University of Northern Philippines Embryology PDF

- Extends from the beginning of 9 th week (3rd month) until birth...

- Extends from the beginning of 9 th week (3rd month) until birth OUTLINE - Further sub-divided into two parts: a. Pre-embryonic period I. INTRODUCTION b. Embryonic period proper A. Embryology B. Neuroembryology Embryological Divisions of Prenatal period: C. Clinical Correlations 1. Pre-embryonic period II. DEVELOPMENT OF SPINAL CORD - Extends from conception (fertilization) to the end of 2nd A. Histogenesis of Neural Tube week of intrauterine life (IUL) B. Formation of Vertical Slit - Morphogenetic events: C. Development of Functional Columns a. Fertilization D. Positional Changes of the SC b. Transportation of zygote through the uterine III. DEVELOPMENT OF THE BRAIN tube A. End of 4th Week c. Mitotic divisions/cleavage B. 5th Week d. Implantation C. Three Flexures of the Brain e. Formation of primordial embryonic tissues IV. DEVELOPMENT OF THE VENTRICULAR SYSTEM 2. Embryonic period A. Cavities of the Brain Vesicle - Extends from beginning of the 3 rd week to the end of the B. Four Ventricles of the Brain 8 th week of IUL C. Clinical Correlations - Morphogenetic events : V. PRIMARY BRAIN VESICLES a. Differentiation of germ layers become into A. Prosencephalon specific body organs B. Mesencephalon b. Formation of placenta, umbilical cord & C. Rhombencephalon extraembryonic membranes D. Clinical Correlations 3. Fetal period - Extends from beginning of the 9 th week to birth (+) tremendous growth & specialization of body structures I. INTRODUCTION those organs that are formed now tend to function accordingly EMBRYOLOGY Postnatal Development - Development of an individual continues even after birth Is based actually on neuroanatomy, it is neuroanatomy oriented and up to 20’s or age of 25 years it is the first system to start and develop before the development of the embryo. NEUROEMBRYOLOGY Science that deals with the development & growth of an individual within the female organ the uterus from fertilization occur of an Nervous System -derived from the ectoderm except its blood vessels ovum to birth & some neuroglial elements Benefits: Neural Ectoderm a. Understand the rationale of the structure and function of each - Specific cell population of early ectoderm body system - Gives rise to entire NS & special sense organs b. Understand the factors causing many congenital anomalies - Differentiates into three structures: pertaining to neuroembryology 1. Neural tube Prenatal development - Gives rise to the CNS. The central nervous system (CNS) ○ Period of development from fertilization to birth appears at the beginning of the third week as a slipper ○ Before birth the individual is developing; begins with a shaped plate of thickened ectoderm, the neural plate, in single cell called zygote (fertilized ovum) & culminates or the mid-dorsal region in front of the primitive node. Its end after 9 months (38 weeks or 266 days) with a more lateral edges soon elevate to form the neural folds. With complex organism call newborn (made up of billion cells) further development, the neural folds continue to elevate, approach each other in the midline, and finally fuse, Morphogenesis forming the neural tube. ○ Process in prenatal development which includes: cell division 2. Neural crest cells transformation or specialization migration - Form the PNS and to several non-neural cell types, apoptosis/program cell death including smooth muscle cells of the cardiovascular ○ during this process, the genetic or environmental factors system, pigment cells in the skin, and craniofacial bones, may affect the normal development of baby & cause cartilage, and connective tissue. congenital anomalies 3. Ectodermal placodes Clinical Divisions of Prenatal period: - Contribute to CN sensory ganglia, hypophysis & inner ear. 1. Embryonic period These structures form at stereotypical positions reflecting - Extends from fertilization to the end of 8 th the importance of localized cues in their development and week of IUL subsequently undergo differential morphogenesis to - Developing organism is called an embryo generate key components of the cranial sensory 2. Fetal period apparatus. The ectodermal placodes comprise the adenohypophyseal, the olfactory, the lens, the ophthalmic Page 1 of 10 [EMBRYOLOGY] 1.15 EMBRYONIC DEVELOPMENT OF THE CENTRAL NERVOUS SYSTEM – Dr. Steve Arellano and maxillomandibular