Characterization of cation-rich follicle cells in vitellogenic follicles of Drosophila melanogaster

Abstract. Cations in the ovarian follicles of Drosophila were precipitated with potassium antimonate and the distribution of the precipitates in the follicle cells was studied. In three stage 10 follicles, the precipitate density in the different follicle cells was determined by counting the number of precipitates per unit area in an electron microscope. Among the follicle cells, those migrating centripetally between the nurse cells and oocyte were found to differ widely in their precipitate density. When stage 10B follicles at different substages were compared, the number of precipitaterich follicle cells was found to increase with developmental age. At middle and late stage 10B, groups of cation-rich follicle cells extended from the region of the ring canals to the outer face of the follicle. We also found changes in the precipitate density of the border cells during vitellogenesis. The precipitate-rich follicle cells may play a role in the postulated electrical current flow.     

Localized synthesis of specific proteins during oogenesis and early embryogenesis inDrosophila melanogaster

Summary Protein synthesis in egg follicles and blastoderm embryos ofDrosophila melanogaster has been studied by means of two-dimensional gel electrophoresis. Up to 400 polypeptide spots have been resolved on autoradiographs. Stage 10 follicles (for stages see King, 1970) were labelled in vitro for 10 to 60 min with³⁵S-methionine and cut with tungsten needles into an anterior fragment containing the nurse cells and a posterior fragment containing the oocyte and follicle cells. The nurse cells were found to synthesize a complex pattern of proteins. At least two proteins were detected only in nurse cells but not in the oocyte even after a one hour labelling period. Nurse cells isolated from stages 9, 10 and 12 follicles were shown to synthesize stage specific patterns of proteins. Several proteins are synthesized in posterior fragments of stage 10 follicles but not in anterior fragments. These proteins are only found in follicle cells. No oocyte specific proteins have been detected. Striking differences between the protein patterns of anterior and posterior fragments persist until the nurse cells degenerate. In mature stage 14 follicles, labelled in vivo, no significant differences in the protein patterns of isolated anterior and posterior fragments could be detected; this may be due to technical limitations. At the blastoderm stage localized synthesis of specific proteins becomes detectable again. When blastoderm embryos, labelled in vivo, are cut with tungsten needles and the cells are isolated from anterior and posterior halves, differences become apparent. The pole cells located at the posterior pole are highly active in protein synthesis and contribute several specific proteins which are found exclusively in the posterior region of the embryo. In this study synthesis of specific proteins could only be demonstrated at those developmental stages which are characterized by the presence of different cell types within the egg chamber, while no differences were detected when stage 14 follicles were cut and anterior and posterior fragments analyzed separately. The differences in the pattern of protein synthesis by pole cells and blastoderm cells indicate that even the earliest stages of determination are reflected by marked changes at the biochemical level.     

F-actin distribution in the ovaries of pre-vitellogenic and vitellogenic black blowflies,Phormia regina (Meigen) (Diptera : Calliphoridae)

The spatial distribution of F-actin microfilaments in the ovaries of previtellogenic and vitellogenic female black blowflies, Phormia regina (Diptera : Calliphoridae), as the females shift from a sugar to a liver diet, is determined using rhodamine-labelled phalloidin (rh-phalloidin). During the pre-vitellogenic stages of ovarian development (i.e. corresponding to a sugar diet) a single bright fluorescent layer marks the interface between follicle cells and the oocyte. Fluorescence is also most evident at the inner surface of the ring canals of the nurse cells. This is observed in the nurse cells both in the distal part of the germarium, and in the vitellogenic growing oocyte. However, when liver-fed (i.e. necessary for vitellogenesis), 2 bright fluorescent layers are observed at the follicle cell-oocyte interface. In addition, the cytoplasm of the nurse cells during vitellogenesis appears full of fluorescent microfilaments and the actin rings are found to increase in size and thickness. The changing organization of the F-actin microfilaments in the follicles during the process of both egg chamber and oocyte formation is discussed and possible functions considered.     

