top of page

Group

Public·15 members
Owen Turner
Owen Turner

Family By Fate [v0.3]



During this time, Kent found the helmet becoming more and more possessive of him, so rather than abandoning his battle against chaos, Kent created the half helm. Although Kent's powers were severely limited, he still had the ability of flight, invulnerability and super strength.[29][30] However, Kent donned the helmet one last time in order to find the missing Spectre. Dr. Fate discovered that the Spectre was under the control of Kulak and in a titanic battle with the Spectre, he was defeated and the Helmet of Nabu was lost somewhere in the netherverse. It would not be until the early 1960s that Kent Nelson would somehow recover the helmet of Nabu and become Dr. Fate again. In the early 1950s Kent Nelson had retired his fate persona and became a physician, but some time later he had recovered the Helmet of Nabu under never explained events, and in 1955 he fought Khalis, the mad Egyptian priest who once used his Amulet of Anubis.[5] It was not until the 1963 that Kent would rejoin the Justice Society of America[31]. As their Justice Society comrades aged, Kent and Inza seemed immortal. The magic fate held over them, virtually stopped their aging process. Inza and Kent also received a small portion of Ian Karkull's power which gave them even more vitality.




Family By Fate [v0.3]



islet/tailup EVOLUTIONARY HOMOLOGS part 3/3 Expression of Islet Homologs in Chickens and Mammals: Brain and Spinal Cord MotoneuronsMotor neurons located at different positions in the embryonic spinal cord innervate distinct targets inthe periphery, establishing a topographic neural map. The topographic organization of motor projectionsdepends on the generation of subclasses of motor neurons that select specific paths to their targets. A family of LIM homeobox genes has been cloned in the chick. The combinatorialexpression of four of these genes, Islet-1, Islet-2, Lim-1, and Lim-3, defines subclasses of motorneurons that segregate into columns in the spinal cord and select distinct axonal pathways. Thus the combination of LIM domain proteins serve to code motor neuron identity in the spinal cord (Tsuchida, 1994).Sonic hedgehog (Shh) is strongly implicated in the development of ventral structures in the nervous system. Addition of Sonic hedgehog proteinto chick spinal cord explants induces floor plate and motoneuron development. Whether Shh acts directly to induce these cell types or whethertheir induction is mediated by additional factors is unknown. To further investigate the role of Shh in spinal neuron development, low-density cultures of murine spinal cord precursor cells were used. Shh stimulates neuronal differentiation; however, it does not increase the proportion ofneurons expressing the first postmitotic motoneuron marker Islet-1. Moreover, Shh induces Islet-1 expression in neural tube explants,suggesting that it acts in combination with neural tube factors to induce motoneurons. Another factor implicated in motoneuron development isneurotrophin 3 (NT3): when assayed in isolated precursor cultures, it has no effect on Islet-1 expression. However, the combination ofN-terminal Shh and NT3 induces Islet-1 expression in the majority of neurons in low-density cultures of caudal intermediate neural plate.Furthermore, in explant cultures, Shh-mediated Islet-1 expression is blocked by an anti-NT3 antibody. Previous studies have shownexpression of NT3 in the region of motoneuron differentiation and that spinal fusimotor neurons are lost in NT3 knock-out animals. Takentogether, these findings suggest that Shh can act directly on spinal cord precursors to promote neuronal differentiation, but induction of Islet-1expression is regulated by factors additional to Shh, including NT3 (Dutton, 1999). Different neuronal subpopulations derived from in vitro differentiation of embryonic stem (ES) cells have been characterized using as markers the expression of several homeodomain transcription factors. Following treatment of embryo-like aggregates with retinoic acid (RA), Pax-6, a protein expressed by ventral central nervous system (CNS) progenitors, is induced. In contrast, Pax-7 expressed in vivo by dorsal CNS progenitors, and erbB3, a gene expressed by neural crest cells and its derivatives, are almost undetectable. CNS neuronal subpopulations generate expressed combination of markers characteristic of somatic motoneurons (Islet-1/2, Lim-3, and HB-9); cranial motoneurons (Islet-1/2 and Phox2b) and interneurons (Lim-1/2 or EN1). Molecular characterization of neuron subtypes generated from ES cells should considerably facilitate identification of new genes expressed by restricted neuronal cell lineages (Renoncourt, 1998).These LIM homeodomain proteins are expressed prior to the formation of distinct motor axon pathways and before motor columnsappear. Depending on their arrangement in columns and eventual synaptic targets, motor neurons of the