Ovulation and egg segregation in the tunic of a colonial ascidian,Diplosoma listerianum (Tunicata,Ascidiacea)

Summary The process of egg segregation in the tunic of the ovoviviparous ascidian Diplosoma listerianum was studied by light and electron microscopy. One egg at a time was seen to mature in each zooid. The eggs had large yolk and grew on the ovary wall enveloped in four layers: (1) outer follicle cells (OFC), long and rich in RER (rough endoplasmic reticulum) and with dense granules in the Golgi region; (2) flat inner follicle cells (IFC); (3) a loosely fibrillar vitelline coat (VC); (4) test cells encased on the egg surface. The growing egg protrudes from the ovary wall and presses on the contiguous epidermis. Granulocytes enter the space between the epidermis and the egg and insinuate cytoplasmic protrusions, disrupting the continuity of the OFC layer. At ovulation, OFC and IFC are discharged and form a post-ovulatory follicle (corpus luteum). The epidermis shrinks and closes, possibly by activation of microfilaments, causing the egg to be completely surrounded by the tunic. In the zooid, the wound caused by the passage of the egg is repaired both by contraction of the epidermis and by phagocytic activity. Altered spermatozoans are found in phagocytosing cells in the lumen of the ovary. These are presumably remnants of those which entered to fertilize the egg before segregation.     

Studies on the formation of germ cells in a compound ascidian Botryllus,primigenus oka

Gametogenesis of a compound ascidian Botryllus primigenus was studied histologically. On either side of the zooid (stage 9), in the gonadal space between the epidermis and the atrial epithelium, either a single testis or a complex of an egg follicle and a testis can be formed. The egg follicle consists of a single ovum (occasionally two ova) and its accessory cells and is connected with the atrial epithelium by the follicle stalk. The egg follicle is always accompanied by the brood pouch, a diverticulum of the atrial cavity. The testis is equipped with a vestigial spermiduct and is attached to the atrial epithelium. Buds of stage 8 comprise, besides the developing testes and, egg follicles, loose aggregations of hemoblasts and oocytes of early developmental stages, which are already accompanied by primary follicular cells. Both the oocytes and the primary follicular cells seem to arise from the hemoblasts. The young oocytes are isolated in the gonadal space of the buds nnd are transferred to buds of the succeeding generations until they finally mature. In the bud of stage 3, a compact mass of cells appears, attaching to tbe inner vesicle on either side of the body. It is derived from the hemoblasts lodged there in the preceding generation and presumably also from the circulating hemoblasts. When the cell mass receives a large oocyte derived from the preceding generation, part of the cell mass differentiates into egg envelopes, forming an egg follicle, and a follicle stalk and the remainder into a testis. When the cell mass receives no oocyte, it differentiate as a whole into a testis. In the egg follicle thus formed the outer and inner follicular cells increase in number by mitotic division. Subsequently, initial test cells are derived from the inner follicle by migration across the developing chorion; then they increas2 in number by mitosis. In the testis, meiosis and spermiogenesis take place.     

Development of the calyx and lateral oviduct during oogenesis in Aedes aegypti

Summary The lateral oviduct and calyx of nulliparous Aedes aegypti on a sucrose diet are both flattened sacs, lacking a well defined lumen. Both are formed of an inner epithelial and an outer muscular layer, each one cell thick. The lateral oviduct is surrounded by a circular muscle sheath which is continuous with the ovarian sheath. Each ovariolar sheath is continuous with the outer layer of the calyx. The structure of both the lateral oviduct and the calyx is greatly modified after the initial blood meal. A distinct lumen develops; there is an extensive development of the outer muscular layers, and the inner epithelial layers become invaginated forming deep crypts lined with extensive microvilli. The follicular stem, which joins the primary follicle to the calyx in each ovariole, is not hollow and does not mark the opening into the calyx through which the mature egg can pass. The eggs gain access to the oviductal system after the calyx extends around the follicular epithelium of the primary follicle, when breaks appear in the calyx wall opposed to the follicular epithelium, until the mature eggs, eventually lie in a highly distended thin-walled sac of calyx from which they have direct and easy access to the lateral oviduct. After oviposition, this sac contracts to occupy once more a compact axial position in the ovary. Remnants of the follicular epithelium, containing many lysosomes are attached to the calyx at this time.     

