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67-529: SVZ can refer to: Subventricular zone , a structure in the mammalian brain South Volcanic Zone , a volcanic arc in southwestern South America Slovak Air Force of WWII (Slovak: Slovenské vzdušné zbrane ) Schweriner Volkszeitung , a German newspaper Steve van Zandt , an American musician Topics referred to by the same term [REDACTED] This disambiguation page lists articles associated with

134-576: A body plan from a common ancestor, and that taxa were branches of a single tree of life . The word homology, coined in about 1656, is derived from the Greek ὁμόλογος homologos from ὁμός homos 'same' and λόγος logos 'relation'. Similar biological structures or sequences in different taxa are homologous if they are derived from a common ancestor . Homology thus implies divergent evolution . For example, many insects (such as dragonflies ) possess two pairs of flying wings . In beetles ,

201-416: A speciation event ( orthologs ) or a duplication event ( paralogs ). Homology among proteins or DNA is inferred from their sequence similarity. Significant similarity is strong evidence that two sequences are related by divergent evolution from a common ancestor. Alignments of multiple sequences are used to discover the homologous regions. Homology remains controversial in animal behaviour , but there

268-465: A gene in an organism is duplicated, the two copies are paralogous. They can shape the structure of whole genomes and thus explain genome evolution to a large extent. Examples include the Homeobox ( Hox ) genes in animals. These genes not only underwent gene duplications within chromosomes but also whole genome duplications . As a result, Hox genes in most vertebrates are spread across multiple chromosomes:

335-565: A network of functionally correlated Glial Fibrillary Acid Protein ( GFAP )-positive astrocytic processes that are linked to junctional complexes, yet lack cell bodies except for the rare neuronal somata. While the function of this layer is yet unknown in humans, it has been hypothesized that the astrocytic and ependymal interconnections of Layer I and II may act to regulate neuronal functions, establish metabolic homeostasis , and/or control neuronal stem cell proliferation and differentiation during development. Potentially, such characteristics of

402-419: A non-evolutionary context by the anatomist Richard Owen in 1843. Homology was later explained by Charles Darwin 's theory of evolution in 1859, but had been observed before this from Aristotle's biology onwards, and it was explicitly analysed by Pierre Belon in 1555. A common example of homologous structures is the forelimbs of vertebrates , where the wings of bats and birds , the arms of primates ,

469-477: A pattern of gene expression in the growing zones ( meristems ) is described by the ABC model of flower development . Each of the four types of flower parts is serially repeated in concentric whorls, controlled by a small number of genes acting in various combinations. Thus, A genes working alone result in sepal formation; A and B together produce petals; B and C together create stamens; C alone produces carpels. When none of

536-421: A self-renewal capacity. It was therefore proposed that a small population of BTSCs with such self-renewal capabilities were maintaining tumors in diseases such as leukemia and breast cancer . Several characterizing factors lead to the proposed idea of neuronal stem cells (NSCs) being the origin for BTSCs, as they share several features. These features are shown in the figure. This group provides evidence of

603-542: A valuable source of information regarding the SVZ and its structure-to-function relationship, the human model will prove significantly different. Epigenetic DNA modifications have a central role in regulating gene expression during differentiation of neural stem cells . The conversion of cytosine to 5-methylcytosine (5mC) in DNA by DNA methyltransferase DNMT3A appears to be an important type of epigenetic modification occurring in

670-419: Is a known site of neurogenesis and self-renewing neurons in the adult brain , serving as such due to the interacting cell types, extracellular molecules, and localized epigenetic regulation promoting such cellular proliferation. Along with the subgranular zone of the dentate gyrus , the subventricular zone serves as a source of neural stem cells (NSCs) in the process of adult neurogenesis . It harbors

737-487: Is a synapomorphy for fleas. Patterns such as these lead many cladists to consider the concept of homology and the concept of synapomorphy to be equivalent. Some cladists follow the pre-cladistic definition of homology of Haas and Simpson, and view both synapomorphies and symplesiomorphies as homologous character states. Homologies provide the fundamental basis for all biological classification, although some may be highly counter-intuitive. For example, deep homologies like

