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PDF Full Text Edorium J Neurol 2016;3:17–25. Cuoco et al. 17 www.edoriumjournalofneurology.com REVIEW ARTICLE PEER REVIEWED | OPEN ACCESS Neuroembryology and congenital disorders of the nervous system: A primer for medical students Joshua A. Cuoco, Christopher L. Hoehmann, Dane M. Edwards ABSTRACT How to cite this article Embryonic development of the nervous system Cuoco JA, Hoehmann CL, Edwards DM. is a challenging topic within the medical school Neuroembryology and congenital disorders of the curriculum. Nonetheless, it is essential for future nervous system: A primer for medical students. physicians to have a basic knowledge of this Edorium J Neurol 2016;3:17–25. subject as it provides context for the congenital disorders affecting the nervous system. Supplemented with numerous illustrations, this Article ID: 100009N06JC2016 article provides medical students a basic context of homeostatic neuroembryology of the brain, spinal cord and cranial nerves. Furthermore, ********* we provide a brief overview of the common congenital disorders that can occur as a result doi:10.5348/N06-2016-9-RA-3 of disruptions in these normal developmental processes. Keywords: Medical Education, Neurodevelop- ment, Neuroembryology, Neurophobia INTRODUCTION Neuroembryology is a topic of the neurosciences that medical students have historically found one of the most challenging to comprehend [1–12]. Nevertheless, the subject is essential for future physicians to learn as it Joshua A. Cuoco1, Christopher L. Hoehmann2, Dane M. provides a framework into the understanding of human Edwards3 development [12]. Furthermore, it provides critical Affiliations: 1MS,Third year medical student, Department context to better appreciate congenital disorders of the of Biomedical Sciences, New York Institute of Technology brain and spinal cord. College of Osteopathic Medicine, Old Westbury, New York A recent review of brainstem vascular syndromes 11568 USA; 2BS,Third year medical student, Department of demonstrated a simplified yet effective learning Anatomy, New York Institute of Technology College of Oste- methodology for medical and health professional students 3 opathic Medicine, Old Westbury, New York 11568 USA; BS, [13]. In this brief review, we provide medical students an Third year medical student, Faculty of Medicine, University overview of the normal embryology of the nervous system of British Columbia, Vancouver, Canada. as well as describe the common disorders that can occur Corresponding Author: Joshua Aaron Cuoco MS, 6 Manor when this process fails. Place, Glen Clove, New York 11542; E-mail: jcuoco@nyit. edu GENERAL DEVELOPMENT OF THE Received: 06 September 2016 CENTRAL NERVOUS SYSTEM Accepted: 08 November 2016 Published: 10 December 2016 The nervous system consists of three components. The brain and spinal cord comprise the central nervous system (CNS). The peripheral nervous system (PNS) consists of Edorium Journal of Neurology, Vol. 3; 2016. Edorium J Neurol 2016;3:17–25. Cuoco et al. 18 www.edoriumjournalofneurology.com nerves outside of the CNS and includes the cranial nerves as well as spinal nerves. The autonomic nervous system is derived from both the CNS and PNS and consists of neurons innervating glands, cardiomyocytes and smooth musculature, among other structures. At approximately day-16 of embryological development, the notochord arises from a primary germ cell layer known as mesoderm [14–16]. The notochord induces the overlying ectoderm, another primary germ cell layer, to begin to form the neural plate [16]. Beginning in the third week, this neural plate represents the earliest form of the CNS, which will eventually separate into the brain and spinal cord (Figure 1A) [15, 16]. The edges of the neural plate rise forming two parallel folds appropriately termed the neural folds (Figure 1B) [15, 16]. Continued elevation of the neural folds will result Figure 1: Development of the neural tube from ectoderm (A) in fusion and formation of the neural tube (Figure 1C) During early development, the trilaminar structure of the [15, 16]. Fusion of the neural folds occurs first in its embryo undergoes a metamorphosis as it begins to differentiate middle and then proceeds to fuse in both the cranial into its functional components. The ectoderm undergoes a series of steps, becoming the neural plate, neural folds and and caudal directions giving rise to an opening at each neural tube before finally maturing into the brain and spinal end termed the cranial and caudal neuropores [14]. cord. The notochord and somites are derived from mesoderm Complete closure of the cranial neuropore occurs on while the neural plate is derived from ectoderm. In fact, it is the day-25 followed by closure of the caudal neuropore on notochord itself that induces a phenotypic