Boundary cells of endodermal origin define the mouth of Hydra vulgaris (Cnidaria)

We investigated morphology, dynamics and origin of cells surrounding the mouth of Hydra vulgaris using the monoclonal antibody L96. This antibody recognises a one cell-thick ring of endodermal epithelial cells exactly at the boundary between endoderm (gastrodermis) and ectoderm (epidermis). L96⁺ cells can stretch considerably without any cell rupture during mouth opening. Thus, our data prove the existence of a distinct cell population defining hydra's mouth. A model for mouth opening is proposed and the significance of L96⁺ cells for boundary formation between ectoderm and endoderm is discussed.     

Fine structure study on the development of trabeculae in the siphonous green algaCaulerpa simpliciuscula C. Ag.

Summary Bundles of fibrils and tubular structures were found to be associated with growing trabeculae ofCaulerpa simpliciuscula. In the rhizome tips, the bundles had an average diameter of 0.1 to 0.3 m, and a length greater than 10 m. The fibrils in the bundles were oriented in a strictly parallel fashion, with an individual thickness of 3–8 nm. The development of trabeculae started with the apposition of material of low electron density onto the bundles, which in this way became the inner skeleton of the trabeculae.Although fibre bundles with the same internal structure also occurred in the frond tip, these rarely contributed to trabecula formation. In the frond tips a different type of bundle with paracrystalline structure was found associated with the trabecular surface, forming a temporary connection between adjacent trabeculae. Permanent connection was achieved by deposition of further layers of trabecular material. These bundles in the frond tip consisted of two layers of tubular elements with a wall thickness of 80 Å and an inner diameter of 20–25 nm.Both fibre bundles and tubular bundles appear to contribute to trabecula formation. The similarity of these structures to the vacuolar inclusions observed in other siphonous algae is discussed.     

Uptake of Lucifer Yellow CH into intact barley roots: Evidence for fluid-phase endocytosis

Intact barley (Hordeum vulgare L.) roots have been shown to take up the highly fluorescent dye Lucifer Yellow CH (LYCH) into their cell vacuoles. In the apical 1 cm of root tip, differentiating and dividing cells showed a prolific uptake of LYCH into their provacuoles. The LYCH was retained during fixation, apparently becoming bound to electron-dense material in the vacuoles. The dye freely entered the apoplast of roots in which the Casparian band was not developed, being taken up into the vacuoles of cells in both the cortex and stele. However, when LYCH was applied to a 1-cm zone approx. 6 cm behind the root tip the Casparian band on the radial walls of the endodermis completely prevented the dye from entering the cells of the stele, only the cell walls and vacuoles of the cortical cells taking up the dye. The inability of LYCH to cross the plasmalemma of the endodermal cells and enter the stele via the symplast substantiates previous claims that the dye is unable to cross the plasmalemma of plant cells. The results are discussed in the light of recent demonstrations that LYCH is a particularly effective marker for fluid-phase endocytosis in animal and yeast cells. A calculation of the energetic requirements for LYCH uptake into barley roots supports the contention that LYCH is taken up into the vacuoles of plant cells by fluid-phase endocytosis.Abbreviation LYCH Lucifer Yellow CH     

Ultrastructural observations on the cuticular envelope in salt glands ofFrankenia pauciflora

Summary During five developmental stages in differentiation of salt glands in leaves ofFrankenia pauciflora, details of the deposition of incrusting material in the cuticular envelope,i.e., hydrophobic suberin and/or cutin, have been observed by means of transmission electron microscopy.Around each transfusion area in the cuticular envelope a conspicuous lamellate ring structure is found. Thin 3.5 nm lamellae associated with the plasmalemma around each ring structure appear to participate in the formation of tripartite structures showing a reversed contrast compared to the plasmalemma. Below the ring structure the cuticular envelope is divided into an outer fibrillar and an inner amorphous zone. A gradual transition between the tripartite lamellae of the ring structure and the amorphous material of the envelope is evident. Very dense material present between the lamellae appear to accumulate in the transition zone. The results are discussed in relation to basic structural features of various incrustations of lipid nature.     