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804-549: Is defined in terms of shared ancestry. Two segments of DNA can have shared ancestry because of either a speciation event ( orthologs ) or a duplication event ( paralogs ). Homology among proteins or DNA is typically inferred from their sequence similarity. Significant similarity is strong evidence that two sequences are related by divergent evolution of a common ancestor. Alignments of multiple sequences are used to indicate which regions of each sequence are homologous. Homologous sequences are orthologous if they are descended from

871-437: Is different, however, from the rodent SVZ in two distinct ways; the first is that the astrocytes of humans are not in close juxtaposition to the ependymal layer, rather separated by a layer lacking cell bodies; the second is that the human SVZ lacks chains of migrating neuroblasts seen in rodent SVZ, in turn providing for a lesser number of neuronal cells in the human than the rodent. For this reason, while rodent SVZ proves as

938-449: Is one of two places where neurogenesis has been found to occur in the adult mammalian brain. Adult SVZ neurogenesis takes the form of neuroblast precursors of interneurons that migrate to the olfactory bulb through the rostral migratory stream . The SVZ also appears to be involved in the generation of astrocytes following a brain injury. The innermost layer (Layer I) contains a single layer (monolayer) of ependymal cells lining

1005-402: Is sensitive to any deleterious effects. Therefore, the SVZ can recover itself following mild injury, and potentially provide for replacement cell therapy to other affected regions of the brain. In an attempt to characterize and analyze the mechanism concerning the proliferation of neuronal cells within the subventricular zone, Decressac et al. observed the proliferation of neural precursors in

1072-443: Is shared due to common ancestry. Primary homology may be conceptually broken down further: we may consider all of the states of the same character as "homologous" parts of a single, unspecified, transformation series. This has been referred to as topographical correspondence. For example, in an aligned DNA sequence matrix, all of the A, G, C, T or implied gaps at a given nucleotide site are homologous in this way. Character state identity

1139-427: Is similarity in anatomical structures or genes between organisms of different taxa due to shared ancestry , regardless of current functional differences. Evolutionary biology explains homologous structures as retained heredity from a common ancestor after having been subjected to adaptive modifications for different purposes as the result of natural selection . The term was first applied to biology in

1206-440: Is suggestive evidence that, for example, dominance hierarchies are homologous across the primates . Homology was noticed by Aristotle (c. 350 BC), and was explicitly analysed by Pierre Belon in his 1555 Book of Birds , where he systematically compared the skeletons of birds and humans. The pattern of similarity was interpreted as part of the static great chain of being through the mediaeval and early modern periods: it

1273-450: Is taken to be homologous. As implied in this definition, many cladists consider secondary homology to be synonymous with synapomorphy , a shared derived character or trait state that distinguishes a clade from other organisms. Shared ancestral character states, symplesiomorphies, represent either synapomorphies of a more inclusive group, or complementary states (often absences) that unite no natural group of organisms. For example,

1340-404: Is the hypothesis that the particular condition in two or more taxa is "the same" as far as our character coding scheme is concerned. Thus, two Adenines at the same aligned nucleotide site are hypothesized to be homologous unless that hypothesis is subsequently contradicted by other evidence. Secondary homology is implied by parsimony analysis , where a character state that arises only once on a tree

1407-594: The pax6 genes that control the development of the eyes of vertebrates and arthropods were unexpected, as the organs are anatomically dissimilar and appeared to have evolved entirely independently. The embryonic body segments ( somites ) of different arthropod taxa have diverged from a simple body plan with many similar appendages which are serially homologous, into a variety of body plans with fewer segments equipped with specialised appendages. The homologies between these have been discovered by comparing genes in evolutionary developmental biology . Among insects,

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1474-402: The rostral migratory stream to the olfactory bulb (confirming previous experiments) and to the striatum . Such data supports the author's hypothesis in that neurogenesis would be stimulated through introduction of such a peptide . As NPY is a 36 amino acid peptide associated with many physiological and pathological conditions, it has multiple receptors that are broadly expressed in

1541-465: The stinger of the female honey bee is a modified ovipositor , homologous with ovipositors in other insects such as the Orthoptera , Hemiptera , and those Hymenoptera without stingers. The three small bones in the middle ear of mammals including humans, the malleus , incus , and stapes , are today used to transmit sound from the eardrum to the inner ear . The malleus and incus develop in