trigeminal, the otic, lateral line located at the two lateral edges of the central nervous system in contact with and the epibranchial placodes the ectodermal epidermis (a). During neurulation, these border territories approach one another along the dorsal line (b) when the neural plate closes to form the neural tube (c). After neural tube closure, the NCC leave the central nervous system (i.e., delamination) via an epithelial to mesenchymal transition (EMT) that is accompanied by an alteration of cell contact with neural plate cells and enhanced migration capability (d). Figure 1. Organogenesis Figure 4. Genesis of the Nervous System and Special Sense Organs 20th day of embryonic life - Fusion of neural folds begins in the midline or middle part (4th somite region) - Simultaneously proceeds in the cephalic & caudal directions going to the head and tail region - Fusion at the cranial & caudal ends of neural tube is somewhat delayed, forming small openings called anterior & posterior neuropores - Neural tube & overlying amniotic cavity remain temporarily in open Figure 2. Prenatal Development communication with each other through these pores: a. Anterior Neuropore Formation of the Neural Tube - Closes (middle of 4th week) at 18-20 somite 16th day of embryonic life stage or region. - Ectoderm thickness in midline forming neural plate - Closure of the cranial neuropore proceeds - Somatic mesoderm develops on either side of the notochord cranially from the initial closure site in the - Margins of neural plate are elevated as neural folds cervical region and from a site in the forebrain - Center of the plate sinks creating neural groove that forms later. This latter site proceeds - Neural folds gradually move together toward the midline & fuse to cranially, to close the rostral most region of the form a cylindrical neural tube that loses its connection with the neural tube, and caudally to meet advancing surface ectoderm closure from the cervical site - Some of the neural crest cells will migrate somewhere on the b. Posterior Neuropore periphery of the neural tube - Closes (end of 4th week) at 25 somite stage - Process of neural tube formation is called Neurulation - By the time the neural tube is completely closed, it is divided into an enlarged cranial part & an enlarged cranial part & an elongated caudal part, which gives rise to the brain & spinal cord Figure 5. Neural plate and neural tube formation. This diagram shows the neural plate and neural tube of human embryos at 19 days pc (a), 20 days pc (b), and 22 days pc (c) showing folding of the neural groove to produce the neural tube. The first point of fusion between the neural folds is at the hindbrain/spinal cord Figure 3. Formation of the Neural tube. Anatomical embryological development junction. of neural crest (in green) from neural ectoderm plate (in blue) at the contact of (*Note the rapid progression of somite formation, the appearance of the neural epidermal ectoderm (gray) up to detachment from neural tube (stage a-d). fold, and the deepening of the neural groove from day 19 to day 20 in Neural crest cells are generated in the ectodermal germ layer during development.) gastrulation and initially reside in two neural plate border territories, which are Page 2 of 10 [EMBRYOLOGY] 1.15 EMBRYONIC DEVELOPMENT OF THE CENTRAL NERVOUS SYSTEM – Dr. Steve Arellano CLINICAL CORRELATION Anencephaly (Craniorachischisis) ○ Occurs in 1/5000 births (more common in females than in males) ○ Failure of the cephalic part of the neural tube to close ○ Due to defective development of the neural tube ○ Associated defective development of the vault of the skull ○ Could be prevented by having women take 400 microgram of folic acid per day, before and during early pregnancy Characteristics features: Figure 6. Changes in the morphology of the embryo in the embryonic period a. Vault of the skull is absent b. Brain is represented by a mass of degenerated tissue exposed to the Formation of Neural Crest Cells surface - When neural folds come together & fuse, cells at the tips of neural c. Cord is open in the cervical region folds break away from the neuroectoderm to form the Neural Crest d. Child: bulging eyes, (chin is continuous with chest due to) absence of cells neck - Surface ectoderm of one side becomes continuous with surface ectoderm of the opposite side over the neural tube. - Neural crest cells originate in the ectoderm at the margins of the neural