Skeletal Ultrastructure in some Cyclostome Bryozoans of the Family Lichenoporidae

The ultrastructure of the skeleton is described in six species of lichenoporid cyclostome bryozoans using field emission SEM. Both interior walls (vertical, interzooidal walls, and brood chamber roofs and floors) and exterior walls (basal walls) are initially secreted as tiny wedge-shaped crystallites without a strong preferred orientation. These are seeded directly onto pre-existing crystallites in the case of interior walls, but onto the organic cuticle in exterior walls, the bases of the crystallites forming a tightly packed mosaic against the cuticle. With growth the wedges become longer, broader and relatively flatter, developing into platey crystallites. These crystallites grow predominantly distally (i.e. parallel to wall growth direction) and are closely imbricated in a foliated fabric. Local disruptions to this pattern occur, especially in association with crystallite division along “divergent zones”. The pattern also breaks down in old walls where crystallite growing edges become less evident and imbrication is poorly developed. Although conforming to this general model, some differences exist between species of lichenoporids, and in the patterns found in different parts of the skeleton (e.g. apertural spines). Lichenoporid ultrastructure differs from that of both cinctiporid and hornerid cyclostomes: notably, lichenoporids lack the layer of transverse fibres found in cinctiporids, and their predominant distal growth direction of crystallites contrasts with the proximal direction found in hornerids.     

Micro‐ and nano‐scale ultrastructure of cell walls in Cryogenian microfossils: revealing their biological affinity

Moczyd?owska, M., Schopf, J.W. & Willman, S. 2009: Micro‐ and nano‐scale ultrastructure of cell walls in Cryogenian microfossils: revealing their biological affinity. Lethaia, Vol. 43, pp. 129–136. Recently established protocols and methods in advanced microscopy and spectrometry applied to studies of ancient unicellular organic‐walled microfossils of uncertain biological affinities (acritarchs) provide new evidence of the fine ultrastructure of cell walls and their biochemistry that support the interpretation of some such microfossils as photosynthesizing microalgae. The micro‐scale and nanoscale ultrastructure of the cell walls of late Cryogenian sphaeromorphic acritarchs from the Chichkan Formation (Kazakhstan) revealed by the advanced techniques and studied originally by Kempe et al. (2005) is here further analysed and compared with that of modern microalgal analogues. On the basis of such comparison, we interpret the preserved cell wall ultrastructure to reflect original layering and lamination within sub‐layers of the fossil wall, rather than being a result of taphonomic and diagenetic alteration. The outer thick layer represents the primary wall and the inner layer the secondary wall of the cell, whereas the laminated amorphous sub‐layers, 10–20 nm in thickness and revealed by transmission electron and atomic force microscopy, are recognized as trilaminar sheath structure. Because two‐layered cell walls, trilaminar sheaths and the position of the TLS within the fossil cell wall are characteristic of the mature developmental state in cyst morphogenesis in modern microalgae, we infer that the Chichkan sphaeromorphs are probably resting cells (aplanospores) of chlorophyceaen green microalgae from the order Volvocales. □Biological affinity, cell wall, Cryogenian, microfossils, ultrastructure.     

Skeletal Ultrastructure in Some Articulate Cyclostome Bryozoans

The ultrastructure of the calcareous skeleton is described in 11 species of articulate cyclostome bryozoans with elastic joints. Ten species have interior walls comprising semi-nacreous and pseudofoliated fabrics without a precursory granular layer. Exterior walls consist of outer, finely granular and planar spherulitic layers, succeeded by semi-nacreous and pseudofoliated fabrics like those of interior walls. Outer fabrics are calcified as longitudinal strips, each corresponding to a planar sphcrulitic unit. Articulation surfaces comprise ring diaphragms of very fine granular fabric with concentric laminations. The semi-nacre of walls adjacent to ring diaphragms contains minute holes. Crisulipora occidentalis is unique in having interior walls of transverse fibres succeeded by pseudofoliated fabric, articulation surfaces festooned with deep pits but lacking well-differentiated ring diaphragms, and pseudopores containing sieve-like closure plates. The ultrastructure of most articulates resembles tubuliporine cyclostomes with dominantly semi-nacreous walls, although the lack of precursory granular fabric in the interior walls and the presence of subcircular tablets of semi-nacre (without six-fold sectoring) may be peculiar to articulates. In contrast, Crisulipora is more similar to other tubuliporines with transverse fibres. evidence which, together with other skeletal characters, suggests that Crisulipora evolved jointing independently of the rest of the articulate cyclostomes.     