Comparative investigations on the differentiation of the endodermis and the development of the Hartig net in mycorrhizae of Picea abies and Larix decidua

Summary The differentiation of the endodermis of mycorrhizal roots of Picea abies and Larix decidua was investigated by means of light and transmission electron microscopy and with fluorescence techniques. The initiation and differentiation of the Hartig net were recorded. Differences between the two tree species were found, as were differences between the two tree species and angiosperms. The Casparian band developed immediately after the origin of endodermal cells from the meristem in mycorrhizae of both tree species. In L. decidua only the primary endodermis was present in most mycorrhizal laterals. The secondary structure of the endodermis was restricted to main roots and proximal parts of larch mycorrhizae. In P. abies mycorrhizae, however, the secondary stage of the endodermis developed soon after the primary endodermis and was characterized by regular alternation of short, active passage cells and elongated, rapidly degenerating cells, the inner surface of which was covered by a thick suberin layer. Hartig net development started in P. abies short roots only after the differentiation of endodermis into the secondary stage, whereas in L. decidua, the Hartig net was already initiated at the primary endodermal stage. Differences were specific for tree species.     

Gastrulation in the sea anemone Nematostella vectensis occurs by invagination and immigration: an ultrastructural study

The sea anemone Nematostella vectensis has recently been established as a new model system for the understanding of the evolution of developmental processes. In particular, the evolutionary origin of gastrulation and its molecular regulation are the subject of intense investigation. However, while molecular data are rapidly accumulating, no detailed morphological data exist describing the process of gastrulation. Here, we carried out an ultrastructural study of different stages of gastrulation in Nematostella using transmission electron microscope and scanning electron microscopy techniques. We show that presumptive endodermal cells undergo a change in cell shape, reminiscent of the bottle cells known from vertebrates and several invertebrates. Presumptive endodermal cells organize into a field, the pre-endodermal plate, which undergoes invagination. In parallel, the endodermal cells decrease their apical cell contacts but remain loosely attached to each other. Hence, during early gastrulation they display an incomplete epithelial–mesenchymal transition (EMT). At a late stage of gastrulation, the cells eventually detach and fill the interior of the blastocoel as mesenchymal cells. This shows that gastrulation in Nematostella occurs by a combination of invagination and late immigration involving EMT. The comparison with molecular expression studies suggests that cells expressing snailA undergo EMT and become endodermal, whereas forkhead/brachyury expressing cells at the ectodermal margin of the blastopore retain their epithelial integrity throughout gastrulation.     

A unique pattern of F-actin organization supports cytokinesis in vacuolated cells of Macrocystis pyrifera (Phaeophyceae) gametophytes

Summary. The organization of actin filaments and their role in cytokinesis was studied in regenerating protoplasts and thallus cells of gametophytes of the brown alga Macrocystis pyrifera. Before the onset of cytokinesis, a ring of actin filaments appeared on the putative cytokinetic plane just under the plasmalemma. Light and electron microscopy of cytokinetic cells revealed that large vacuoles occupy the space between the daughter nuclei, which very often are eccentrically positioned at the cell cortex. By the progress of cytokinesis, actin filament bundles emanating from the cytokinetic ring tend to form an actin plate that enters cytoplasmic pockets in which the cytokinetic diaphragm develops. The mechanism of this cytokinetic pattern that has not been reported so far for brown algae is discussed. Correspondence and reprints: Department of Botany, Faculty of Biology, University of Athens, Athens 157 84, Greece.     

Substructural morphogenesis of the spindle apparatus in meiotic basidia ofCoprinus micaceus (Agaricales)

The spindle apparatus ofCoprinus micaceus begins to develop from the diglobular polar body outside the nucleus. During both meiotic divisions it operates inside the nuclear envelope and consists of two amorphous poles, a central bundle of interpolar microtubules, and chromosomal microtubules. A metaphase plate cannot exist because the interpolar strand of fibers is persistent throughout the division process. Within the spindle axis more than 100 microtubules can be estimated. They are encircled by a ring of chromatic structures. During the telophase the former spindle pole is evaginated from the nuclear envelope and contacts the plasmalemma near the cell wall.     

Eyespot membranes in newly released zoospores of the green algaChlorosarcinopsis gelatinosa (Chlorosarcinales) and their fate during zoospore settlement

Summary Eyespot membranes in zoospores of the green algaChlorosarcinopsis gelatinosa were studied with the freeze-fracture technique. The PF of the plasmalemma overlying the eyespot lipid globules contains significantly greater numbers of intramembraneous particles (IMP; 8,200 IMP/m²) compared to other areas of the plasmalemma (2,100 IMP/m²). In the eyespot area the EF of the plasmalemma reveals no IMP, but regularly arranged depressions corresponding to the PF particles. Sizes of PF particles are not significantly different between the eyespot area and other areas of the plasmalemma. Zoospore settlement starts approximately two hours after release and involves in sequence, rounding up of the cells, retraction of the flagella and secretion of a cell wall. Eyespot membrane specializations on the PF of the plasmalemma disappear during flagellar retraction and before cell wall secretion.The functional significance of eyespot membrane specializations is discussed in accordance with the view that these membranes are engaged in photoreception and primary sensory transduction relating to green algal phototaxis.     

