Final Division Research Articles

Dynamic Filopodia Transmit Intermittent Delta-Notch Signaling to Drive Pattern Refinement during Lateral Inhibition

The organization of bristles on the Drosophila notum has long served as a popular model of robust tissue patterning. During this process, membrane-tethered Delta activates intracellular Notch signaling in neighboring epithelial cells, which inhibits Delta expression. This induces lateral inhibition, yielding a pattern in which each Delta-expressing mechanosensory organ precursor cell in the epithelium is surrounded on all sides by cells with active Notch signaling. Here, we show that conventional models of Delta-Notch signaling cannot account for bristle spacing or the gradual refinement of this pattern. Instead, the pattern refinement we observe using live imaging is dependent upon dynamic, basal actin-based filopodia and can be quantitatively reproduced by simulations of lateral inhibition incorporating Delta-Notch signaling by transient filopodial contacts between nonneighboring cells. Significantly, the intermittent signaling induced by these filopodial dynamics generates a type of structured noise that is uniquely suited to the generation of well-ordered, tissue-wide epithelial patterns.

EKLF Directly Activates the p21WAF1/CIP1 Gene by Proximal Promoter and Novel Intronic Regulatory Regions during Erythroid Differentiation

The switch from proliferation to differentiation during the terminal stages of erythropoiesis is a tightly controlled process that relies in part on transcription factor-mediated activation of cell cycle components. EKLF is a key transcription factor that is necessary for the initial establishment of the red cell phenotype. Here, we find that EKLF also plays a role during the subsequent differentiation process, as it induces p21(WAF1/CIP1) expression independent of p53 to regulate the changes in the cell cycle underlying erythroid maturation. EKLF activates p21 not only by directly binding to an EKLF site within a previously characterized GC-rich region in the p21 proximal promoter but also by occupancy at a novel, phylogenetically conserved region that contains consensus CACCC core motifs located downstream from the p21 TATA box. Our findings demonstrate that EKLF, likely in coordination with other transcription factors, directly contributes to the complex set of events that occur at the final erythroid cell divisions and accentuates terminal differentiation directly by activation of CDK inhibitors such as p21.

The zebrafish flotte lotte mutant reveals that the local retinal environment promotes the differentiation of proliferating precursors emerging from their stem cell niche

It is currently unclear how intrinsic and extrinsic mechanisms cooperate to control the progression from self-renewing to neurogenic divisions in retinal precursor cells. Here, we use the zebrafish flotte lotte (flo) mutant, which carries a mutation in the elys (ahctf1) gene, to study the relationship between cell cycle progression and neuronal differentiation by investigating how proliferating progenitor cells transition towards differentiation in a retinal stem cell niche termed the ciliary marginal zone (CMZ). In zebrafish embryos without Elys, CMZ cells retain the capacity to proliferate but lose the ability to enter their final neurogenic divisions to differentiate as neurons. However, mosaic retinae composed of wild-type and flo cells show that despite inherent cell cycle defects, flo mutant cells progress from proliferation to differentiation when in the vicinity of wild-type retinal neurons. We propose that the differentiated retinal environment limits the proliferation of precursors emerging from the CMZ in a manner that explains the spatial organisation of cells in the CMZ and ensures that proliferative retinal progenitors are driven towards differentiation.

PU.1 Directly Regulates cdk6 Gene Expression, Linking the Cell Proliferation and Differentiation Programs in Erythroid Cells

Cell proliferation and differentiation are highly coordinated processes during normal development. Most leukemia cells are blocked from undergoing terminal differentiation and also exhibit uncontrolled proliferation. Dysregulated expression of transcription factor PU.1 is strongly associated with Friend virus-induced erythroleukemia. PU.1 inhibits erythroid differentiation by binding to and inhibiting GATA-1. PU.1 also may be involved in controlling proliferation of erythroid cells. We reported previously that the G(1) phase-specific cyclin-dependent kinase 6 (CDK6) also blocks erythroid differentiation. We now report that PU.1 directly stimulates transcription of the cdk6 gene in both normal erythroid progenitors and erythroleukemia cells, as well as in macrophages. We propose that PU.1 coordinates proliferation and differentiation in immature erythroid cells by inhibiting the GATA-1-mediated gene expression program and also by regulating expression of genes that control progression through the G(1) phase of the cell cycle, the period during which the decision to differentiate is made.

