Larval Development and Metamorphosis in Sipuncula

In a brief review of development of the phylum Sipuncula, fourpatterns of development are recognized: (1) direct with no pelagicstage; (2) one larval stage, a lecithotrophic trochophore; (3)two larval stages, a lecithotrophic trochophore and a lecithotrophicpelagosphera; (4) two larval stages, a lecithotrophic trochophoreand a planktotrophic pelagosphera. Larval types and their metamorphosesare described, with special attention to the development andmorphology of the larval cuticle. In the majority of speciesstudied, the egg envelope is transformed into the larval cuticleat metamorphosis of the trochophore. The cuticle of many planktotrophicpelagosphera larvae is characterized by surface papillae ofdiverse form and pattern. The underlying cuticle in some speciesis composed of layers of fibers at right angles to one another.     

A scanning electron microscopic study of larval development in the marine polychaete,Galeolaria caespitosa Lamarck (Serpulidae)

Summary Gametes and developing larvae of the polychaete Galeolaria caespitosa were examined by scanning electron microscopy. The sperm display a primitive morphology. When treated with 0.33 M CaCl₂, they release a branched acrosomal process. At spawning, the polygonal oocytes have a granular surface made up of spherules and the tips of microvilli. The oocyte coat develops a ridged appearance as the oocyte rounds up. At fertilization, the microvilli are withdrawn from the coat surface. Microvilli again appear on the coat surface during the trochophore stage, but the egg coat appears to be retained as the larval cuticle until the demersal stage. The surface of the larva now shows many microvilli. Details of the organization of several ciliary structures are clarified. Moreover, the present study shows rapid, sequential development of paired setal sacs, with the most anterior pair appearing first.     

Ultrastructure of the epithelium from the ovary wall of Dendrochirus brachypterus (Pteroidae,Teleostei)

Summary The inner epithelium of the ovary wall in Dendrochirus brachypterus does not take part in egg production, as it does in other teleosts. The epithelium consists of columnar cells rich in mitochondria and secretory organelles. The distal ends of these cells hang free in the ovary lumen, separated from each other and densely covered by large microvilli. During reproduction the epithelium secretes large amounts of mucus that forms an envelope around the eggs produced from a spongy stroma of the ovary, and keeps the spawn afloat for 24 h.Thanks are due to Mrs. H. Segal and Mr. N. Sharon for technical assistance, and Mr. D. Fridman, for help during the collection of the fish. Also appreciation is given to the technicians of the Electron Microscopy Laboratory of Tel-Aviv University for help in preparation of electron and scanning microscope figures     

Follicular placenta of the viviparous fish,Heterandria formosa: II. Ultrastructure and development of the follicular epithelium

Embryos of the viviparous poeciliid fish, Heterandria formosa, develop to term in the ovarian follicle where they undergo a 3,900% increase in embryonic dry weight. Maternal-embryonic nutrient transfer occurs across a follicular placenta that is formed by close apposition of the embryonic surface (i.e., the entire body surface during early gestation and the pericardial amnionserosa during mid-late gestation) to the follicular epithelium. To complement our recent study of the embryonic component of the follicular placenta, we now describe the development and fine structure of the maternal component of the follicular placenta. Transmission electron microscopy reveals that the ultrastructure of the egg envelope and the follicular epithelium that invests vitellogenic oocytes is typical of that described for teleosts. The egg envelope is a dense matrix, penetrated by microvilli of the oocyte. The follicular epithelium consists of a single layer of cuboidal cells that lack apical microvilli, basal surface specializations, and junctional complexes. Follicle cells investing the youngest embryonic stage examined (Tavolga's and Rugh's stage 5–7 for Xiphophorus maculatus) also lack apical microvilli and basal specializations, but possess junctional complexes. In contrast, follicle cells that invest embryos at stage 10 and later display ultrastructural features characteristic of transporting epithelial cells. Apical microvilli and surface invaginations are present. The basal surface is extensively folded. Apical and basal coated pits are present. The cytoplasm contains a rough endoplasmic reticulum, Golgi complexes, and dense staining vesicles that appear to be lysosomes. The presence of numerous apically located electron-lucent vesicles that appear to be derived from the apical surface further suggests that these follicle cells may absorb and process follicular fluid. The egg envelope, which remains intact throughout gestation and lacks perforations, becomes progressively thinner and less dense as gestation proceeds. We postulate that these ultrastructural features, which are not present in the follicles of the lecithotrophic poeciliid, Poecilia reticulata, are specializations for maternal-embryonic nutrient transfer and that the egg envelope, follicular epithelium, and underlying capillary network form the maternal component of the follicular placenta. © 1994 Wiley-Liss, Inc.     

