anterior pole of the oocyte. Our findings provide novel insights into the evolution of the follicle cell patterning and functioning in dipterans. A critical analysis of these processes in different dipteran groups strongly indicates that in Tachypeza, follicular epithelium diversification follows a distinct pattern, novel for higher dipterans.     

Formation of multiple-oocyte follicles in culture

Basement membranes are found in every organ of the body. They provide structure and a selective filter for molecules. The ovary is no different with the follicular basal lamina (FBL) separating the granulosa and theca cells, facilitating regulation of the changing follicular environment providing appropriate conditions for the developing oocyte. The FBL is modified in C1galt1 Mutant mice (C1galt1 ^FF:ZP3Cre) resulting from oocyte-specific deletion of C1galt1. Changes in the FBL lead to follicles joining to generate multiple-oocyte follicles (MOFs); where two or more oocytes are contained within a single follicle. This study aimed to determine if single-oocyte follicles could join in culture to become MOFs by co-culturing preantral follicles from Control or Mutant mice. Co-cultured follicles from both Control and Mutant follicles could superficially fuse (73% of Control follicle pairs; 84% of Mutant). Confocal microscopy revealed alterations in the organization of the space between follicles but was unable to discern MOFs. When co-cultured follicle pairs were embedded, sectioned and stained with haematoxylin, it was revealed that MOFs had formed from 50% of Mutant follicle pairs but none from Control follicle pairs. In conclusion, MOFs can form from C1galt1 Mutant follicles in culture and this model is a useful tool to elucidate the role of the oocyte in follicle development and the generation and function of the FBL. Furthermore, understanding the relationship between oocyte function and FBL generation will likely provide insight into optimizing conditions for follicle culture, which is important for fertility treatments and ART.     

Gap junctions in ovarian follicles of Drosophila melanogaster: inhibition and promotion of dye-coupling between oocyte and follicle cells

The analysis of chimeras has shown that communication between germ-line and soma cells plays an important role during Drosophila oogenesis. We have therefore investigated the intercellular exchange of the fluorescent tracer molecule, Lucifer yellow, pressure-injected into the oocyte of vitellogenic follicles of Drosophila. The dye reached the nurse cells via cytoplasmic bridges and entered, via gap junctions, the somatic follicle cells covering the oocyte. The percentage of follicles showing dye-coupling between oocyte and follicle cells was found to increase with the developmental stage up to stage 11, but depended also on the status of oogenesis, i.e., the stage-spectrum, in the respective ovary. During late stage 10B and stage 11, dye-coupling was restricted to the follicle cells covering the anterior pole of the oocyte. No dye-coupling was observed from stage 12 onwards. During prolonged incubation in vitro, the dye was found to move from the follicle cells back into the oocyte; this process was suppressable with dinitrophenol. Dyecoupling was inhibited when prolonged in vitro incubation preceded the dye-injection. Moreover, dye-coupling was inhibited with acidic pH, low [K⁺], high intracellular [Ca²⁺], octanol, dinitrophenol, and NaN₃, but not with retinoic acid, basic pH, or high extracellular [Ca²⁺]. Dyecoupling was stimulated with a juvenile hormone analogue and with 20-hydroxyecdysone. Thus, gap junctions between oocyte and follicle cells may play an important role in intercellular communication during oogenesis. We discuss the significance of our findings with regard to the electrophysiological properties of the follicles, and to the coordinated activities of the different cell types during follicle development and during the establishment of polarity in the follicle.     

Evidence for Follicle Wall Involvement in Ovulation and Progesterone Production by Frog (Rana pipiens) Follicles in vitro