Skeletal Ultrastructures in some Cerioporine Cyclostome Bryozoans

Abstract The ultrastructure of the calcareous skeleton is described in nine species of Recent cyclostome bryozoans belonging to the suborder Cerioporina. Two species of Heteropora have interior zooecial walls comprising a granular precursory layer followed by a thick layer of transverse fibres and a subordinate foliated fabric with, in mature proximal walls, a semi-nacreous layer. The remaining seven species have interior walls with no transverse fibres and instead predominantly comprise a distally-imbricated, regularly foliated fabric overlying a granular precursory layer. Older, proximal surfaces often have abundant screw dislocations, but true semi-nacre is absent. Basal walls comprise an outer finely granular precursory fabric and planar spherulitic layer, succeeded by the same ultrastructural succession seen in the interior zooecial walls of the respective groups. Exterior walled diaphragms, peristomes and gonozooids similarly comprise an external fabric of planar spherulitic calcite, lined internally by the predominant fabric seen in the interior walls. Ultrastructurally, therefore, cerioporines may be split into two groups with different fabric suites, the first resembling cinctiporids and many tubuliporines in having interior walls with fabrics of transverse fibres, foliated crystallites and semi-nacre; and the second resembling the rectangulates Lichenopora and Disporella in having interior walls comprising only the foliated fabric. These findings support the close phylogenetic relationship between cerioporines and other cyclostomes but suggest that the cerioporines may constitute either a diphyletic or a paraphyletic group.     

Ultrastructure and Development of the Ooecial Walls in Some Calloporid Bryozoans (Gymnolaemata: Cheilostomata)

In the brood chambers (ovicells) of six calloporid cheilostomes studied each skeletal wall consists of four calcified layers: (1) a very thin superficial layer of planar spherulitic crystallites, (2) an upper (outer) layer with wall-perpendicular prismatic ultrastructure, (3) an intermediate lamellar layer, and (4) a lower (inner) wall-perpendicular prismatic layer. Comparative studies of both the ovicell wall ultrastructure and early ovicell formation showed a hypothetical opportunity for evolving complex (multilayered) skeletal walls by fusion of the initially separated gymnocystal and cryptocystal calcifications in Cheilostomata. In two species studied, a bilobate pattern in the final stage of the formation of the ooecial roof was encountered in specimens with the cuticle preserved. A possible explanation to this finding is discussed – the bilobate pattern is suggestive of the hypothetical origin of the brood chamber from (1) two flattened spines, or (2) reduction in spine number of an originally multispinous ovicell.     

Fine structure of isolated and non-isolated potato tuber periderm

Cell walls of the periderm of native potato tuber (Solanum tuberosum L. cv. Primura) consist of a primary wall, a suberized secondary wall and a tertiary wall. With a mixture of pectinase and cellulase intact periderm membranes can be isolated. Isolation does not affect fine structure. It is suggested that the lignin in the middle lamellae and primary walls prevents the enzymes from digesting pectinaceous materials and cellulose. In specimens fixed with OsO₄, the suberized walls appear as alternating electrondense and electron-lucent lamellae. This lamellar architecture is not altered by extraction with chloroform. Therefore, the current view that the electronlucent lamellae consist of soluble lipids (waxes) can no longer be maintained. It is argued that the lamellation is a property of the suberin itself, and the suberized wall consists of alternating layers of suberins differing in polarity. A hypothesis of suberin assembly from sub-units is advanced and the subunits are shown for the first time.     