chick brain stem are designated as belonging to somatic motor (sm) visceral motor (vm), or branchiomotor (bm) classes or to the ipsilateral or contralateral vestibuloacoustic effect neuronal population. Sm neurons innervate muscle derived from the paraxial mesoderm and prechordal plate mesoderm. Bm, vm and vestibuloacoustic axons extend dorsolaterally for some distance through the neuroepithelium before converging on large single exit points within the dorsal neural tube (alar plate). Bm neurons innervate muscle derived from paraxial mesoderm within the branchial arches, while vm neurons innervate parasympathetic ganglia associated with lacrimal and salivary glands or neuronal plexuses that innervate smooth muscle; vestibuloacoustic efferent neurons innervate the hair cells of the inner ear. Subpopulations of spinal motor neurons within specific locations in the spinal cord and distinct targets in the periphery express different combinations of LIM homeobox genes. Sm neurons of the medial division of the median motor column express Islet-1, Isl-2, and Lim-3, while those of the lateral division of the median motor column and the medial division of the later motor column express Isl-1 and Isl-2. Sm neurons of the lateral division of the lateral motor column express Lim-1 and Isl-2. Since the lateral motor column is present only at limb levels, Lim-1 expression is restricted to these levels of the neuraxis. At early stages, visceral motor neurons express both Isl-1 and Isl-2, but after their migration to form the column of Terni, only a subset of these neurons continues to express Isl-1. These genes are good candidates to confer target specificity upon motor neuron classes, since they are expressed at times before the motor columns have fully segregated and before axons have reached their targets (Tsuchida, 1994). Motor neuron differentiation in the mouse is accompanied by the expression of a LIM homeodomain transcription factor, Islet1(ISL1). Motor neurons arenot generated without ISL1, although many other aspects of cell differentiation in the neural tube occur normally.A population of interneurons that express Engrailed1 (Drosophila homolog: Engrailed), however, also fails to differentiate in Isl1 mutantembryos. The differentiation of EN1+ interneurons can be induced in both wild-type and mutant neural tissue byregions of the neural tube that contain motor neurons. These results show that ISL1 is required for the generationof motor neurons and suggest that motor neuron generation is required for the subsequent differentiation ofcertain EN1 expressing interneurons (Pfaff, 1996). The generation of distinct classes of motor neurons is an early step in the control of vertebrate motor behavior. To study the interactions that control the generation of motor neuron subclasses in the developing avian spinal cord in vivo grafting studies were performed in which either the neural tube or flanking mesoderm were displaced between thoracic and brachial levels. The positional identity of neural tube cells and motor neuron (MN) subtype identity was assessed by Hox and LIM homeodomain protein expression. Brachial (B) levels of the median motor column (MMC) are organized into three columns: neurons of the medial MMC (MMCM) co-express Isl1, Isl2 and Lim3, neurons of the medial lateral motor column (LMCM) co-express Isl1 and Isl2, and motoneurons of the lateral LMC (LMCL) coexpress Isl2 and Lim1. At thoracic (T) levels motoneurons are also organized into three columns: MMCM neurons; lateral MMC neurons that coexpress Isl1 and Isl2 but not Lim3, and dorsomedially positioned Column of Terni (CT) neurons that express only Isl1. Grafts of 13-15 segment quail T neural tube were placed rostrally at the B level of 12-15 segment chick hosts. Marker and morphological analysis reveals that grafted neural cells divert their normal T fates and their neuronal progeny acquire the molecular properties of B MNs. These changes in the neural tube are restricted to a limited time frame. The rostrocaudal identity of neural cells is plastic at the time of neural tube closure and is sensitive to positionally restricted signals from the paraxial mesoderm. Such paraxial mesodermal signals appear to control the rostrocaudal identity of neural tube cells and the columnar subtype identity of motor neurons. Analysis of neural Hoxc8 expression provides evidence that the change in cell identity after neural tube displacement is not restricted to the MNs; the change occurs in a graded manner along the rostrocaudal axos of the spinal cord, and is associated with both a rostral and caudal respecification in cell fate. In contrast, neural tube grafts between B and T levels do not change the pattern of Hoxc8 expression in the flanking paraxial mesodem. These results suggest that the generation of motor neuron subtypes in the developing spinal cord involves the integration of distinct rostrocaudal and dorsoventral patterning signals that