Ariadne’s Thread in the Developing Cerebral Cortex: Mechanisms Enabling the Guiding Role of the Radial Glia Basal Process during Neuron Migration

Radial neuron migration in the developing cerebral cortex is a complex journey, starting in the germinal zones and ending in the cortical plate. In mice, migratory distances can reach several hundreds of microns, or millimeters in humans. Along the migratory path, radially migrating neurons slither through cellularly dense and complex territories before they reach their final destination in the cortical plate. This task is facilitated by radial glia, the neural stem cells of the developing cortex. Indeed, radial glia have a unique bipolar morphology, enabling them to serve as guides for neuronal migration. The key guiding structure of radial glia is the basal process, which traverses the entire thickness of the developing cortex. Neurons recognize the basal process as their guide and maintain physical interactions with this structure until the end of migration. Thus, the radial glia basal process plays a key role during radial migration. In this review, we highlight the pathways enabling neuron-basal process interactions during migration, as well as the known mechanisms regulating the morphology of the radial glia basal process. Throughout, we describe how dysregulation of these interactions and of basal process morphology can have profound effects on cortical development, and therefore lead to neurodevelopmental diseases.

Two new species of Cerapanorpa (Mecoptera, Panorpidae) from the Qinling and Minshan mountains

Two new species of Cerapanorpa Gao, Ma & Hua, 2016 are described from the Qinling and Minshan mountains, respectively. Cerapanorpa qinlingensissp. nov. can be readily distinguished from its congeners by the elongate hypovalves and the extremely developed basal process of gonostylus in male genitalia. Cerapanorpa minshanasp. nov. is characterized by its bifurcated parameres and a cluster of long black bristles on the inner apex of the gonocoxite. The number of species of Cerapanorpa is raised to 21. An updated key to species of Cerapanorpa is presented.

Photoreceptor progenitor dynamics in the zebrafish embryo retina and its modulation by primary cilia and N-cadherin

BackgroundPhotoreceptors of the vertebrate neural retina are originated from the neuroepithelium, and like other neurons, must undergo cell body translocation and polarity transitions to acquire their final functional morphology, which includes features of neuronal and epithelial cells.MethodsWe analyzed this process in detail on zebrafish embryos using in vivo confocal microscopy and electron microscopy. Photoreceptor progenitors were labeled by the transgenic expression of EGFP under the regulation of the photoreceptor-specific promoter crx, and genes of interest were knocked-down using morpholino oligomers.ResultsPhotoreceptor progenitors detached from the basal retina at pre-mitotic stages, rapidly retracting a short basal process as the cell body translocated apically. They remained at an apical position indefinitely to form the outer nuclear layer (ONL), initially extending and retracting highly dynamic neurite-like processes, tangential to the apical surface. Many photoreceptor progenitors presented a short apical primary cilium. The number and length of these cilia was gradually reduced until nearly disappearing around 60 hpf. Their disruption by knocking-down IFT88 and Elipsa caused a notorious defect on basal process retraction. Time-lapse analysis of N-cadherin knock-down, a treatment known to cause a severe disruption of the ONL, showed that the ectopic photoreceptor progenitors initially migrated in an apparent random manner, profusely extending cell processes, until they encountered other cells to establish cell rosettes in which they stayed acquiring the photoreceptor-like polarity.ConclusionAltogether, our observations indicate a complex regulation of photoreceptor progenitor dynamics to form the retinal ONL, previous to the post-mitotic maturation stages.

Photoreceptor progenitor dynamics in the zebrafish embryo retina and its modulation by primary cilia and N-cadherin

Photoreceptors of the vertebrate neural retina are originated from the neuroepithelium, and like other neurons, must undergo cell body translocation and polarity transitions to acquire their final functional morphology, which includes features of neuronal and epithelial cells. We analyzed this process in detail on zebrafish embryos using in vivo confocal microscopy and electron microscopy. Photoreceptor progenitors were labeled by the transgenic expression of EGFP under the regulation of the photoreceptor-specific promoter crx, and structures of interest were disrupted using morpholino oligomers to knock-down specific genes. Photoreceptor progenitors detached from the basal retina at pre-mitotic stages, rapidly retracting a short basal process as the cell body translocated apically. They remained at an apical position indefinitely to form the outer nuclear layer (ONL), initially extending and retracting highly dynamic neurite-like processes, tangential to the apical surface. Many photoreceptor progenitors presented a short apical primary cilium. The number and length of these cilia was gradually reduced until nearly disappearing around 60 hpf. Their disruption by knocking-down ift88 and elipsa caused a notorious defect on basal process retraction. To assess the role of cell adhesion in the organization of photoreceptor progenitors, we knocked-down cdh2/N-cadherin and observed the cell behavior by time-lapse microscopy. The ectopic photoreceptor progenitors initially migrated in an apparent random manner, profusely extending cell processes, until they encountered other cells to establish cell rosettes in which they stayed acquiring the photoreceptor-like polarity. Altogether, our observations indicate a complex regulation of photoreceptor progenitor dynamics to form the retinal ONL, previous to the post-mitotic maturation stages.

A new species of Phonotimpus Gertsch & Davis, 1940 (Araneae: Phrurolithidae) from Mexico

The genus Phonotimpus Gertsch & Davis, 1940 includes four species all known only from Mexico (World Spider Catalog 2018). P. separatus and P. eutypus were described by Gertsch & Davis (1940) from San Luis Potosí and from Nuevo León, respectively. Phonotimpus pennimani and P. talquian, described by Chamé-Vázquez, Ibarra-Núñez & Jiménez (2018), are the southernmost species in this genus (Chamé-Vázquez et al. 2018), and have restricted allopatric geographic distributions in Chiapas. This genus differs from other American phrurolithid genera by the following combination of characters: posterior eye row recurved, posterior median eyes closer to posterior lateral eyes than to each other, posterior median eyes smaller than posterior lateral eyes, females with dorsal scutum on the opisthosoma; palpi of males with retrolateral and dorsal tibial apophyses not joined at the base, and copulatory bulb with a conductor, embolus and embolar basal process (Gertsch & Davis 1940; Penniman 1985; Chamé-Vázquez et al. 2018). In this paper, a new species of Phonotimpus is described and illustrated, based on specimens collected in soil of a pine-oak forest in the state of Mexico.