Involvement of different cellular investments of the amphibian ovarian follicle wall in the ovulatory process, progesterone production, and oocyte maturation was investigated. Following microdissection, to selectively remove one or more layers (surface epithelium, theca, follicle cells) of the follicle wall, dissected and undirected ovarian follicles were treated with frog pituitary homogenate (FPH) or progesterone. Intact follicles ovulated in response to pituitary homogenate and this was associated with contractions of the follicle wall. Ovulation and follicular contractions were not observed following removal of the surface epithelium without removing the thecal layer. Oocyte maturation occured in response to FPH following removal of the surface epithelium alone or together with the theca, but not in the absence of the follicle cells. Intact follicles were most responsive to FPH with respect to progesterone production, and removal of all somatic cells from oocytes obliterated FPH stimulated progesterone production. Oocytes, regardless of wether any or all follicular wall layers were removed, matured but did not ovulate following exposure to progesterone. The results suggest that the surface epithelium, but not the theca, is required for FPH-induced extrusion (ovulation) of the oocyte from ovarian follicle wall. Additionally, the somatic tissue rather than the oocyte appears to be the cells producing progesterone following FPH treatment. The results indicate that separate cellular layers (individually and/or as a result of interactions) of the follicle wall carry out different functions during follicular differentiation and mediation of ovulation. Data provide functional evidence for a role of the surface epithelium in controlling the process of ovulation and follicular contraction.     

Ion physiology of vitellogenic follicles

The ion physiology of vitellogenic follicles from a lepidopteran (Hyalophora cecropia) and a hemipteran (Rhodnius prolixus) are compared. Similarities that can be expected to occur in vitellogenic follicles of many other insects include: (1) gap junctions, which unite the cells of a follicle into an integrated electrical system, (2) transmembrane K⁺ and H⁺ gradients that account for over 60% of follicular membrane potentials, (3) absence of a Cl⁻ potential, (but the opening of channels to this anion when vitellogenesis terminates in H. cecropia), (4) an electrogenic proton pump that supplements follicular membrane potentials, (5) Ca²⁺ action potentials evoked by injecting depolarizing currents into oocytes, and (6) the use of osmotic pressure to control epithelial patency. Differences include: a Na⁺/K⁺-ATPase that accounts for about 20% of the follicular resting potential in R. prolixus but is absent from H. cecropia, and an intrafollicular Ca²⁺ current that moves from oocyte to nurse cells through cytoplasmic bridges in H. cecropia. Evidence is also summarized for two promising mechanisms that require further substantiation: (1) transmission via gap junctions of a follicle cell product that promotes endocytosis in the oocyte; and (2) transport of the proton pump back and forth between cell surface and endosomes as the membrane that carries it recycles through successive rounds of vitellogenin uptake.     

A Follicle Rupture Assay Reveals an Essential Role for Follicular Adrenergic Signaling in Drosophila Ovulation

Ovulation is essential for the propagation of the species and involves a proteolytic degradation of the follicle wall for the release of the fertilizable oocyte. However, the precise mechanisms for regulating these proteolytic events are largely unknown. Work from our lab and others have shown that there are several parallels between Drosophila and mammalian ovulation at both the cellular and molecular levels. During ovulation in Drosophila, posterior follicle cells surrounding a mature oocyte are selectively degraded and the residual follicle cells remain in the ovary to form a corpus luteum after follicle rupture. Like in mammals, this rupturing process also depends on matrix metalloproteinase 2 (Mmp2) activity localized at the posterior end of mature follicles, where oocytes exit. In the present study, we show that Mmp2 activity is regulated by the octopaminergic signaling in mature follicle cells. Exogenous octopamine (OA; equivalent to norepinephrine, NE) is sufficient to induce follicle rupture when isolated mature follicles are cultured ex vivo, in the absence of the oviduct or ovarian muscle sheath. Knocking down the alpha-like adrenergic receptor Oamb (Octoampine receptor in mushroom bodies) in mature follicle cells prevents OA-induced follicle rupture ex vivo and ovulation in vivo. We also show that follicular OA-Oamb signaling induces Mmp2 enzymatic activation but not Mmp2 protein expression, likely via intracellular Ca²⁺ as the second messenger. Our work develops a novel ex vivo follicle rupture assay and demonstrates the role for follicular adrenergic signaling in Mmp2 activation and ovulation in Drosophila, which is likely conserved in other species.     

Follicle cells and germ line cells both affect polarity in dicephalic chimeric follicles of Drosophila

Summary In aberrant egg follicles of the pattern mutant dicephalic (dic) the oocyte is wedged in between two groups of nurse cells, and this condition may give rise to embryos which express anterior traits at both ends. We have analysed the role of the dic genotype of the germ line cells and the surrounding somatic follicle cells in the formation of the dic follicular phenotype. By means of pole cell transplantations into Fs (1) K 1237 hosts (this cell-autonomous mutation causes degeneration of the host's germ line cells early in oogenesis), we constructed chimeras in which either the follicle cells, the germ line cells, or both were homozygous for the dic mutation. In all three combinations the dic phenotype was expressed but not in controls with dic ⁺ in both germ line cells and follicular epithelium. Since follicles with the dic phenotype may be produced if either the germ line cells or the follicle cells lack dic ⁺ gene activity we suggest that cellular interactions between both cell types are required for the correct positioning of the oocyte at the follicle's posterior pole.     

