The ultrastructure of the cell wall of normal and apolar embryos ofFucus vesiculosus

Summary Structure and composition of the walls of normal and apolar embryos ofFucus vesiculosus L. were studied. Fucoidin was found in an amorphous outer layer and in an inner fibrillar layer of the wall, mainly at the rhizoid pole. Also in apolar embryos this inner layer was present; it was markedly thickened at the presumptive site of rhizoid formation.We suggest that initiation and extension of the rhizoid is accompanied by apposition of new fibrillar wall material containing sulphated polysaccharides on the inner side of the wall at the rhizoid pole. In apolar embryos this material accumulates at this pole.     

An electron microscopy study of wall expansion during Candida albicans yeast and mycelial growth using concanavalin A-ferritin labelling of mannoproteins

Depending upon growth temperature, Candida albicans can exhibit two different morphologies, a budding yeast or a mycelium. By studying the distribution of concanavalin A-ferritin particles on the cell wall surface during bud and germ tube formation, we have elucidated the way cell wall extension occurs. Both processes initially require the localized lysis of the wall in order to allow the incorporation of the newly synthesized material. Later on, the cell wall behaves as an elastic structure, allowing extension by an intosusception process and, as a consequence, cell growth.Abbreviation Con A concanavalin A     

Growth,cell wall formation and differentiation in the protonema of the moss,Funaria hygrometrica: Effects of plasmolysis on the developmental program and its expression

Summary Cultivation ofFunaria protonemata under plasmolytic or slightly subplasmolytic conditions initially causes a cessation of growth which is accompanied by a transient disappearance (or strong reduction in frequency, respectively) of putative cellulose synthesizing particle rosettes in the plasma membrane. Simultaneously, the formation and exocytosis of cell wall materialsecreting Golgi vesicles is slowed down. The latter process does not become apparent for several hours, though the reduction in activity can be proved indirectly. As a consequence of the imbalance between exocytosis, cell wall material accumulates in the plasmolytic space, generally at the cell tip. This indicates that the pattern of local, polar deposition of cell wall formation and cell elongation, membrane debris as well as wall material is maintained for some time. Later, however, the whole protoplast may become covered by new wall layers. Potentially growing filament tips and the distal region of nontip cells increase in diameter after longer cultivation in subplasmolytic conditions. It is suggested that normal wall growth results from a softening of the existing wall, its stretching and simultaneous stabilization by the apposition of new wall layers. We believe that the swelling is caused by a change in the equilibrium between the obviously less affected softening process and the imperfect stabilization by new wall layers because the wall layers which are formed at reduced turgor pressure are looser than normal and may have a changed composition.Kinetin-induced buds do not develop under plasmolytic conditions. Instead, spiral filaments are formed which readily give rise to buds when the osmotic value of the (kinetin-containing) medium is normalized. The results show that plasmolysis affects the expression of the developmental program rather than its initiation or maintenance.     

CELL DIVISION AND THE ROLE OF THE PRIMARY WALL IN THE FILAMENTOUS DESMID ONYCHONEMA LAEVE (CHLOROPHYTA)1

Cell division and the role of the primary wall in filament formation in the desmid Onychonema laeve Nordst. were investigated by transmission and scanning electron microscopy. In addition, sequential chemical extractions and enzyme treatments were performed, on cell walls of intact filaments. Interphase cells are deeply constricted, consisting of two semicells, each elliptical in front view and circular in side view. In addition to two short lateral spines, each semicell has two apical processes that originate on opposite sides at an angle of about 15° from the central axis and overlap the adjacent cell. Division is initiated as in other desmids by a slight separation of the semicells and development of a girdle of primary wall material at the isthmus. In O. laeve the girdle of primary wall expands to form a spherical vesicle (termed a division vesicle) between the separating semicells. Nuclear division and septum formation occur in this vesicle when it is nearly the full diameter of the filament. Morphogenesis of the apical processes begins with completion of the septum, before the secondary wall appears. At maturity each apical process is surrounded by a thick layer of both secondary and primary wall, except that its capitate tip protrudes through the shroud of primary wall. Sequential treatment with hot ammonium oxalate, 4% NaOH, 17.5% NaOH and 10% chromic acid or various enzyme solutions did not cause filament breakage. SEM and TEM views of O. laeve after these treatments show intact secondary walls and intact primary wall material covering and connecting the apical processes of adjacent cells. It is the persistence of the primary wall between cells and around the apical processes that maintains the long, unbranched filamentous morphology of Onychonema laeve.     