Mutation of the Variant α-Tubulin TUBA8 Results in Polymicrogyria with Optic Nerve Hypoplasia

The critical importance of cytoskeletal function for correct neuronal migration during development of the cerebral cortex has been underscored by the identities of germline mutations underlying a number of human neurodevelopmental disorders. The proteins affected include TUBA1A, a major alpha-tubulin isoform, and microtubule-associated components such as doublecortin, and LIS1. Mutations in these genes are associated with the anatomical abnormality lissencephaly, which is believed to reflect failure of neuronal migration. An important recent observation has been the dependence of cortical neuronal migration upon acetylation of alpha-tubulin at lysine 40 by the histone acetyltransferase Elongator complex. Here, we describe a recognizable autosomal recessive syndrome, characterized by generalized polymicrogyria in association with optic nerve hypoplasia (PMGOH). By autozygosity mapping, we show that the molecular basis for this condition is mutation of the TUBA8 gene, encoding a variant alpha-tubulin of unknown function that is not susceptible to the lysine 40 acetylation that regulates microtubule function during cortical neuron migration. Together with the unique expression pattern of TUBA8 within the developing cerebral cortex, these observations suggest a role for this atypical microtubule component in regulating mammalian brain development.

Expressing the diphtheria toxin A subunit from the HAP2(GCS1) promoter blocks sperm maturation and produces single sperm-like cells capable of fertilization.

After meiosis, the male germline of flowering plants undergoes two mitoses, producing two sperm that are carried within a pollen tube to an ovule. One sperm fuses with the egg to form the zygote and the other fuses with the central cell to form the primary endosperm. The mechanisms that control male germline development and gene expression, and ensure that sperm properly fuse with female gametes are just beginning to be understood. Expression of the potent translation inhibitor, diphtheria toxin A subunit, from the Arabidopsis (Arabidopsis thaliana) HAP2(GCS1) promoter blocked sperm development before the final cell division, resulting in pollen tubes that carried a single sperm-like cell rather than two sperm. These pollen tubes targeted ovules and fertilized either the egg or the central cell, producing seeds with either endosperm or an embryo, but not both. Endosperm-only seeds significantly outnumbered embryo-only seeds, suggesting that single sperm-like cells preferentially fuse with the central cell. These experiments show that de novo translation is required for completion of sperm development, that the HAP2(GCS1) promoter is very tightly controlled, and that disruption of gene expression can result in male germ cells with a bias for gamete fusion.

MicroRNA-mediated switching of chromatin-remodelling complexes in neural development

One of the most distinctive steps in the development of the vertebrate nervous system occurs at mitotic exit when cells lose multipotency and begin to develop stable connections that will persist for a lifetime. This transition is accompanied by a switch in ATP-dependent chromatin-remodelling mechanisms that appears to coincide with the final mitotic division of neurons. This switch involves the exchange of the BAF53a (also known as ACTL6a) and BAF45a (PHF10) subunits within Swi/Snf-like neural-progenitor-specific BAF (npBAF) complexes for the homologous BAF53b (ACTL6b) and BAF45b (DPF1) subunits within neuron-specific BAF (nBAF) complexes in post-mitotic neurons. The subunits of the npBAF complex are essential for neural-progenitor proliferation, and mice with reduced dosage for the genes encoding its subunits have defects in neural-tube closure similar to those in human spina bifida, one of the most serious congenital birth defects. In contrast, BAF53b and the nBAF complex are essential for an evolutionarily conserved program of post-mitotic neural development and dendritic morphogenesis. Here we show that this essential transition is mediated by repression of BAF53a by miR-9* and miR-124. We find that BAF53a repression is mediated by sequences in the 3' untranslated region corresponding to the recognition sites for miR-9* and miR-124, which are selectively expressed in post-mitotic neurons. Mutation of these sites led to persistent expression of BAF53a and defective activity-dependent dendritic outgrowth in neurons. In addition, overexpression of miR-9* and miR-124 in neural progenitors caused reduced proliferation. Previous studies have indicated that miR-9* and miR-124 are repressed by the repressor-element-1-silencing transcription factor (REST, also known as NRSF). Indeed, expression of REST in post-mitotic neurons led to derepression of BAF53a, indicating that REST-mediated repression of microRNAs directs the essential switch of chromatin regulatory complexes.