Embryonic integument and "molts" in Manduca sexta (Insecta,Lepidoptera)

In Manduca sexta the germ band is formed 12 h post-oviposition (p.o.) (=10% development completed) and is located above the yolk at the egg surface. The cells show a polar organization. They are engaged in the uptake and degradation of yolk globules, pinched off from the yolk cells. This process can be observed in the integumental cells during the first growth phase of the embryo that lasts until "katatrepsis," an embryonic movement that takes place at 40% development completed. At 37% development completed, the ectoderm deposits a thin membrane at its apical surface, the first embryonic membrane, which detaches immediately before katatrepsis. The second period of embryonic growth--from katatrepsis to 84 h p.o. (70% development completed)--starts with the deposition of a second embryonic membrane that is somewhat thicker than the first one and shows a trilaminar, cuticulin-like structure. Whereas the apical cell surface is largely smooth during the deposition of the first embryonic membrane, it forms microvilli during deposition of the second one. At the same time, uptake of formed yolk material ceases and the epidermal cells now contain clusters of mitochondria below the apical surface. Rough endoplasmic reticulum (RER) increases in the perinuclear region. The second embryonic membrane detaches about 63 h p.o. At 69 h p.o., a new generation of microvilli forms and islands of a typical cuticulin layer indicate the onset of the deposition of the larval cuticle. The third growth phase is characterized by a steady increase in the embryo length, the deposition of the larval procuticle, and by cuticular tanning at about 100 h p.o. Beginning at that stage, electron-lucent vesicles aggregate below the epidermal surface and are apparently released below the larval cuticle. Manduca sexta is the first holometabolous insect in which the deposition of embryonic membranes and cuticles has been examined by electron microscopy. In correspondence with hemimetabolous insects, the embryo of M. sexta secretes three covers at approximately the same developmental stage. A marked difference: the second embryonic cover, which in Hemimetabola clearly exhibits a cuticular organization, has instead a membranous, cuticulin-like structure. We see the difference as the result of an evolutionary reductional process promoted by the redundancy of embryonic covers in the egg shell. Embryonic "molts" also occur in noninsect arthropods; their phylogenetical aspects are discussed.     

Studies on Pogonophora. 4. Fine structure of the cuticle and epidermis

The fine structure of the integument in several species of Pogonophora has been examined by electron microscopy. The cuticle over the main body is composed of several layers of orthogonally arranged fibres embedded in an amorphous matrix. It is regularly traversed by microvilli from underlying epidermal cells. Toothed bristles of the annuli and setae of the anchor are composed of closely packed fibrous cylinders wrapped in a cortical material. In fine structure the cuticle, setae, toothed bristles (or setae) and setal sacs forming the setae closely resemble the corresponding structures in annelids. The cuticle is maximally thick over the forepart (protosome + mesosome) ; it is very thin and non-fibrous over the surface of the metameric papillae and over extensive areas of post-metameric trunk. The possibilities of a collagenous nature of the cuticle fibres and their mode of secretion by the epidermal cells are discussed. The organization of various cell-types forming the epidermis over the entire animal is examined. Possible functions of these cell-types are discussed. Notable amongst these are 'possible zymogen cells' and some absorptive cells. The intriguing question of nutrition in these gut-less tubiculous animals is re-examined in the light of present observations.     