tube and, after a phase of epithelial-mesenchymal transition and extensive migration, settle down in different parts of the body to contribute to the formation of a plethora of different tissues and organs. Neural Crest cells differentiate to form: a. Cells of DRG (Dorsal Root Ganglion) b. Sensory ganglia of CN (Cranial Nerve) c. Autonomic ganglia d. Adrenal medulla Figure 8. Infant with Anencephaly e. Chromaffin tissue f. Melanocytes Rachischisis g. Schwann cells ○ Severe form of spina bifida ○ Incomplete closure of caudal neuropore Formation of Ectodermal Placodes ○ (+) defective development of the neural tube - Arise from common pan placodal ectoderm (PPE) a ○ Defective development of associated vertebral arches horseshoe-shaped region of ectoderm surrounding the anterior malformation neural plate and neural crest. Each placode then differentiates to eventually have different developmental fates. These sensory Characteristic features: placodes will later contribute key components of each of our special a. Failure of dorsal portions of the vertebral arches to fuse with each senses (vision, hearing and smell). other - Prior to the neural tube closure, the neural fold contains two types b. Neural tissue is widely exposed to the surface of cell populations of Neural Folds: c. Neural tissue shows considerable overgrowth which becomes 1. Neural Crest cells necrotic shortly before or after birth 2. Neuroepithelial cells During Neurulation - Neural crest cells are detached - Neuroepithelial cells get incorporated into the surface ectoderm - Areas of neuroepithelium within the surface ectoderm are termed Ectodermal Placodes Figure 7. Neural Crest Cells Differentiation Figure 9. Congenital anomalies of the NS: Neural tube defects Page 3 of 10 [EMBRYOLOGY] 1.15 EMBRYONIC DEVELOPMENT OF THE CENTRAL NERVOUS SYSTEM – Dr. Steve Arellano II. DEVELOPMENT OF SPINAL CORD Spinal cord develops from the caudal elongated part of the neural tube. During the neural groove stage and immediately after closure of the tube, they divide rapidly, producing more and more neuroepithelial cells. Collectively, they constitute the neuroepithelial layer or neuroepithelium. A. HISTOGENESIS OF THE NEURAL TUBE Neural tube increases in thickness due to repeated mitosis the division of the cells of its epithelial lining 5th week of embryonic life - Transverse section of recently closed neural tube - Neural Tube layer zones (accdg. to Classical Theory): Figure 11. A transverse section through the developing spinal cord of a human a. Matrix (ependymal) zone embryo 4 weeks old. (A) A schematic drawing of the real slide (B). Note the roof b. Mantle zone or alar plate and the floor or basal plate. c. Marginal zone MANTLE ZONE Surrounds neuroepithelial layer Formed by neuroblasts Later forms the gray matter of the SC Once the neural tube closes, neuroepithelial cells begin to give rise to another cell type characterized by a large round nucleus with pale nucleoplasm and a dark-staining nucleolus. These are the primitive nerve cells, or neuroblasts. They form the mantle layer, a zone around the neuroepithelial layer. The mantle layer later forms the gray matter of the spinal cord. MARGINAL ZONE Outermost layer of the SC Contains fibers emerging from Neuroblasts in the Mantle zone Myelination of the Nerve Fiber gives Marginal zone a WHITE appearance (from the white matter of the SC) B. FORMATION OF THE VERTICAL SLIT NOTE: On cross section, the cavity of the neural tube appears like a vertical slit. Gives dorsal and ventral walls of the Neural Tube a thin appearance (Roof and Floor Plates) Figure 10. Neural tube layer zone Lateral walls get thickened and demarcated into dorsal and ventral regions by SULCUS LIMITANS MATRIX (EPENDYMAL) ZONE ○ Sulcus Limitans Lines the enclosed cavity (neurocele) inner longitudinal sulcus Cells undergoing mitosis produce: a. Neuroblasts → future neurons b. Spongioblasts → future neuroglial cell Neuroblasts migrate to the mantle zone (future SC gray matter) & their axons enter the external marginal zone (future SC white matter) Some central processes of DRG ascend in the marginal zone while other synapse with neurons in the mantle zone Once histogenesis is complete, remaining matrix cells differentiate into ependymal cells lining the central canal of your spinal cord or the counterpart in the ventricles of the brain according to the classical theory According to Current Theory: - Closed neural tube