Desmozoon lepeophtherii n. gen., n. sp., (Microsporidia: Enterocytozoonidae) infecting the salmon louse Lepeophtheirus salmonis (Copepoda: Caligidae).

BackgroundA microsporidian was previously reported to infect the crustacean parasite, Lepeophtheirus salmonis (Krøyer, 1837) (Copepoda, Caligidae), on farmed Atlantic salmon (Salmo salar L.) in Scotland. The microsporidian was shown to be a novel species with a molecular phylogenetic relationship to Nucleospora (Enterocytozoonidae), but the original report did not assign it to a genus or species. Further studies examined the development of the microsporidian in L. salmonis using electron microscopy and re-evaluated the molecular findings using new sequence data available for the group. Here we report a full description for the microsporidian and assign it to a new genus and species.ResultsThe microsporidian infects subcuticular cells that lie on the innermost region of the epidermal tissue layer beneath the cuticle and along the internal haemocoelic divisions. The mature spores are sub-spherical with a single nucleus and an isofilar polar filament with 5-8 turns in a double coil. The entire development is in direct contact with the host cell cytoplasm and is polysporous. During early merogony, a diplokaryotic nuclear arrangement exists which is absent throughout the rest of the developmental cycle. Large merogonial plasmodia form which divide to form single uninucleate sporonts. Sporogonial plasmodia were not observed; instead, binucleate sporonts divide to form two sporoblasts. Prior to final division, there is a precocious development of the polar filament extrusion apparatus which is associated with large electron lucent inclusions (ELIs). Analyses of DNA sequences reveal that the microsporidian is robustly supported in a clade with other members of the Enterocytozoonidae and confirms a close phylogenetic relationship with Nucleospora.ConclusionThe ultrastructural findings of the precocious development of the polar filament and the presence of ELIs are consistent with those of the Enterocytozoonidae. However, the confirmed presence of an early diplokaryotic stage and a merogonial plasmodium that divides to yield uninucleate sporonts instead of transforming into a sporogonial syncitium, are features not currently associated with the family. Yet, analyses of DNA sequence data clearly place the microsporidian within the Enterocytozoonidae. Therefore, due to the novelty of the copepod host, the ultrastructural findings and the robust nature of the phylogenetic analyses, a new genus should be created within the Enterocytozoonide; Desmozoon lepeophtherii n. gen. n. sp. is proposed.

Changes in Cell-Cycle Kinetics Responsible for Limiting Somatic Growth in Mice

In mammals, the rate of somatic growth is rapid in early postnatal life but then slows with age, approaching zero as the animal approaches adult body size. To investigate the underlying changes in cell-cycle kinetics, [methyl-H]thymidine and 5'-bromo-2'deoxyuridine were used to double-label proliferating cells in 1-, 2-, and 3-wk-old mice for four weeks. Proliferation of renal tubular epithelial cells and hepatocytes decreased with age. The average cell-cycle time did not increase in liver and increased only 1.7 fold in kidney. The fraction of cells in S-phase that will divide again declined approximately 10 fold with age. Concurrently, average cell area increased approximately 2 fold. The findings suggest that somatic growth deceleration primarily results not from an increase in cell-cycle time but from a decrease in growth fraction (fraction of cells that continue to proliferate). During the deceleration phase, cells appear to reach a proliferative limit and undergo their final cell divisions, staggered over time. Concomitantly, cells enlarge to a greater volume, perhaps because they are relieved of the size constraint imposed by cell division. In conclusion, a decline in growth fraction with age causes somatic growth deceleration and thus sets a fundamental limit on adult body size.

Division of the right posterior attachments of the head of the pancreas with a linear stapler during pancreaticoduodenectomy: vascular and oncological considerations based on an anatomical cadaver-based study