A fine structural analysis of the epidermis and cuticle of the oligochaete aeolosoma Bengalense stephenson

The fine structure of the epidermis and cuticle has been described for the oligochaete Aeolosoma bengalense. The epidermis is a pseudostratified epithelium and consists of the following cell types: ciliated and nonciliated supportive cells, pigment cells and associated satellite cells, mucous cells, basal cells, and ciliated non-supportive columnar cells. Overlying and restricted to the supportive cells is a delicate cuticle composed of: (a) a discontinuous layer of membrane-bounded surface particles; (b) a thin filamentous layer of moderate electron density just under the surface particles; (c) a thicker inner filamentous layer of low electron density. Digestion with pronase effectively removes the cuticle. This, together with the fact that it stains with alcian blue and ruthenium red, indicates that the cuticle contains an acid mucopolysaccharide. Regeneration of the cuticle, following pronase treatment, is marked by the elaboration of numerous microvilli by the supportive cells. Most of the microvilli are transitory and evidence supports a microvillar origin for the cuticular surface particles. The presence of cuticular surface particles may be a characteristic shared in common by all oligochaetes and, perhaps, some polychaetes.     

An ultrastructural study of polyembryonic parasitoid embryo and host embryo cell interactions

The morula‐stage embryo of the polyembryonic egg‐larval parasitoid Copidosoma floridanum forms outside the host embryo and secondarily invades the host body. Electron microscopic analyses of cellular interactions between the extraembryonic syncytium of the parasitic morula and the host embryonic epithelial cells showed that morula penetration into the host embryo did not cause obvious damage to the host cells, except for the abrasion of the embryonic cuticle. Epithelial cells of the host embryo extended microvilli toward the invading C. floridanum morula and also adjacent host cells in the same way. Shortly after settlement of the morula within the host body cavity, gap junctions and adherens junctions with host cells were formed. The morula was then surrounded by a cyst comprised of host cells into which host tracheoles were invaginated. J. Morphol., 2010. © 2010 Wiley‐Liss, Inc.     

The coelomic envelope of Xenopus laevis eggs: a quick-freeze, deep-etch analysis

The extracellular matrix of Xenopus laevis oocytes was analyzed before and after meiotic maturation using quick-freeze, deep-etch, rotary-shadow electron microscopy. The perivitelline space (PS) of the meiotically immature oocyte contains a filamentous network which connects microvilli (MV) and follicle cell macrovilli to the folded oocyte surface below. The envelope overlying the PS is composed of bundles of large fibers which course between the tips of the MV. Spaces between these bundles contain smaller fibrils which secure the egg envelope to the microvillar tips. Meiotic maturation is accompanied by flattening of the oocyte plasma membrane, formation of an orderly array of MV, and elevation of the egg envelope. In the coelomic eggs, the reorganized envelope is composed of loosely bundled large fibers which course above the microvillar tips rather than between them. The spaces between these bundles contain small fibers similar to those seen in the meiotically immature oocyte. This reorganized envelope, however, will not bind sperm; further modifications must transpire during passage through the oviduct to render it sperm receptive.     

Fine structure of the investing layer on the gymnoblastic hydroid Syncoryne tenella (farquhar, 1895)

The hydranth of the gymnoblastic hydroid Syncoryne tenella is invested by a cuticle approximately 530 mμ thick which is continuous with the periderm of the hydrocaulus. The ectodermal cells of the hydranth possess regularly spaced microvilli orientated with their long axis perpendicular to the ectodermal surface. The microvilli project into the cuticle, and probably serve to anchor the cuticle to the ectoderm. In the hydrocaulus the periderm is loosely applied to the ectoderm: in this region microvilli are absent from ectodermal cells. The periderm is a layered structure composed of finely filamentous material. No structural basis is found for the previously reported differential staining of peridermal layers in the hydrocaulus.     

