Invagination and evagination: a hydromechanical model

It has been proposed that processes of invagination and evagination be considered in the context of a single, "hydromechanical," model. The model is based on the assumption that the structures demonstrating invaginations and evaginations are closed systems capable of changing their intracavitary pressure in an autonomous regime. The mass, which occupies the cavity, should overcome the resistance of the surrounding membrane during its growth. In places of the weakest resistance, the inner mass expands especially quickly: it bulges out. If the pressure under the membrane is less than the exterior one, the process goes in the opposite direction: the outer mass intrudes (invaginates) into the cavity, causing its wall to sag in the weakest regions.     

Gastrulation in the sea urchin embryo is accompanied by the rearrangement of invaginating epithelial cells

The second phase of gastrulation in the sea urchin embryo, secondary invagination, involves a dramatic elongation of the tube-like gut rudiment. The cells in the wall of the rudiment, which are organized as a monolayered epithelium, change their arrangement during this process. The number of cells in the wall of the gut rudiment at any given level along its long axis decreases markedly as determined by light microscopy of serial cross sections and by scanning electron microscopy, an observation that can be accounted for only if some of the cells exchange nearest neighbors during secondary invagination. Transmission electron microscopy reveals that cell rearrangement takes place despite the continued presence of typical intercellular junctional complexes. In addition to undergoing rearrangement, the cells in the wall of the gut rudiment change their shape during secondary invagination, becoming more flattened. These data raise the possibility that mechanisms other than the contraction of the filopodia of the presumptive secondary mesenchyme cells contribute to the second phase of invagination in the sea urchin embryo. In addition, the observation that cells in the wall of the gut rudiment undergo rearrangement during secondary invagination provides additional evidence that epithelial sheets can exhibit fluid-like properties during morphogenesis.     

Cell rearrangement and cell division during the tissue level morphogenesis of evaginating Drosophila imaginal discs

The evagination of Drosophila imaginal discs is a classic system for studying tissue level morphogenesis. Evagination involves a dramatic change in morphology and published data argue that this is mediated by cell shape changes. We have reexamined the evagination of both the leg and wing discs and find that the process involves cell rearrangement and that cell divisions take place during the process. The number of cells across the width of the ptc domain in the wing and the omb domain in the leg decreased as the tissue extended during evagination and we observed cell rearrangement to be common during this period. In addition, almost half of the cells in the region of the leg examined divided between 4 and 8 h after white prepupae formation. Interestingly, these divisions were not typically oriented parallel to the axis of elongation. Our observations show that disc evagination involves multiple cellular behaviors, as is the case for many other morphogenetic processes.     

Btk-dependent epithelial cell rearrangements contribute to the invagination of nearby tubular structures in the posterior spiracles of Drosophila

The Drosophila respiratory system consists of two connected organs, the tracheae and the spiracles. Together they ensure the efficient delivery of air-borne oxygen to all tissues. The posterior spiracles consist internally of the spiracular chamber, an invaginated tube with filtering properties that connects the main tracheal branch to the environment, and externally of the stigmatophore, an extensible epidermal structure that covers the spiracular chamber. The primordia of both components are first specified in the plane of the epidermis and subsequently the spiracular chamber is internalized through the process of invagination accompanied by apical cell constriction. It has become clear that invagination processes do not always or only rely on apical constriction. We show here that in mutants for the src-like kinase Btk29A spiracle cells constrict apically but do not complete invagination, giving rise to shorter spiracular chambers. This defect can be rescued by using different GAL4 drivers to express Btk29A throughout the ectoderm, in cells of posterior segments only, or in the stigmatophore pointing to a non cell-autonomous role for Btk29A. Our analysis suggests that complete invagination of the spiracular chamber requires Btk29A-dependent planar cell rearrangements of adjacent non-invaginating cells of the stigmatophore. These results highlight the complex physical interactions that take place among organ components during morphogenesis, which contribute to their final form and function.     

Kinetics and hydrodynamics of Agapanthus umbellatus pollen-tube growth: A structural and stereological study

Summary In vitro pollen germination of Agapanthus umbellatus follows a logistic-type curve. It has a lag phase, which corresponds to pollen grain (PG) hydration, followed by an exponential phase — initial pollentube (PT) growth. The lag phase is characterized by an increase of about 40% in the volume of the PG as a result of the hydration process. During the exponential phase the PT emerges, and 40 min later it possesses an ultrastructural organization with a typical two-layer wall and four well-defined zones: the apical, sub-apical, nuclear and vacuolar zones. In this period the material transported by the Golgi vesicles seems to be mostly incorporated into the pollen-tube wall (PTW). Stereological analysis showed that the increase in tube volume is correlated with the increase in the vacuolar compartment at the PG level. The decrease in the relative volume occupied by the mitochondria, generative cell and vegetative nucleus in the PG suggests that these organelles move to the PT. A correlation between the disappearance of lipid droplets in the lag phase and the metabolic reactions that take place during hydration is suggested.Abbreviations PT Pollen tube - Pg pollen grain - PTW pollen-tube wall     