Origin and Differentiation of the Inner Follicular Cells during Oogenesis in Molgula pacifica (Urochordata), an Ascidian without Test Cells

In Molgula pacifica small previtellogenic oocytes are found between cells of the ovarian epithelium. Each oocyte subsequently grows within a compartment of the epithelium known as a primary follicle. The wall of the primary follicle is composed of outer follicular epithelial cells. While growing from about 15–70 μm in diameter, each oocyte gradually recruits a set of about 950 non-epithelial inner follicular cells. These cells co-differentiate in sets with each oocyte, but test cells never appear. The first filamentous components of the vitelline coat appear on the surface of an oocyte in places where it is in contact with undifferentiated (stage 2) inner follicular cells. Each fully differentiated inner follicular cell stores adhesive precursors in a large compartment of the endoplasmic reticulum and probably secretes components of the vitelline coat. There is no evidence that the outer follicular epithelial cells transform into inner follicular cells by dedifferentiation as has often been assumed. Inner follicular cells, in stage 1, are nearly identical to hemoblasts. Hemoblasts may form the inner follicular cells, but to do this they would have to cross the outer follicular epithelium and this phenomenon has not yet been seen.     

Egg release and germling development in Sargassum horneri (Fucales, Phaeophyceae)

The mature female conceptacle of Sargassum horneri (Turner) C. Agardh has an ostiole filled with a gelatinous plug. The oogonium in the conceptacle has cell walls that can be differentiated into a dense outer and a less dense inner microfibrillar layer. Just prior to egg release, stalk material is produced inside the outer layer and the inner layer disappears. At this stage the gelatinous plug is extruded and mucilage is released through the ostiole. The released eggs are retained on the receptacle by the stalk and are surrounded by a large amount of the mucilage. Three-celled germlings form a primary wall with a polylamellated structure of microfibril layers. In multicellular germlings that have differentiated into thallus and rhizoids, the peripheral thallus cells have an outer cell wall consisting of a microfibril layer under the primary wall, while the cell wall of the rhizoid tip has an amorphous structure. The germlings are released from the stalk and become attached to the substratum by an adhesive substance secreted from rhizoidal cells.     

Post-fertilization changes in the zona pellucida glycoproteins of rat eggs

The zona pellucida (ZP) is the extracellular coat surrounding the mammalian egg. Numerious evidence supports the role of ZP carbohydrate residues as the specific sperm receptors. In this study we used lectins to study different distribution patterns of carbohydrate residues in the rat ZP, and to follow changes at fertilization. ZP were collected from follicular, ovulated, and fertilized eggs, incubated with one of 11 different biotin-labeled lectins, followed by avidin-fluorescein isothiocyanate (FITC) complex, and visualized by epifluorescent microscopy. For electron microscope (EM) histochemistry, eggs were embedded in LR white and ultrathin sections were stained with the complexRicinus communis lectin (RCA-1)-colloidal gold. Some lectins (RCA-I,Glycine max) bound to the entire ZP while others were restricted to the inner or outer zones [Griffonia simplicifolia, Concanovalia ensiformis, Triticum vulgaris (WGA), succinyl-WGA]. Other lectins (Lens culinaris, Ulex europhaeus) were totally excluded. The RCA-1 binding pattern changed following sperm penetration, from homogeneous in ZP of ovulated eggs (57%) to uneven in ZP of fertilized (71%) or activated (68%) eggs. Our results demonstrate an uneven distribution of different sugar residues in the rat ZP, and a post-fertilization change in the distribution of β-galactose, which is specifically recognized by RCA-I, presumably correlated with other changes in the ZP that lead to the block to polyspermy. This work is in partial fulfillment of the requirements for the PhD degree of Tamar Raz at the Sackler School of Medicine, Tel Aviv University     

Comparative studies on the structure of reproductive organs of four botryllid ascidians