consists of one cell type: Pluripotent Figure 12. Roof and floor plate. Neuroepithelial cells - Cells form a pseudostratified neuroepithelium C. DEVELOPMENT OF THE FUNCTIONAL COLUMNS - Zonal appearance reflects different phases of their proliferative cycle, the sequence being termed ALAR LAMINA (ALAR PLATES) Interkinetic Migration Cells of dorsal region Afferent or Sensory Alar Plates are dorsal thickening, which forms the sensory areas BASAL LAMINA (BASAL PLATES) Efferent or Motor ○ Spinal Nerves Page 4 of 10 [EMBRYOLOGY] 1.15 EMBRYONIC DEVELOPMENT OF THE CENTRAL NERVOUS SYSTEM – Dr. Steve Arellano Formed from Axons of basal lamina that leave ○ SC extends along the entire length of vertebral canal the cord as Ventral Roots joined with ○ Spinal nerves exit intervertebral foramina at the level of Peripheral Process of DRG their origin Basal Plates are ventricular thickening, which contains ventral motor ○ intervertebral foramina DOES NOT remain at the level of horn cells, from motor areas of the spinal cord spinal nerves ○ Spinal nerves are forced to go down and exit in an oblique NOTE: Cells of both Alar and Basal Laminae are arranged into LONGITUDINAL direction COLUMNS. A longitudinal groove, the sulcus limitans, marks the boundary In the third month of development, the spinal cord extends the between the alar and basal Plates. entire length of the embryo and the spinal nerves pass through the intervertebral foramina at their level of origin. COLUMNS OF THE LAMINA 24th week 1. Two (2) afferent columns of alar/dorsal lamina ○ During this time, vertebral column grows more rapidly - Receive axons from DRG than the spinal cord a. General Somatic Afferent column (GSA) ○ LOWER part of SC ends at S1 Vertebrae Found throughout the SC At Birth Receives impulses from: ○ LOWER part of SC ends at the level of L3 Vertebra ○ Superficial (Cutaneous) Receptors In Adults ○ Deep (Proprioceptive) Receptors ○ LOWER part of SC ends at eh LOWER BORDER of L1 b. General Visceral Afferent column (GVA) Vertebrae Found only in: ○ Thoracolumbar Region ○ Sacral Region Receives impulses from: ○ Viscera ○ Blood Vessels 2. Two (2) efferent columns of basal lamina - Gives rise to Motor Fibers a. General Visceral Efferent column (GSE) Found throughout the SC Provides fibers that innervate: ○ Skeletal Muscles Makes the skeletal muscle to contract b. General Somatic Efferent column (GVE) Found only in: Figure 14. Development of the spinal cord on the 8th and the 24th week, at ○ Thoracolumbar Region birth, and in adults. ○ Sacral Region Provides preganglionic fibers to: TERMINATION OF SPINAL CORD IN RELATION TO VERTEBRAL CANAL AT ○ Viscera VARIOUS STAGE OF DEVELOPMENT ○ Blood Vessels Nerve Root ○ Glands ○ From: LUMBAR, SACRAL and COCCYGEAL ○ Descends below the conus medullaris Conus Medullaris Lower end of the SC Surrounded by Filum Terminalis ○ Filum Terminalis Thin thread-like prolongation of Pia mater from the tip of conus medullaris Marks the tract of regression of the spinal cord as well as provides support for the cord (the part covered by dura and extending from S2 to the coccyx is also called coccygeal ligament) Forms cauda equina (horsetail-like structure in the spinal canal of subarachnoid space) Cauda equina is made up of dorsal and ventral roots of spinal nerves below the end of the cord at L2-L3. Figure 13. Cross section of a developing spinal cord showing four (4) longitudinal cell columns. Everything that is in dorsal region will be afferent/sensory columns Everything that is in ventral region will be efferent/motor columns D. POSITIONAL CHANGES OF THE SPINAL CORD 8th week ○ This time, the vertebral canal is filled with spinal cord ○ Lengths of SC and vertebral column are EQUAL Page 5 of 10 [EMBRYOLOGY] 1.15 EMBRYONIC DEVELOPMENT OF THE CENTRAL NERVOUS SYSTEM – Dr. Steve Arellano Figure 16. Primary and Secondary Brain Vesicles. The prosencephalon consists o f the telencephalon, which forms the cerebral hemispheres, and the diencephalon, which forms the optic cup and stalk, pituitary, thalamus, hypothalamus, and epiphysis. Differentiation of Prosencephalon into: Figure 15. Illustrations showing the conus medullaris, filum terminale, and a. Telencephalon cauda equina. - Rostral part - Develops Lateral Diverticula by EVAGINATIONS III. DEVELOPMENT OF THE BRAIN - Enlarges, overgrows and covers the caudal diencephalon to form CEREBRAL HEMISPHERE b. Diencephalon (Interbrain) Brain - Caudal part - Develops from an