1608-415: The striatum ) type. Along with the effects of NPY injection on striatal dopamine , GABA and glutamate parameters to regulate neurogenesis in the subventricular zone (previous study), this finding is still under consideration as it could be a secondary modulator of the aforementioned neurotransmitters . As is necessary for all research, this group conducted its experiments with a broad perspective on

1675-423: The "same organ in different animals under every variety of form and function", and contrasting it with the matching term "analogy" which he used to describe different structures with the same function. Owen codified 3 main criteria for determining if features were homologous: position, development, and composition. In 1859, Charles Darwin explained homologous structures as meaning that the organisms concerned shared

1742-486: The 1830 Cuvier-Geoffroy debate . Geoffroy stated the principle of connections, namely that what is important is the relative position of different structures and their connections to each other. Embryologist Karl Ernst von Baer stated what are now called von Baer's laws in 1828, noting that related animals begin their development as similar embryos and then diverge: thus, animals in the same family are more closely related and diverge later than animals which are only in

1809-496: The HoxA–D clusters are the best studied. Some sequences are homologous, but they have diverged so much that their sequence similarity is not sufficient to establish homology. However, many proteins have retained very similar structures, and structural alignment can be used to demonstrate their homology. It has been suggested that some behaviours might be homologous, based either on sharing across related taxa or on common origins of

1876-723: The SVZ function in the rodent brain has been, to a certain extent, examined and defined for its abilities. With such research, it has been found that the dual-functioning astrocyte is the dominant cell in the rodent SVZ; this astrocyte acts as not only a neuronal stem cell, but also as a supporting cell that promotes neurogenesis through interaction with other cells. This function is also induced by microglia and endothelial cells that interact cooperatively with neuronal stem cells to promote neurogenesis in vitro, as well as extracellular matrix components such as tenascin-C (helps define boundaries for interaction) and Lewis X (binds growth and signaling factors to neural precursors). The human SVZ

1943-552: The SVZ stem cells are the cell of origin for brain tumors such as gliomas, there is strong evidence that suggests increased tumor aggressiveness and mortality in those patients whose high-grade gliomas infiltrate or contact the SVZ. In prostate cancer, tumor-induced neurogenesis is characterized by the recruitment of neural progenitor cells (NPC) from SVZ. NPCs infiltrate the tumor where they differentiate into autonomic neurons (adrenergic neurons mainly) that stimulate tumor growth. Homology (biology) In biology , homology

2010-470: The SVZ's apparent role in tumorigenesis as demonstrated by the possession of mitogenic receptors and their response to mitogenic stimulation, specifically type C cells that express the epidermal growth factor receptor (EGFR), making them highly proliferative and invasive. Additionally, the existence of microglia and endothelial cells within the SVZ was found to enhance neurogenesis , as well as providing for some directional migration of neuroblasts from

2077-402: The SVZ, yet repair capacity of the SVZ was observed despite the lack of white matter necrosis ; this occurred likely because the SVZ was able to gradually replace the neuroglia of the brain. Chemotherapeutics were also tested for their effects on the SVZ, as they are currently used for many diseases yet lead to complications within the central nervous system . To do so, methotrexate (MTX)

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2144-584: The SVZ. In addition, some current theories propose that the SVZ may also serve as a site of proliferation for brain tumor stem cells (BTSCs), which are similar to neural stem cells in their structure and ability to differentiate into neurons , astrocytes , and oligodendrocytes . Studies have confirmed that a small population of BTSCs can not only produce tumors, but they can also maintain it through innate self-renewal and multipotent abilities. While this does not allow for inference that BTSCs arise from neural stem cells, it does raise an interesting question as to

2211-607: The SVZ. Recently, the human SVZ has been characterized in brain tumor patients at phenotypic and genetic level. These data reveal that in half of the patients the SVZ is an exact site of tumorigenesis whereas in the remaining patients it represents an infiltrated region. Thus, it is distinctly possible that in humans a relationship exists between the NSC generation of the region and the consistently self-renewing cells of primary tumors that give way to secondary tumors once removed or irradiated. While it remains to be definitely proven whether

2278-476: The application of their findings, which they claimed could potentially benefit potential candidates for endogenous brain repair through stimulation of the subventricular zone neural stem cell proliferation. This natural molecular regulation of adult neurogenesis would be adjunct with therapies of appropriate molecules such as the tested NPY and Y1 receptor, in addition to pharmacological derivatives, in providing for manageable forms of neurodegenerative disorders of