Reproductive organs of four botryllid ascidians, Botryllus primigenus, Botryllus schlosseri, Botrylloides violaceus and Botrylloides leachi, were studied histologically. In every species, the egg follicle consisting of an egg and its inner and outer follicles, is attached to the follicle stalk, the vesicle being composed of a flat epithelium, which in its turn is connected to the atrial epithelium or to the brood pouch specialized from it. In B. schlosseri, the egg is ovulated into the atrial cavity and remains there held by the brood cup, of which the inner epithelium is derived from the follicle stalk and the outer one from the atrial epithelium. In B. primigenus, the brood pouch develops as a diverticulum of the atrial cavity, around the entrance of which a fold differentiates from the atrial epithelium and closes the pouch during embryogenesis. In both species of Botrylloides, the brood pouch is formed by the outgrowth of the thickened atrial epithelium into the blood space, the entrance of which is closed during embryogenesis. The discarded outer follicle completely disintegrates soon after ovulation in B. schlosseri, but part of it remains throughout embryogenesis in the blood space in B. primigenus or projecting into the interior of the brood pouch in Botrylloides. In primigenus, the testis, when it accompanies the egg follicle, is placed at the bottom of the brood pouch and the sperm is shed through the pouch prior to ovulation. In B. schlosseri and the Botrylloides species, the testis is located independently from the egg follicle and the sperm matures after ovulation.     

Structure and formation of the chorion in the butterfly, Calpodes

The hemispherical eggshell of Calpodes consists of a domed dorsal surface of thin inner endochorion and a thick, lamellate, outer exochorion, and a flat bottom of predominantly unlamellated endochorion. The endochorion is traversed by small pores and the exochorion by larger ones, which are formed by the withdrawal of processes from the follicular cells. The presence of a phenoloxidase in the ovariolar ducts, lateral and common oviducts and in ovulated eggs, and the stability of ovulated eggs suggest that stabilization of the eggshell is accomplished through quinone tanning. However, the endochorion, which is soluble in sodium dodecyl sulphate (SDS), is more likely cross-linked by di- and tri-tyrosyls through the action of a peroxidase, present in the cells of the ovariolar duct and in the endochorion.     

Modulation of spermatozoa and zona pellucida properties by the soluble acrosome reaction-inducing factor of the ovulated egg-cumulus complex

Three sources of hamster periovulatory fluids (± heat inactivation at 56°C), with bovine serum albumin (BSA) as control, were tested for effects on penetration of three classes of eggs by hamster sperm precapacitated in BSA. These fluids were a soluble extract of cumulus oophorus fluid (COF) from the ovulated hamster egg-cumulus complex, serum, and follicular fluid. Egg types were ovulated, salt-stored (ovulated), and follicular. In both COF and serum, there were significant differences among egg types in mean penetration, and significant effects of fluid addition. In contrast, there was no effect of follicular fluid and no differences between follicular and stored eggs. For the follicular eggs (combined data, normalized, ranked), patterns of response to the three factors (± heating) were different: only unheated COF and heated serum increased penetration significantly above BSA control levels (average rank 20.2, 41.4, 38, for BSA, COF (unheated), serum (heated), respectively). This indicated that the active component in COF was heat labile, not present in either serum or follicular fluid, and, therefore, of oviductal origin. Oviduct and/or COF exposure of eggs and sperm was tested for effects as an acrosome reaction inducing factor (ARIF) for acrosome reactions (AR; zonabound and free-swimming sperm) and on sperm:zona binding and penetration. The COF ARIF for free-swimming sperm AR was heat stable. Penetration of follicular eggs increased after incubation in COF prior to sperm addition, but a greater response occurred when COF was added to eggs with sperm. In kinetic experiments, 25 min following sperm attachment, follicular eggs had lost 41% of initially bound sperm, vs. 23% for ovulated eggs, and had only 16 AR sperm/egg, vs. 26 for ovulated. Follicular eggs incubated in COF (then washed three times) had the same number of bound AR sperm as ovulated eggs. Acid solubilized zona pellucida (ASZP) from ovulated eggs was more effective as an ARIF per zona than ASZP from follicular eggs. Zonae of follicular eggs, as eivdenced by dissolution times in β-mercaptoethanol (β-MEOH), were not “harder” than those of ovulated eggs. There were differences in lectin binding antigens on zonae of both fresh and stored, follicular and ovulated, eggs. We conclude that multiple biological factors orchestrate sperm:egg interactions in the ampulla. Our data are consistent with the presence of at least three effective components: (1) the oviductal lectin-binding antigen (ZPO or oviductin), (2) an additional heat-labile component, and (3) the heat-stable ARIF for free-swimming sperm. © 1994 Wiley-Liss, Inc.     