Side branch formation and orientation in the caulonema of the moss,Funaria hygrometrica: Experiments with inhibitors and with centrifugation

Summary Colchicine treatment ofFunaria caulonemata, usually does not inhibit initiation of a side branch or its incipient elongation but does prevent movement of chloroplasts and the nucleus into the outgrowth. After colchicine and after cytochalasin B treatment side branches are formed about at the normal age of the cells; because of the inhibition of the apical cell they arise at an abnormal position,i.e., not in the third but in the second cell of a filament. After D₂O treatment the organelles are dislocated toward the basal cross wall. The site of side branch formation is then obviously determined by the position of the nucleus. Cells with an irreversibly reversed longitudinal polar axis can be found; by centrifugation in proximal direction the sites of side branch initiation likewise are displaced into the proximal region of the cell, especially if the remigration of the nucleus is inhibited by colchicine. High concentrations of Ca²⁺ ions induce the formation of side branch cells, without any outgrowth. The calcium ionophore A 23 187 influences the position of the nucleus and of the side branch only slightly. After these various treatments intercalary divisions frequently occur. The role and interrelationship of the nucleus and peripheral cytoplasm in establishing and maintaining the polar axes, and the role of microtubules are discussed.     

Description of Chytridiopsis Trichopterae N. Sp. (Microspora, Chytridiopsidae), A Microsporidian Parasite of the Caddis Fly Polycentropus Flavomaculatus (Trichoptera, Polycentropodidae), With Comments On Relationships Between the Families Chytridiopsidae and Metchnikovellidae

ABSTRACT. The microsporidium Chytridiopsis trichopterae n. sp., a parasite of the midgut epithelium of larvae of the caddis fly Polycentropus flavomaculatus found in southern Sweden, is described based on light microscopic and ultrastructural characteristics. All life cycle stages have isolated nuclei. Merogonial reproduction was not observed. the sporogony comprises two sequences: one with free spores in parasitophorous vacuoles, the other in spherical, 5.6-6.8 μm wide, sporophorous vesicles which lie in the cytoplasm. the free sporogony yields more than 20 spores per sporont. the vesicle-bound sporogony produces 8, 12 or 16 spores. the envelope of the sporophorous vesicle is about 82 nm thick and layered. the internal layer is the plasma membrane of the sporont; the surface layer is electron dense with regularly arranged translucent components. Both spore types are spherical. They have an ～ 35-nm thick spore wall, with a plasma membrane, an electron-lucent endospore, and an ～ 14-nm thick electron-dense exospore. the polar sac is cup-like and lacks a layered anchoring disc. the polar filament is arranged in two to three isofilar coils in the half of the spore opposite the nucleus. the coupling between the polar sac and the polar filament is characteristic. the surface of the polar filament is covered with regularly arranged membraneous chambers resembling a honeycomb. There is no polaroplast of traditional type. the cytoplasm lacks polyribosomes. the nucleus has a prominent, wide nucleolus. the two spore types have identical construction, but differ in dimensions and electron density. Free living spores are about 3.2 μm wide, the diameter of the polar filament proper is 102-187 nm, the chambers of the honeycomb are 70-85 nm high, and the polar sac is up to 425 nm wide. Living spores in the vesicle-bound sporogony are about 2.1 μm wide, the polar filament measures 69-102 nm, the chambers of the honeycomb are about 45 nm high, and these spores are more electron dense. Comparisons of cytology (especially the construction of the spore wall and the polar filament and associated structures) and life cycles reveal prominent differences among the Chytridiopsis-like microsporidia, and close relationships between the families Chytridiopsidae and Metchnikovellidae.     