EGY1 plays a role in regulation of endodermal plastid size and number that are involved in ethylene-dependent gravitropism of light-grown Arabidopsis hypocotyls

Egy1 was isolated as an ethylene-dependent gravitropism-deficient Arabidopsis mutant. Molecular studies reveal that EGY1 gene encodes a 59-kDa plastid-targeted metalloprotease. It is actively expressed in hypocotyl tissue and targets to endodermal and cortex plastid. Its protein level is up-regulated by both ethylene and light. CAB protein accumulation and chlorophyll level is severely reduced in hypocotyls and endodermal cells, respectively. Sucrose is able to restore the severely reduced starch and lipid contents as well as the deficient endodermal plastid size found in light-grown egy1 hypocotyls yet it fails to rescue the reduced plastid number and chlorophyll level in egy1 endodermal cells. The loss-of-function egy1 mutation results in a smaller size (1.9 ± 0.3 μm in diameter) and less number (5 ± 1) of plastids in endodermal cells, which are nearly 50% of the wild-type. EGY1 is specially required for the development of full-size endodermal plastid in seedlings that are grown on sucrose-free media under light. It plays a direct role in controlling the light-induced chlorophyll production, grana formation and plastid replication in endodermal cell. However, it plays an indirect role in regulation of endodermal plastid size. It is likely that the ethylene-dependent gravitropism-deficient phenotype of egy1 hypocotyls may result from the smaller size and less number of endodermal plastids. Gravicurvature assays performed on ethylene-insensitive mutants, etr1-1, etr2-1, ers2-1, ein4-1 and ein2-5, have clearly demonstrated the necessary role for ethylene in vigorous gravitropism of light-grown hypocotyls. The degree of ethylene-dependent gravicurvature is positively correlated with the combined state of endodermal plastid mass and number. Neither ethylene nor EGY1-regulated full-size endodermal plastid is sufficient for promotion of vigorous hypocotyl gravitropism. Presence of 4 full-size plastids per endodermal cell together with ethylene pretreatment of hypocotyls becomes sufficient to trigger vigorous gravicurvature in light-grown seedlings. A model is therefore proposed to address the role of EGY1 in regulation of endodermal plastid size and number as well as the stimulatory effect of ethylene on hypocotyl gravitropism.     

An AGP epitope distinguishes a central metaxylem initial from other vascular initials in theArabidopsis root

Summary The developmental expression of an arabinogalactan protein (AGP) recognised by a monoclonal antibody, JIM 13, is described inArabidopsis roots. It is expressed in the single initial of the central metaxylem vessel that lies immediately above the four central cells of the quiescent centre. AGP expression spreads in a non-clona] fashion to neighbouring cell files as they mature. The AGP first appears in other pre-metaxylem elements and subsequently in the layer of parenchyma cells on either side of the xylem axis. Later, it spreads to the entire ring of eight endodermal cell files and to those pericycle cell files next to the xylem pole. It is postulated that this epitope reflects an underlying set of cell signalling events that may be involved in defining local positional values of cells important in establishing cellular patterns in the root.     

The development and ultrastructure of gum ducts inCitrus plants formed as a result of brown-rot gummosis

Summary Young stems ofCitrus plants were infected with the fungusPhytophthora citrophthora. The effect of the infection on gum duct development was studied. The following sequence of structural changes was observed in the cambial zone: 1. The middle lamellae between layers of xylem mother cells dissolve forming duct cavities. 2. The cells around the duct cavities differentiate into epithelial cells rich in cytoplasm. 3. The amount of Golgi bodies and associated vesicles increases. The vesicles and small vacuoles, some of which seem to originate from the fusion of Golgi vesicles, contain fibrillar material that stains for polysaccharides. Vesicles and vacuoles appear to fuse with the plasmalemma. Material staining positively for polysaccharides accumulates between the plasmalemma and cell wall, and penetrates the latter. 4. The protoplast shrinks and the space below the cell wall, which contains polysaccharides, increases in volume. 5. After a period of 10 days or more the gum ducts become embedded in the xylem, and the activity of the epithelial cells ceases. The cell walls of many of them break, and the gum still present in the cells is released.     