Heterologous gap junctions between oocytes and follicle cells of an insect,Dysdercus intermedius,and their potential role as ion current pathways

Summary In telotrophic insect ovaries, the oocytes develop in association with two kinds of supporting cells. Each ovary contains five to seven ovarioles. An ovariole consists of a single strand of several oocytes. At the apex of each ovariole is a syncytium of nurse cells (the tropharium), which connects by strands of cytoplasm (the trophic cords) to four or more previtellogenic oocytes. In addition, each oocyte is surrounded by an epithelium of follicle cells, with which it may form gap junctions. To study the temporal and spatial patterns of these associations, Lucifer yellow was microinjected into ovaries of the red cotton bug, Dysdercus intermedius. Freeze-fracture replicas were examined to analyze the distribution of gap junctions between the oocyte and the follicle cells. Dye-coupling between oocytes and follicle cells was detectable early in previtellogenesis and was maintained through late vitellogenesis. It was restricted to the lateral follicle cells. The anterior and posterior follicle cells were not dye-coupled. Freeze-fracture analysis showed microvilli formed by the oocyte during mid-previtellogenesis, and the gap junctions became located at the tips of these. As the microvilli continued to elongate until late vitellogenesis, gap junction particles between them and follicle cell membranes became arranged in long arrays. The morphological findings raise questions about pathways for the intrafollicular phase of the ion currents known to surround the previtellogenic and vitellogenic growth zones of the ovariole.Supported by the Deutsche Forschungsgemeinschaft (Schwerpunkt Differenzierung)     

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.     

Yolk synthesis in ovarian follicles ofDrosophila

Summary The autonomous synthesis of yolk proteins in ovarian follicles ofDrosophila melanogaster was analyzed. Vitellogenic follicles were labelled with³⁵S-methionine in vitro and the newly synthesized yolk proteins were separated by SDS-polyacrylamide gel electrophoresis. Possible contamination of the follicle preparations caused by adhering fat body cells could be excluded by culturing follicles in males prior to labelling in vitro. When labelled follicles were cut at the nurse cell/oocyte border the three yolk proteins (YP1, YP2, YP3) were found only in posterior fragments containing ooplasm and follicle cells, whereas two radioactive protein bands (A and B) were detected in nurse cells (anterior fragments). The yolk proteins of these five bands were characterized by peptide mapping. Band A protein, migrating a little more slowly than YP2, is closely related to both YP1 and YP2 while band B contains a yolk protein which is very similar to YP3. Hence, the nurse cells have been identified as a site of vitellogenin synthesis within the ovary ofDrosophila.Supported by the Deutsche Forschungsgemeinschaft, SFB 46     

Distribution of yolk polypeptides in the follicle cells during the differentiation of the follicular epithelium in Sarcophaga bullata egg follicles

In S. bullata, the ovaries contribute to the synthesis of yolk polypeptides. A specific antiserum for yolk polypeptides was used to visualize the presence of yolk polypeptides in the follicle cells during their differentiation. After vitellogenesis has started, all follicle cells contain yolk polypeptides. The squamous follicle cells covering the nurse cells and the border cells lose yolk polypeptides before mid-vitellogenesis, whereas the follicle cells over the oocyte contain yolk polypeptides until after late vitellogenesis. All follicle cells are immunonegative afterwards. In vitro translation of poly(A)+ RNA demonstrated that the presence of yolk polypeptide mRNA correlates well with follicle cell immunopositivity for yolk polypeptides. This suggests that the follicle cells synthesize the ovarian yolk polypeptides. Differences in cellular and nuclear morphology, total and poly(A)+ RNA synthesis and the rate of yolk polypeptide synthesis were shown to be correlated with the presence or absence of yolk polypeptides in the differentiating follicular epithelium. The possible relationship between these different aspects of follicle cell differentiation, follicle cell polyploidy and the extracellular current pattern around follicles are discussed.     