change of the section day-28 [15]. During the fourth week, the cranial end of of the overlying ectoderm to become the neural plate. (B) Proper the neural tube exhibits three expansions known as the development of the neural plate is of critical importance as it primary brain vesicles (Figure 2A) [16]. The primary gives rise to the neural tube and ultimately the brain and spinal vesicles include the prosencephalon, mesencephalon, cord. The edges of the neural plate elevate into two neural folds. and rhombencephalon that will eventually form the (C) As these folds continue to elevate them eventually meet to form the neural tube. Complete closure of the neural tube forebrain, midbrain and hindbrain, respectively [14–16]. ensues. Following the development of these primary vesicles, the secondary vesicles arise during the fifth week; that is, Table 1: Alar and basal plate distribution in the brainstem. the three primary brain vesicles divide further into five Brainstem Nuclei Cranial Innervation Type secondary brain vesicles (Figure 2B) [16]. Derivative Nerve Metencephalon GSE III, IV, VI Striated muscle (somatic) NEUROEMBRYOLOGY OF THE BRAIN SVE V, VII Striated muscle of the pharynx The rhombencephalon or hindbrain consists of the SA VII, IX Taste metencephalon and myelencephalon. The metencephalon SA VII Balance and hearing forms the pons and cerebellum and contains the upper portion of the fourth ventricle. The myelencephalon GSA V, VII, IX Head and neck sensation forms the medulla oblongata and contains the lower portion of the fourth ventricle. The alar plates of the Myelencephalon GSE XII Striated muscle (somatic) metencephalon and myelencephalon, which exist on the dorsal aspect of the neural tube, each consist of three SVE IX, X Striated muscle of groups of sensory nuclei. These groups of sensory nuclei the pharynx include general visceral afferents, special afferents, and GVE IX, X Smooth muscle general somatic afferents (Table 1) [15]. The basal plates GVA X Visceral structures of the metencephalon and myelencephalon, which form SA VII, IX Taste on the ventral aspect of the neural tube, each consist GSA V, VII, IX Head and neck of three groups of motor nuclei. These groups of motor sensation nuclei include general visceral efferents, special efferents, Mesencephalon GVE III Parasympathetics and general somatic efferents [15]. of the eye Unlike the rhombencephalon and prosencephalon, Abbreviations: GSE General Somatic Efferent, SVE Special the mesencephalon is a primary brain vesicle that Visceral Efferent, SA Special Afferent, GSA General Somatic does not divide further; rather, it forms the midbrain Afferent, GVE General Visceral Efferent, GVA General Visceral containing the aqueduct of Sylvius. The basal plates of Afferent. Edorium Journal of Neurology, Vol. 3; 2016. Edorium J Neurol 2016;3:17–25. Cuoco et al. 19 www.edoriumjournalofneurology.com Figure 2: Development of the brain vesicles (A) - During the fourth week of development, the embryologic brain exists as a series of vesicular expansions at the cranial end of the neural tube. (B) The three primary brain vesicles further develop into five secondary brain vesicles, which will eventually develop into mature brain structures. Each vesicle will mature into a specific section of cerebral tissue and the corresponding cerebrospinal fluid containing ventricle. The first primary brain vesicle, known as the prosencephalon or primitive forebrain, will evolve Figure 3: Chiari malformations and associated syringomyelia. into two secondary brain vesicles known as the telencephalon Arnold-Chiari malformations are characterized by a downward and diencephalon. The telencephalon gives rise to the cerebral displacement and herniation of the cerebellar tonsils through hemispheres and lateral ventricles, with the diencephalon the foramen magnum of the cranium. It can clinically present becoming the thalamus, hypothalamus and third ventricle. The with a range of symptoms including neck pain, ataxia, sleep mesencephalon is the second primary brain vesicle and does apnea and respiratory infections. Chiari malformations can not separate but develops into the midbrain with the cerebral be highly correlated with other pathologies. Type 1 Chiari aqueduct. The rhombencephalon or hindbrain is the primary malformations are associated with syringomyelia, which is an brain vesicle that will form the secondary brain vesicles known accumulation of fluid in the central canal or spinal cord tissue. as the metencephalon and myelencephalon. The metencephalon Syringomyelia can be either acquired or congenital and presents continues development into the pons, cerebellum and upper with progressive neurologic symptoms such as weakness, pain portion of the fourth ventricle and the myelencephalon
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