Invagination and Evagination: A Hydromechanical Model

It has been proposed that processes of invagination and evagination be considered in the context of a single, hydromechanical, model. The model is based on the assumption that the structures demonstrating invaginations and evaginations are closed systems capable of changing their intracavitary pressure in an autonomous regime. The mass, which occupies the cavity, should overcome the resistance of the surrounding membrane during its growth. In places of the weakest resistance, the inner mass expands especially quickly: it bulges out. If the pressure under the membrane is less than the exterior one, the process goes in the opposite direction: the outer mass intrudes (invaginates) into the cavity, causing its wall to sag in the weakest regions.     

The role of Irf6 in tooth epithelial invagination

Thickening and the subsequent invagination of the epithelium are an important initial step in ectodermal organ development. Ikkα has been shown to play a critical role in controlling epithelial growth, since Ikkα mutant mice show protrusions (evaginations) of incisor tooth, whisker and hair follicle epithelium rather than invagination. We show here that mutation of the Interferon regulatory factor (Irf) family, Irf6 also results in evagination of incisor epithelium. In common with Ikkα mutants, Irf6 mutant evagination occurs in a NF-κB-independent manner and shows the same molecular changes as those in Ikkα mutants. Irf6 thus also plays a critical role in regulating epithelial invagination. In addition, we also found that canonical Wnt signaling is upregulated in evaginated incisor epithelium of both Ikkα and Irf6 mutant embryos.     

An electron microscopic study on the mating tube formation in the heterobasidiomycete Tremella mesenterica

Ultrastructure of the mating tube formed in yeast haplont of the heterobasidiomycete Tremella mesenterica was studied by electron microscopy. Cell wall of the mating tube emerged as evagination of the inner layers, rupturing outer layers of the mother cell wall. Comparison with budding cells suggested that the tube emergence place at bud scar and the process of tube emergence was the same as that of bud emergence. Electron transparent vesicles of 0.1 m diameter were scattered in the cytoplasm of the mating tube. Nucleus-associated organelle was located at one side of the nuclear envelope which extended towards the mating tube. A few microtubules were detected in the mating tube, but their association with a nucleus was not clear. The cytoplasmic structure of the mating tube was discussed in comparison with that of hyphae of the filamentous fungi.     

A model to explain spermatophore implantation in cephalopods (Mollusca: Cephalopoda) and a discussion on its evolutionary origins and significance

Male coleoid cephalopods produce spermatophores that can attach autonomously on the female's body during a complex process of evagination called the ‘spermatophoric reaction’, during which the ejaculatory apparatus and spiral filament of the spermatophore are everted and exposed to the external milieu. In some deepwater cephalopods, the reaction leads to the intradermal implantation of the spermatophore, a hitherto enigmatic phenomenon. The present study builds upon several lines of evidence to propose that spermatophore implantation is probably achieved through the combination of (1) an ‘evaginating‐tube’ mechanism performed by the everting ejaculatory apparatus and (2) the anchorage provided by the spiral filament's stellate particles. The proposed theoretical model assumes that, as it is exposed to the external milieu, each whorl of the spiral filament anchors to the surrounding tissue by means of its sharp stellate particles. As the ejaculatory apparatus tip continues evaginating, it grows in diameter and stretches lengthwise, enlarging the diameter of the whorl and propelling it, consequently tearing and pushing the anchored tissue outward and backward, and opening space for the next whorl to attach. After the ejaculatory apparatus has been everted and has perforated tissue, the cement body is extruded, possibly aiding in final attachment, and the sperm mass comes to lie inside the female tissue, encompassed by the everted ejaculatory apparatus tube. It is proposed that this unique, efficient spermatophore attachment mechanism possibly evolved in intimate relationship with the adoption of an active mode of life by coleoids. The possible roles of predation pressure and sperm competition in the evolution of this mechanism are also discussed. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 105 , 711–726.     