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.     

Patterns of ionic current through Drosophila follicles and eggs

Large steady electrical currents traverse Drosophila follicles in vitro as well as permeabilized eggs. During the period of main follicle growth (stages 9-11), these currents enter the anterior or nurse cell end of the follicles. This inward current acts like a sodium ion influx with some calcium involvement. During the period of chorion formation (stages 12-14), foci of inward current also appear at the posterior, posterodorsal, and anterodorsal regions of follicles in vitro. In stage 14, the posterior in current acts like a chloride ion efflux. In preblastoderm eggs substantial currents continue to enter their anterior end; while weaker and less frequent ones enter their posterior end. We present models in which the currents during follicle growth are driven by the plasma membrane of the oocyte nurse cell syncitium; the external currents during choriogenesis are driven by the follicular epithelium; while the currents through the preblastoderm egg are driven by its plasma membrane. Measurements of pole-to-pole resistances and voltages across preblastoderm eggs indicate that the transcellular currents normally maintain a steady extracellular voltage gradient along the perivitelline space, with the anterior pole kept negative by perhaps 4 or 5 mV. The developmental significance of these currents is discussed.     

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.     

Electrotonic junctions in cecropia moth ovaries.

The steady-state potential of the oocyte, resistance between the ooplasm and the medium, and electronic coupling between oocytes in adjacent follicles were examined in vitellogenic ovarioles of Hyalophora cecropia. The steady-state potential had a constant value of ?40 mV throughout the 100-fold volume increase accompanying yolk deposition, while membrane resistance decreased gradually with increasing size. Resistance rose steeply with the onset of chorion deposition, but did not detectably change with either nurse cell collapse or termination of vitellogenesis. Nonrectified electrical coupling was found between oocytes in adjacent follicles, and fluorescein ions injected into the ooplasm moved readily from follicle to follicle. Large surface area and low membrane resistance made coupling difficult to detect electrically between more mature oocytes, but interfollicular fluorescein migration was found to persist until the end of vitellogenesis. Migration of fluorescein from the oocyte to the follicular epithelium could also be visualized and fingers of ooplasm that cross the vitelline envelope and terminate in dome-shaped attachments to the epithelial cells were implicated in this transfer. The termination of interfollicular coupling coincided with the termination of epithelial-oocyte coupling, and is proposed to result from thickening of the vitelline envelope and withdrawal of the ooplasmic processes.     

Development and physiology of follicular atresia during ovarian growth in the house fly, Musca domestica

Atretic follicles regularly occur in the ovary of the house fly, Musca domestica. The frequency of ovarian follicular atresia and the proportion of atretic follicles per ovary are related to the stage of oögenesis and to the age of the females. Only vitellogenic follicles may become atretic. The atresia may occur at any stage of vitellogenesis, though most follicles become atretic in mid-vitellogenesis. Atretic follicles are completely resorbed within 24–36 hr. The follicle cells may play a synthesizing role during growth and disintegrating one during follicle resorption. The induction of glycogen synthesis by the cessation of RNA and protein synthesis and by vitellogenesis in normal follicles is discussed. The same processes occur prematurely in the atretic follicle which can be thus distinguished by a high content of glycogen.     

Experimental study of follicle formation in suppressed parathyroid glands of Mongolian gerbils.

Parathyroid follicle formation was studied in Mongolian gerbils subjected to different concentrations of calcium in vivo and in vitro, using light and electron microscopic methods, including the potassium pyroantimonate technique and x-ray microanalysis for identification of cations. Follicles were frequent at high calcium concentration, but sparse at intermediate and low levels of calcium. Two main types of follicle were differentiated: "degenerative follicles" containing cellular debris and lined by smooth-surfaced epithelium which occasionally showed degenerative changes; and "secretory follicles" characterized by amorphous and granular contents, and an epithelium possessing microvilli and cytoplasmic projections. Amorphous masses were also seen in dilated intercellular spaces and in dilated cisterns of rough endoplasmic reticulum in the follicle epithelium. Calcium-containing precipitates were found in degenerating chief cells, and between degenerating cells and follicles. Parathyroid follicles are believed to be formed by degeneration of suppressed chief cells (degenerative follicles), and by secretion of hormonal and/or other substances into dilated intercellular spaces which progressively increase in size to form follicular cavities (secretory follicles), thereby possibly reducing the level of metabolically active parathyroid hormone. Functional suppression is believed to underlie the development of parathyroid follicles.     