Ovary of the seahorse,Hippocampus erectus

The ovary of the seahorse, Hippocampus erectus, is a cylindrical tube bounded by an outer layer consisting of a mesothelium and muscular wall and by an inner luminal epithelium, with a single row of developing follicles sandwiched between the two layers. Follicles are produced by a germinal ridge, which contains oogonia, early oocytes, and prefollicle cells, and which runs along the length of the ovary. The germinal ridge is an outpocketing of the luminal epithelium, as indicated by a continuous underlying basal lamina. Prefollicle cells invest diplotene oocytes and the complex eventually pinches off the germinal ridge as a primordial follicle surrounded by a basal lamina derived from the germinal ridge. Subsequent investment of the primordial follicle by elements of the theca complete the process of folliculogenesis. H. erectus has two ovaries and each ovary has two dorsally located germinal ridges. Thus, in each ovary the derived follicular lamina is bilaterally symmetrical: two temporally and spatially arranged sequences of developing follicles are produced, with the largest follicles found along the ventral midline of the ovary. The advantages of developmental, kinetic, and systemic analyses of these unusual ovaries are indicated.     

Role of epidermal cilia in development of the Australian lungfish,Neoceratodus forsteri (Osteichthyes: Dipnoi)

In common with the embryos of other anamniotes, young of the Australian lungfish, Neoceratodus forsteri, have ciliated cells in the epidermis. These first appear at stage 28, ˜ 10 days before hatching, and develop progressively to a peak in numbers and in activity at stage 44, just after hatching. After this point, ciliary action in the epidermal cells slowly declines, and cilia disappear completely from the outer surface of the hatchling by stage 52. Cilia are lost earlier from the oral epithelium, between stages 45 and 46, and from the epithelium covering the gills and lining the operculum at stage 51, although they are retained in the nares and in the cavity of the olfactory organ. To assess possible functions for the ciliated epidermis in lungfish hatchlings, the presence of cilia in the epidermis of young N. forsteri is compared with landmarks of development. The ciliated epidermal cells are not associated with movements of the embryo within the egg capsule, nor are they a part of a feeding mechanism. They are not related to oxygen uptake. The ciliated epidermis appears to function as a mechanism for clearing the animal of particles and settling organisms before hatching, when the egg membranes have developed holes, and after hatching, when the young fish is living among the submerged rootlets of trees growing on the river bank or in dense stands of aquatic plants. The function of a ciliated epidermis in N. forsteri hatchlings in relation to microhabitat is discussed. © 1996 Wiley-Liss, Inc.     

Oogenesis in a paedogenetic dipteran insect under normal conditions and after experimental elimination of the follicular epithelium

Summary Paedogenetically developing eggs of the gall midgeHeteropeza pygmaea are not deposited, but develop in the hemocoel of the mother larva. The nurse chamber remains present in the cleaving egg, and the follicular epithelium does not form a chorion but envelops the growing egg during embryonic development. It is possible to obtain naked eggs, i.e. eggs lacking the follicular epithelium, which are able to develop up to the blastoderm stage but remain spherical instead of assuming an elongated shape. Oogenesis of normal and naked eggs has been studied at the ultrastructural level with special reference to the nurse chamber. It is shown that the nurse chamber nuclei develop large nucleoli during oogenesis, indicating that the nurse chamber supplies the oocyte with ribosomal RNA (rRNA). The dense bodies in the nurse chamber may represent an intermediate stage in the transport of the rRNA from the nurse chamber to the oocyte; they are probably not related to the polar granules in the oocyte. It is also shown that the intercellular bridge joining the nurse chamber to the oocyte disappears shortly before cleavage initiation. During egg cleavage the follicular epithelium surrounds the nurse chamber, which degenerates and is gradually absorbed by the growing egg plasmodium. Naked cleaving eggs are never attached to a nurse chamber or to relics of it. Naked oocytenurse chamber complexes frequently aggregate, which may indicate a role of the follicular epithelium in follicle separation during normal development.     

