Homeobox genes and development of the vertebrate CNS

The discovery of homeobox genes in vertebrates may allow analysis of a basic problem in developmental neurobiology: how regional differences in CNS organization are specified during development. This view is based on the roles defined for homologous genes in Drosophila development, and is supported by studies of the patterns of homeobox gene expression in vertebrate embryos. Homeobox genes comprise a multigene family, members of which are expressed in different spatially restricted domains along the anterior-posterior axis of the CNS. These observations are consistent with homeobox genes having roles in the positional specification of CNS organization, and experimental tests of this should be forthcoming shortly.     

Specification of dopaminergic and serotonergic neurons in the vertebrate CNS

The early specification of dopaminergic and serotonergic neurons during vertebrate CNS development relies on signals produced by a small number of organizing centers. Recent studies have characterized these early organizing centers, the manner in which they may be established, the inductive signals they produce, and candidate signaling systems that control the later development of the dopaminergic system.     

Mechanisms of cell migration in the vertebrate embryo

In vertebrate embryos, many cells are involved in active and passive movements before they regroup into defined tissues. Oriented migration is controlled by different mechanisms, which may include chemotaxis, galvanotaxis, haptotaxis, contact guidance, contact inhibition of movement, and population pressure. A given cell type may utilize different mechanisms in different species and even in the same species when segregating into different lineages. Most of these processes are not yet understood at the molecular level. An even greater difficulty is faced by the molecular embryologist in attempting to unravel the mechanisms governing epithelium-mesenchyme interconversion, which can regulate the initiation and termination of migration. Cells migrating in the extracellular matrix interact directly with fibronectin, although this glycoprotein does not induce the egress of cells from epithelia. Recent studies on the molecular mechanism of intercellular adhesion have led to the identification and characterization of several surface molecules (CAM). Cell surface modulation of such cell adhesion molecules throughout development should contribute to the shaping of the embryo.     

Do different populations of GABA-receptors exist in the vertebrate CNS?

Studies of the possible multiplicity of GABA-receptors that have been conducted with ligand-binding techniques and with certain electrophysiological and behavioral methods have been reviewed and analyzed. It seems evident that different populations of GABA binding sites exist in subcellular preparations of CNS tissues, but it is not yet certain that these sites reflect different populations of GABA-receptors that might exist in vivo.     

Mechanisms of ventral patterning in the vertebrate nervous system

Dorsoventral patterning of the neural tube has a crucial role in shaping the functional organization of the CNS. It is well established that hedgehog signalling plays a key role in specifying ventral cell types throughout the neuroectoderm, and major progress has been made in elucidating how hedgehog signalling works in this ventral specification. In addition, other molecular pathways, including nodal, retinoic acid and fibroblast growth factor signalling, have been identified as important molecular cues for ventral patterning of the spinal cord, telencephalon and eye. Here, we discuss recent advances in this field, highlighting the emerging interplay of these signalling pathways in the molecular specification of ventral patterning at different rostrocaudal levels of the CNS.     