Primary Invagination of the Vegetal Plate During Sea Urchin Gastrulation

The initial phase of echinoid gastrulation, primary invagination,involves an inpocketing of a monolayered epithelium. To gaininformation about the nature of the mechanical forces that areresponsible for primary invagination, several experimental approacheshave been taken, using the transparent embryos of the sea urchin,Lytechinus pictus, as the principal material. Vegetal platesisolated microsurgically well before the onset of gastrulationwill invaginate normally, demonstrating that the forces responsiblefor primary invagination are generated by the cells in the vegetal to of the embryo. As shown by serial reconstructions of L.pictus embryos, relatively few cells (about 100) take part inprimary invagination. Both the number of cells and the totalvolume of tissue in the wall of the archenteron increase withtime. Even so, it can be shown that very little movement ofcells over the lip of the blastopore takes place during primaryinvagination, and this process is best viewed as a simple inpocketingof the vegetal epithelium. The cells in the wall of the archenteronhave a distinctive shape; they are elongated along their apico-basalaxes and frequently have enlarged, rounded, basal ends. However,they do not undergo any dramatic changes in shape during primaryinvagination. In particular, there is only a slight decreasein the height of the cells (length along the apico-basal axis),a result that is inconsistent with the hypothesis that invaginationis due to cell rounding (Gustafson and Wolpert, 1967). Examinationof L. pictus and Strongylocentrotus purpuratus gastrulae bytransmission electron microscopy reveals that cells in the wallof the archenteron continue to be joined by typical junctionalcomplexes during primary invagination. In addition, the morphologyof the junctional complex at the gastrula stage is more elaboratethan previously described. Sparse bands of micronlaments areassociated with the plasma membrane at the level of the junctionalcomplexes in both endodermal and ectodermal cells. These andother relevant data on early echinoid gastrulation are discussedin relation to several possible mechanisms of epithelial morphogenesis.     

Presentation by scanning electron microscopy of the life cycle of microsporidia of the genus Encephalitozoon

This paper presents, for the first time, documentation by detailed scanning electron microscopy of the life cycle of microsporidia of the genus Encephalitozoon. Phase 1 is represented by the extracellular phase with mature spores liberated by the rupture of host cells. To infect new cells the spores have to discharge their polar filament. Spores with everted tubes show that these are helically coiled. When the polar tubules have started to penetrate into a host cell they are incomplete in length. The infection of a host cell can also be initiated by a phagocytic process of the extruded polar filament into an invagination channel of the host cell membrane. After the penetration process, the tube length is completed by polar tube protein which passes through the tube in the shape of swellings. A completely discharged polar tube with its tip is also shown. The end of a polar tube is normally hidden in the cytoplasm of the host cell. After completion of the tube length the transfer of the sporoplasm occurs and phase 2 starts. Phase 2 is the proliferative phase, or merogony, with the intracellular development of the parasite that cannot be documented by scanning electron microscopy. The subsequent intracellular phase 3, or sporogony, starts when the meronts transform into sporonts, documented as chain-like structures which subdivide into sporoblasts. The sporoblasts finally transform directly into spores which can be seen in their host cell, forming bubble-like swellings in the cell surface.     

The growth of the pollen tube wall in Oenothera organensis.

The growth of the pollen tube wall of Oenothera is effected by the expulsion of fibrillar material from the cytoplasm into the developing wall. This material may also be seen in the cytoplasm, contained in membrane-bound vesicles. It is not clear how the content of the vesicles is discharged, but it appears not to involve the participation of microtubules. The source of the cytoplasmic fibrillar bodies depends upon the stage of development of the pollen tube. The earilest growth is derived from the inclusion into the wall of vesicles containing pre-formed materials present in the grain on pollination. During the next stage of growth the wall is derived from the content of double-membraned inclusions also present in the pollen. The content of the former vesicles is not so similar to the wall as the latter, but intermediates between the 2 types of vesicle may be seen in the cytoplasm, indicating that the former are formed from the latter. Most of the tube wall is derived from the products of dictyosomes in the pollen grain or tube. These dicytosomes are few in number and they must be exceedingly active. This, and the observation that dictyosome vesicles are frequently associated with banked complexes of mitochondria, indicates that some steps in the metabolism of the vesicular content, perhaps phosphorylation, take place distant from the dicytosomes. These different sources of fibrillar material presumably permit the rapid starting of tube growth, without any attendant metabolism. However, it would be impossible to include enough pre-formed wall material in the grain to enable the full growth of the tube, so once started, it seems that the tube then relies on the elaboration of simple reserves for the contruction of its wall. These reserves are likely to be held in the pollen, and may be the large numbers of starch grains characteristic of the pollen cytoplasm.     