2345-538: The behaviour in an individual's development; however, the notion of homologous behavior remains controversial, largely because behavior is more prone to multiple realizability than other biological traits. For example, D. W. Rajecki and Randall C. Flanery, using data on humans and on nonhuman primates , argue that patterns of behaviour in dominance hierarchies are homologous across the primates. As with morphological features or DNA, shared similarity in behavior provides evidence for common ancestry. The hypothesis that

2412-757: The brain parenchyma . It is identified by a high presence of myelin in the region. Four cell types are described in the SVZ: 1. Ciliated Ependymal Cells (Type E): are positioned facing the lumen of the ventricle, and function to circulate the cerebrospinal fluid . 2. Proliferating Neuroblasts (Type A): express PSA-NCAM ( NCAM1 ), Tuj1 ( TUBB3 ), and Hu, and migrate in line order to the olfactory bulb 3. Slow Proliferating Cells (Type B): express Nestin and GFAP , and function to ensheathe migrating Type A Neuroblasts 4. Actively Proliferating Cells or Transit Amplifying Progenitors (Type C): express Nestin, and form clusters interspaced among chains throughout region The SVZ

2479-404: The developing cerebral cortex , which resides in the dorsal telencephalon , the SVZ and VZ are transient tissues that do not exist in the adult. However, the SVZ of the ventral telencephalon persists throughout life. The adult SVZ is composed of four distinct layers of variable thickness and cell density as well as cellular composition. Along with the dentate gyrus of the hippocampus , the SVZ

2546-503: The developing and mature rodent brain. However, given in vivo studies performed by this group, the Y1 receptor displayed specifically mediated neuroproliferative effects through the induction of NPY with increased expression in the subventricular zone. Identification of the Y1 receptor also sheds light on the fact that the phenotype of expressed cells from such mitotic events are actually cells that are DCX + ( neuroblasts that migrate directly to

2613-482: The embryo from structures that form jaw bones (the quadrate and the articular) in lizards, and in fossils of lizard-like ancestors of mammals. Both lines of evidence show that these bones are homologous, sharing a common ancestor. Among the many homologies in mammal reproductive systems , ovaries and testicles are homologous. Rudimentary organs such as the human tailbone , now much reduced from their functional state, are readily understood as signs of evolution ,

2680-510: The embryos develop. The implication that the ancestors of snakes had hind legs is confirmed by fossil evidence: the Cretaceous snake Pachyrhachis problematicus had hind legs complete with hip bones ( ilium , pubis , ischium ), thigh bone ( femur ), leg bones ( tibia , fibula ) and foot bones ( calcaneum , astragalus ) as in tetrapods with legs today. As with anatomical structures, sequence homology between protein or DNA sequences

2747-472: The explanation being that they were cut down by natural selection from functioning organs when their functions were no longer needed, but make no sense at all if species are considered to be fixed. The tailbone is homologous to the tails of other primates. In many plants, defensive or storage structures are made by modifications of the development of primary leaves , stems , and roots . Leaves are variously modified from photosynthetic structures to form

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2814-455: The first pair of wings has evolved into a pair of hard wing covers , while in Dipteran flies the second pair of wings has evolved into small halteres used for balance. Similarly, the forelimbs of ancestral vertebrates have evolved into the front flippers of whales , the wings of birds , the running forelegs of dogs , deer , and horses , the short forelegs of frogs and lizards , and

2881-410: The front flippers of whales , and the forelegs of four-legged vertebrates like horses and crocodilians are all derived from the same ancestral tetrapod structure. In developmental biology , organs that developed in the embryo in the same manner and from similar origins, such as from matching primordia in successive segments of the same animal, are serially homologous . Examples include

2948-445: The genes are active, leaves are formed. Two more groups of genes, D to form ovules and E for the floral whorls, complete the model. The genes are evidently ancient, as old as the flowering plants themselves. Developmental biology can identify homologous structures that arose from the same tissue in embryogenesis . For example, adult snakes have no legs, but their early embryos have limb-buds for hind legs, which are soon lost as