Leaf structure in relation to solute transport and phloem loading in Zea mays L.

Small and intermediate (longitudinal) vascular bundles of the Zea mays leaf are surrounded by chlorenchymatous bundle sheaths and consist of one or two vessels, variable numbers of vascular parenchyma cells, and two or more sieve tubes some of which are associated with companion cells. Sieve tubes not associated with companion cells have relatively thick walls and commonly are in direct contact with the vessels. The thick-walled sieve tubes have abundant cytoplasmic connections with contiguous vascular parenchyma cells; in contrast, connections between vascular parenchyma cells and thin-walled sieve tubes are rare. Connections are abundant, however, between the thin-walled sieve tubes and their companion cells; the latter have few connections with the vascular parenchyma cells. Plasmolytic studies on leaves of plants taken directly from lighted growth chambers gave osmotic potential values of about-18 bars for the companion cells and thin-walled sieve tubes (the companion cell-sieve tube complexes) and about-11 bars for the vascular parenchyma cells. Judging from the distribution of connections between various cell types of the vascular bundles and from the osmotic potential values of those cell types, it appears that sugar is actively accumulated from the apoplast by the companion cell-sieve tube complex, probably across the plasmalemma of the companion cell. The thick-walled sieve tubes, with their close spatial association with the vessels and possession of plasmalemma tubules, may play a role in retrieval of solutes entering the leaf apoplast in the transpiration stream. The transverse veins have chlorenchymatous bundle sheaths and commonly contain a single vessel and sieve tube. Parenchymatic elements may or may not be present. Like the thick-walled sieve tubes of the longitudinal bundles, the sieve tubes of the transverse veins have plasmalemma tubules, indicating that they too may play a role in retrieval of solutes entering the leaf apoplast in the transpiration stream.     

Ultrastructural study of the relationship between generative and vegetative cells inMagnolia ×soulangeana Soul.-Bod. pollen grains

Summary InMagnolia ×soulangeana pollen grains the generative cell (GC) does not become totally free within the vegetative cell (VC), at least until the pollen tube emergence. Due to a deviation in its detachment process from the sporoderm, the opposing ends of the VC plasmalemma do not fuse themselves when the GC moves away from the intine. Consequently, the interplasmalemmic space surrounding the GC does not become isolated but rather maintains continuity with the sporoderm through a complex formation that we have called plasmalemmic cord. The real existence of this formation was confirmed through serial sectioning showing the plasmalemmic cord to consist of the VC plasmalemma. In its initial portion it is occupied by a reasonably accentuated wall ingrowth of the inner layer of the intine (intine 3). In the remainder portion, neither of the cytochemical tests used in this work have revealed the presence of a significant amount of wall material. However, ultrathin sections of samples processed either chemically or by cryofixation showed the existence of an intricate system of tubules and vesicles, some of which are evaginations of the VC plasmalemma. The hypothesis that the plasmalemmic cord may have a role in the complex interactions between the two pollen cells is discussed.     

The neck constriction in plasmodesmata

Simple plasmodesmata between mesophyll and bundle sheath cells in actively expanding leaves of Salsola kali L. and roots of Epilobium hirsutum L. are shown to possess specialized structures, called sphincters, around their neck regions. The sphineters are made visible by the combined effects of tannic acid and heavy metal staining; they are localized just outside that area of the plasmalemma, which forms the collar around the entrance to each plasmodesmos. This localization corresponds to a very active area of the plasmodesmos/olasmalemma complex (i.e. enzyme activity and/or presence of strongly reducing substances).Evidence is presented that these ring structures are structural equivalents to hypothetical sphincters performing some valve function; i.e. participating in the control of rates and directions of symplastic transport of solutes through plasmodesmata. The middle layer of the plasmalemma in the neck region is composed of closely-packed, globular subunits appearing in negative contrast. Apparently, these subunits correspond to particle clusters observed at the plasmodesmatal entrance in freeze-fracture preparations. They appear similar to particle clusters in animal tight junctions, and their possible function in providing electrical coupling via low resistance junctions between plant cells is discussed.     