enlarged cranial part of the neural tube - Becomes hidden in the lower parts of the cerebral hemisphere - Sometimes divided into the: - Forms: Brainstem: (direct continuation of spinal cord; has distinct Thalamus basal (motor) & alar (sensory) plates) Divided by a groove, hypothalamic sulcus ○ Myelencephalon Hypothalamus ○ Pons from Metencephalon Epithalamus ○ Mesencephalon c. Mesencephalon - Gives rise to the MIDBRAIN Higher Centers: (reflect almost none of this basic pattern, - Not much changes in Early development and, instead, show accentuation of the alar plates and - Its cavity gets progressively narrowed to form regression of the basal plates) CEREBRAL AQUEDUCT/SYLVIAN AQUEDUCT ○ Cerebellum d. Rhombencephalon ○ Cerebral Hemispheres - Differentiates into: Metencephalon End of 4th Week - Rostral part Enlarged cephalic part shows three (3) distinct dilations called: - Develops into PONS (derived from basal plates; pathway Primary Brain Vesicles for nerve fibers between SC & cerebral & cerebellar Cavities form ventricular system of adult brain cortices) and CEREBELLUM (derived from the alar plates; coordination center for posture & movement) Primary Brain Vesicles: Myelencephalon a. Prosencephalon (Forebrain) - Caudal part b. Mesencephalon (Midbrain) - Gives rise to MEDULLA OBLONGATA (a transitional zone - 2 groups of motor nuclei in its basal plate: between the brain and spinal cord; differs from the SC in 1. medial somatic efferent group that its lateral walls are everted). 2. small general visceral efferent group - Initially its alar plates appear as 2 longitudinal elevations separated by shallow midline depression. With further development, a IV. DEVELOPMENT OF THE VENTRICULAR SYSTEM transverse groove divides each elevation into: 1. anterior (superior) colliculus C. CLINICAL CORRELATIONS 2. posterior (inferior) colliculus Hydrocephalus c. Rhombencephalon (Hindbrain) ○ Clinical condition characterized by dilatation of ventricles - consist of the myelencephalon, the most caudal of the brain vesicles, due to excess accumulation of CSF within them. and the metencephalon, which extends from the pontine flexure to ○ In most cases, hydrocephalus in the newborn is due to an the rhombencephalon isthmus. obstruction of the aqueduct of Sylvius (aqueductal stenosis). This prevents the CSF of the lateral and third 5th Week ventricles from passing into the fourth ventricle and from Both the prosencephalon (forebrain) and rhombencephalon there into the subarachnoid space, where it would be (hindbrain) subdivide into two (2) vesicles producing: FIVE (5) resorbed. Secondary Brain Vesicles ○ Occurs either due to overproduction of CSF or obstruction to its circulation, or there is failure of reabsorption of the CSF ○ features: fluid accumulates in the lateral ventricles and presses on the brain and bones of the skull. Because the cranial sutures have not yet fused, spaces between them widen as the head expands. In extreme cases, brain tissue and bones become thin and the head may be very large. Page 6 of 10 [EMBRYOLOGY] 1.15 EMBRYONIC DEVELOPMENT OF THE CENTRAL NERVOUS SYSTEM – Dr. Steve Arellano ○ “setting-sun” appearance of the eyes ○ disproportionately large sized head V. PRIMARY BRAIN VESICLES 1. PROSENCEPHALON (FOREBRAIN) - The prosencephalon consists of the telencephalon, which forms the cerebral hemispheres, and the diencephalon, which forms the optic cup and stalk, pituitary, thalamus, hypothalamus, and epiphysis. A. DIENCEPHALON develops from the median portion of the prosencephalon has its cavity called 3RD VENTRICLE consists of two thick lateral walls, a thin roof and floor plates ROOF PLATE AND EPIPHYSIS The roof plate of the diencephalon consists of a single layer of ependymal cells covered by vascular mesenchyme which these layers give rise to the choroid plexus of the third ventricle. The most caudal part of the roof plate develops into the pineal body, or epiphysis. In the adult, calcium is frequently deposited in the epiphysis and Figure 17. Child with severe hydrocephalus. Because the cranial sutures had not then serves as a landmark on radiographs of the skull. closed, pressure from the accumulated CSF enlarged the head, thinning the bones of the skull and cerebral cortex. ALAR PLATE, THALAMUS, AND HYPOTHALAMUS The alar plates form the lateral walls of the diencephalon. Dandy-Walker Syndrome HYPOTHALAMIC SULCUS ○ Due to atresia and blockage of the apertures (foramina of sulcus