An ultrastructural investigation of the hypocerebral organ of the adult Euperipatoides kanangrensis (Onychophora, Peripatopsidae)

The hypocerebral organs of Euperipatoides kanangrensis are a pair of spherical vesicles located ventral to the cerebral ganglia. They develop in the embryo from the most anterior pair of ventral organs, in the antennal segment. The wall of each hypocerebral organ is a dense epithelium of elongate cells with peripheral nuclei. The cytoplasm of the cells includes numerous mitochondria, Golgi bodies and microtubules. The small lumen, located eccentrically within the organ, contains concentrically layered electron-dense material resembling cuticle.Each hypocerebral organ is enclosed by a layer of extracellular matrix continuous with that surrounding the adjacent cerebral ganglion. There are no nerve connections between ganglion and organ, but cellular connections traverse the intervening matrix and could serve as a communication pathway. The ultrastructure of the hypocerebral organs indicates that they are glands.     

The involvement of <Emphasis Type="Italic">engrailed</Emphasis> and <Emphasis Type="Italic">wingless</Emphasis> during segmentation in the onychophoran <Emphasis Type="Italic">Euperipatoides kanangrensis</Emphasis> (Peripatopsidae: Onychophora) (Reid 1996)

As the putative sister group to the arthropods, onychophorans can provide insight into ancestral developmental mechanisms in the panarthropod clade. Here, we examine the expression during segmentation of orthologues of wingless (Wnt1) and engrailed, two genes that play a key role in defining segment boundaries in Drosophila and that appear to play a role in segmentation in many other arthropods. Both are expressed in segmentally reiterated stripes in all forming segments except the first (brain) segment, which only shows an engrailed stripe. Engrailed is expressed before segments are morphologically visible and is expressed in both mesoderm and ectoderm. Segmental wingless expression is not detectable until after mesodermal somites are clearly distinct. Early engrailed expression lies in and extends to both sides of the furrow that first demarcates segments in the ectoderm, but is largely restricted to the posterior part of somites. Wingless expression lies immediately anterior to engrailed expression, as it does in many arthropods, but there is no precise cellular boundary between the two expression domains analogous to the overt parasegment boundary seen in Drosophila. Engrailed stripes extend along the posterior part of each limb bud, including the antenna, while wingless is restricted to the distal tip of the limbs and the neurectoderm basal to the limbs. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Fate and nature of the onychophoran mouth–anus furrow and its contribution to the blastopore

The ancestral states of bilaterian development, and which living groups have conserved them the most, has been a controversial topic in biology for well over a hundred years. In recent years, the idea that gastrulation primitively proceeded via the formation of a slit-like blastopore that then evolved into either protostomy or deuterostomy has gained renewed attention and some molecular developmental support. One of the key pieces of evidence for this ‘amphistomy’ theory comes from the onychophorans, which form a clear ventral groove during gastrulation. The interpretation of this structure has, however, proved problematic. Based on expression patterns of forkhead (fkh), caudal (cad), brachyury (bra) and wingless (wg/Wnt1), we show that this groove does not correspond to the blastopore, even though both the mouth and anus later develop from it. Rather, the posterior pit appears to be the blastopore; the posterior of the groove later fuses with it to form the definitive anus. Onychophoran development therefore represents a case of ‘concealed’ deuterostomy. The new data from the onychophorans thus remove one of the key pieces of evidence for the amphistomy theory. Rather, in line with other recent results, it suggests that ancestral bilaterian development was deuterostomic.     

Ultrastructure of mesoderm in embryos of Opisthopatus roseus (Onychophora, Peripatopsidae): revision of the "long germ band" hypothesis for Opisthopatus

In previous studies, an unusual pattern of development which resembles the "long germ band" development of some insects has been described in the onychophoran Opisthopatus cinctipes. This pattern has been proposed to be a characteristic of the genus Opisthopatus. To test this assumption, the ultrastructure of embryos of O. roseus, the sister species of O. cinctipes, was examined. Two kinds of paired, segmentally arranged coelomic cavities were found in the embryos studied: 1) dorsolateral coelomic cavities lined by extremely thin epithelia, and 2) ventral coelomic cavities situated within the anlagen of ventrolateral body appendages. Only the dorsolateral coelomic cavities can be considered "somites," since they occur earlier during embryogenesis. This is in contrast with the previous view that suggested a ventral position of "somites" in O. cinctipes. In addition, an anterior-to-posterior gradient occurs in the development of O. roseus. Based on our findings, we reevaluated the previous data on O. cinctipes. From this survey, no evidence in support of a "long germ band" hypothesis in Opisthopatus was found. Instead, the embryogenesis in representatives of Opisthopatus is more similar to that in other onychophorans than expected.     