Permeability and Self-Induction as Factors in Water Transport through Bean Roots

Water uptake of root systems of Phaseolus vulgaris was measured in a Scholander pressure cylinder at a constant pressure. Water uptake rises gradually till a steady state is reached after 15 to 60 minutes. This time course can be described as a transport process with the property of a self-induction. The latter was not affected by temperature. It is concluded that the property of self-induction is located in the cytoplasm or at the interface between cytoplasm and plasmalemma of the endodermal cells. It is suggested that cytoplasmic streaming controls the time course of water transport.     

Scanning electron microscopy of the muscle system of Hydra magnipapillata

The fine structure of the ectodermal and endodermal muscle layers of Hydra magnipapillata has been analyzed by scanning electron microscopy after hydrolytic removal of the mesoglea with NaOH and subsequent exposure of the basal and lateral aspects of the layers by mechanical dissection. The ectodermal muscle layer consists of fibrous processes of epithelial cells extending longitudinally to the body axis, whereas the endodermal muscle layer comprises cells with hexagonal bases and several strands of myonemes oriented circularly. In each layer, the muscular elements tightly interdigitate, extending a continuous muscle sheet along the mesoglea. The ectodermal and endodermal muscle sheets communicate with each other via foliate microprojections penetrating the mesoglea. On the lateral aspect of the ectodermal epithelium, spiny nerve fibers run along the upper surface of the muscle processes. The spines are often attached to muscle processes, suggesting that the former monitor muscle contraction. Nerve fibers occasionally come into contact with the mesoglea through narrow gaps between the muscle processes. In the hypostomal ectoderm, a small spindle-shaped cell, probably sensory in nature, extends an apical cilium and a long basal process.     

Morphology and ultrastructure ofOlisthodiscus luteus (Raphidophyceae) with special reference to the taxonomy

A flattened discoid flagellate collected from the Seto Inland Sea, Japan, has been examined by light and electron microscopy. This alga agrees well withClisthodiscus luteus Carter. It has two heterodynamic flagella emerging from a furrow on the upward side of the cell that contains six to 13 yellow-green parietal chloroplasts. It does not rotate but smoothly glide while swimming. The cell has a thin periplast lying between the plasmalemma and chloroplasts. Neither lipid bodies nor mucocysts are seen in the periplast. The pyrenoid matrix being free from thylakoids is penetrated by several cytoplasmic canals from various directions. There are no vesicles of periplastidal network in the narrow space between chloroplast envelope and chloroplast ER. The ultrastructural features ofO. luteus are unique, sharing certain characters with the raphidophycean algae but others withPseudopedinella pyriformis, a unique member of the Chrysophyceae.     

Biomechanical properties across trabeculae from the proximal femur of normal and ovariectomised sheep

The elastic behaviour of trabecular bone is a function not only of bone volume and architecture, but also of tissue material properties. Variation in tissue modulus can have a substantial effect on the biomechanical properties of trabecular bone. However, the nature of tissue property variation within a single trabecula is poorly understood. This study uses nanoindentation to determine the mechanical properties of bone tissue in individual trabeculae. Using an ovariectomised ovine model, the modulus and hardness distribution across trabeculae were measured. In both normal and ovariectomised bone, the modulus and hardness were found to increase towards the core of the trabeculae. Across the width of the trabeculae, the modulus was significantly less in the ovariectomised bone than in the control bone. However, in contrast to this hardness was found not to differ significantly between the two groups. This study provides valuable information on the variation of mechanical material properties in healthy and diseased trabecular bone tissue. The results of the current study will be useful in finite element modelling where more accurate values of trabecular bone modulus will enable the prediction of the macroscale behaviour of trabecular bone.     

Direct cytoplasm-cytoplasm connection: an unusual host-parasite interaction of the tremelloid mycoparasiteTetragoniomyces uliginosus

Summary The cellular interaction ofTetragoniomyces uliginosus andRhizoctonia sp. was restudied by transmission electron microscopy. During the first stages of interaction a body of medium electron density is visible at the center of the haustorial apex in close association with the plasmalemma. A single micropore is produced between the haustorial filament and the host cell. Cytoplasmic connection via the pore always occurred. The pore membrane is continuous with the plasmalemma of both cells. The protoplasts of both the haustorium and the host cell fuse via the micropore. An electron transparent to dense body occlude the pore. Among basidiomycetes, direct connection between the parasite and host protoplasts represents a hitherto unknown type of parasitic interaction.Part 65 of the series Studies in Heterobasidiomycetes.