[NEURO]LEC_003_DEVELOPMENT-OF-THE-NERVOUS-SYSTEM (4).pdf

(003) DEVELOPMENT OF THE NERVOUS SYSTEM DR. J. VIADO | 09/30/2020 OUTLINE II. CARNEGIE STAGES I. INTRODUCTION II. CARNEGIE STAGES III. DEVELOPMENTAL STAG...

(003) DEVELOPMENT OF THE NERVOUS SYSTEM DR. J. VIADO | 09/30/2020 OUTLINE II. CARNEGIE STAGES I. INTRODUCTION II. CARNEGIE STAGES III. DEVELOPMENTAL STAGES OF THE NERVOUS SYSTEM A. Embryogenesis B. Gastrulation C. Neurulation IV. FORMATION OF CENTRAL NERVOUS SYSTEM A. Primary and Secondary Vesicles B. Formation of Flexures V. NEURAL DEVELOPMENT. C. Six Cellular Processes I. INTRODUCTION Development of the Nervous System – mechanisms by which CNS and PNS develops. > Brain is the first part of the body to develop. Any defect in the CNS will result to irreversible damage. > Brain cells/ neurons does not regenerate. > Week 3 post conception: development of CNS Gestational Age - Commonly used in clinical practice, beginning with the first day of the last menstrual period (Preterm, term, post-term) - Usually, the number of menstrual or gestational weeks exceeds the number of post fertilization weeks by 2. - Neural tube: anterior and posterior - Age(days) 23-25: Closure of anterior tube usually happens. Anterior gives rise to the brain. A maldevelopment results to anencephaly. PREPARED AND EDITED BY: TRANS GROUP 6  (003) DEVELOPMENT OF THE NERVOUS SYSTEM DR. J. VIADO | 09/30/2020 - Age(days) 25-28: If the mother got sick with viral infection or took some abortifacient, the posterior neuropore closure will be affected resulting to meningocele, tethered cord syndrome, etc. - Congenital malformation may be explained through the development of the nervous system (ex. Nasoethmoidal meningocele- maybe casued by the mother having viral infection or taking medication during pregancy) III. DEVELOPMENTAL STAGES OF THE NERVOUS SYSTEM A. Embryogenesis Sperm cell and egg cell meet forms Totipotent cells, Blastomers, ZYGOTE Multiple Cell divisions without growing through a process called CLEAVAGE forms a MORULA formation of BLASTOCYST EMBRYOBLAST AND TROPHOBLAST Embryoblast HYPOBLAST AND EPIBLAST Hypoblast forms ENDODERM Epiblast forms ECTODERM – where Nervous system is derived Trophoblast CYTOTROPHOBLAST, SYNCYTIOTROPHOBLAST Week 01 of fertilization: During the trophoblast and embryoblast time, it will attach to endometrium. EMBRYOGENESIS PROCESS 1. The process of embryogenesis begins with an egg or ovum being FERTILIZED by a sperm cell to form a ZYGOTE. The zygote is surrounded by a strong membrane made up of glycoproteins, which the sperm has managed to penetrate. The zygote is formed 24 hours after the egg and sperm nuclei B. GASTRULATION fuse. The zygote is a single diploid cell. Process whereby the bilaminar embryonic disc 2. The zygote undergoes a number of cell undergoes reorganization to form a trilaminar disc. divisions, a process referred to as CLEAVAGE. Once the embryo has The first cells to invaginate through the primitive reached the 8-cell stage, it undergoes groove form the definitive ENDODERM. another process called -the inner most layer which gives rise to the COMPACTATION, which involves the gastrointestinal tract, lungs, and the liver - tight binding of the cells to create a ENDODERM compact sphere. After compactation, the The remaining cells of the epiblast are called the embryo is made up of 16 cells, which is ECTODERM. referred to as the MORULA. -the outermost layer, formed of columnar 3. A cavity referred to as the blastocele epithelium which gives rise to the entire nervous develops in the morula. Cells inside the system blastocele compact and flatten to form a Cells that remain in the space between the structure referred to as the blastocyst. ectoderm and definitive endoderm form a layer This process is called BLASTULATION. called the MESODERM 4. The blastocyst moves towards the womb, -the middle layer which gives rise to the muscle, where it implants in the lining. Blastocyst connective tissues and the vascular system will then give rise to Embryoblasts (inner cell mass) and Trophoblasts GASTRULATION PROCESS (outer layer). 