Cellular Compensatory Mechanisms in the CNS of Dysmyelinated Rats

Loss or absolute lack of myelin in the CNS results in remarkable compensation at the cellular level. In this study on the natural progression of neuropathology in the CNS in 2 related but distinct long-lived dysmyelinated rats, total lack of myelin was associated with remarkable glial cell proliferation and ineffective myelinating activity throughout life in Long Evans Bouncer (LE-bo) rats; conversely, in Long Evans Shaker (LES) rats, futile myelinating activity ceased when rats were advanced in age. Progressively severe astrogliosis separates individual axons from each other and coincides with widespread, abundant axonal sprouting throughout the life in both rat strains. Severely dysmyelinated Long Evans rats can serve as excellent models to elucidate the cellular and molecular mechanisms of neuroglial compensation to lack or loss of myelin in vivo and to study axonal plasticity in the adult demyelinated CNS.Abbreviations: ³H-TdR, [³H]-thymidine; LES, Long Evans Shaker; LE-bo, Long Evans BouncerCompensatory cellular responses to loss of myelin in multiple sclerosis, experimental allergic encephalomyelitis or spinal cord injury, and both loss and absolute lack of myelin in dysmyelinated CNS are not well understood. Morphologic studies indicate that recent demyelinated lesions show proliferation of oligodendrocyte-type cells coinciding with repopulation of demyelinated areas and precocious remyelination.^34,36,38 Newly formed myelin sheaths are thin and may form shortened internodes that can be interspersed by demyelinated axonal segments.³⁵ Compensatory remyelination in human patients affected by multiple sclerosis^26,34,41 involves oligodendrocyte progenitor cells that populate the adult CNS and can be active in areas of demyelination.^45,51,52 However, remyelination is incomplete and fails altogether after recurrent bouts of demyelination.¹ Demyelinated plaques in sclerotic lesions can have surviving axons arranged in bundles which separated from each other by hypertrophied astrocyte processes.¹Cellular mechanisms of compensation in response to demyelination can be studied systematically and in detail in animal models of experimental allergic encephalomyelitis^5,39 or viral encephalitis, such as intracerebral inoculation of Theiler virus in CD1 mice.⁴³ Remyelination in areas of chronic demyelination requires increased mitotic activity, resulting in generation of astrocyte and oligodendrocyte progenitors and their migration into demyelinating areas with subsequent myelination and astrogliosis.^8,37,40 After immunoglobulin treatment of Theiler virus-inoculated SJL/J mice, their demyelinated lesions can have proliferation and maturation of oligodendrocyte progenitors followed by spontaneous remyelination.⁴⁴ Glial cell proliferation near lymphocytes and hypertrophied astrocytes suggested a beneficial role of at least some proinflammatory mechanisms in remyelination. Intracerebral inoculation of C57Bl/6N mice with mouse hepatitis virus produced demyelination followed by transient increase in proliferation of glial progenitors, which differentiated into oligodendrocytes and astrocytes with remyelination and astrogliosis.¹⁵ Other experimental methods of demyelination, such as oral administration of cuprizone resulting in demyelination of cerebellar peduncles in CD1 mice²⁵ and intraspinal injection of ethidium bromide in rats,¹⁰ have been used. Consistently demyelination was followed by increased mitotic activity with subsequent generation of astrocytes and oligodendrocytes and spontaneous remyelination and astrogliosis.^10,25,27Paracrine signaling involving oligodendrocytes and axons has been implicated in determination of the number of oligodendrocytes required to myelinate a population of axons in an area of CNS;³ therefore, dysmyelinated rodent models can afford us insight into cellular mechanisms of compensation to hypomyelination.^4,49 The failure of the jimpy (jp) mouse, which carries a mutation in proteolipid protein,⁴⁷ to generate a normal amount of myelin results in severe hypomyelination due to oligodendrocyte dysfunction^19,29 and coincides with increased proliferation of cells of oligodendrocyte lineage that is balanced by increased oligodendrocyte death.^2,9,18,47,53 Vigorous proliferation of glial cell progenitors in the spinal cord and optic nerve of normal mice declines postpartum and is arrested or negligible by the third week of life.