Ultrastructure of the Cell Wall and Cell Division of Unicellular Blue-green Algae

The fine structure of the cell wall and the process of cell division were examined in thin sections of two unicellular blue-green algae grown under defined conditions. Unilateral invagination of the photosynthetic lamellae is the first sign of cell division in the rod-shaped organism, Anacystis nidulans. Symmetrical invagination of the cytoplasmic membrane and inner wall layers follows. One wall layer, which appears to be the mucopolymer layer, is then differentially synthesized to form the septum; the outer wall layers are not involved in septum formation. Centripetal splitting of the inner layer separates the two daughter cells. A second division, in a plane parallel to the first, usually occurs before the first daughter cells are separated. In the coccoid organism, Gleocapsa alpicola, the features of cell division are broadly similar; however, unilateral invagination of the lamellae is not observed and the second division takes place in a plane perpendicular to the plane of the previous division.     

ZipA Is Required for FtsZ-Dependent Preseptal Peptidoglycan Synthesis prior to Invagination during Cell Division

Rod-shaped bacteria grow by a repetitive cycle of elongation followed by division, and the mechanisms responsible for these two processes have been studied for decades. However, little is known about what happens during the transition between the two activities. At least one event occurs after elongation ends and before division commences, that being the insertion of new cell wall peptidoglycan into a narrowly circumscribed ribbon around midcell where septation is destined to take place. This insertion does not depend on the presence of the septation-specific protein PBP3 and is therefore known as PBP3-independent peptidoglycan synthesis (PIPS). Here we report that only FtsZ and ZipA are required to generate PIPS in wild-type Escherichia coli. PIPS does not require the participation of other members of the divisome, the MreB-directed cell wall elongation complex, alternate peptidoglycan synthases, the major peptidoglycan amidases, or any of the low-molecular-weight penicillin binding proteins. ZipA-directed PIPS may represent an intermediate stage that connects cell wall elongation to septal invagination and may be the reason ZipA is essential in the gammaproteobacteria.     

Light microscopy and scanning electron microscopy of the In vitro evagination process of Echinococcus multilocularis protoscolices

Marchiondo A. A. and Andersen F. L. 1984. Light microscopy and scanning electron microscopy of the in vitro evagination process of Echinococcus multilocularis protoscolices. International Journal for Parasitotogy14:151–157. During histogenesis of the protoscolices of Echinococcus multilocularis, the apical portion of the protoscolex consisting of the suckers, rostellum and hook region develops as an introversion and invagination within the tissue of the basal portion. In vitro incubation of protoscolices in evagination fluid stimulates the emergence of the apical portion. The initiation of evagination is first detected by a surface change in the basal portion. The smooth contour of this surface which lacks microtriches becomes transformed into tegumental indentations that form transverse and longitudinal furrows within the basal tegument as the protoscolices contract and expand, respectively. An orifice formed at the site or junction where the apical portion is invaginated begins to expand laterally in order to allow emergence of the suckers. The hooks are arranged within the invaginated protoscolex with blades directed towards the basal orifice, the handles directed towards the peduncle and the guards directed laterally. This arrangement persists throughout the evagination of the suckers and rostellum until the apical dome of the hook region emerges, thereby rotating the blades laterally in the direction of the peduncle and rotating the handles and guards medially to assume a coronal arrangement. Evagination is an asynchronous event and therefore allows observation of individual protoscolices in various stages of emergence.     

Viperous fangs: development and evolution of the venom canal

Fangs are specialised long teeth that contain either a superficial groove (Gila monster, Beaded lizard, some colubrid snakes), along which the venom runs, or an enclosed canal (viperid, elapid and atractaspid), down which the venom flows inside the tooth. The fangs of viperid snakes are the most effective venom-delivery structures among vertebrates and have been the focus of scientific interests for more than 200 years. Despite this interest the questions of how the canal at the centre of the fang forms remains unresolved. Two different hypotheses have been suggested. The mainstream hypothesis claims that the venom-conducting canal develops by the invagination of the epithelial wall of the developing tooth germ. The sides of this invagination make contact and finally fuse to form the enclosed canal. The second hypothesis, known as the "brick chimney", claims the venom-conducting canal develops directly by successive dentine deposition as the tooth develops. The fang is thus built up from the tip to the base, without any folding of the tooth surface. In an attempt to cast further light on this subject the early development of the fangs was followed in a pit viper, Trimeresurus albolabris, using the expression of Sonic hedgehog (Shh). We demonstrate that the canal is indeed formed by an early folding event, resulting from an invagination of epithelial cells into the dental mesenchyme. The epithelial cells proliferate to enlarge the canal and then the cells die by apoptosis, forming an empty tube through which the poison runs. The entrance and discharge orifices at either end of the canal develop by a similar invagination but the initial width of the invagination is very different from that in the middle of the tooth, and is associated with higher proliferation. The two sides of the invaginating epithelium never come into contact, leaving the orifice open. The mechanism by which the orifices form can be likened to that observed in reptiles with an open groove along their fangs, such as the boomslang. It is thus tempting to speculate that the process of orifice formation in viperids represents the ancestral pleisomorphic state, and that enclosed canals developed by a change in the shape and size of the initial invagination.     