Expression of the decapentaplegic ortholog in embryos of the onychophoran Euperipatoides rowelli

The gene decapentaplegic (dpp) and its homologs are essential for establishing the dorsoventral body axis in arthropods and vertebrates. However, the expression of dpp is not uniform among different arthropod groups. While this gene is expressed along the dorsal body region in insects, its expression occurs in a mesenchymal group of cells called cumulus in the early spider embryo. A cumulus-like structure has also been reported from centipedes, suggesting that it might be either an ancestral feature of arthropods or a derived feature (=synapomorphy) uniting the chelicerates and myriapods. To decide between these two alternatives, we analysed the expression patterns of a dpp ortholog in a representative of one of the closest arthropod relatives, the onychophoran Euperipatoides rowelli. Our data revealed unique expression patterns in the early mesoderm anlagen of the antennal segment and in the dorsal and ventral extra-embryonic tissue, suggesting a divergent role of dpp in these tissues in Onychophora. In contrast, the expression of dpp in the dorsal limb portions resembles that in arthropods, except that it occurs in the mesoderm rather than in the ectoderm of the onychophoran limbs. A careful inspection of embryos of E. rowelli revealed no cumulus-like accumulation of dpp expressing cells at any developmental stage, suggesting that this feature is either a derived feature of chelicerates or a synapomorphy uniting the chelicerates and myriapods.     

In Vivo Expression of EphrinA5-Fc in Mice Results in Cephalic Neural Crest Agenesis and Craniofacial Abnormalities

Eph receptors and their ligands ephrins have been implicated in guiding the directed migration of neural crest cells (NCCs). In this study, we found that Wnt1-Cre-mediated expression of ephrinA5-Fc along the dorsal midline of the dien- and mesencephalon resulted in severe craniofacial malformation of mouse embryo. Interestingly, expression of cephalic NCC markers decreased significantly in the frontonasal process and branchial arches 1 and 2, which are target areas for the migratory cephalic NCCs originating in the dien- and mesencephalon. In addition, these craniofacial tissues were much smaller in mutant embryos expressing ephrinA5-Fc. Importantly, EphA7-positive cephalic NCCs were absent along the dorsal dien- and mesencephalon of mutant embryos expressing ephrinA5-Fc, suggesting that the generation of cephalic NCCs is disrupted due to ephrinA5-Fc expression. NCC explant experiments suggested that ephrinA5-Fc perturbed survival of cephalic NCC precursors in the dorsal midline tissue rather than affecting their migratory capacity, which was consistent with our previous report that expression of ephrinA5-Fc in the dorsal midline is responsible for severe neuroepithelial cell apoptotic death. Taken together, our findings strongly suggest that expression of ephrinA5-Fc decreases a population of cephalic NCC precursors in the dorsal midline of the dien- and mesencephalon, thereby disrupting craniofacial development in the mouse embryos.     

When an epithelium ceases to exist – an ultrastructural study on the fate of the embryonic coelom in Epiperipatus biolleyi (Onychophora, Peripatidae)

It is an accepted fact that fusion between the coelomic cavities and the primary body cavity occurs during development in the Arthropoda. However, such a fusion is much disputed in the Onychophora. In order to clarify this subject, the fate of embryonic coelomic cavities has been studied in an onychophoran. Ultrastructural investigations in this paper provide evidence that embryonic coelomic cavities fuse with spaces of the primary body cavity in Epiperipatus biolleyi. During embryogenesis, the somatic and splanchnic portions of the mesoderm separate and the former coelomic linings are transformed into mesenchymatic tissue. The resulting body cavity therefore represents a mixture of primary and secondary (coelomic) body cavities, i.e. the ‘mixocoel’. The nephridial anlage is already present, when the ‘mixocoel’ is formed, although there is no trace of a sacculus yet. The lumen of the nephridial anlage, thus, communicates with the newly formed ‘mixocoel’. Accordingly, the lumen of the nephridial sacculus cannot be regarded as a kind of ‘persisting coelomic cavity’ in E. biolleyi. Our findings support the hypothesis that the ‘mixocoel’ was already present in the common stem species of the Onychophora and Euarthropoda.     

A note on the structure of synapses in the ventral nerve cord of the onychophoran peripatoides leuckarti

Summary The fine structure of synapses, their distribution and arrangement in the ventral nerve cord of Peripatoides leuckarti (phylum Onychophora) is described. The asymmetric synaptic junctions show a well developed synaptic cleft (300Å) and pre- and subsynaptic electron dense apposition. They frequently show an array of presynaptic projections and a subsynaptic cisterna of endoplasmic reticulum. The onychophoran synapses differ from those of annelids and insects.Supported by the Deutsche Forschungsgemeinschaft, grant Schu 374/1