⁴⁶ In contrast, immature jimpy glial cells show even more robust proliferation in the neonatal life, as measured by intranuclear internalization of tritiated thymidine ([³H]-TdR); this proliferation declines, as in glial cells of normal mice, but is still remarkable by the week 3.⁴⁶ In the spinal cord of myelin-deficient (md) rats, another severely dysmyelinated mutant strain with abnormal proteolipid protein, the proliferation of glial cells predominantly of oligodendrocyte lineage was increased as in jimpy cells, and inhibition of this proliferation was delayed until the third week postpartum.²⁴ Despite increased proliferation of oligodendrocytes during the postnatal period, the number of oligodendrocytes markedly declined in relation to oligodendrocyte apoptosis in a longer-surviving substrain of myelin-deficient rats¹⁴ suggesting the lack or insufficient proliferation of oligodendrocyte progenitors to counteract cell death. In the optic nerve of the Long Evans Shaker (LES) rat, a severely dysmyelinated but long-surviving rat with a mutation in myelin basic protein,³⁰ proliferation of glial cells is enhanced and their inhibition delayed, resulting in increased numbers of glial cells predominantly of oligodendrocyte morphology.²² Inhibition of proliferation of oligodendrocyte progenitors was delayed in the spinal cord of shiverer (shi) mouse, another rat mutant in myelin basic protein, coinciding with a remarkable increase in oligodendrocyte numbers.⁶ Although the molecular mechanisms regulating increased proliferation of oligodendrocyte progenitors and delayed inhibition of this proliferation in the postnatal period are unknown, a failure of myelination is considered to induce the proliferative response⁴In severely dysmyelinated mutants such as the shiverer mouse and LES rat, oligodendrocytes develop pathology characterized by accumulation of a membraneous material which forms vesicles limited by a pentalamellar membrane where at regular intervals 3 electrodense lines are separated by 2 electrolucent lines.^13,22 Analysis of electron micrographs of degenerating oligodendrocytes in cases of Pelizaeus–Merzbacher disease revealed that vesicles have limiting membrane with multiple dense lines regularly spaced at 5.8 nm ⁵⁰ Similar vesicles in degenerating oligodendrocytes have been observed in jimpy mice and LES and Long Evans Bouncer (LE-bo) rats, in which the dense lines in the vesicle walls are spaced at approximately 5-nm intervals.^21,22 Although the membraneous material has not been analyzed chemically yet, its morphology including the regular periodicity of dense lines and formation of vesicles suggest a lipid-rich material whose hydrophobic properties induce it to form vesicles in the aqueous environment⁴⁸ of the mutated oligodendrocytes. In comparison, the periodicity of dense lines in the CNS myelin sheath averages 15 nm.²³Lack of myelin in the CNS coincides with astrogliosis, resulting in the enveloping of small bundles of naked axons by hypertrophied astrocyte processes, which effectively separates the axonal bundles from each other in jimpy¹⁹ and shiverer mice^19,33,42 and LES²² and old taiep rats (which progressively lose CNS myelin).²⁸ Review of electron micrographs in published literature revealed that small (less than 0.3 μm) structures containing neurotubules and neurofilaments, which are considered to be axonal sprouts, are common in dysmyelinated CNS in jimpy¹⁹ and shiverer^17,33 mice and LES³² and old taiep²⁸ rats. Although the precise mechanisms of astrogliosis in which hypertrophied astrocyte processes separate bundles of naked axons are unknown, this reaction can be considered compensatory for lack of myelin and an attempt to isolate naked axons. Axonal sprouting, indicative of axonal plasticity in a severely dysmyelinated environment, has been demonstrated in the optic nerve of mature LES rats.³² This observation supports the notion of inhibition of axonal plasticity and regeneration in presence of CNS myelin^7,11 but perhaps not in presence of hypertrophied astroglial processes; this finding also indicates the usefulness of dysmyelinated animals in in vivo studies of axonal regeneration relevant to spinal cord injury.The present study was undertaken to analyze the natural progression of neuropathology in the CNS of 2 severely dysmyelinated long-lived mutant rat strains, LES and LE-bo, and to characterize the mechanisms of cellular response to lack of myelin.     