Micrasterias cells as a model system for research on morphogenesis.

Micrasterias species have been the subject of numerous experimental studies on cell shape formation in the last 40 years. Chemical and physical treatment during different developmental stages, as well as investigations of ultrastructure by means of various different preparation methods, have yielded information about some principles of morphogenesis in the symmetric, highly ornamented Micrasterias cell. The basic symmetry of a Micrasterias cell is determined prior to mitosis and is established without nuclear control thereafter. Normal cell development, however, may occur only under the conditions of continuous protein synthesis throughout the cell cycle. A prepattern for the later cell shape seems to be present at the plasma membrane at the early stages of septum formation. It is realized by a local, patterned distributed incorporation of cell wall material that is delivered by Golgi-produced vesicles. The areas where fusions take place between the primary wall material containing vesicles and the plasma membrane are defined by inward ionic currents that are carried at least in part by calcium. These areas develop into lobes during the following course of cell growth. Cell shaping in Micrasterias cells is thus mediated by both an enhanced extension of the cell wall and an additional incorporation of wall material in the areas of the lobes. Numerous studies have indicated that actin plays an important role in morphogenesis, whereas microtubules do not participate in this process but are involved mainly in nuclear migration. The present review shows that although a wealth of details concerning Micrasterias morphogenesis has already been elucidated, two main questions, i.e., the method of septum formation and the splitting of the lobes, remain to be answered.     

Changes in wall and internal structure of allomyces-resistant sporangia during germination

The electron microscope was used to examine changes which take place in wall, as well as in internal, structure during germination of mature resistant sporangia of Allomyces neo-moniliformis. When the resistant sporangia are first placed in water to initiate germination, nuclei, mitochondria, and endoplasmic reticulum are not evident, though after the sporangia have been in water for more than 30 min all of these structures become visible. At this time no cracks are evident in the resistant sporangial wall and the cell membrane appears highly convoluted. Within the next 30 min the outer wall splits and the inner wall expands considerably as the protoplast increases in volume. At the same time the cell membrane straightens out, apparently in response to the protoplasmic expansion. The “cementing substances” begin to dissolve about this time so that 1 1/2 hr after placement in water the outer wall is completely separated from the inner wall which now acts as the cell wall. Cleavage appears to be initiated by the invagination of the cell membrane and by the appearance of segments of endoplasmic reticulum with filled vesicles at one end. Between 2 1/2 and 3 hr after placement in water zoospores are released.     

小麦离体花药中花粉核无丝分裂的电子显微镜观察

用电子显微镜观察到,在离体培养的小麦花药中,花粉细胞在脱分化分裂时,除了进行有丝分裂之外,还存在着两种类型的无丝分裂——劈裂(cleavage)和碎裂(fragmentation)。它们都是通过核膜的内陷实现的。认为劈裂式无丝分裂导致游离核花粉的形成,碎裂式无丝分裂导致微核花粉的形成,两者都可引起亚倍体和非整倍体植株的产生。     

The germ tubes of Candida albicans hyphae and pseudohyphae show different patterns of septin ring localization

The location of the septin ring in the germ tubes of Candida albicans hyphae and pseudohyphae was studied using an antibody to Saccharomyces cerevisiae Cdc11p. In pseudohyphae induced by growth at 35 degrees C in YEPD or Lee's medium, a septin ring formed at or near (mean 1.8 microm) the neck between the mother cell and the germ tube. This became double later in the cycle, and the first mitosis took place across the plane of this double ring. A septin ring also formed at the germ tube neck of developing hyphae induced by serum or growth on Lee's medium at 37 degrees C. However, at later times, this ring became disorganized and disappeared. A second double ring then appeared 10-15 microm (mean 12.5 microm) along the length of the germ tube. The nucleus subsequently migrated out of the mother cell into the germ tube, and the first mitosis took place across the plane of this second septin ring. The relocation of the septin ring in developing hyphae provides a clear-cut molecular distinction between hyphae and pseudohyphae. Commitment to one type of septin localization and mitosis was shown to occur early in the first mitotic cycle, well before evagination. Germ tubes of hyphae and pseudohyphae also have different widths. A point of commitment to germ tube width was also demonstrated, but occurred later in the cycle, approximately coincident with the time of evagination.