Hormonal dissociation of ovulation and maturation of oocytes: Ovulation of immature amphibian oocytes by prostaglandin

Prostaglandin involvement in ovulation and maturation of amphibian (Rana pipiens) ovarian follicular oocytes was investigated using in vitro-cultured ovarian follicles. Exposure of follicles to PGF₂α during culture stimulated variable but generally low levels of ovulation without concomitant induction of maturation. Addition of PGF₂α to cultured follicles markedly enhanced the incidence of ovulation in follicles exposed to progesterone or frog pituitary homogenate (FPH). Onset of the ovulatory process was further accelerated following addition of PGF₂α to FPH-treated follicles. PGE, in contrast to PGF₂α, exhibited no stimulatory effects on ovulation and consistently inhibited ovulation induction by FPH and progesterone. Cytological analysis of follicles undergoing ovulation revealed that ovulation of immature oocytes induced by PGF₂α varied markedly from that seen following FPH or progesterone stimulation of follicles in vivo or in vitro. Immature oocytes in contrast to maturing oocytes were typically ovlulated with follicle cells still attached to the vitelline membrane. The observations indicate that PGF₂α effected follicle rupture and contraction of the follicular epithelium and theca without prior separation of the follicle cells from the oocyte. Selective inhibitors of steroid synthesis (cyanoketone) and protein synthesis (cycloheximide) inhibited FPH-induced ovulation and maturation. PGF₂α reversed the inhibitory effects of cyanoketone and cycloheximide on FPH-induced ovulation but not maturation of oocytes. Neither prostaglandins alone or in combination with progesterone or FPH induced ovulation of oocytes following removal of the follicular epithelium. Ovulatory effects of PGF₂α appear to be mediated through the follicular epithelium. Results indicate that ovulation and maturation of amphibian oocytes can be induced independently of each other by separate classes of hormones. Normal synchronization of ovulation and maturation of oocytes may require the combined action of prostaglandins and steroids acting within different follicular compartments.     

Cellular and molecular markers of anteroposterior and dorsoventral organisation in the vitellogenic follicles of adult Sarcophaga bullata (Diptera) and dorsoventral orientation of follicles in the ovary

Summary Polar organisation in the follicles of adult Sarcophaga bullata is reflected in the nurse cell-oocyte axis and in the orientation of the two polar cell pairs in the follicular epithelium. The internal organisation of the nurse cell chamber contributes to polarity but not to dorsoventral asymmetry. Dorsoventral asymmetry is correlated with the eccentric position of the germinal vesicle and the orientation of the polar cell pairs; no other follicle cell specialisations are seen. In an ovary, follicles are preferentially orientated with the dorsal side to the centre of the ovary. Cytoskeletal and some haemolymph proteins are molecular markers of polarity. Thus, in pre-vitellogenic stages, tubulin immunoreactivity is higher in the oocyte than in the nurse cells, actin immunoreactivity is the same over the cystocytes and larval serum proteins are restricted to the poles. During vitellogenesis, both actin and tubulin become more concentrated in the nurse cells and larval serum protein 1 accumulated in the polar cells during border cell migration when yolk polypeptides also accumulate in the oocyte. At the end of vitellogenesis a lipophorin is taken up by the oocyte. No molecular marker of dorsoventral asymmetry was identified.     

Synthesis and secretion of egg-specific protein from follicle cells of the silkworm,Bombyx mori

The synthesis and secretion of egg-specific protein (ESP) were investigated using the follicle cells isolated from the developing ovary of the silkworm, Bombyx mori. The follicle cells were isolated manually from a follicle into a cell layer by thoroughly extruding the oocyte contents through a small hole. Whole follicles and isolated follicle cells were incubated in vitro with [³⁵S]methionine, and ESP and its precursors were immunochemically isolated using antiserum raised to ESP. The isolated follicle cells incorporated label into ESP but the incorporation rate was about one-fifth of that found in whole follicles. About 20% of the total radioactivity of ESPs were recovered from the incubation medium of the isolated follicle cells while only trace activity (<2%) was found in the incubation medium of whole follicles. These results clearly showed that follicle cells synthesize and release ESP to be taken up by the developing oocyte.     