Section of the right posterior attachment (RPA) of the pancreatic head from the adventicia of the superior mesenteric artery (SMA) is the last step of pancreaticoduodenectomy. This procedure might be technically demanding, time consuming and potentially dangerous. A method using a stapler has been proposed to simplify this step of the pancreaticoduodenectomy. To evaluate the potential consequences of RPA section using this new method, we used an experimental model of pancreaticoduodenectomy in 20 cadavers. After RPA stapling, the residual areolar tissue located between the staple line and SMA was removed (by sub adventicial dissection) and weighed. This allowed an evaluation of: (1) the theoretical risk of SMA injury and (2) the potential carcinological consequences of using stapling division, in comparison with the lymphadenectomy obtained with the conventional sub-adventicial dissection. A right hepatic artery was accidentaly injured in 10% of cases because of the lack of arterial dissection and arterial anatomy recognition. Forty-three percent of the RPA containing nerves and lymphatics remains in place. Using the stapling technique for RPA division theoretically increases the risk of micro or macroscopically positive margin after surgical resection. The result of this human cadaver-based dissection suggests that the use of the mechanical stapling device for final division of the RPA might increase the risk of SMA injury. The risk of vascular injury has been reported in surgical literature and is confirmed by this study. Such results (10% of accidentaly injury) may not be representative in leaving human patients, thanks to the use of a detectable palpable pulse of the SMA. The latter is important in avoiding vascular injury during RPA division. This study also shows that this new technique may increase the risk of incomplete tumor resection.

Cellular footprints of plant evolution—news from the blepharoblast

The microtubular cytoskeleton of higher plants diverges considerably from its animal counterpart. This divergence involves a fundamentally different organization with microtubule arrays, which are specific to higher plants, such as cortical microtubules or the phragmoplast. On the other hand, centrioles, which are central organizers of microtubules in cells of animals and lower plants, have been progressively reduced in the course of plant evolution, and are now absent, as in most seed plants. The role of centrioles as major microtubule-organizing centers (MTOC) apparently has been taken over by the nuclear envelope. However, the spermatogenous cells of pteridophytes and some of the primitive gymnosperms are still endowed with centriolar structures, which is probably related to the fact that the spermatozoids move by flagella. In other words: these specialized cells represent something like a “missing link” between the ancestral situation, where microtubules are nucleated from persistent microtubule rods that are organized in a specialized organelle, and a derived situation, where microtubules emerge from more or less diffuse MTOCs. In the pteridophytes, the centriolar structure is known as the blepharoplast and arises in the last two division cycles. It organizes the interphase array of microtubules along with the spindle for the two final divisions. Prior to each of these mitotic events, the blepharoplast divides itself and moves to the poles, serving as the focus for an astral spindle. After the final division, it re-organizes to a much looser structure and basal bodies are produced that will eventually organize the sperm flagellar apparatus. The work by Vaughn and Bowling in the present issue highlights this evolutionarily interesting transition. Using spermatogenous cells of the fern Ceratopteris richardii, they investigate the effect of transient elimination of microtubules. They use oryzalin, an inhibitor that sequesters tubulin heterodimers from being incorporated into the growing microtubule, such that microtubules shorten and eventually disappear as consequence of their innate turnover. The blepharoplast contains barrel-like structures that serve as centriolar templates embedded in an electronopaque pericentriolar matrix. Upon treatment with oryzalin they observe that this pericentriolar material forms clear gaps that correspond in their dimension to the typical microtubular diameter and stain with γ-tubulin antibodies. Despite the absence of microtubules, the division of the blepharoblast is observed to continue—this means that the division of the MTOC seems to be independent of the presence of microtubules. This represents an evolutionary “modern” trait heralding the situation in higher plant cells, where microtubule arrays cycle although there are no centriolar microtubule asters. The gaps might be morphological manifestations of the γ-tubulin ring complexes that nucleate microtubules in different phyla. After washout, the microtubule arrays showed malformations or multiplications that mirrored the behaviour of the blepharoblasts during drug treatment. Therefore, in terms of cell biology, the spermatogenous cells combines an evolutionarily ancient situation, namely, dependence of microtubule organization from a centriolar organelle, with an evolutionarily derived trait, independence of microtubule organizers from preexisting microtubules. These cells are therefore exciting models to follow the cellular footprints of plant evolution. Protoplasma (2008) 233:175 DOI 10.1007/s00709-008-0009-4

V2a and V2b neurons are generated by the final divisions of pair-producing progenitors in the zebrafish spinal cord