Activities of auxin polar transport in inflorescence axes of flower mutants ofArabidopsis thaliana: Relevance to flower formation and growth

Relationships between the activity of auxin polar transport and flower formation were studied using several flower mutants ofArabidopsis thaliana. The activity of auxin polar transport in the upper portion of inflorescence axis of wildtype plants ofArabidopsis thaliana was significantly lower than that of the basal part. The activities of auxin polar transport in the upper portion of inflorescence axes ofap1 andclv1 mutants were significantly higher than that of wild-type plant. However, those of other flower mutants tested,ap3-1, ag, pi, Fl-40, Fl-54, Fl-89 andpin-formed, were extremely low as compared with that of wild one. We got some evidence that the reduction of the activity of auxin polar transport is concerned with the growth and development of plants. We could mimic it by the removal of all flowers and pods including mature or immature seeds. Moreover, artificial pollination inap3-1 andpi mutants, in which no seeds are found naturally, resulted in the partial recovery of the activity of auxin polar transport in inflorescence axis. Considering these results in this study together with the fact that inhibitors of auxin polar transport generated almost same disruptions ofpin-formed orpinoid mutants which normally had no flowers in inflorescence axis (Okadaet al. 1991, Uedaet al. 1992, Bennettet al. 1995), the systern of auxin polar transport and its activity in inflorescence axis seems to be essential for the development of flower bud in early stage ofArabidopsis thaliana, and the activity of auxin polar transport is also regulated by the formation of flowers and seeds in inflorescence axis.     

Lipid polymers accumulate in the epidermis and mestome sheath cell walls during low temperature development of winter rye leaves

Summary Winter rye (Secale cereale L cv. Puma) was grown at 20 °C and at 5 °C and the development of epidermal and mestome sheath cells of leaves from plants grown at both temperatures was compared by electron microscopy. At 5 °C, the cells became densely packed with cytoplasm and small vacuoles after 41 days of growth. By day 56 at 5 °C, epidermal and mestome sheath cells were small in diameter and multivacuolate with asymmetrically thickened walls. By day 76 at 5 °C, a new developmental stage had been reached in epidermal and mestome sheath cells. The cells were larger in diameter although the thickened cell walls and multivacuolate cytoplasm were still present. As epidermal and mestome sheath cell walls thickened during low temperature growth of winter rye, an increase in cuticle thickness and the deposition of a lamellar layer could be observed in epidermal and mestome sheath cells, respectively. The lipid-derived polymers from the leaves of rye plants grown at 20 °C were shown by reductive depolymerization and GC-MS analysis to be comprised of 18-hydroxy-9, 10-epoxyoctadecanoic acid (47%) and dihydroxyhexa-decanoic acid (29%). The leaves of plants grown at 5 °C had two to four times as much lipid-derived polymeric material as those grown at 20 °C and the proportion of the major monomer, 18-hydroxy-9,10-epoxyoctadecanoic acid, increased to 73% of the polymeric material. Physical isolation of both epidermal tissue and vascular bundles followed by GC-MS analysis of the monomeric components released by reduction of the respective lipid polymers showed that 18-hydroxy-9,10 epoxyoctadecanoic acid was the major monomer in the polymer of both the epidermis and the mestome sheaths. The presence of this epoxide monomer in both the cuticles and mestome sheath cell walls of rye leaves was confirmed and visualized by using an epoxide-specific staining reaction.     

Morphology and anionic polymer content in the cell wall of a glycerol-requiring mutant ofBacillus subtilis

A glycerol-requiring mutant ofBacillus subtilis formed irregular spheres and showed disturbed septum formation, when subjected to growth limitation by the supply of glycerol. Under phosphate limitation the cells were also round and developed asymmetric septa. In magnesium-limited cultures the cells contained a thickened wall, as compared with that of the parent strain grown under the same conditions. Chemical analysis revealed the presence of teichoic acid as the major anionic polymer in the wall of the glycerol-, as well as the magnesium-limited cells of the glycerol-requiringB. subtilis mutant.Under phosphate limitation teichuronic acid was the only anionic polymer present in the wall. Thus, in this respect, there were no apparent differences between mutant organisms and the parent strain when grown under magnesium and phosphate limitation, respectively and the observed morphological deviations could not be correlated with an altered anionic polymer content of the wall.     

Zero order kinetics of cell wall turnover inStaphylococcus aureus

Cells exponentially grown from four strains ofS. aureus (SG 511, H, 52A5G, and248 PN-1) and uniformly labeled in their walls with³H-N-acetylglucosamine, were found to turn over their old walls at constant rates of up to 25% per generation. Wall turnover was not observed to follow first order kinetics, thus ruling out the implication that maintenance of normal wall thickness was achieved by a random distribution of new wall components in the old wall. Instead, wall turnover in all cases strictly followed zero order kinetics, indicating that newly synthesized wall material was placed layer by layer beneath the inner surface of the old cell wall. This finding correlates with evidence obtained from earlier electron microscopic investigations into the regeneration of the staphylococcal cell wall after chloramphenicol treatment. Based on the experimental data presented, a simplified model for wall turnover of the growing staphylococcal cell was proposed. The model also takes into account the finding, derived from additional experiments with strainSG 511, that the total cell wall turned over at a somewhat higher rate than the old portions of the wall. The rates of cell wall turnover found inS. aureus SG 511 are the highest reported to date for pathogenic bacteria. The medical implications of this finding were discussed.     