Skeletal Ultrastructure in some Tubuliporine Cyclostome Bryozoans

The ultrastructure of the calcareous skeleton is described in twenty–one species of recent tubuliporine cyclostome bryozoans, using field emission SEM. The succession of skeletal fabrics in interior walls may be classified into four different fabric suites. The first–formed part of the calcitic skeleton in all species for which it has been observed is a precursory fabric of tiny, wedge–shaped crystallites. This is succeeded in about half of the species studied by a fabric of transverse fibres, followed by foliated fabric and often semi–nacre (fabric suite 1). Most of the remaining species lack transverse fibres and have interior walls largely comprising semi–nacre (fabric suite 2). A few species have skeletons consisting of predominantly distally–oriented, irregularly or regularly foliated fabric (fabric suite 3). A single species has a skeleton of proximally–oriented foliated fabric (fabric suite 4). Basal exterior walls in all species have a precursory fabric of tiny wedge–shaped crystallites without a strong preferred orientation, deposited directly upon the organic cuticle, followed by a layer of planar spherulitic structure, which in turn is succeeded by a similar fabric to that developed in the interior wall of the species concerned. Outermost layers of frontal exterior walls exhibit one of the following combinations of three fabrics: an outer layer of (1) finely granular or wedge–shaped crystallites; a thin dense granular layer followed by (2) distally accreting planar spherulitic fabric., or (3) obliquely accreting planar spherulitic fabric growing partly towards the midline of the frontal wall. Terminal diaphragms usually have outer layers dominated by planar spherulitic ultrastructure with centripetal growth directions. The fabric suites present in tubuliporines encompass most known fabrics found in the other cyclostome suborders and support the notion that this species–rich suborder occupies a central position in cyclostome evolution.     

Skeletal ultrastructure and phylogeny of cyclostome bryozoans

Recent research on the ultrastructure of the calcareous skeleton in the bryozoan order Cyclostomata is summarized and updated, based on field emission SEM studies of 87 species. Six fundamental ultrastructural fabrics are recognized which differ in the crystallographic orientations, shapes and prevailing growth directions of the constituent crystallites. During the growth of individual walls a succession of fabrics is secreted, defining a fabric suite. Five fabric suites are described in interior walls and four in exterior walls. Nine ultrastructural characters were combined with 37 other skeletal characters in a PAUP analysis of the relationships between 28 post-Palaeozoic cyclostomes chosen to include representatives of all suborders. A single tree of length 142 steps was found. Comparison of tree statistics for three categories of characters showed ultrastructural characters to be more homoplastic than zooidal characters, and the latter more homoplastic than colonial characters. Rooting the tree on the paleotubuliporine Cuffeyella gave four transitions from fixed- to free-walled organization and no reversals. With respect to the five extant suborders of cyclostomes, this first, preliminary analysis implies that Rectangulata and Cancellata are monophyletic groups, whereas Articulata are diphyletic, and both Tubuliporina and Cerioporina paraphyletic.     

LIGHT AND ELECTRON MICROSCOPE OBSERVATIONS OF ALGIROSPHAERA ROBUSTA (PRYMNESIOPHYCEAE)1

The coccolithophore Algirosphaera robusta (Lohmann) R. E. Norris was isolated into laboratory culture for the first time. This species is of particular interest as the first deep photic coccolithophore to be cultured, the only member of the Rhabdosphaeraceae to have been successfully isolated, and the first coccolithophore with a coccolith structure including a complex disjunct central area to have been studied in detail. Observations on the culture strain supported the previous inference that the commonly recognized species A. robusta, A. oryza Schlauder, and A. quadricornu (J. Schiller) R. E. Norris are conspecific. However, A. meteora (Müller) R. E. Norris and A. cucullata (Lecal‐Schlauder) J. R. Young, Probert et Kleijne were recognized as discrete species. Coccolith rim formation in A. robusta follows the pattern of biomineralization documented in other heterococcoliths and was suggested to be universal. However, the prominent central hood had a unique ultrastructure and appeared to be formed by a distinctively different biomineralization mode. We suggest that this can provide a key to reinterpreting homology in coccolith structure and that this species is a promising target for comparative biochemical and genomic studies of biomineralization. In terms of cell ultrastructure, A. robusta exhibited marked similarities to Syracosphaera pulchra Lohmann, and a close evolutionary relationship between the families Rhabdosphaeraceae and Syracosphaeraceae is suggested.     