Regeneration of sialic acid on the surface of Chinese hamster cells in culture. I. General characteristics of the replacement process

Electropotential differences between the cell interior and the external medium have been studied with intracellular microelectrodes in ovarian oocytes, ovulated unfertilized eggs and fertilized eggs of R. pipiens. In ovarian oocytes the cytoplasm was 50 to 80 mV negative, relative to isotonic Ringer's solution. In contrast, electrode penetration of the oocyte nucleus in situ indicated that the nucleoplasm was about 25 mV positive, relative to the cytoplasm. After ovulation, the cortical cytoplasm became 20 to 50 mV positive with regard to an external solution of 0.1 strength Ringer's solution (ca. pond water). Penetration of the cytoplasm at levels from 0.3 to 0.6 mm below the egg surface revealed an inner zone with a potential which was about 15 mV negative, relative to the cortical cytoplasm. A slow hyperpolarization of the cortical membrane occurred at activation, with the potential returning to that of the ovulated unfertilized egg within ten minutes. After fertilization, the egg cytoplasm remained positive until the first cleavage. As division proceeded, the cytoplasm slowly depolarized and became 50 to 60 mV negative, relative to 0.1 strength Ringer's solution.     

Topological Control of Life and Death in Non-Proliferative Epithelia

Programmed cell death is one of the most fascinating demonstrations of the plasticity of biological systems. It is classically described to act upstream of and govern major developmental patterning processes (e.g. inter-digitations in vertebrates, ommatidia in Drosophila). We show here the first evidence that massive apoptosis can also be controlled and coordinated by a pre-established pattern of a specific ‘master cell’ population. This new concept is supported by the development and validation of an original model of cell patterning. Ciona intestinalis eggs are surrounded by a three-layered follicular organization composed of 60 elongated floating extensions made of as many outer and inner cells, and indirectly spread through an extracellular matrix over 1200 test cells. Experimental and selective ablation of outer and inner cells results in the abrogation of apoptosis in respective remaining neighbouring test cells. In addition incubation of outer/inner follicular cell-depleted eggs with a soluble extract of apoptotic outer/inner cells partially restores apoptosis to apoptotic-defective test cells. The 60 inner follicular cells were thus identified as ‘apoptotic master’ cells which collectively are induction sites for programmed cell death of the underlying test cells. The position of apoptotic master cells is controlled by topological constraints exhibiting a tetrahedral symmetry, and each cell spreads over and can control the destiny of 20 smaller test cells, which leads to optimized apoptosis signalling.     

Ultrastructure of the eggshell and its formation in Planipapillus mundus (Onychophora: Peripatopsidae)

Although the majority of onychophorans are viviparous or ovoviviparous, oviparity has been described in a number of species found exclusively in Australia and New Zealand. Light microscopy and scanning and transmission electron microscopy were used to examine developing eggs and the reproductive tract of the oviparous Planipapillus mundus. Deposited eggs and fully developed eggs dissected from the terminal end of the uteri have an outer thick, slightly opaque chorion, and an inner thin, transparent vitelline membrane. The chorion comprises an outermost extrachorion, sculptured with domes equally spaced over the surface; a middle exochorion, with pores occurring in a pattern of distribution equivalent to that of the domes of the extrachorion above; and an innermost, thick endochorion consisting of a spongelike reticulum of cavities comparable to the respiratory network found in insect eggs. The vitelline membrane lies beneath the chorion, from which it is separated by a fluid‐filled space. The vitelline membrane tightly invests the developing egg. Examination of oocytes in the ovary and developing eggs at various stages of passage through the uterus indicate that the majority of chorion deposition occurs in the midregion of the uterus, where vast networks of endoplasmic reticulum are present in the columnar epithelium. The vitelline membrane, however, is believed to begin its development as a primary egg membrane, surrounding the developing oocytes in the ovary. The vitelline membrane is transformed after fertilization, presumably by secretions from the anterior region of the uterus; hence, it should be more accurately referred to as a fertilization membrane. Aspects of the reproductive biology of P. mundus are also included. J. Morphol., 2008. © 2008 Wiley‐Liss, Inc.     