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.     

Structural and functional dynamics of oogenesis in Glossina austeni: vitellogenesis with special reference to the follicular epithelium.

Morphological changes of the oocyte, follicle cells and nurse cells of the ovaries of the viviparous fly Glossina austeni during vitellogenesis and postvitellogenesis are outlined. During vitellogenesis, material is pinocytosed and incorporated into yolk spheres by subsequent fusions. Various lines of evidence are presented that indicate much of this material is derived from the follicular epithelium. The ultrastructure of the follicular cells throughout the 9 day cycle and their role in protein synthesis is presented. Subsequent to vitellogenesis, the follicle cells synthesize the secondary envelopes.     

Studies on the ovotestis of the slug Agriolimax reticulatus (Müller)

Summary The follicle cells, nurse cells and germinal epithelia, which are closely associated with the oocyte of Agriolimax reticulatus (Müller) during its development in the ovotestis, have been studied using light and electron microscopy. The various secretory, digestive and phagocytic activities of these cells have also been investigated using electron cytochemical tests for oxidisable polysaccharide, acid phosphatase and electron-opaque tracer molecules. The oocyte lies initially between the germinal epithelia and a layer of nurse cells but, as oocyte vitellogenesis proceeds, it becomes encapsulated by a layer of follicle cells. Both the follicle and the nurse cells are active in secretion and digestion and contain Golgi apparatus, granular endoplasmic reticulum and acid phosphatase-rich digestive vacuoles. The significance of these activities is discussed in relation to oocyte vitellogenesis, secondary envelope formation and the digestion and recycling of cellular material.     

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.     

Mitochondrial development during Drosophila oogenesis: distribution,density and in situ RNA hybridizations

The changes in distribution and density of mitochondria and the level of mitochondrial RNA during Drosophila oogenesis were studied simultaneously in the 3 cell types ie follicle cells, nurse cells and oocyte, making up the egg chamber. Up to stage 6, mitochondrial density (mitochondrial and cellular areas ratio) was elevated and increased similarly in both follicle and nurse cells. Thereafter the mitochondrial density of follicle cells continued to increase and that of the nurse cells declined markedly while the nurse cell mitochondria assembled in dense groups and decreased in size. This can be related to a transfer of nurse cell cytoplasm, including mitochondria, to the oocyte. In the oocyte from stage 4 to stage 7 we observed a significant decrease of the mitochondrial density due to the absence of mitochondrial biogenesis. Then the cytoplasm transfer caused mitochondrial density to increase up to the level found in the nurse cells at the end of oogenesis. The mature oocyte contains enough mitochondria to supply 15 000 somatic cells. Our results strongly suggest that the variations in size, distribution and density of mitochondria relate to the particular energetic requirements of the different cell types during the first half of oogenesis. Later they relate to the developmental requirements of the nurse cells and the oocyte, in particular the storage of mitochondria in the oocyte. The level of mitochondrial RNA was studied through in situ hybridization. Throughout oogenesis the follicle and nurse cell RNA evolved similarly. Up to stage 9, there was no change in RNA densities in these cells, suggesting a correlation with the cell volume and/or the nuclear DNA content. Thereafter the cellular RNA concentration declined rapidly. In the oocyte the RNA concentration evolved differently especially from stage 10 to the end, the RNA density being stabilized. This can be related to the injection of nurse cell mitochondria, followed by their assignment to reserve status. Our results suggest that the mt RNA density is under extramitochondrial control mechanisms.     