3015-561: The grasping hands of primates including humans. The same major forearm bones ( humerus , radius , and ulna ) are found in fossils of lobe-finned fish such as Eusthenopteron . The opposite of homologous organs are analogous organs which do similar jobs in two taxa that were not present in their most recent common ancestor but rather evolved separately . For example, the wings of insects and birds evolved independently in widely separated groups , and converged functionally to support powered flight , so they are analogous. Similarly,

3082-597: The insect-trapping pitchers of pitcher plants , the insect-trapping jaws of the Venus flytrap , and the spines of cactuses , all homologous. Certain compound leaves of flowering plants are partially homologous both to leaves and shoots, because their development has evolved from a genetic mosaic of leaf and shoot development. The four types of flower parts, namely carpels , stamens , petals , and sepals , are homologous with and derived from leaves, as Goethe correctly noted in 1790. The development of these parts through

3149-451: The largest population of proliferating cells in the adult brain of rodents, monkeys and humans. In 2010, it was shown that the balance between neural stem cells and neural progenitor cells (NPCs) is maintained by an interaction between the epidermal growth factor receptor signaling pathway and the Notch signaling pathway. While it has yet to have been studied in-depth in the human brain,

3216-526: The layer may act as a remainder of early developmental life or pathway for cellular migration given similarity to a homologous layer in bovine SVZ shown to have migratory cells common only to higher order mammals. The third layer (Layer III) forms a ribbon of astrocyte cell bodies that are believed to maintain a subpopulation of astrocytes able to proliferate in vivo and form multipotent neurospheres with self-renewal abilities in vitro. While some oligodendrocytes and ependymal cells have been found within

3283-506: The legs of a centipede , the maxillary and labial palps of an insect , and the spinous processes of successive vertebrae in a vertebrate's backbone . Male and female reproductive organs are homologous if they develop from the same embryonic tissue, as do the ovaries and testicles of mammals , including humans . Sequence homology between protein or DNA sequences is similarly defined in terms of shared ancestry. Two segments of DNA can have shared ancestry because of either

3350-423: The mouse subventricular zone through injection of the neuropeptide Y (NPY). NPY is a commonly expressed protein of the central nervous system that has previously been shown to stimulate proliferation of neuronal cells in the olfactory epithelium and hippocampus . The peptide ’s effects were observed through BrdU labeling and cell phenotyping that provided evidence for the migration of neuroblasts through

3417-400: The presence of wings is a synapomorphy for pterygote insects, but a symplesiomorphy for holometabolous insects. Absence of wings in non-pterygote insects and other organisms is a complementary symplesiomorphy that unites no group (for example, absence of wings provides no evidence of common ancestry of silverfish, spiders and annelid worms). On the other hand, absence (or secondary loss) of wings

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3484-413: The region. Additionally, the neural stem cell population within the SVZ is likely responsible for this injury response. The effects of irradiation on the SVZ provided for a recognition of the amount or dose of radiation that can be given is determined mostly by the tolerance of the normal cells near the tumor . As described, the increasing dose of radiation and age led to decrease in three cell types of

3551-412: The relationship that exists from our own cells to those that can cause so much damage. There are currently many different aspects of the SVZ being researched by individuals in the public and private sectors. Such research interests range from the role of the SVZ in neurogenesis , directed neuronal migration, to the previously mentioned tumorigenesis , as well as many others. Below there are summaries of

3618-503: The ribbon, they not only serve an unknown function, they are uncommon by comparison to the population of astrocytes that reside in the layer. The astrocytes present in Layer III can be divided into three populations through electron microscopy , with no unique functions yet recognizable; the first type is a small astrocyte of long, horizontal, tangential projections mostly found in Layer II;

3685-414: The same order and have fewer homologies. Von Baer's theory recognises that each taxon (such as a family) has distinctive shared features, and that embryonic development parallels the taxonomic hierarchy: not the same as recapitulation theory . The term "homology" was first used in biology by the anatomist Richard Owen in 1843 when studying the similarities of vertebrate fins and limbs, defining it as

3752-423: The same ancestral sequence separated by a speciation event: when a species diverges into two separate species, the copies of a single gene in the two resulting species are said to be orthologous . The term "ortholog" was coined in 1970 by the molecular evolutionist Walter Fitch . Homologous sequences are paralogous if they were created by a duplication event within the genome. For gene duplication events, if

