Generation of electromotor neurons in Sternarchus albifrons: Differences between normally growing and regenerating spinal cord
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Ependymal Cell
Abstract The morphology of spinal cord in the caudal‐most spinal segments of normal adult Sternarchus albifrons is different from that of more rostral adult cord. The caudal segments are strikingly similar to the regenerating spinal cord observed after amputation of the tail in Sternarchus . In the caudal‐most vertebral segment of normal spinal cord, ependymal cells are radially enlarged and are more numerous than in more rostral adult cord. Large processes of the ependymal cells extend into the central canal, which also contains a prominent Reissner's fiber. Invaginations of the outer surface of the spinal cord, with the associated basal lamina, are common. Lateral to the immediate ependymal layer, extracellular spaces contain longitudinally oriented neurites. Cell bodies and cell processes filled with dense‐cored vesicles occur throughout the caudal‐most segment of spinal cord, and are especially concentrated in the ventral half, interspersed with numerous capillaries. In all these respects the caudal‐most segments of normal adult spinal cord in Sternarchus closely resemble regenerating spinal cord of Sternarchus . In both regions, at least some of the ependymal cells retain the ability to divide and generate new neurons and glial cells.
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The thickness of the ventral midline of the spinal cord was determined in 9 human embryos aged five weeks (developmental stages 13-15). This part of the spinal cord consists of floor plate, mantle and marginal layers. The floor plate ependymal cells form pseudostratified columnar epithelium. The thickness of the investigated structure varied from 20 to 50 micrometers at different levels of the spinal cord.
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Secondary neurulation is a common feature of vertebrate development, which in non-mammalian and non-anuran vertebrates, results in the formation of a caudal spinal cord. The present study was undertaken to describe the terminal end of the caudal spinal cord in a crocodylian, a group chosen for their unique status of a living-tailed archosaur. The caudal spinal cord of Alligator mississippiensis terminates near the intervertebral joint between the fourth and fifth terminal vertebrae. Prior to this termination, the dorsal root ganglia get proportionately larger, then stop before the termination of the spinal cord; and the gray matter of the spinal cord is lost producing an unusual morphology in which an ependymal-lined central canal is surrounded by only white matter which is not divided into a cauda equina. The inner layer of the meninges (the pia-arachnoid) courses over the distal end of the spinal cord and forms a ventral attachment, reminiscent of a very short Filum terminale; there is no caudal cistern. The dura extends beyond the termination of the spinal cord, continuing for at least the length of the fourth terminal vertebra, forming a structure herein termed the distal meningeal sheath. During its course, the distal meningeal sheath surrounds a mass of mesothelial cells, then terminates as an attachment on the dorsal surface of the vertebra.
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Part of the development and maturation of the central nervous system (CNS) occurs through interactions with the environment. Through physical activities and interactions with the world, an animal receives considerable sensory information from various sources. These sources can be internally (proprioceptive) or externally (such as touch and pressure) generated senses. Ample evidence exists to demonstrate that the sensory information originating from large diameter afferents (Ia fibers) have an important role in inducing essential functional and morphological changes for the maturation of both the brain and the spinal cord. The Ia fibers transmit sensory information generated by muscle activity and movement. Such use or activity-dependent plastic changes occur throughout life and are one reason for the ability to acquire new skills and learn new movements. However, the extent and particularly the mechanisms of activity-dependent changes are markedly different between a developing nervous system and a mature nervous system. Understanding these mechanisms is an important step to develop strategies for regaining motor function after different injuries to the CNS. Plastic changes induced by activity occur both in the brain and spinal cord. This paper reviews the activity-dependent changes in the spinal cord neural circuits during both the developmental stages of the CNS and in adulthood.
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For many decades, the inability of lesioned central neurons to regrow was accepted almost as a "law of nature", and on the clinical level, spinal cord and brain lesions were seen as being irreversible. Today we are starting to understand the mechanisms of neuronal regeneration in the central nervous system and its presence in the periphery. There is now a rapid expansion in this field of neuroscience. Developmental neurobiology has produced tools and concepts that start to show their impact on regeneration research. This is particularly true for the availability of antibodies and factors and for the rapidly growing cellular and molecular understanding of crucial aspects of neurite growth, guidance, target finding, and synapse stabilization. New cell biological concepts on the mechanisms of neuron survival and death and on the interaction of inflammatory cells with the central nervous system also find their way into the field of spinal cord and brain lesions and have, indeed, led already to new therapeutic approaches. This review briefly summarizes the current knowledge on the mechanisms involved in degeneration and tissue loss and in axonal regeneration subsequent to spinal cord lesions, particularly in mammals and humans.
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Abstract Urodele amphibians, newts and salamanders, can regenerate lesioned spinal cord at any stage of the life cycle and are the only tetrapod vertebrates that regenerate spinal cord completely as adults. The ependymal cells play a key role in this process in both gap replacement and caudal regeneration. The ependymal response helps to produce a different response to neural injury compared with mammalian neural injury. The regenerating urodele cord produces new neurons as well as supporting axonal regrowth. It is not yet clear to what extent urodele spinal cord regeneration recapitulates embryonic anteroposterior and dorsoventral patterning gene expression to achieve functional reconstruction. The source of axial patterning signals in regeneration would be substantially different from those in developing tissue, perhaps with signals propagated from the stump tissue. Examination of the effects of fibroblast growth factor and epidermal growth factor on ependymal cells in vivo and in vitro suggest a connection with neural stem cell behavior as described in developing and mature mammalian central nervous system. This review coordinates the urodele regeneration literature with axial patterning, stem cell, and neural injury literature from other systems to describe our current understanding and assess the gaps in our knowledge about urodele spinal cord regeneration. Developmental Dynamics 226:295–307, 2003. © 2003 Wiley‐Liss, Inc.
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The ability of peripheral nervous system (PNS) but not central nervous system (CNS) neurons to regenerate their axons is a striking peculiarity of higher vertebrates. Much research has focused on the inhibitory signals produced by CNS glia that thwart regenerating axons. Less attention has been paid to the injury-induced loss of trophic stimuli needed to promote the survival and regeneration of axotomized neurons. Could differences in the mechanisms that control CNS and PNS neuronal survival and growth also contribute to the disparity in regenerative capacity? Here we review recent studies concerning the nature of the signals necessary to promote neuronal survival and growth, with an emphasis on their significance to regeneration after CNS injury.
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