Mitochondrial development during Drosophila oogenesis: distribution, density and in situ RNA hybridizations.

The changes in distribution and density of mitochondria and the level of mitochondrial RNA during Drosophila oogenesis were studied simultaneously in the 3 cell types ie follicle cells, nurse cells and oocyte, making up the egg chamber. Up to stage 6, mitochondrial density (mitochondrial and cellular areas ratio) was elevated and increased similarly in both follicle and nurse cells. Thereafter the mitochondrial density of follicle cells continued to increase and that of the nurse cells declined markedly while the nurse cell mitochondria assembled in dense groups and decreased in size. This can be related to a transfer of nurse cell cytoplasm, including mitochondria, to the oocyte. In the oocyte from stage 4 to stage 7 we observed a significant decrease of the mitochondrial density due to the absence of mitochondrial biogenesis. Then the cytoplasm transfer caused mitochondrial density to increase up to the level found in the nurse cells at the end of oogenesis. The mature oocyte contains enough mitochondria to supply 15,000 somatic cells. Our results strongly suggest that the variations in size, distribution and density of mitochondria relate to the particular energetic requirements of the different cell types during the first half of oogenesis. Later they relate to the developmental requirements of the nurse cells and the oocyte, in particular the storage of mitochondria in the oocyte. The level of mitochondrial RNA was studied through in situ hybridization. Throughout oogenesis the follicle and nurse cell RNA evolved similarly. Up to stage 9, there was no change in RNA densities in these cells, suggesting a correlation with the cell volume and/or the nuclear DNA content. Thereafter the cellular RNA concentration declined rapidly. In the oocyte the RNA concentration evolved differently especially from stage 10 to the end, the RNA density being stabilized. This can be related to the injection of nurse cell mitochondria, followed by their assignment to reserve status. Our results suggest that the mt RNA density is under extramitochondrial control mechanisms.     

Structure of the ovary and the differentiation of follicular epithelium in the pig louse, Haematopinus suis (Insecta: Phthiraptera)

The female reproductive system of the pig louse, Haematopinus suis (Insecta: Phthiraptera) is composed of paired ovaries, lateral oviducts, and a common oviduct that leads into a vagina. Clusters of mycetocytes (= cells filled with symbiotic organisms) are associated with lateral oviducts. Each ovary is composed of five loosely arranged ovarioles of the polytrophic-meroistic type. An individual ovariole is covered by a basal lamina and is composed of a terminal filament, germarium, and vitellarium. The terminal filament is composed of large, disc-shaped cells that are orientated perpendicularly to the long axis ofthe ovariole. The basal part of the terminal filament is separated from the germarium by a well-developed transverse septum. The germarium is short and filled with clusters of oogonial cells. In each cluster the cells arejoined by intercellular bridges, filled with fusomal material. Within the cluster, only one cell, the future oocyte, enters the prophase of the first meiotic division; the other cells differentiate into nurse cells. The basal part ofthe germarium is filled with the somatic prefollicular cells. The boundary between the germarium and the vitellarium is not distinct. The vitellarium contains linearly arranged ovarian follicles in subsequent stages of oogenesis (previtellogenesis, vitellogenesis and choriogenesis). Each follicle consists of an oocyte and 7 nurse cells and is surrounded by follicular cells. During oogenesis the follicular cells diversify, so that ultimately, five morphologically distinct subpopulations of these cells can be distinguished: (1) cells in contact with the nurse cells, (2) anterior cells, (3) mainbody cells, (4) posterior cells, and (5) interfollicular cells. Interestingly, the follicular cells associated with the anterior part of the oocyte, i.e. located in space at the oocyte/nurse cell border (fold cells) are mitotically active throughout previtellogenesis. It might be suggested, in this context, that the separation of the oocyte from the nurse cell compartment is brought about by mitotic divisions, consequent multiplication and centripetal migration of these cells.