Multizooidal Budding in Parasmittina trispinosa (Johnston) (Bryozoa,Cheilostomata)

Two principally different wall types occur in the bryozoan colony: Exterior walls delimiting the super-individual, the colony, against its surroundings and interior walls dividing the body cavity of the colony thus defined into units which develop into sub-individuals, the zooids. In the gymnolaemate bryozoans generally, whether uniserial or multiserial, the longitudinal zooid walls are exterior, the transverse (proximal and distal) zooid walls interior ones. The radiating zooid rows grow apically to form “tubes” each surrounded by exterior walls but subdivided by interior (transverse) walls. The stenolaemate bryozoans show a contrasting mode of growth in which the colony swells in the distal direction to form one confluent cavity surrounded by an exterior wall but internally subdivided into zooids by interior walls. In the otherwise typical gymnolaemate Parasmittina trispinosa the growing edge is composed of a series of “giant buds” each surrounded by exterior walls on its lateral, frontal, basal and distal sides and forming an undifferentiated chamber usually 2–3 times as broad and 3 or more times as long as the final zooid. Its lumen is subdivided by interior walls into zooids 2–3, occasionally 4, in breadth. This type of zooid formation is therefore similar to the “common bud” or, better-named, “multizooidal budding” characteristic of the stenoleamates but has certainly evolved independently as a special modification of the usual gymnolaemate budding.     

Ultrastructural study of spore wall morphogenesis inOphioglossum thermale kom. var.nipponicum (Miyabe et Kudo) nishida

Spore wall morphogenesis ofOphioglossum thermale var.nipponicum was examined by transmission electron microscopy. The spore wall of this species consists of three layers: endospore, exospore, and perispore. The spore wall development begins at the tetrad stage. At first, the outer undulating lamellar layer of the exospore (Lo) is formed on the spore plasma membrane in advance of the inner accumulating lamellar layer (Li) of the exospore. Next, the homogeneous layer of the exospore (H) is deposited on the outer lamellar layer. Both lamellar layers may be derived from spore cytoplasm; and the homogeneous layer, from the tapetum. Then the endospore (EN) is formed. It may be derived from spore cytoplasm. The membranous perispore (PE), derived from the tapetum, covers the exospore surface as the final layer. Though the ornamentation of this species differs distinctly from that ofO. vulgatum, the results mentioned above are fundamentally in accordance with the data obtained fromO. vulgatum (Lugardon, 1971). Therefore, the pattern of spore wall morphogenesis appears to be very stable in the genusOphioglossum.     

CELL WALL CHEMISTRY AND ULTRASTRUCTURE OF CHLOROCOCCUM OLEOFACIENS (CHLOROPHYCEAE)1,2

Cell walls of Chlorococcum oleofadens Trainor & Bold were examined ultrastructurally and chemically. The wall of zoospores has a uniform 30 nm width and a regular lamellar pattern. Zoospores and young vegetative cell walk exhibit periodicities, consisting of 20 nm ridges on the outer layer. Vegetative cell walls have a variable thickness of Up to 800 nm and are composed of multiple layers of electron dense material. Further, vegetative walk contain a microfibrillar material composed predominantly of glucose and presumed to be cellulose. Except for this cellulose, vegetative cell wall chemistry is very similar to that of Chlamydomemas being composed of glycoprotein rich in hydroxyproline. The hydroxyproline in Chlorococcum walls is linked glycosidically to a mixture of hetrooligosaccharides composed of arabinose and galactose, and in one instance, an unknown 6-deoxyhexose. Altogether, the glycoprotein complex accounts for at least 52% of the wall. The amino acid composition of the walls is stikingly similar to those of widely different plant species. Indirect evidence indicates zoospore cell walls are also chemically similar to those of Chlamydomonas, and like them, are cellulose free. Thus a major chemical difference between zoospore and vegetative cell walk of Chlorococcum is the presence of cellulose in the latter. The contribution of this microfibrillar cellulose to the physical properties of the vegetative wall is discussed.     