The p2 progenitor domain in the ventral spinal cord gives rise to two interneuron subtypes: V2a and V2b. Delta-Notch-mediated cell-cell interactions between postmitotic immature neurons have been implicated in the segregation of neuron subtypes. However, lineage relationships between V2a and V2b neurons have not been reported. We address this issue using Tg[vsx1:GFP] zebrafish, a model system in which high GFP expression is initiated near the final stage of p2 progenitors. Cell fates were followed in progeny using time-lapse microscopy. Results indicate that the vast majority, if not all, of GFP-labeled p2 progenitors divide once to produce V2a/V2b neuron pairs, indicating that V2a and V2b neurons are generated by the asymmetric division of pair-producing progenitor cells. Together with evidence that Notch signaling is involved in the cell fate specification process, our results strongly suggest that Delta-Notch interactions between sister cells play a crucial role in the final outcome of these asymmetric divisions. This mechanism for determining cell fate is similar to asymmetric divisions that occur during Drosophila neurogenesis, where ganglion mother cells divide once to produce distinct neurons. However, unlike in Drosophila, the divisional axes of p2 progenitors in zebrafish were not fixed. We report that the terminal division of pair-producing progenitor cells in vertebrate neurogenesis can reproducibly produce two distinct neurons through a mechanism that may not depend on the orientation of the division axis.

Hormonal and synaptic influences of serotonin on adult neurogenesis

New neurons are incorporated into the adult brains of a variety of organisms, from humans and higher vertebrates, to non-vertebrates such as crustaceans. In virtually all of these systems serotonergic pathways appear to provide important regulatory influences over the machinery producing the new neurons. We have developed an in vitro preparation where adult neurogenesis can be maintained under highly controlled conditions, and are using this to test the influence of hormones on the production of neurons in the crustacean ( Homarus americanus) brain. Serotonin levels have been manipulated in this in vitro preparation, and the resulting effects on the rate of neurogenesis have been documented. In addition we have compared in vitro influences of serotonin with results acquired from in vivo exposure of whole animals to serotonin. These experiments suggest that there are multiple mechanisms and pathways by which serotonin may regulate neurogenesis in the crustacean brain: (1) serotonin is effective in regulating neurogenesis at levels as low as 10 −10 M, suggesting that circulating serotonin may have hormonal influences on neuronal precursor cells residing in a vascular niche or the proliferation zones; (2) contrasting effects of serotonin on neurogenesis (up- vs. down-regulation) at high concentrations (10 −4 M), dependent upon whether eyestalk tissue is present or absent, indicate that serotonin elicits the release of substances from the sinus glands that are capable of suppressing neurogenesis; (3) previously demonstrated (Beltz, B.S., Benton, J.L., Sullivan, J.M., 2001. Transient uptake of serotonin by newborn olfactory projection neurons. Proc. Natl. Acad. Sci. USA 98, 12730–12735) serotonergic fibers from the dorsal giant neuron project directly into the proliferation zone in Cluster 10, suggest synaptic or local influences on neurogenesis in the proliferation zones where the final cell divisions and neuronal differentiation occur. Serotonin therefore regulates neurogenesis by multiple pathways, and the specific mode of influence is concentration-dependent.

Biological Fabrication of Photoluminescent Nanocomb Structures by Metabolic Incorporation of Germanium into the Biosilica of the Diatom Nitzschia frustulum

Diatoms are single-celled algae that make microscale silica shells or "frustules" with intricate nanoscale features such as two-dimensional pore arrays. In this study, the metabolic insertion of low levels of germanium into the frustule biosilica of the pennate diatom Nitzschia frustulum by a two-stage cultivation process induced the formation of frustules which strongly resembled double-sided nanocomb structures. The final product from the two-stage cultivation process contained 0.41 wt % Ge in biosilica and consisted of an equal mixture of parent frustule valves possessing a normal two-dimensional array of 200 nm pores and daughter valves possessing the nanocomb structure. The nanocomb structures had overall length of 8 mum, rib width of 200 nm, rib length of 500 nm, and slit width of 100 nm. Each slit of the nanocomb was most likely formed by a directed morphology change of a row of 200 nm pores to a single open slit following Ge incorporation into the developing frustule during the final cell division. The frustules possessed blue photoluminescence at peak wavelengths between 450 and 480 nm, which was attributed to contributions from nanostructured biosilica in both the parent valves and in the nanocomb daughter valves. This is the first reported study of using a cell culture system to biologically fabricate a photoluminescent nanocomb structure.