derive, respectively, from paraxial and axial mesodermal cell groups (Ensini, 1998).The diversification of neuronal cell types in the vertebrate central nervous system depends on inductive signalsprovided by local organizing cell groups of both neural and nonneural origin.The link between neuronal birth date, migratory pattern, and identity is also evident in the generation of motor neurons in thespinal cord. These conserved features are particularly apparent for motor neurons of the lateral motor column (LMC). Thisclass of motor neurons is generated selectively at brachial and lumbar levels of the spinal cord, and their axons innervatetarget muscles in the limb. Within the LMC, motor neurons can befurther divided into two subclasses: medial LMC neurons that project to ventrally derived limb muscles, and lateral LMCneurons that project to dorsally derived limb muscles. Motorneurons destined to form the medial LMC leave the cell cycle before lateral LMC neurons; as a consequence, prospectivelateral LMC neurons emerge from the ventricular zone and migrate past medial LMC neurons to their final position. The timeof generation and the distinct migratory environment represent two prominent differences between the development of lateralLMC neurons and other motor neurons. In addition, the total number of motor neurons generated at limb levels of the spinalcord is greater than that at nonlimb levels, presumably to accommodate the formation ofthe LMC (Sockanathan, 1998 and references). All somatic motor neurons initially express Isl1 and Isl2,and most maintain the expression of these genes. Lateral LMC neurons, however,extinguish Isl1 and initiate Lim1 expression as they begin to migrate past medial LMC neurons, thus acquiring a uniqueLIM homeobox gene code. Studies of LIM homeobox gene function in vertebrates andinvertebrates have provided evidence that this genefamily has a role in motor neuron differentiation and axon pathfinding. The diversification of motor neuron subtypes is initiated by inductive signals from the axial and paraxial mesoderm thatoperate along the dorsoventral and rostrocaudal axes of the neural tube. However,medial and lateral LMC motor neurons are generated from progenitor cells that occupy the same dorsoventral androstrocaudal positions, and thus it is unlikely that mesodermal signals impose this distinction. The late birth date of lateralLMC neurons and their migration past early-born LMC neurons prompted a consideration of whether the fate of lateral LMCneurons might be directed by signals provided by early-born LMC neurons. This hypothesis invokes the idea thatLMC motor neurons generated at early stages express a local but non-cell-autonomous signal that induces the lateral LMCphenotype in late-born LMC neurons. A retinoid-mediated signal provided by one subset of early-born spinal motor neurons (the medial) imposes alocal variation in the number of motor neurons generated at different axial levels and also specifies the identity of alater-born subset of motor neurons (the lateral). Thus, in the vertebrate central nervous system the distinct fates of late-bornneurons may be acquired in response to signals provided by early-born neurons (Sockanathan, 1998). To begin to define the contribution of retinoid signaling to motor neuron differentiation, the pattern ofexpression of retinaldehyde dehydrogenase 2 (RALDH2) in the developing spinal cord was examined. At brachial levels, RALDH2 expression is first detected at stage 19, and at this and subsequent stages, expression in the ventral spinal cord appears to be restricted to motorneurons. By stage 27, when the medial motor column (MMC) and LMC have segregated, expression of RALDH2 is restricted toLMC neurons. Within the LMC, RALDH2 is expressed by both medial andlateral LMC neurons. A similar LMC-specific pattern of RALDH2 expression is detected atlumbar levels. Consistent with the restriction of RALDH2 expression to LMC neurons, no expression ofthe gene is detected in motor neurons at thoracic levels. The expression of RALDH2 in motor neurons atbrachial and lumbar levels persists until at least stage 35, although from stage 29 onward, expression gradually becomesrestricted to specific motor neuron pools. The only other site of RALDH2 expression in thespinal cord is in the roof plate, both at limb and nonlimb levels. These selective results show that (1) RALDH2 expression is initiated during the early phase of motor neuron generation atbrachial levels of the spinal cord; (2) RALDH2 expression distinguishes developing LMC neurons from other somatic orvisceral motor neurons, and (3) RALDH2 expression precedes the appearance of Isl2+, Lim1+ lateral LMC neurons (Sockanathan, 1998). The number of Isl+ motor neurons was counted inbrachial ventral/floor plate (vf) explants grown either alone or with retinol (Rol), a metabolic precursor of retinoic acid, or with all-trans retinoicacid (RA). The number of Isl+ motor neurons in [vf] explants grown in