1. By the end of the SECOND WEEK of 5. The mass of cells rapidly divides, giving development, the bilaminar embryonic disc, rise to a disc-shaped structure that has consisting of the hypoblast and epiblast are two layers. One of the layers goes on to formed. become the EMBRYO AND THE 2. Throughout the THIRD WEEK of AMNIOTIC CAVITY, while the other development, the bilaminar disc layer develops into the YOLK SAC. differentiates to establish three primary germ layers in a process known as GASTRULATION. PREPARED AND EDITED BY: TRANS GROUP 6  (003) DEVELOPMENT OF THE NERVOUS SYSTEM DR. J. VIADO | 09/30/2020 3. Around 15 days after fertilization, a C. NEURULATION thickened structure along the midline in the Marks the beginning of the formation of the epiblast, near the caudal end of the bilaminar central nervous system and is the process embryonic disc. This is called the PRIMITIVE whereby the neural plate forms into a neural tube. STREAK, including the cranial end (towards -Fusion starts at about the midpoint along the the head) and caudal ends (towards the tail), groove and extends cranially and caudally so that as well as the left and right sides of the in the earliest stage, the cavity of the tube embryo. remains in communication with the amniotic cavity 4. At the cranial end of the embryonic disc, the through the Anterior and Posterior Neuropores. primitive streak expands to create a -The anterior neuropore closes first and 2 days PRIMITIVE NODE, which contains a circular after, the posterior neuropore closes. Thus neural depression, known as the PRIMITIVE PIT. tube closure is complete within 28 days. 5. This depression continues along the midline Neural tube – becomes Central Nervous of the epiblast, towards the caudal end of the System, which consists of the brain and spinal streak, forming a PRIMITIVE GROOVE. cord. 6. Once formed, cells of the epiblast migrate The cavity of the tube (neural cavity) – inwards towards the streak, detach from the becomes the ventricles of the brain and central epiblast, and slip beneath it, into the canal of the spinal cord. INTERIOR OF THE EMBRYO. This process Neural crest cells is known as INVAGINATION. o becomes the neurons of the 7. The first cells to invaginate through the Peripheral Nervous System that primitive groove invade the HYPOBLAST, have their cell bodies located in the and displace its cells. ganglia. 8. The hypoblast cells are eventually completely o Becomes neurolemmocytes replaced by a new proximal layer, which is (Schwann cells) of the PNS referred to as the DEFINITIVE ENDODERM. o Becomes adrenal medulla cells, 9. By day 16: The majority of the hypoblast has melanocytes of skin, and a variety been replaced. The remaining cells of the of structures in the face. epiblast are now referred to as the ectoderm, NEURULATION PROCESS and forms the MOST EXTERIOR, DISTAL Where the CNS and PNS comes from LAYER. Week 3: Division of ectoderm, mesoderm 10. Some of the invaginated epiblast cells remain and endoderm. in the space between the ectoderm and newly formed definitive endoderm. These cells form a germ layer, known as the MESODERM. 11. Once formation of the definitive endoderm and mesoderm is complete, epiblast cells no longer migrate towards the primitive streak. 12. Throughout gastrulation, the ECTODERM continues to form the cranial to the caudal end of the embryo, establishing THREE DISTINCT PRIMARY GERM LAYERS throughout the whole embryonic disc, completing the gastrulation process. The Nervous system comes from the germ layer Week 04: Development of neural ECTODERM. groove from the ectoderm 1. Neuroectodermal tissues differentiate from the ectoderm and thicken into the neural plate. The neural plate border separates the ectoderm from the neural plate. 