Cell biology and genetics of root hair formation inArabidopsis thaliana

Summary In this review we integrate the information available on the cell biology of root hair formation with recent findings from the analysis of root hair mutants ofArabidopsis thaliana. The mature Arabidopsis root epidermis consists of root-hair-producing cells and non-root-hair-producing cells. Root hair growth begins with a swelling of the outer epidermal wall. It has been postulated that this is due to a pH-mediated localised cell wall loosening. From the bulge a single root hair emerges which grows by tip growth. The root hair tip consists of a vesicle-rich zone and an organelle-rich subapical zone. The vesicles supply new plasma membrane and cell wall material for elongation. The cytoskeleton and its associated regulatory proteins such as profilin and spectrin are proposed to be involved in the targeting of vesicles. Ca²⁺ influxes and gradients are present in hair tips, but their function is still unclear. Mutants have been isolated with lesions in various parts of the root hair developmental pathway from bulge identity and initiation, to control of tip diameter and extent and polarity of elongation.Abbreviations [Ca²⁺]_c cytosolic calcium concentration - MT microtubule - PM plasma membrane - VRZ vesicle-rich zone - WT wild type Dedicated to Professor Brian E. S. Gunning on the occasion of his 65th birthday     

Preliminary observations on organogenesis inTaraxacum officinale tissue cultures

Summary Tissue cultures ofTaraxacum officinale have been isolated from the secondary thickened root. Callus development and leaf and root formation occur on a basal medium supplemented with coconut milk and IAA or NAA, and the addition of kinetin to these media enhances callus growth and organogenesis. Cultures grown on the basal medium with coconut milk and 2,4-D show only callus growth, but organogenesis is induced by the substitution of IAA for 2,4-D. In the 2,4-D grown callus a layer of secondary meristematic tissue is present and organogenesis apparently occurs from localized regions of this tissue which have undergone de-differentiation to the primary meristematic condition.     

Studies ofNajas pollen tubes. Fine structure and the dependence of chlorotetracycline fluorescence on external free ions

Summary The distribution of membrane-associated calcium was investigated in pollen grains and tubes of the underwater pollinated angiospermNajas marina L. using chlorotetracycline (CTC). Tubes grown in distilled water (pH 6) showed the highest fluorescence in a subapical region that tapered basally into a fluorescent strand centrally located in the tube and extending back towards the pollen grain. The apical cap had low fluorescence as did the cytoplasm surrounding the fluorescent strand, the tube base and the pollen grain. Tubes grown in different pond waters (pH 8) revealed no intracellular CTC fluorescence. Instead there was an external fluorescence forming a distinct layer around the whole tube, frequently enhanced in a subapical region to form an external collar.Modification of the patterns of fluorescence could be induced by manipulation pH of the growth media and content of specific ions. For example tubes grown in distilled water with 10^–3 M Mg²⁺ salts showed a similar CTC fluorescence as those grown in pond water. In contrast, Ca²⁺ enrichment had no visible influence on the patterns of fluorescence. The pattern of fluorescence displayed by tubes grown in distilled water, could be reproduced in pond water if the pH was artificially reduced to pH 6.Ultrastructurally, there was no detectable difference in the markedly polar distribution of organelles between pollen tubes grown in the various growth media. The secretory vesicles found in the pollen grain prior to germination become distributed throughout the pollen tube but are least concentrated in regions that show highest internal CTC fluorescence. These regions appear to have large amounts of endoplasmic reticulum and include mitochondria.These results are discussed in relation to the significance of calcium gradients for tip growth and limitations in the use of CTC.Abbreviations CTC chlorotetracycline - SV secretory vesicle - ER endoplasmic reticulum - PIXE proton induced X-ray emissions     

Ultrastructure of infection-thread development during the infection of soybean by Rhizobium japonicum