the presence of either Rol or RA isincreased by 60%. The detection of an increase in motor neuron number with Rol, as well as withRA, indicates that explants grown in medium with no added retinoid are deprived of the metabolic substrate required forsynthesis of RA by RALDH2. To examine further the involvement of RALDH2 activity in the control of motor neuronnumber, thoracic [vf] explants, which do not express RALDH2, were exposed to Rol or RA and the number ofIsl+ motor neurons measured. In contrast to results obtained with brachial level explants, exposure of thoracic [vf] explants to Rol doesnot increase motor neuron number, whereas RA similarly induces a 60% increase in Isl+ motorneurons. Taken together, these results provide evidence that (1) retinoids increase the number of motor neurons;(2) the increase in motor neuron number detected after exposure of brachial [vf] explants to Rol is correlated with thesynthesis of active retinoids by RALDH2 activity, and (3) the apparent requirement for RALDH2-generated retinoids can beovercome by exogenous RA. The retinoid-induced increase in motor neuron number at brachial levelsappears to result from an increase in the number of progenitor cells. These experiments suggest that, at limb levels, aRALDH2-generated LMC source of retinoids acts non-cell-autonomously to increase the number of motor neuronprogenitors and, consequently, postmitotic motor neurons. Studies using an RAR antagonist show that retinoid receptor activation is required for the generation of lateral LMC neurons and for thecontrol of motor neuron number. Maintenance of the lateral LMC phenotype appears to require ongoingretinoid signaling over the period that these neurons are migrating to their lateral position (Sockanathan, 1998). RALDH2-dependent induction of lateral LMC neurons requires non-autonomous RA signaling. The onset of RALDH2 and Lim1 expression by lateral LMCneurons was examined. At stage 23, many Isl2+, Lim1+ lateral LMC neurons are still located medial to Isl1+, Isl2+ medial LMC neurons. These Isl2+, Lim1+ neurons do not express RALDH2, suggesting that their lateralLMC phenotype has not been acquired through cell-intrinsic RALDH2 activity. Many of the motor neurons that are locatedin an even more medial position, distant from RALDH2+ neurons, will populate the lateral LMC, but at this stage theseneurons express Isl1/2 but not Lim1. These observations support the idea that the lateral phenotype of LMCneurons is acquired by virtue of the proximity of the neurons to a RALDH2-dependent signal provided by earlier-born LMC neurons. The late birth date of lateral LMC neurons requires that they migrate past early-born neurons to reach their final position. What role might this inside-out program of neuronal migration have in the establishment of the lateral LMCphenotype? The detection of late-born Isl2+, Lim1+ lateral LMC neurons in positions adjacent but medial to early-bornRALDH2+ medial LMC neurons provides evidence that proximity to early-born neurons is sufficient to achieve a lateralLMC identity. The failure of late-born LMC neurons to migrate past medial LMC neurons might, however, have theconsequence that some LMC neurons fail to be exposed to retinoid signals before they lose competence to respond. In thisview, the migration of prospective lateral LMC neurons through early-born LMC neurons would achieve a rapid intermixingof inductive and responsive neurons and ensure that the entire population of late born LMC neurons efficiently encounters alocal source of retinoid signals (Sockanathan, 1998). In mammals, Pax-6 (Drosophila homolog: Eyeless)is expressed in severaldiscrete domains of the developing CNS and has been implicated in neuraldevelopment, although its precise role remains elusive. A novel Small eye ratstrain (rSey2) was found with phenotypes similar to mouse and rat Small eye. Analyses of thePax-6 gene reveals one base (C) insertion in an exon encoding the regiondownstream of the paired box of the Pax-6 gene (resulting in the rSey2 mutation), resulting in the generation of a truncatedprotein due to the frame shift. rSey2/rSey2 mutant rats exhibit abnormal development of motor neurons in the hindbrain. TheIslet-1-positive motor neurons are generated just ventral to the Pax-6-expressingdomain, both in the wild-type and mutant embryos. However, two somatic motor (SM)nerves, the abducens and hypoglossal nerves, are missing in homozygous embryos.No SM-type axonogenesis (ventrallygrowing) is found in the mutant postotic hindbrain, though branchiomotor and visceral motor(BM/VM)-type axons (dorsally growing) are observed within the neural tube. Todiscover whether the identity of these motor neuron subtypes is changed in themutant, expression of LIM homeobox genes Islet-1, Islet-2 and Lim-3 were examined.At the postotic levels of the hindbrain, SM neurons express all the three LIM gene


About

Welcome to the group! You can connect with other members, ge...
bottom of page