Selective Vulnerability of Neurons in Primary Cultures and in Neurodegenerative Diseases

Primary neuronal cultures are commonly used to dissect the molecular and cellular mechanisms that underlie human brain diseases. Neurons dissociated from an embryonic brain and grown in culture dishes are almost by definition different from those residing inside a living brain. Not only are the individual cells stripped of their normal chemical and physical contacts, but the cellular composition of the cultures (the ratio of cell types) can be affected by many intrinsic and extrinsic factors, including brain region, neuronal birthday, gender, genetic background and in vitro age. Changes in any of these factors may have a strong impact on the outcome of the experiment. In a recent study, Romito-DiGiacomo et al. /54/ demonstrated that when neurons were harvested from murine embryonic cortex, the typical protocol favored cells that were just finishing cell division at the time of harvest. By taking advantage of the fact that the date of the final cell division (birthday) of a neuron correlates with its final position in the cortical plate they were able to assay deeper layer neurons (layers V-VI) separately from the more superficial layers (layers II-III). They reported that while the superficial cells were sensitive to the toxic effect of beta-amyloid, the deeper layer neurons were virtually resistant to death in its presence. The findings recapitulate selective vulnerability in the neocortex of Alzheimer's disease. This is a beautiful example of how to turn the apparent weakness of primary cultures into strength through experimental design and data interpretation. Selective vulnerability is a common feature of neurodegenerative disease, thus it is critical to use the right primary culture. Do you know what is in your culture?

The cell cycle control protein cdc25C is present, and phosphorylated on serine 214 in the transition from germinal vesicle to metaphase II in human oocyte meiosis

Cdc25C is a dual specificity phosphatase essential for dephosphorylation and activation of cyclin-dependent kinase 1 (cdk1), a prerequisite step for mitosis in all eucaryotes. Cdc25C activation requires phosphorylation on at least six sites including serine 214 (S214) which is essential for metaphase/anaphase transit. Here, we have investigated S214 phosphorylation during human meiosis with the objectives of determining if this mitotic phosphatase cdc25C participates in final meiotic divisions in human oocytes. One hundred forty-eight human oocytes from controlled ovarian stimulation protocols were stained for immunofluorescence: 33 germinal vesicle (GV), 37 metaphase stage I (MI), and 78 unfertilized metaphase stage II (MII). Results were stage dependent, identical, independent of infertility type, or stimulation protocol. During GV stages, phospho-cdc25C is localized at the oocyte periphery. During early meiosis I (MI), phosphorylated cdc25C is no longer detected until onset of meiosis I. Here, phospho-cdc25C localizes on interstitial microtubules and at the cell periphery corresponding to the point of polar body expulsion. As the first polar body reaches the periphery, phosphorylated cdc25C is localized at the junction corresponding to the mid body position. On polar body expulsion, the interior signal for phospho-cdc25C is lost, but remains clearly visible in the extruded polar body. In atresic or damaged oocytes, the polar body no longer stains for phospho-cdc25C. Human cdc25C is both present and phosphorylated during meiosis I and localizes in a fashion similar to that seen during human mitotic divisions implying that the involvement of cdc25C is conserved and functional in meiotic cells.

Gene Expression During Development of Fetal and Adult Leydig Cells

In rats and mice, Leydig cells are formed as two morphologically and functionally different generations. The first generation develops in utero, from undifferentiated stem Leydig cells (SLCs) that differentiate into fetal Leydig cells (FLCs). After birth, SLCs that may differ from the fetal SLCs undergo lineage-specific commitment and give rise to adult Leydig cells (ALCs). The intermediates of ALCs first become apparent by day 11 postpartum. These first-appearing intermediates, progenitor Leydig cells (PLCs), are spindle shaped and identifiable as steroidogenic because they express luteinizing hormone receptor (LHR) and 3beta-hydroxysteroid dehydrogenase (3betaHSD). The next step in the transition of PLCs to ALCs is the appearance of the immature Leydig cells (ILCs), most commonly seen in the testis during days 28 to 56 postpartum. ILCs have a more abundant smooth endoplasm reticulum (SER), the network of membranes providing a scaffold for steroidogenic enzyme localization, compared to PLCs, but are considered immature because they secrete higher levels of 5alpha-reduced androgen than testosterone. ILCs undergo a final division before ALC steroidogenic function matures by postnatal day 56. ALCs mark the point of maximum differentiation, and at this stage, the Leydig cell secretes testosterone at the highest rate. In this review, trends of gene expression during development of the two Leydig-cell generations, and recent information from gene profiling by microarray, are evaluated. The expression profiles are distinct, indicating that FLCs and ALCs may originate from separate pools of stem cells.