20-羟基蜕皮甾酮处理后舞毒蛾外部形态和幼虫体壁超微结构的变化

为了探明20-羟基蜕皮甾酮对昆虫蜕皮过程中体壁的表皮层、 皮细胞及其细胞器的具体影响过程, 本研究利用透射电镜技术研究了20-羟基蜕皮甾酮对舞毒蛾Lymantria dispar （Linnaeus）5龄幼虫体壁超微结构的变化。结果表明, 用高浓度20-羟基蜕皮甾酮溶液浸过的白桦叶片饲喂幼虫, 处理6 h, 摄入约400 μg 20-羟基蜕皮甾酮后, 幼虫停止取食; 处理12 h时表皮细胞顶膜上的微绒毛减少, 在皮细胞与旧表皮之间形成蜕皮间隙, 旧头壳从幼虫头部脱离; 处理24 h时蜕皮间隙继续增大, 旧表皮与皮细胞进一步分离, 新表皮质层开始形成; 处理36 h时皮细胞顶膜形成较短的微绒毛, 胞质区域出现数量较多的电子疏松泡, 新表皮由上表皮、 外表皮及8层左右内表皮片层组成; 处理48 h时顶膜与内表皮界限模糊, 内表皮继续合成至16层左右; 72 h时细胞内出现大面积电子疏松泡, 内表皮合成至20层左右。 处理96 h时, 与对照组相比, 皮细胞细胞器较少, 核仁周围出现小部分空白区域, 胞质区域内含物减少; 虫体发黑缩小, 即将死亡; 内表皮层数仍旧保持20层左右。对照组幼虫6-96 h虫体活跃, 正常取食, 外部观察及透射电镜结果均未显现蜕皮现象; 表皮层由上表皮、 外表皮及内表皮组成; 皮细胞顶膜微绒毛密度高; 表皮细胞分泌活动旺盛, 胞质区域细胞界限明显, 内含物丰富; 细胞器典型而且活跃; 内表皮片层随时间不断增加至50层左右。结果提示, 外源20-羟基蜕皮甾酮能够导致舞毒蛾5龄幼虫的致死性蜕皮。     

Filamentous structure of the external surface of abalone eggs revealed by quick-freeze deep-etch technique

The external surface of abalone eggs was examined by thin section and quick-freeze, deep-etch electron microscopy. In thin sections, networks of fine filaments were found interconnecting the adjacent microvilli on the surface of unfertilized eggs. Quick-freeze, deep-etch electron microscopy revealed the three-dimensional structure of these networks of filaments on the external surface of the egg. Mainly two networks of filaments were identified; one was composed of thicker (14–19 nm) filaments interconnecting with the neighboring microvilli nearly horizontally, and the other was composed of thinner (8–14 nm) branched filaments closely surrounding the microvilli surface as well as highly interconnecting neighboring microvilli in a polygonal pattern. The overall structure of the filamentous network on the egg surface showed no distinct alteration after fertilization. These networks of filaments observed on the egg surface may play a key role in sperm–egg interaction.     

Novel surface specialization on a sea anemone egg: “Spires” of actin-filled microvilli

The ultrastructure of the “spiny” surface of Tealia crassicornis eggs is examined in detail by scanning and transmission electron microscopy in order to understand its function. Long microvilli are clustered together in spiral aggregates of 50–75 microvilli called “spires.” There are about 15,000 spires per egg. Dense bundles of microfilaments making up the cores of these microvilli are shown to be composed of actin by staining with the fluorescent dye nitrobenzoxadiazole (NBD)-phallacidin. It is postulated that the bundles of actin and the spires of microvilli are stiff and provide reinforcement to the egg surface. Such postulated properties would provide physical protection for these large eggs which, unlike the eggs of most invertebrates, appear to lack all extracellular investing coats.     