Loss of inhibin alpha uncouples oocyte-granulosa cell dynamics and disrupts postnatal folliculogenesis

Targeted disruption of the inhibin α gene (Inha^-/-) in mice results in an ovarian phenotype of granulosa cell tumors that renders the animals infertile. Little is known about the reproductive defects prior to tumor development. Here, we report novel data on early follicle dynamics in Inha^-/- mice, which demonstrate that inhibin α has important consequences upon follicle development. Morphological changes in both germ and somatic cells were evident in postnatal day 12 ovaries, with Inha^−/− mice exhibiting numerous multilayered follicles that were far more advanced than those observed in age-matched controls. These changes were accompanied by alterations in follicle dynamics such that Inha^−/− ovaries had fewer follicles in the resting pool and more committed in the growth phase. Absence of inhibin α resulted in advanced follicular maturation as marked by premature loss of anti-Müllerian hormone (AMH) in secondary follicles. Additionally, gene expression analysis revealed changes in factors known to be vital for oocyte and follicle development. Together, these data provide key evidence to suggest that regulation of the inhibin/activin system is essential for early folliculogenesis in the prepubertal mouse ovary.     

Structural modifications of the nuclear components during lizard oogenesis in relation to the differentiation of the follicular epithelium

This paper deals with an electron microscope study of nucleolar ultrastructural modifications that occur in the oocytes of the lizard Podarcis sicula during ovarian follicle differentiation. In small diplotene oocytes around which a monolayered follicular epithelium forms, the nucleolus appears as a fibrillo-granular structure. Afterwards, simultaneously with the beginning of pyriform cell differentiation inside the granulosa, the nucleolus progressively condenses and breaks into fragments, forming dense spherical bodies. In larger follicles, with well differentiated pyriform cells, a typical nucleolus is no longer detectable in the oocyte nucleus. These ultrastructural modifications suggest a possible impairment of the oocyte nucleolus in ribosome organization. A possible involvement of pyriform cells in supplying ribosomes to the growing oocyte is discussed.     

PERMEABILITY OF THE OVARIAN FOLLICLE OF AEDES AEGYPTI MOSQUITOES

The passage of tracers of various molecular weights into resting and vitellogenic ovarian follicles of Aedes aegypti mosquitoes was studied ultrastructurally. The outermost layer of the follicular sheath (the basement lamina) is a coarse mechanical filter. It is freely permeable to particles with molecular weights ranging from 12,000 to 500,000 (i.e. cytochrome c, peroxidase, hemoglobin, catalase, ferritin, immunoglobulin (IgG)-peroxidase, iron dextran and Thorotrast) that have dimensions less than 110 A. Molecules as large as carbon (300–500 A) are totally excluded. Whereas proteins and polysaccharide tracers permeate the basement lamina with apparent ease, certain inert particles (e.g. Thorotrast, Fellows-Testager Div., Fellows Mfg. Co., Inc., Detroit, Mich.) penetrate more slowly. With respect to the tracers tested, resting follicles are as permeable as vitellogenic follicles. The follicle epithelium of resting or vitellogenic follicles is penetrated by narrow intercellular channels. Our observations suggest that these spaces are lined with mucopolysaccharide material. After permeating the basement lamina, exogenous tracers fill these channels, while the bulk of material accumulates in the perioocytic space. Within 3 hr after imbibing blood, the pinocytotic mechanism of the oocyte is greatly augmented. Pinocytosis is not selective with regard to material in the perioocytic space, since double tracer studies show that exogenous compounds are not separated, but are incorporated into the same pinocytotic vesicle. During later stages of vitellogenesis, 36–48 hr after the blood-meal, the pinocytotic mechanism of the oocyte is diminished. Simultaneously, the intercellular channels become occluded by desmosomes, and the vitelline membrane plaques separate the oocyte and follicle epithelium.