Changing of the cell division axes drives vulva evolution in nematodes

Vulval epithelial tubes invaginate through concerted cell migration, ring formation, stacking of rings and intra-ring cell fusion in the nematodes Caenorhabditis elegans, Oscheius tipulae and Pristionchus pacificus. The number of rings forming the invaginations is invariantly seven, six, and eight, respectively. We hypothesize that each ring is formed from pairs of symmetrically positioned primordial vulval cells following three premises: If the final cell division is left–right, the daughters will fuse, migrate and form only one ring. If these cells do not divide, one ring will form. If the final division is anterior–posterior, two rings will form. We test the ring hypothesis and found coincidence between the patterns of vulva cell divisions and the number of rings for 12 species. We find heterochronic variations in the timing of division, migration and fusion of the vulval cells between species. We report a unique ring-independent pathway of vulva formation in Panagrellus redivivus. C. elegans lin-11(n389) mutation results in cell fate transformations including changes in the orientation of vulval cell division. lin-11 animals have an additional ring, as predicted by the ring hypothesis. We propose that the genetic pathway determining how vulval cells invaginate evolves through ring-dependent and ring-independent mechanisms.

The Pleiomorphic Plant MTOC: An Evolutionary Perspective

Abstractγ‐Tubulin is an essential component of the microtubule organizing center (MTOC) responsible for nucleating microtubules in both plants and animals. Whereas γ‐tubulin is tightly associated with centrosomes that are inheritable organelles in cells of animals and most algae, it appears at different times and places to organize the myriad specialized microtubule systems that characterize plant cells. We have traced the distribution of γ‐tubulin through the cell cycle in representative land plants (embryophytes) and herein present data that have led to a concept of the pleiomorphic and migratory MTOC. The many forms of the plant MTOC at spindle organization constitute pleiomorphism, and stage‐specific “migration” is suggested by the consistent pattern of redistribution of γ‐tubulin during mitosis. Mitotic spindles may be organized at centriolar centrosomes (only in final divisions of spermatogenesis), polar organizers (POs), plastid MTOCs, or nuclear envelope MTOCs (NE‐MTOCs). In all cases, with the possible exception of centrosomes in spermatogenesis, the γ‐tubulin migrates to broad polar regions and along the spindle fibers, even when it is initially a discrete polar entity. At anaphase it moves poleward, and subsequently migrates from polar regions (distal nuclear surfaces) into the interzone (proximal nuclear surfaces) where interzonal microtubule arrays and phragmoplasts are organized. Following cytokinesis, γ‐tubulin becomes associated with nuclear envelopes and organizes radial microtubule systems (RMSs). These may exist only briefly, before establishment of hoop‐like cortical arrays in vegetative tissues, or they may be characteristic of interphase in syncytial systems where they serve to organize the common cytoplasm into nuclear cytoplasmic domains (NCDs).

Cdk5 is required for multipolar-to-bipolar transition during radial neuronal migration and proper dendrite development of pyramidal neurons in the cerebral cortex

The mammalian cerebral cortex consists of six layers that are generated via coordinated neuronal migration during the embryonic period. Recent studies identified specific phases of radial migration of cortical neurons. After the final division, neurons transform from a multipolar to a bipolar shape within the subventricular zone-intermediate zone (SVZ-IZ) and then migrate along radial glial fibres. Mice lacking Cdk5 exhibit abnormal corticogenesis owing to neuronal migration defects. When we introduced GFP into migrating neurons at E14.5 by in utero electroporation, we observed migrating neurons in wild-type but not in Cdk5(-/-) embryos after 3-4 days. Introduction of the dominant-negative form of Cdk5 into the wild-type migrating neurons confirmed specific impairment of the multipolar-to-bipolar transition within the SVZ-IZ in a cell-autonomous manner. Cortex-specific Cdk5 conditional knockout mice showed inverted layering of the cerebral cortex and the layer V and callosal neurons, but not layer VI neurons, had severely impaired dendritic morphology. The amount of the dendritic protein Map2 was decreased in the cerebral cortex of Cdk5-deficient mice, and the axonal trajectory of cortical neurons within the cortex was also abnormal. These results indicate that Cdk5 is required for proper multipolar-to-bipolar transition, and a deficiency of Cdk5 results in abnormal morphology of pyramidal neurons. In addition, proper radial neuronal migration generates an inside-out pattern of cerebral cortex formation and normal axonal trajectories of cortical pyramidal neurons.