Purified trout egg vitelline envelope proteins VEbeta and VEgamma polymerize into homomeric fibrils from dimers in vitro

The rainbow trout egg vitelline envelope (VE) is composed of three proteins, called VEalpha ( approximately 58-60kDa Mr), VEbeta ( approximately 52kDa Mr), and VEgamma ( approximately 47kDa Mr). Each of these proteins is related to mouse egg zona pellucida (ZP) glycoproteins, called ZP1, ZP2, and ZP3, and possesses a ZP domain that has been implicated in the polymerization of the proteins into long, interconnected fibrils or filaments. Here, trout egg VEbeta and VEgamma were purified to homogeneity and analyzed under various experimental conditions (SDS-PAGE, Blue Native-(BN-)PAGE, size-exclusion chromatography, and transmission electron microscopy) to determine whether individual VE proteins would polymerize into fibrils in vitro. Such analyses revealed that in the presence of 6M urea each VE protein is present primarily as monomers and as small oligomers (dimers, tetramers, etc.). However, either a reduction in urea concentration or a complete removal of urea results in the polymerization of VEbeta and VEgamma dimers into very large oligomers. Mixtures of VEbeta and VEgamma also give rise to large oligomers. Under these conditions, VE proteins are visualized by transmission electron microscopy as aggregates of long fibrils, with each fibril composed of contiguous beads located periodically along the fibril. The relationship between the behavior of fish egg VE proteins and mouse ZP glycoproteins, as well as other ZP domain-containing proteins, is discussed.     

The primate median eminence II

Summary The ultrastructure of the normal median eminence of the male rhesus monkey (Macaca mulatta) is described using high-voltage electron microscopy. Surface specializations of ependymal cells lining the infundibular recess included cilia, apical extrusions, and microvilli. Supraependymal cells were predominantly macrophage-like, but examples of lymphocytic types were also seen. Tanycytes had long, branching, basal processes filled with numerous microtubules, some lipid droplets, and granules. The zona interna was composed of large unmyelinated neurosecretory fibers. A few myelinated fibers were also seen, but their character as neurosecretory fibers could not be established. The zona externa was composed of densely-packed profiles of neurosecretory fibers of small diameter, was well-vascularized and contained the terminations of tanycytes. Perivascular glial cells, vesiculated elements, pituicytes, and cellular elements common to connective tissue were observed. The intricate relationships between both the cellular and fibrous elements of the median eminence can be appreciated with the capability of high-voltage electron microscopy to discern ultrastructure in sections 10 times thicker than those used for low-voltage electron microscopy. The median eminence of this primate species has an ultrastructural organization similar to that described for most other species.Supported by USPHS Program Project Grant NS-11642 and USPHS HD-08867. The authors appreciate the excellent technical advice and assistance of Mr. George Wray in operation of the HVEMCareer Development Awardee K04-GM-70001     

Ultrastructure of the Anterior End in Three Ontogenetic Stages of Nectonema munidae (Nematomorpha)

Abstract The structure of the anterior end of three ontogenetically successive stages of Nectonema munidae (Nematomorpha) is investigated by light and transmission electron microscopy. During development, an adult cuticle is formed under a larval cuticle, which is subsequently moulted. Only one moult can be documented for Nectonema. The brain has a main subpharyngeal portion and a weak suprapharyngeal commissure. It is circumpharyngeal only in early developmental stages, the dorsal commissure is reduced in the adult stage. Four giant cells and a cerebral cavity are adult features. Although the morphology of the giant cells is elucidated, their function remains unclear, but a sensory function is probable. A septum marks the posterior border of the anterior end and divides a cerebral cavity from a body cavity. A precursor of the septum is present in the first stage observed, but it lies next to the epidermis and does not separate a cerebral cavity. Cuticular structures in the pre-pharyngeal region of the early stages are interpreted as remnants of the larval boring organ. They are moulted together with the larval cuticle. The morphology of the pharynx and the anterior part of the intestine is shown.     

Etude ultrastructurale de l'évolution du tégument de la Sangsue Hirudo medicinalis L. (Annélide,Hirudinée) au cours d'un cycle de mue / Developmental changes in the integument of the leech Hirudo medicinalis L. (Annelida,Hirudinea) during a molting cycle. An ultrastructural study

Summary

The integument of the leech Hirudo medicinalis is mainly composed of a single layer of cuticle-secreting epidermal cells. The cuticle is made up of collagen fibers which support a layer of membrane-bound epicuticular projections.