3819-426: The second type is found between Layers II and III as well as within the astrocyte ribbon, characterized by its large size and many organelles; the third type is typically found in the lateral ventricles just above the hippocampus and is similar in size to the second type but contains few organelles. The fourth and final layer (Layer IV) serves as a transition zone between Layer III with its ribbon of astrocytes and

3886-562: The striatal area. In an attempt to characterize the role of the subventricular zone in potential tumorigenesis , Quinones-Hinojosa et al. found that brain tumor stem cells (BTSCs) are stem cells that can be isolated from brain tumors by similar assays used for neuronal stem cells. In forming clonal spheres similar to neurospheres of neuronal stem cells, these BTSCs were able to differentiate into neurons , astrocytes and oligodendrocytes in vitro , yet more importantly capable of initiating tumors at low cell concentrations, providing

3953-432: The three groups. Thus, in the pterosaurs, the "wing" involves both the forelimb and the hindlimb. Analogy is called homoplasy in cladistics , and convergent or parallel evolution in evolutionary biology. Specialised terms are used in taxonomic research. Primary homology is a researcher's initial hypothesis based on similar structure or anatomical connections, suggesting that a character state in two or more taxa share

4020-493: The title SVZ . If an internal link led you here, you may wish to change the link to point directly to the intended article. Retrieved from " https://en.wikipedia.org/w/index.php?title=SVZ&oldid=1256207805 " Category : Disambiguation pages Hidden categories: Articles containing Slovak-language text Short description is different from Wikidata All article disambiguation pages All disambiguation pages Subventricular zone In

4087-484: The uncommitted progenitor cells acting as the proliferating population following ischemia . Mechanical brain injury also induces cell migration and proliferation, as was observed in rodents, and it may also increase cell number, negating the previously held notion that no new neuronal cells can be generated. In conclusion, this group was able to determine that cells in the SVZ are able to produce new neurons and glia throughout life, given it does not suffer damage as it

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4154-417: The ventricular cavity; these cells possess apical cilia and several basal expansions that may stand in either parallel or perpendicular to the ventricular surface. These expansions may interact intimately with the astrocytic processes that are interconnected with the hypocellular layer (Layer II). The secondary layer (Layer II) provides for a hypocellular gap abutting the former and has been shown to contain

4221-436: The wings of a sycamore maple seed and the wings of a bird are analogous but not homologous, as they develop from quite different structures. A structure can be homologous at one level, but only analogous at another. Pterosaur , bird and bat wings are analogous as wings, but homologous as forelimbs because the organ served as a forearm (not a wing) in the last common ancestor of tetrapods , and evolved in different ways in

4288-462: The work of three different lab groups focusing primarily on one aspect of the SVZ; these include the role of SVZ in cell replacement after brain injury, simulation of NSC proliferation, and role in various tumorigenic cancers. In their review, Romanko et al. characterized the impact of acute brain injury on the SVZ. Overall, the authors determined that moderate insults to the SVZ allowed for recovery while more severe injuries caused permanent damage to

4355-424: Was found to also decrease the cell count of the SVZ by 20%, with 50% of neurons in the striatum and neocortex being destroyed, but the cell types of the SVZ killed were as non-uniform as the region itself. Upon subsequent testing, it was found that a different portion of each cell was eliminated, yet the medial SVZ cell population remained mostly alive. This may provide for a certain resiliency of such cells, with

4422-787: Was not then seen as implying evolutionary change. In the German Naturphilosophie tradition, homology was of special interest as demonstrating unity in nature. In 1790, Goethe stated his foliar theory in his essay "Metamorphosis of Plants", showing that flower parts are derived from leaves. The serial homology of limbs was described late in the 18th century. The French zoologist Etienne Geoffroy Saint-Hilaire showed in 1818 in his theorie d'analogue ("theory of homologues") that structures were shared between fishes, reptiles, birds, and mammals. When Geoffroy went further and sought homologies between Georges Cuvier 's embranchements , such as vertebrates and molluscs, his claims triggered

4489-418: Was used alone and in combination with radiation to find that roughly 70% of the total nuclear density of the SVZ had been depleted, yet given loss of neuroblast cells ( progenitor cells), it was remarkable to find that SVZ NSCs would still generate neurospheres similar to subjects that did not receive such treatment. In relation to interruption of blood supply to the brain, cerebral hypoxia / ischemia (H/I)

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