present in each lateral wall of the diencephalon Luschka and Magendie) in the roof of 4th ventricle appears to be rostral continuation of sulcus limitans syndrome consists of the following divides lateral wall into dorsal (THALAMUS) and ventral ○ dilatation of 4th ventricle (black area in the MRI of the regions (HYPOTHALAMUS) patient) (See Figure 14) The hypothalamus, forming the lower portion of the alar ○ agenesis of cerebellar vermis or the mid area of your plate, differentiates into a number of nuclear areas that cerebellum regulate the visceral functions, including sleep, digestion, ○ occipital meningocele body temperature, and emotional behavior. ○ agenesis of the splenium of corpus callosum instead of a hardened body like your corpus callosum, it is filled with HYPOPHYSIS OR PITUITARY GLAND CSF The hypophysis, or pituitary gland, develops from two completely different parts: ○ an ectodermal outpocketing of the stomodeum (primitive oral cavity) immediately in front of the oropharyngeal membrane, known as Rathke pouch, and ○ a downward extension of the diencephalon, the infundibulum NEUROHYPOPHYSIS downgrowth from the floor of the anterior hypothalamus joins an upgrowth from the stomodeum (ADENOHYPOPHYSIS) to form HYPOPHYSIS CEREBRI (PITUITARY GLAND) EPITHALAMUS comprising pineal gland and habenular nuclei Figure 18. Normal MRI compared to an MRI of a patient with Dandy-Walker develops posteriorly in the roof plate Malformation showing dilatation of 4th ventricle. PINEAL GLAND grows posteriorly from the roof plate at its junction with the midbrain lies on the dorsal surface of midbrain between the two superior colliculi B. TELENCEPHALON consists of a median part and two lateral diverticular or cerebral vesicles MEDIAN PART ○ forms a small anterior part of the 3rd ventricle and lamina terminalis Figure 19. Dandy-Walker Syndrome The Telencephalon is over the diencephalon. The Telencephalon is your cerebral hemispheres. LAMINA TERMINALIS represents the cephalic end of the primitive neural tube Page 7 of 10 [EMBRYOLOGY] 1.15 EMBRYONIC DEVELOPMENT OF THE CENTRAL NERVOUS SYSTEM – Dr. Steve Arellano corresponds with the closure of anterior neuropore The birth prevalence of NTDs, including spina bifida and anencephaly, varies among different populations and may be as high LATERAL DIVERTICULA as 1/200 births in some areas, such as northern China. represent rudiments of cerebral hemispheres The birth prevalence of NTDs in the United States has decreased by cavities of hemispheres (lateral ventricles) communicate approximately 25% to 1/1,500 births since fortification of flour with with the cavity of diencephalon (3rd ventricle) through folic acid was instituted in 1998. the interventricular foramen of Monro developing cerebral hemisphere enlarges forward, SPINA BIFIDA upward and backward in that order so that it will cover A general term for NTDs affecting the spinal region, consists of the diencephalon or the thalamus splitting of the vertebral arches and may or may not involve as the vesicle grows backward it overlaps successively underlying neural tissue. diencephalon, mesencephalon and cerebellar rudiments lowest parts of medial walls of hemispheres remain very 3 Types of Spina bifida: thin due to disproportionate growth of various parts of 1) Spina bifida occulta the hemispheres ○ A defect in the vertebral arches that is covered by skin choroid plexus of 3rd ventricle protrudes laterally through and normally does not involve underlying neural tíssue. this thin wall into the lateral ventricle along a line ○ The defect occurs in the sacral región [S1- S2) and is (CHOROID FISSURE) sometimes marked by a patch of hair overlying the above the choroid fissure, the medial wall of the affected region. hemisphere thickens to from HIPPOCAMPUS that will be ○ It is due to a lack of fusion of the vertebral arches, affects part of your limbic system about 10% of otherwise normal people. The malformatíon subsequent massive expansion of the cerebral is not usually detected at birth and does not cause hemispheres (neocortex), the hippocampus is displaced disability. postero-inferiorly into the lateral ventricle ○ Often, the defect is first noticed as an incidental finding FORNIX is drawn out as an efferent tract on its medial diagnosed when an x-ray of the back is performed. aspect choroid fissure also becomes curved, interposed between 2) Meningocele the fornix and diencephalon ○ Fluid-filled meninges