Shedding of the old cuticle is preceded by the formation of a new cuticle. The epicuticular projections are the first to develop: they originate from the tips of numerous microvilli of the epidermal cells. As soon as it appears, the newly-formed collagen layer is firmly attached to the epidermal cells by numerous hemidesmo-somes, whereas the old cuticle is no longer connected with the epidermal surface. The epidermal cells exhibit marked characteristics of secretory activity during the laying down of the new cuticle.

The observations are discussed in connexion with recent findings of high ecdysteroid levels in leeches at the beginning of the molting cycle.     

Interactions between the egg and larval parasitoids of a gall-forming midge and their impact on the host

1. The gall‐forming midge Rhopalomyia californica was exposed experimentally to parasitism and predation during only the egg stage, during only the larval stage, during neither stage, or during both stages. 2. The combined action of natural enemies that attack during both the egg stage and the larval stage led to the lowest number of midges and total insects (midges + parasitoids) in the next generation, and the highest percentage parasitism. 3. The larval parasitoid killed a large fraction of hosts without producing new parasitoid offspring, while there is some indication that the egg parasitoid on its own tended to produce the most parasitoid offspring. The contrasting implications of host mortality versus parasitoid production for biological control are discussed. 4. Exposure to larval parasitoids resulted in a reduction in the number of egg parasitoid offspring produced, but exposure to the egg parasitoid did not affect the number of larval parasitoid offspring produced significantly.     

Ultrastructure of the tube-foot of an ophiuroid echinoderm, Hemipholis elongata

The morphology of the tube-foot of the brittlestar, Hemipholis elongata was examined by transmission and scanning electron microscopy. The entire surface of the tubefoot is covered by microvilli and a thick cuticle layer. At the tip of the tube-foot the cuticle layer is thinner and sensory-secretory complexes are seen. These are composed of two secretary cells and a central ciliated sensory cell. Scanning electron microscopy shows that the cilium which extends from the receptor cell ends flush with the surrounding microvilli. These studies suggest that the tube-feet in ophiuroids are important structures in sensory-reception, gasexchange as well as locomotion. The hemoglobin containing cells within the lumen of the tubefeet may serve in oxygen transport and storage.     

Segmental organisation of the tail region in the embryo of Drosophila melanogaster

Summary The segmental organisation of the tail region in the embryo of Drosophila melanogaster, which is defined here as the epidermal region posterior to the boundary between abdominal segments A7 and A8, has been investigated by means of ultraviolet (UV) laser fate-mapping and phenotypic analysis of embryonic mutants that alter the segmental pattern of the larval cuticle. Wild-type embryos were irradiated in the presumptive tail region with a UV- laser microbeam of 20 m diameter at the blastoderm stage. The ensuing defects were scored in the cuticle pattern of the tail region of the first-instar larva, which is described in detail in this paper. The spatial distribution of defect frequencies was used to construct a blastoderm fate-map of the cuticle structures of the larval tail region. The segmental origin of the larval tail structures was inferred from the phenotypic analysis of segmentation and homoeotic mutants, which revealed pattern repetition throughout the embryonic tail region corresponding to four segment anlagen, A8 to A11, and a non-segmental telson. These data enabled the transformation of the blastoderm fate-map of cuticle structures into a map of tail segment anlagen. The tail anlage occupies about 10% of the egg length (EL), bounded by segment A7 anteriorly at 20% EL and by the proctodaeum posteriorly at 10% EL, as measured from the posterior pole. The anlagen of segments A8 and A9 appear to be narrow dorso-ventral strips of blastoderm cells similar to the anlagen of the trunk segments, whereas the anlagen of A10 and A11 are smaller and produce fewer pattern elements. The telson is represented in the cuticle by the tuft which derives from a very dorsal posterior position. The antero-posterior axis of the entire tail anlage appears curved upward posteriorly. Differences in the mode of development between tail and trunk segments are discussed, as are similarities of larval and imaginal tail development in Drosophila. Comparison with tail development in other insects suggests that, during evolution, the transition from semi-long-germ to long-germ development modified the organisation of the tail region without affecting its primary subdivision into metameric units.