The location and topography of infection sites in soybean (Glycine max (L.) Merr.) root hairs spot-inoculated with Rhizobium japonicum have been studied at the ultrastructural level. Infections commonly developed at sites created when the induced deformation of an emerging root hair caused a portion of the root-hair cell wall to press against an adjacent epidermal cell, entrapping rhizobia within the pocket between the two host cells. Infections were initiated by bacteria which became embedded in the mucigel in the enclosed groove. Infection-thread formation in soybean appears to involve degradation of mucigel material and localized disruption of the outer layer of the folded hair cell wall by one or more entrapped rhizobia. Rhizobia at the site of penetration are separated from the host cytoplasm by the host plasmalemma and by a layer of wall material that appears similar or identical to the normal inner layer of the hair cell wall. Proliferation of the bacteria results in an irregular, wall-bound sac near the site of penetration. Tubular infection threads, bounded by wall material of the same appearance as that surrounding the sac, emerge from the sac to carry rhizobia roughly single-file into the hair cell. Growing regions of the infection sac or thread are surrounded by host cytoplasm with high concentrations of organelles associated with synthesis and deposition of membrane and cell-wall material. The threads follow a highly irregular path toward the base of the hair cell. Threads commonly run along the base of the hair cell for some distance, and may branch and penetrate into subjacent cortical cells at several points in a manner analagous to the initial penetration of the root hair.     

The Hippo pathway controls polar cell fate through Notch signaling during Drosophila oogenesis

During Drosophila oogenesis, the somatic follicle cells form an epithelial layer surrounding the germline cells to form egg chambers. In this process, follicle cell precursors are specified into polar cells, stalk cells, and main-body follicle cells. Proper specification of these three cell types ensures correct egg chamber formation and polarization of the anterior–posterior axis of the germline cells. Multiple signaling cascades coordinate to control the follicle cell fate determination, including Notch, JAK/STAT, and Hedgehog signaling pathways. Here, we show that the Hippo pathway also participates in polar cell specification. Over-activation of yorkie (yki) leads to egg chamber fusion, possibly through attenuation of polar cell specification. Loss-of-function experiments using RNAi knockdown or generation of mutant clones by mitotic recombination demonstrates that reduction of yki expression promotes polar cell formation in a cell-autonomous manner. Consistently, polar cells mutant for hippo (hpo) or warts (wts) are not properly specified, leading to egg chamber fusion. Furthermore, Notch activity is increased in yki mutant cells and reduction of Notch activity suppresses polar cell formation in yki mutant clones. These results demonstrate that yki represses polar cell fate through Notch signaling. Collectively, our data reveal that the Hippo pathway controls polar cell specification. Through repressing Notch activity, Yki serves as a key repressor in specifying polar cells during Drosophila oogenesis.     

CONJUGATION IN SPIROGYRA1

Conjugating filaments of Spirogyra were examined with both light and electron microscopes. Initially 2 or more filaments of Spirogyra were attached by mucilagenous material. Papillae appeared first in one filament and then in adjacent positions on the other filament. Subsequent growth of papillae separated the conjugating filaments; wall microtubules disappeared in papillae as they elongated. Golgi activity then increased markedly only in the male filament; mucilage production by these Golgi coincided with contraction of the male gamete from its cell wall and may be responsible for its subsequent migration. The end walls separating papillae dissolved to form the conjugation tube, allowing gamete union. The male protoplast then migrated through the tube and further cytoplasmic condensation formed an elliptical-shaped zygote. During the migration phase, zygote wall formation was initiated and numerous active Golgi apparently contributed material to it. Early zygote maturation was characterized by rapid wall formation and an increase in lipid droplets.     

Enforced growth-rate fluctuation causes pectin ring formation in the cell wall of Lilium longiflorum pollen tubes

Normally growing lily (Lilium longiflorum Thunb.) pollen tubes cultured in standard sucrose medium display a relatively steady tip-growth pattern and a rather even pectin sheath in the cell wall. In an attempt to better understand pulsatory growth, observed in some species, e.g., Petunia, and its possible role in causing the formation of thickened cell wall rings, we have imposed marked fluctuations in the growth-rate of lily pollen tubes. The appropriate growth-perturbing conditions were achieved by modulating the medium osmolarity or by applying caffeine, a non-turgor inhibitor, in a specially designed incubation chamber with a controlled medium flow. The relatively non-esterified pectin deposition in the wall of the growth-interrupted pollen tubes was detected by immunofluorescence microscopy using a monoclonal antibody, JIM 5. The observations show that the periods of slow or inhibited growth correspond to the times when the thickened walls are deposited. Since the growth fluctuations were induced by both turgor- and non-turgor-related means, the proposed endogenous regulatory role of turgor pressure is questioned. Other factors, such as the tip-focused Ca²⁺ gradient which was demonstrated by ratiometric ion imaging, and the alteration in the extensibility of the cell wall, which correlated with pectin esterification/de-esterification, emerge as candidates for the regulation of growth fluctuations.     