Epidermal ciliary ultrastructure of adult and larval sipunculids (Sipunculida)

The ultrastructure of the ciliary apparatus of multiciliated epidermal cells in larval and adult sipunculids is described and the phylogenetic implications discussed. The pelagosphera of Apionsoma misakianum has a dense cover of epidermal cilia on the head region. The cilia have a long, narrow distal part and two long ciliary rootlets, one rostrally and one vertically orientated. The adult Phascolion strombus has cilia on the nuchal organ and on the oral side of the tentacles. These cilia have a narrow distal part as in the A. misakianum larva, but the ciliary rootlets have a different structure. The first rootlet on the anterior face of the basal body is very short and small. The second, vertically orientated rootlet is long and relatively thick. The two ciliary rootlets present in the larval A. misakianum are similar to the basal metazoan type of ciliary apparatus of epidermal multiciliated cells and thus likely represent the plesiomorphic state. The minute first rootlet in the adult P. strombus is viewed as a consequence of a secondary reduction. No possible synapomorphic character with the phylogenetically troublesome Xenoturbella was found.     

Studies on the mucilaginous cells in the leaf ofSpartocytisus filipes W.B.

The structure and ultrastructure of epidermal cells with thick mucilaginous inner walls were investigated in leaves ofSpartocytisus filipes. Identification of the main constituents of the wall was attempted by means of histochemistry and polarized light and compared with the ultrastructure of the wall, which showed a mosaic structure alternating with electron dense bands.     

ULTRASTRUCTURE OF THE CORALLINACEAE (RHODOPHYTA) II. VEGETATIVE CELLS OF LITHOTHRIX ASPERGILLUM1

The ultrastructure of the calcareous red coralline alga Lithothrix aspergillum Gray and the development of the various tissue types has been studied. The sub-apical meristematic tissue alternately produces genicular or intergenicular cells. The genicular cells rapidly elongate and their cell walls thicken and become denser as more fibrillar wall material is laid down within the cell wall. These cells contain little cytoplasm and few organelles. The inter genicular cells which elongate only slightly during development have a small vacuole and many free starch grains in the cytoplasm. The peripheral cells in each inter genicular layer remain meristematic and form a cortical cell layer over the genicular cells. These cortical cells and the apical meristematic cells are covered by small epidermal cells which have extensive cell wall ingrowths between the chloroplasts. The inter genicular cells are calcified. Although the CaCO₃ is laid down within the cell walls, there is always a thin layer of CaCO₃-free organic cell wall material between the plasmalemma and the CaCO₃ impregnated wall. Only the distal tips of the genicular cells are calcified. In old genicular tissues of Lithothrix, secondary deposits of CaCO₃ of unknown crystallography are also found in the spaces between the cell walls. Thus there appear to be at least two mechanisms of calcification in this alga.     

The ultrastructure and histochemistry of the spermathecae of Tubifex tubifex (Annelida: Oligochaeta)

The wall of the spermathecal ampulla in Tubifex tubifex consists of epithelial, muscular and peritoneal layers. The epithelial surface contains closely microvilli while lateral and basal plasma membranes are extensively convoluted. Epithelial cytoplasm exhibits a vertical zonation of subcellular components. The distal zone contains filiform secretory particles which are orientated perpendicular to the apical surface; extrusion occurs by their fusion with the plasma membrane between the bases of neighbouring microvilli. Mitochondiral and Golgi zones, the latter containing the nucleus, subtend the distal zone. The basal zone, composed of vertical compartments formed by the folded plasma membrane, is rich in α-glycogen rosettes. The distal epithelium and lumen material contain neutral mucopolysaccharides and carboxylated acid mucopolysaccharides in conjunction with neutral protein. The ultrastructure of the spermathecal duct wall is comparable with that of the ampulla but is characterized by extremely long microvilli and a prominent musculature.     

Microscopic and ultrastructural anatomy of the trachea and bronchi of Melopsittacus undulatus (Aves,Psittaciformes)

Summary The normal microscopic pattern and ultrastructure of the lower trachea and the primary and secondary bronchi of the budgerigar (Melopsittacus undulatus) are described. The trachea is lined by mucociliary pseudostratified columnar epithelium with simple acinar mucous glands; epithelium in primary and secondary bronchi becomes progressively lower and less pseudostratified, and mucous cells less aggregated. The wall structure shows a parallel simplification; tracheal elements are comprised of osseous metaplastic cartilage with bone marrow between cancellous trabeculae, whereas distal secondary bronchial walls are principally comprised of smooth muscle. Mucous cells are similar to those described in mammalian, and other avian respiratory mucosae. Ciliated cells are similar to those known in other avian airways. No brush cells or Clara cells are observed.     