in a sac protrude from the spinal cord CORPUS STRIATUM 3) Myelomeningocele Part of the basal ganglia ○ Neural tissue is included in the sac Develops bilaterally in the floor of telencephalon adjacent ○ Bones of the spine do not completely form to thalami ○ Spina bifida with myeloschisis or rachischisis These areas of gray matter are sensory-motor control ○ the neural folds do not elevate but remain as a flattened centers mass of neural tissue, without a layer of skin covering the Major pathway for descending fibers from the cerebral opening of the spine cortex and ascending fibers from the thalamus Internal Capsule form on each side divide the corpus striatum into two parts: ○ dorsomedial portion (CAUDATE NUCLEUS) ○ ventrolateral portion (LENTIFORM NUCLEUS) THREE MENINGES / MEMBRANES PIA MATER ○ leptomeninges (inner meninges or tissues that cover the brain and spinal cord) ○ derived from neural crest ARACHNOID MATER ○ leptomeninges ○ derived from neural crest DURA MATER ○ pachymeninges / pachymeninx (outer meninges or tissues that cover the brain and spinal cord) ○ derived from mesenchyme surrounding the neural tube Figure 20. Presentation of neural tube defects. CLINICAL CORRELATIONS Mitotic activity is completed during prenatal development. Arnold-Chiari malformation A person is born with all neurons he was destined to have. Herniation of part of the cerebellum into the foramen magnum, Nervous tissue continues to grow and specializes even after birth. which obstructs the flow of cerebrospinal fluid and causes the hydrocephalus. Neural Tube Defects Herniation of the cerebellum occurs because the spinal cord is Most defects of the spinal cord result from abnormal closure of the tethered to the vertebral column due to its abnormal development. neural folds in the third and fourth weeks of development. As the vertebral column lengthens, tethering of the cord pulís the May involve the meninges, vertebrae, muscles, and skin. cerebellum into the foramen magnum, cutting off the flow of Page 8 of 10 [EMBRYOLOGY] 1.15 EMBRYONIC DEVELOPMENT OF THE CENTRAL NERVOUS SYSTEM – Dr. Steve Arellano cerebrospinal fluid. Hydrocephalus can be treated by inserting a caudal part of myelencephalon (rostral part is the ventriculoperitoneal shunt, which allows drainage of the metencephalon) has a central canal and forms the closed cerebro-spinal fluid from one of the cerebral ventricles to the part of medulla peritoneal cavity. rostrally the central canal expands as the cavity of the 4th ventricle 2. MESENCEPHALON (Midbrain) floor of 4th ventricle is derived from myelencephalon Most primitive of the brain vesicles (medulla) and metencephalon (pons) Generally retains a cylindrical form on either side of midline, the floor consists of the basal Its narrowed cavity forms the cerebral aqueduct, which is continuous and alar laminae, which are separated from each other by below the 4th ventricle and above the 3rd ventricle. a longitudinal sulcus called SULCUS LIMITANS During the first month of the embryo, we have the midbrain and it will not differentiate like your prosencephalon and rhombencephalon. Anterior to the cerebral aqueduct, the basal laminae give rise to TEGMENTUM and SUBSTANTIA NIGRA. Figure 23. Posterior view of developing rhombencephalon showing the role of Figure 21. The Midbrain at the embryonic stages of a month and 5 weeks. pontine flexure in the formation of fourth ventricle. CRUS CEREBRI nuclei are arranged in longitudinal columns Formed from the enlargement of the marginal layer of each basal each lamina contains two columns (somatic and visceral) lamina. but in the brainstem: Serve as pathways for nerve fibers descending from the cerebral ○ special branchial columns appears between cortex to the lower centers in pons, medulla and spinal cord. somatic and visceral columns of each lamina Cells of alar laminae invade the roof plate to form bilateral ○ special somatic column appears in the most longitudinal elevations separated by a shallow midline groove. lateral part of the alar lamina to receive Each elevation is subdivided by a transverse groove into upper and impulses of special sensations of hearing and lower parts called SUPERIOR and INFERIOR COLLICULI. balance ○ basal lamina has three columns and alar CORPORA QUADRIGEMINA lamina has four columns Corpora = bodies, quadrigemina = four, develop into the roof plate dorsal to the aqueduct of Sylvius and form the TECTUM. The Pia mater will