Seed coat development, anatomy and scanning electron microscopy of Harpagophytum procumbens (Devil's Claw), Pedaliaceae

Seed coat development of Harpagophytum procumbens (Devil's Claw) and the possible role of the mature seed coat in seed dormancy were studied by light microscopy (LM), transmission electron microscopy (TEM) and environmental scanning electron microscopy (ESEM). Very young ovules of H. procumbens have a single thick integument consisting of densely packed thin-walled parenchyma cells that are uniform in shape and size. During later developmental stages the parenchyma cells differentiate into 4 different zones. Zone 1 is the multi-layered inner epidermis of the single integument that eventually develops into a tough impenetrable covering that tightly encloses the embryo. The inner epidermis is delineated on the inside by a few layers of collapsed remnant endosperm cell wall layers and on the outside by remnant cell wall layers of zone 2, also called the middle layer. Together with the inner epidermis these remnant cell wall layers from collapsed cells may contribute towards seed coat impermeability. Zone 2 underneath the inner epidermis consists of large thin-walled parenchyma cells. Zone 3 is the sub-epidermal layers underneath the outer epidermis referred to as a hypodermis and zone 4 is the single outer seed coat epidermal layer. Both zones 3 and 4 develop unusual secondary wall thickenings. The primary cell walls of the outer epidermis and hypodermis disintegrated during the final stages of seed maturation, leaving only a scaffold of these secondary cell wall thickenings. In the mature seed coat the outer fibrillar seed coat consists of the outer epidermis and hypodermis and separates easily to reveal the dense, smooth inner epidermis of the seed coat. Outer epidermal and hypodermal wall thickenings develop over primary pit fields and arise from the deposition of secondary cell wall material in the form of alternative electron dense and electron lucent layers. ESEM studies showed that the outer epidermal and hypodermal seed coat layers are exceptionally hygroscopic. At 100% relative humidity within the ESEM chamber, drops of water readily condense on the seed surface and react in various ways with the seed coat components, resulting in the swelling and expansion of the wall thickenings. The flexible fibrous outer seed coat epidermis and hypodermis may enhance soil seed contact and retention of water, while the inner seed coat epidermis maintains structural and perhaps chemical seed dormancy due to the possible presence of inhibitors.     

Cleavage and blastoderm formation in normal and experimentally deformed naked eggs of a dipteran insect

Summary The eggs of the gall midgeHeteropeza pygmaea develop parthenogenetically inside of the mother larva. They lack a chorion and remain enveloped by the follicular epithelium. After experimental elimination of the follicular epithelium naked eggs are formed, which reach the blastoderm stage but remain spherical instead of assuming an elongated shape. To analyze this peculiar egg development and the roles of egg shape and envelope during development, the ultrastructure of cleaving normal and naked eggs was investigated. It was shown that the number of elements of Golgi apparatus and endoplasmic reticulum strongly increases during early cleavage. Their association with cleavage furrows and nuclei suggests that these organelles play a dominant role in membrane production. Egg yolk consists of lipids and glycogen, wheareas no proteins are found. Cleaving eggs contain numerous vesicles with lysosomal characteristics, indicating intense autophagic processes. Cleavage furrow formation occurs independently from the positioning of cleavage nuclei. The numerous microtubules, which are associated with cleavage furrows and nuclei and located in the egg periphery, the intercellular bridges, and in the central part of the egg, suggest that the cytoskeleton has an important role in cleavage furrow formation, blastoderm layer establishment, and yolk localization. Since these processes are accurately accomplished in naked spherical eggs, they can be considered as independent of normal egg shape and the follicular epithelium.     