Tube formation by complex cellular processes in Ciona intestinalis notochord

In the course of embryogenesis multicellular structures and organs are assembled from constituent cells. One structural component common to many organs is the tube, which consists most simply of a luminal space surrounded by a single layer of epithelial cells. The notochord of ascidian Ciona forms a tube consisting of only 40 cells, and serves as a hydrostatic “skeleton” essential for swimming. While the early processes of convergent extension in ascidian notochord development have been extensively studied, the later phases of development, which include lumen formation, have not been well characterized. Here we used molecular markers and confocal imaging to describe tubulogenesis in the developing Ciona notochord. We found that during tubulogenesis each notochord cell established de novo apical domains, and underwent a mesenchymal–epithelial transition to become an unusual epithelial cell with two opposing apical domains. Concomitantly, extracellular luminal matrix was produced and deposited between notochord cells. Subsequently, each notochord cell simultaneously executed two types of crawling movements bi-directionally along the anterior/posterior axis on the inner surface of notochordal sheath. Lamellipodia-like protrusions resulted in cell lengthening along the anterior/posterior axis, while the retraction of trailing edges of the same cell led to the merging of the two apical domains. As a result, the notochord cells acquired endothelial-like shape and formed the wall of the central lumen. Inhibition of actin polymerization prevented the cell movement and tube formation. Ciona notochord tube formation utilized an assortment of common and fundamental cellular processes including cell shape change, apical membrane biogenesis, cell/cell adhesion remodeling, dynamic cell crawling, and lumen matrix secretion.     

Loss of capacity for acid-induced wall loosening as the principal cause of the cessation of cell enlargement in light-grown bean leaves

Cell enlargement in primary leaves of bean (Phaseolus vulgaris L.) can be induced, free of cell divisions, by exposure of 10-d-old, red-light-grown seedlings to white light. The absolute rate of leaf expansion increases until day 12, then decreases until the leaves reached mature size on day 18. The cause of the reduction in growth rate following day 12 has been investigated. Turgor calculated from measurements of leaf water and osmotic potential fell from 6.5 to 3.5 bar before day 12, but remained constant thereafter. The decline of growth after day 12 is not caused by a decrease in turgor. On the other hand, Instron-measured cell-wall extensibility decreased in parallel with growth rate after day 12. Two parameters influencing extensibility were examined. Light-induced acidification of cell walls, which has been shown to initiate wall extension, remained constant over the growth period (days 10–18). Furthermore, cells of any age could be stimulated to excrete H⁺ by fusicoccin. However, older tissue was not able to grow in response to fusicoccin or light. Measurements of acid-induced extension on preparations of isolated cell walls showed that as cells matured, the cell walls became less able to extend when acidified. These data indicate that it is a decline in the capacity for acid-induced wall loosening that reduces wall extensibility and thus cell enlargement in maturing leaves.Abbreviations and symbols FC fusicoccin - P turgor pressure - RL red light - WEx wall extensibility - WL white light - P _w leaf water potential - P _s osmotic potential     

MORPHOLOGY AND FINE STRUCTURE OF FISCHERELLA AMBIGUA1

Fischerella ambigua is a branching blue-green alga, the filamentous nature of which is maintained almost entirely by sheath material. Cell division in this organism most closely resembles the septal division found in most unicellular organisms. In all filamentous blue-green algae previously examined with the electron microscope, cell division has resulted from the imagination of the plasma membrane and inner wall layer only; both the middle wall and the outer wall layers remain continuous throughout the length of the filament. In Fischerella, by contrast, the plasma membrane and the inner wall layer invaginate to produce initially 2 cells. However, the middle wall layer, outer wall layer, and sheath also invaginate to separate the daughter cells. The sheath alone remains continuous throughout the length of the filament.