contribute in formation of your choroid plexus These four colliculi, we have the superior and inferior colliculi. that will invaginate in the cavity of your 4th ventricle. This choroid plexus will manufacture CSF Figure 24. Developing fourth ventricle and cerebellum. (MS: median sulcus, SL: sulcus limitans) Figure 22. The corpora quadrigemina of the tectum. B. Functional Columns of Gray Matter in the Brainstem Functional Column in the Basal Lamina of Brainstem 3. RHOMBENCEPHALON (Hindbrain) ○ Somatic Efferent (SE) A. Developing 4th Ventricle ○ Special Visceral Efferent (SVE) ○ General Visceral Efferent (GVE) Page 9 of 10 [EMBRYOLOGY] 1.15 EMBRYONIC DEVELOPMENT OF THE CENTRAL NERVOUS SYSTEM – Dr. Steve Arellano Functional Columns in the Alar Lamina of Brainstem b. Corpora quadrigemina ○ General Visceral Afferent (GVA) c. Crus cerebri ○ Special Visceral Afferent (SVA) d. Lamina terminalis ○ General Somatic Afferent (GSA) 2. Which of the following does not belong to the group? ○ Special Somatic Afferent (SSA) a. Special visceral efferent all motor nuclei of the brainstem are derived from b. Special visceral afferent functional columns of its basal plate c. Somatic efferent all sensory nuclei of the brainstem are derived from d. General visceral efferent functional columns of the alar plate 3. A maldevelopment due to atresia and blockage of the apertures (foramina of Luschka and Magendie) in the roof of 4th ventricle syndrome a. Hydrocephalus b. Dandy-Walker Syndrome c. Craniorachischisis d. Spina Bifida 4. The primitive brain flexure in the middle of rhombencephalon that is convex on its ventral portion a. Cephalic flexure b. Pontine flexure c. Cervical flexure d. Caudal flexure 5. On the 16th day of embryonic life, the ectoderm thickens in the midline forming a. Neural crest Figure 24. Functional columns of gray matter in the brainstem. b. Neural placode c. Neural tube C. Roof of 4th Ventricle, Tela Choroidea, Choroid Plexus, Rhombic Lips 6. It is a clinical division of the prenatal period where it contributes to Roof of 4th Ventricle CN sensory ganglia, hypophysis & inner ear. ○ consists of a single layer of ependymal cells a. Neural crest cells covered by a vascular mesenchyme (pia mater) b. Neural Tube Tela Choroidea c. Cells of dorsal root ganglion ○ consists of pia mater along with the covering d. Ectodermal placodes layer of ependymal cells 7. This primary brain vesicle is located in the lowest parts of medial Choroid Plexus walls of hemispheres remain very thin due to disproportionate ○ sac-like invaginations consisting of tela growth of various parts of the hemispheres choroidea and tuft or bunch of capillaries a. Lateral Diverticula ○ due to active proliferation of vascular b. Lamina Terminalis mesenchyme c. Limbic System Rhombic Lips d. Luschka Foramen ○ formed from the dorsolateral parts of the alar 8. In what day of the embryonic life is where the fusion at the cranial laminae of metencephalon & caudal ends of neural tube is somewhat delayed, forming small ○ extend medially and dorsally openings called anterior & posterior neuropores ○ these meet and fuse in the midline over the a. 16th roof of the 4th ventricle and then grow b. 5th dorsally to form the CEREBELLUM c. 20th d. 13th D. Pons 9. In the development of the spinal cord, what cell closes the neural marginal layer of basal plates of metencephalon expands tube as a bridge for nerve fibers connecting cerebral cortex and a. Neuroepithelial Cells cerebellar cortex (CORTICO- PONTO CEREBELLAR b. Epididymal Cells PATHWAYS), later be known as PONS conduit or c. Pluripotent Neuroepithelial Cells passageway of descending as well as ascending impulses d. Neuroglial Cells from different fibers from the brain to the spinal cord to 10. This is a herniation of the cerebellum occurs because the spinal the cerebellum, and from the spinal cord going up the cord is tethered to the vertebral column due to its abnormal cerebellum and the brain development. a. Herniatus Cerebellatus Leviticus REFERENCES b. Arnold-Chiari malformation c. Rachischisis d. Acute Cerebellar Ataxia Sadler, T. W., et.al. (2019). Langman's medical embryology (14th ed.). Lippincott Williams & Wilkins. ANSWER KEY: Arellano, S. PPT Lecture. 1. A, 2. B, 3. B, 4. B, 5. C, 6. D, 7. A, 8. C, 9. C, 10. B TEST YOUR KNOWLEDGE 1. The floor consists of the basal and alar laminae, which are separated from each other by a longitudinal sulcus called? a. Sulcus limitans Page 10 of 10