Development of the Embryonic Envelopes of Mesocestoides lineatus (Cestoda: Cyclophyllidea)

Summary

Transmission electron microscopy revealed that the eggs of Mesocestoides lineatus consisted of an oncosphere larva surrounded by various coverings. The outermost of these was the embryonic capsule, which appeared as a thin electron-dense membranous sac. The capsule enclosed inner and outer embryonic envelopes, each of which was syncytial and apparently formed from embryonic blastomeres. The envelopes became increasingly vesiculated during embryogenesis, and were attached to each other by desmosomes by the time the larva was fully formed. An electron-dense intracellular embryophore was produced by the inner envelope; it first appeared under the distal plasma membrane as a series of blocks, which grew and fused to form a thick unbroken layer. Early in development, the proximal plasma membrane of the inner envelope was connected to the larval epithelium by a multilaminate membrane complex that was ultrastructurally similar to a continuous junction. At the end of embryogenesis, this appeared to detach from its formative cells on both sides to form the distinctive oncospheral membrane. Several eggs were bound together in clusters by a cluster capsule that was ultrastructurally identical to the individual embryonic capsules. This type of egg packaging has not been described previously for any cestode. Both the cluster and individual capsules broke down by the end of embryogenesis.     

Nervous system of ascidian larvae: Caudal primary sensory neurons

Summary In larvae of Diplosoma macdonaldi one sensory nerve extends along the dorsal midline of the tail and another extends along the ventral midline. Each nerve is composed of 50–70 naked axons lying in a groove in the base of the epidermis, and each projects to the visceral ganglion. The cell bodies of the caudal sensory neurons occur in pairs within the epidermis, and are situated along the courses of the nerves. A single cilium arises from an invagination in the soma of each neuron, passes through the inner cuticular layer of the tunic and enters a tail fin formed by the outer cuticular layer. We propose that these cells are mechanoreceptors. The caudal sensory system is similar in representative species of ten families of ascidians.Abbreviations a axial complex of the tail - ac accessory centriole - ax axon - bb basal body - bl basal lamina - c cilium - cep common epidermal cells - cs ciliary sheath - dcv dense-cored vesicles - dsn dorsal sensory nerve - ec ependymal cells - ep epidermis - gj gap junction - h hemocoel - hc hemocoelic chamber - icl inner cuticular layer of the tunic - m caudal muscle - nc dorsal nerve cord - ncl neurocoel - no notochord - ocl outer cuticular layer of the tunic - sc sensory cell - sn sensory nerve - sv sensory vesicle - vg visceral ganglion - vsn ventral sensory nerve     

The embryonic development of the rhabdocoel flatworm Mesostoma lingua (Abildgaard, 1789)

The embryonic development of the flatworm Mesostoma lingua was studied using a combination of life observation and histological analysis of wholemount preparations and sections (viewed by both light and electron microscopy.) We introduce a series of stages defined by easily recognizable morphological criteria. These stages are also applicable to other platyhelminth taxa that are currently under investigation in our laboratory. During cleavage (stages 1 and 2), the embryo is located in the center of the egg, surrounded by a layer of yolk cells. After cleavage, the embryo forms a solid, disc-shaped cell cluster. During stage 3, the embryo migrates to the periphery of the egg and acquires bilateral symmetry. The side where it contacts the egg surface corresponds to the future ventral surface of the embryo. Stage 4 is the emergence of the first organ primordia, the brain and pharynx. Gastrulation, as usually defined by the appearance of germ layers, does not exist in Mesos-toma; instead, organ primordia emerge ”in situ” from a mesenchymal mass of cells. Organogenesis takes place during stages 5 and 6. Cells at the ventral surface form the epidermal epithelium; inner cells differentiate into neurons, somatic and pharyngeal muscle cells, as well as the pharyngeal and protonephridial (excretory) epithelium. A junctional complex, consisting initially of small septate junctions, followed later by a more apically located zonula adherens, is formed in all epithelial tissues at stage 6. Beginning towards the end of stage 6 and continuing throughout stages 7 and 8, cytodifferentiation of the different organ systems takes place. Stage 7 is characterized by the appearance of eye pigmentation, brain condensation and spindle-shaped myocytes. Stage 8 describes the fully dorsally closed and differentiated embryo. Muscular contraction moves the body in the egg shell. We discuss Mesostoma embryogenesis in comparison to other animal phyla. Particular attention is given to the apparent absence of gastrulation and the formation of the epithelial junctional complex. Received: 10 February 2000 / Accepted: 10 April 2000