CELL WALL CONFORMATION IN DRY SEEDS IN RELATION TO THE PRESERVATION OF STRUCTURAL INTEGRITY DURING DESICCATION

Internal tissues of mature air-dry seeds, prepared anhydrously for observation with the scanning electron microscope, exhibit cell wall structure which is different from that observed in aqueously fixed (hydrated) seed tissues. In a wide range of dry seeds observed (six members of the Cucurbitaceae, two species of Yucca, Hibiscus esculentus, Phaseolus vulgaris, and Helianthus annuus) cell walls exhibit a unique collapsed structure. The manner of cell wall collapse is characteristic for a given species and ranges from a highly regular folding pattern in the Cucurbitaceae to random wrinkling of the walls in Hibiscus. Evidence suggests that the regular patterns of wall folding may result from a mechanism located in the cell wall. Wall collapse in dry seeds is explained as a means of coordinating wall and protoplasmic shrinkage during desiccation and is thought to be essential for preserving the structural integrity of the tissue by conserving intercellular communication and plasmalemma-cell wall association. Implications of these observations may relate to retention of viability in seeds.     

Ultrastructural characteristics of « Diplotaxis erucoides (L.) DC. » suspensor

Abstract

The development and general morphology of Diplotaxis erucoides (L.) DC. suspensor is of the « Onagrad Type », « Alyssum Variation ». Maximum growth of the suspensor occurs from the globular to the early heart stage of embryo development. The suspensor starts then to degenerate disintegrating shortly after the torpedo stage of the embryo.

The wall ingrowths of the long, tapering, basal cell are especially abundant at the cell's micropilar pole which is closely surrounded by well developed wall ingrowths formed by the endosperm. Wall ingrowths and plasmodesmata are present on the suspensor cells cross walls with the exception of the cell closest to the embryo. No such structures in fact are present on the walls separating this last cell both from the embryo and from the rest of the suspensor. Wall ingrowths are generally associated with numerous, large, mitochondria.

The morphological data seem to indicate that absorption and transport of nutrients from the surrounding tissues is a main function of the suspensor. The possibility of an elaborative and secretory function of this structure is discussed.     

SIPHUNCULAR STRUCTURE IN SOME FOSSIL COLEOIDS AND RECENT SPIRULA

Abstract: In Jurassic Phragmoteuthis huxleyi Donovan (Order Phragmoteuthida) the siphuncular wall shows unique structural and morphological features. The septal neck is short, about one‐eighth of chamber length, but the connecting ring is extremely long, extending through 5–6 chambers. The permeable siphuncular wall in each chamber is, therefore, unusually thick and consists of 5–6 consecutive connecting rings. Each connecting ring is calcified and has a highly porous structure in being composed of bundles of spicular crystallites, orientated more or less at right angles to the siphuncular wall, and separated by smaller or larger interspaces. A restudy of the belemnoid Megateuthis gigantea (Schlotheim) and the aulacoceratid Mojsisovicsteuthis? shows that the connecting rings in these taxa are also calcified. Each ring has a length of two chambers and consists of several calcified lamellae that are traversed by minute pores. The permeable siphuncular wall in each chamber therefore consists of two consecutive connecting rings separated by a porous prismatic layer. In Recent Spirula the connecting ring is composed of two layers: an outer spherulitic‐prismatic layer and an inner glycoprotein layer, of which the latter is not preserved in dry shells. The connecting ring structure is here similar to that in Recent Nautilus. Our study shows that at least three different structural types of siphuncular wall occur in coleoids. The phragmoteuthid connecting ring has a primitive structure, unknown in other cephalopods. This indicates that this taxon has no closer relationship with other coleoid taxa. The belemnitid‐aulacoceratid connecting ring is calcified and traversed by numerous pore canals. It shows a certain structural similarity to that in fossil actinoceratid and orthoceratid nautiloids. The spirulid connecting ring is structurally similar to that in Recent Nautilus and fossil nautilitid and tarphyceratid nautiloids. Thus the connecting ring structure indicates that coleoids include several, phylogenetically clearly separated lineages.