Do the species with facies reclinobunoides make up a clade?—A new Lomanius (Opiliones, Podoctidae) from Vietnam and a discussion on its relationships

Lomanius annae sp. nov. is described from southern Vietnam. The species is characterized by the greatly developed dorso-basal process on cheliceral hand of males and by the partial effacement of all mesotergal grooves. The genus Lomanius contains four generic synonyms and currently comprises eight valid species distributed in China, Java, peninsular Malaysia, the Philippines, and Taiwan. The new species displays a general morphology similar to the former genus Paralomanius, with a combination of sexually dimorphic interocular mound (which is very large and strongly leaned back in males) and pedipalpus (which is extremely elongate in males). This morphological suite of features is herein called facies reclinobunoides. The replacement name Metibalonius triceratops nom. nov. is proposed for Trispinibunus abnormis Roewer, 1915, which is a junior secondary homonym of Ibalonius abnormis Strand, 1911. Finally, numerous morphological structures found in Podoctidae are recognized and named: (1) the cheliceral comb, present on cheliceral fingers, (2) the chained tubercular ridges, present on dorsal scutum and (3) several others related to the ocular region. The distribution of these two structures among podoctid species is not fully known, but both are absent in the former Ibaloniinae. We suggest that both structures may be useful to define supra-generic groups in the clade composed of the former Podoctinae and Erecananinae.

Comparative genitalic morphology in ten genera of thread-legged bugs of the tribe Metapterini, and its phylogenetic importance (Hemiptera: Heteroptera: Reduviidae)

The assassin bug tribe Metapterini belongs to the subfamily Emesinae (Hemiptera: Heteroptera: Reduviidae). Morphologically, it is characterized by the conspicuous basal process of the posteroventral series in the foreleg and the presence of wing polymorphism, with a high proportion of the genera with micropterous or apterous species. Here, the male and female ectodermal genitalic structures are documented for ten genera and twenty-three species of Metapterini, including eight species of the speciose genus Ghilianella Spinola, 1850. Descriptions and digital macrophotographs are provided for abdominal segment 8, pygophore, parameres, and phallus of the male, and for tergite 8, tergite 9, gonocoxae, gonapophyses, gonoplac, and bursa copulatrix of the female. The asymmetric male genitalia within Emesinae are discussed. From this morphological documentation sixty six phylogenetic characters are coded, presented as a data matrix and analyzed cladistically, and their potential usefulness for resolving relationships among Metapterini is discussed.

Diodonopsis ramiromedinae (Orchidaceae: Pleurothallidinae), a new species from Colombia

A new species, Diodonopsis ramiromedinae, is described, illustrated, and compared with similar species. A brief history of Masdevallia sect. Pygmaeae and the genus Diodonopsis is provided. Diodonopsis ramiromedinae is most similar to D. anachaeta, but differs by sepals with apices acute or acuminate-triangular vs. narrowly caudate; longer petals with a descending, narrowly linear, rounded basal process vs. with an acute, retrorse basal process; and the larger lip, obovate with the apex very broadly rounded vs. oblong-subpandurate with the apex acute.

Rediscovery and redescription of Laelaps lignicola G. & R. Canestrini, a remarkable myrmecophilous mite of the genus Cosmolaelaps Berlese (Acari: Mesostigmata: Laelapidae) from Italy

Laelaps lignicola G. & R. Canestrini, 1882 is redescribed on the basis of numerous adults and deutonymphs, collected from Italy more than a century after its description. Despite its setiform setae without a discernible asymmetric swelling, this species fits the current concept of the genus Cosmolaelaps Berlese for all remaining diagnostic characters. Males of L. lignicola and Cosmolaelaps vacuus (Michael) share the presence of a basal process bearing seta av on telefemur II. This attribute is apparently not described in other laelapid mites. Also, males of L. lignicola are similar to those of some European species of Cosmolaelaps in having a distinct masticatory ridge on the fixed digit of chelicera. The ecological preference for decaying wood microhabitats, where L. lignicola is closely associated with Lasius emarginatus (Olivier) (Hymenoptera Formicidae), also suggests a close affinity with Cosmolaelaps species. Consequently, this species is provisionally placed in Cosmolaelaps and the new combination C. lignicolus (G. & R. Canestrini, 1882) comb. nov. is proposed.

Independent modes of ganglion cell translocation ensure correct lamination of the zebrafish retina

The arrangement of neurons into distinct layers is critical for neuronal connectivity and function of the nervous system. During development, most neurons move from their birthplace to the appropriate layer, where they polarize. However, kinetics and modes of many neuronal translocation events still await exploration. Here, we investigate ganglion cell (RGC) translocation across the embryonic zebrafish retina. After completing their translocation, RGCs establish the most basal retinal layer where they form the optic nerve. Using in toto light sheet microscopy, we show that somal translocation of RGCs is a fast and directed event. It depends on basal process attachment and stabilized microtubules. Interestingly, interference with somal translocation induces a switch to multipolar migration. This multipolar mode is less efficient but still leads to successful RGC layer formation. When both modes are inhibited, RGCs that fail to translocate induce lamination defects, indicating that correct RGC translocation is crucial for subsequent retinal lamination.