Mechanisms regulating the origins of the vertebrate vascular system

In order to sustain growth, differentiation, and organogenesis, vertebrate embryos must form a functional vascular system early in embryonic development. Intrinsic interest in this process as well as the promise of potential clinical applications has led to significant progress in understanding the mechanisms governing the formation of the vascular system, however the earliest stages of vascular development--the emergence of committed endothelial precursors from the mesoderm--remain unclear. A review of the current literature reveals an unexpected diversity and heterogeneity with respect to where vascular endothelial cells originate in the embryo, when they become committed and the mechanisms governing how endothelial cells acquire their identity. Spatially, a widespread region of the early mesoderm possesses the ability to give rise to vascular endothelial cells; temporally the process is not limited to a small window during embryogenesis, but rather, may continue throughout the lifespan of the organism. On the molecular level, recent findings point to several determinative pathways that regulate, modulate, and extend the scope of the Flk1/VEGF signaling system. An expanding array of novel gene products implicated in endothelial cell type determination appear to act synergistically, with different combinations of factors leading to diverse cellular responses, varying patterns of differentiation, and considerable heterogeneity of endothelial cell types during embryogenesis.     

Pattern formation in the vertebrate nervous system.

In recent years, the classical approaches of experimental embryology have been used in combination with more modern techniques to investigate aspects of neurogenesis. This combination has advanced our knowledge of several areas of neuronal development, including the lineages of neuronal precursors, the segmentation of the nervous system, and the patterning of the neural tube.     

Angiogenic network formation in the developing vertebrate trunk

We have used time-lapse multiphoton microscopy of living Tg(fli1:EGFP)y1 zebrafish embryos to examine how a patterned, functional network of angiogenic blood vessels is generated in the early vertebrate trunk. Angiogenic vascular sprouts emerge from the longitudinal trunk axial vessels (the dorsal aorta and posterior cardinal vein) in two spatially and temporally distinct steps. Dorsal aorta-derived sprouts form an initial primary network of vascular segments, followed by emergence of vein-derived secondary vascular sprouts that interact and interconnect dynamically with the primary network to initiate vascular flow. Using transgenic silent heart mutant embryos, we show that the gross anatomical patterning of this network of vessels does not require blood circulation. However, our results suggest that circulatory flow dynamics play an important role in helping to determine the pattern of interconnections between the primary network and secondary sprouts, and thus the final arterial or venous identity of the vessels in the functional network. We discuss a model to explain our results combining genetic programming of overall vascular architecture with hemodynamic determination of circulatory flow patterns.     

Mechanisms of generation of signals in vertebrate photoreceptors.

The electrical responses of rods are analyzed in different ionic environments. It is shown that the dark level of the membrane potential is predominantly determined by a sodium current, while the peak of responses to bright light is controlled by the concentration of external potassium. The sag from the peak to the plateau of photoresponses seems to be generated by different ionic mechanism. The effects produced by substituting the external calcium with EGTA are also analyzed. It is suggested that calcium plays a role in different mechanisms of generation of electrical responses.     

中枢神经系统发育的不对称细胞分裂机制

无论在无脊椎动物还是脊椎动物中,组成中枢神经系统（CNS）的大多数细胞都是由极性神经祖细胞不对称分裂而来。通过简要综述果蝇（Drosophila melanogaste）成神经母细胞（NB）不对称分裂机制,并与近年来在脊椎动物不对称细胞分裂上取得的研究成果相比较,尝试找出两个系统的相似性和相异性。     

The gastrocoel roof plate in embryos of different frogs

The morphology of the gastrocoel roof plate and the presence of cilia in this structure were examined in embryos of four species of frogs. Embryos of Ceratophrys stolzmanni (Ceratophryidae) and Engystomops randi (Leiuperidae) develop rapidly, provide comparison for the analysis of gastrocoel roof plate development in the slow-developing embryos of Epipedobates machalilla (Dendrobatidae) and Gastrotheca riobambae (Hemiphractidae). Embryos of the analyzed frogs develop from eggs of different sizes, and display different reproductive and developmental strategies. In particular, dorsal convergence and extension and archenteron elongation begin during gastrulation in embryos of rapidly developing frogs, as in Xenopus laevis. In contrast, cells that involute during gastrulation are stored in the large circumblastoporal collar that develops around the closed blastopore in embryos of slow-developing frogs. Dorsal convergence and extension only start after blastopore closure in slow-developing frog embryos. However, in the neurulae, a gastrocoel roof plate develops, despite the accumulation of superficial mesodermal cells in the circumblastoporal collar. Embryos of all four species develop a ciliated gastrocoel roof plate at the beginning of neurulation. Accordingly, fluid-flow across the gastrocoel roof plate is likely the mechanism of left-right asymmetry patterning in these frogs, as in X. laevis and other vertebrates. A ciliated gastrocoel roof plate, with a likely origin as superficial mesoderm, is conserved in frogs belonging to four different families and with different modes of gastrulation.