2. The neural plate bends dorsally, with the two ends eventually joining at the neural plate borders, which are now referred to as the neural crest. 3. The closure of the neural tube disconnects the neural crest from the epidermis. Neural crest cells differentiate to form most of the peripheral nervous system. PREPARED AND EDITED BY: TRANS GROUP 6  (003) DEVELOPMENT OF THE NERVOUS SYSTEM DR. J. VIADO | 09/30/2020 4. The notochord degenerates and only persists as the nucleus pulposus of the intervertebral discs. Other mesoderm cells differentiate into the somites, the precursors of the axial and skeletal muscles. (Due to some action of the notochord, the neural plate will become the neural groove, it will close then become the neural tube.) In summary, neurulation occurs during the third week of embryonic development. The transformation of the neural plate into a neural tube marks the beginning of the formation of the Central Nervous System. A neural plate of thickened cells forms in a cranial to caudal direction. The lateral edges of the neural plate elevate and fuse to form the neural tube Neural crest cells contribute to the formation of the Peripheral Nervous System. Neural tube formation NEURULATION PROCESS Primary Neurulation- neural tube form from neural plate (brain and spinal cord lumbar level) - the proliferation of cells at the cephalic end of the neural (neural plate converges to form the nerual tube and neural crest from tube causes it to dilate and form Three Primary Brain the ectoderm) Vesicles: the Forebrain vesicle (will become the PROSENCEPHALON), the Midbrain vesicle (MESENCEPHALON) and the Hindbrain vesicle (RHOMBOENCEPHALON) -by the 5th week, the forebrain and the hindbrain vesicles divide into two secondary vesicles. The forebrain vesicle forms 1. The TELENCEPHALON with its primitive cerebral hemispheres, and 2. DIENCEPHALON which develops optic vesicles. -the hindbrain vesicles form the 1. METENCEPHALON, the future pons and cerebellum, and the 2. MYELENCEPHALON or the medulla oblongata. -the rest of the tube elongates and remains smaller in diameter, and will form the SPINAL CORD PREPARED AND EDITED BY: TRANS GROUP 6  (003) DEVELOPMENT OF THE NERVOUS SYSTEM DR. J. VIADO | 09/30/2020 Secondary Neurulation- most caudal portion of neural tube (sacral and coccygeal level of the cord) IV. FORMATION OF THE CENTRAL NERVOUS SYSTEM Formation of Neural Tube Initial Development of the Nervous System Induction of the Neural Plate PREPARED AND EDITED BY: TRANS GROUP 6  (003) DEVELOPMENT OF THE NERVOUS SYSTEM DR. J. VIADO | 09/30/2020 Neural Tube formation in relation to Embryo Growth Secondary Neurulation Neural Crest Formation NEURULATION TIMELINE Day 17-18 Neural Plate induction Day 19-20 Neural Plate elongation Day 20-21 Neural folds; neural groove Day 21-22 Neural fold begin to fuse Day 22 Midbrain flexure Day 25 Closure of cranial neuropore Day 27 Closure of caudal neuropore Week 4-8 Secondary neurulation PREPARED AND EDITED BY: TRANS GROUP 6  (003) DEVELOPMENT OF THE NERVOUS SYSTEM DR. J. VIADO | 09/30/2020 A. PRIMARY AND SECONDARY VESICLES V. NEURAL DEVELOPMENT B. FORMATION OF FLEXURES SIX CELLULAR PROCESS/ NEURAL DEVELOPMENT 1. Neurogenesis 2. Cell migration 3. Cell differentiation 4. Synaptogenesis 5. Neuronal cell death (apoptosis) 6. Synaptic rearrangement Four stages of Cell Division that must occur in order to continue neurogenesis: G1 Phase: cells move from the ventricular zone to the Midbrain flexure occurs at the level of the midbrain marginal zone of the neural tube Cervical flexure appears at the junction between the brain and S Phase: cells which are genetically programmed to divide spinal cord return to the ventricular zone Pontine flexure is concave dorsally G2 Phase: cells copy their DNA to prepare for mitosis (cell division) M Phase: mitosis PREPARED AND EDITED BY: TRANS GROUP 6  (003) DEVELOPMENT OF THE NERVOUS SYSTEM DR. J. VIADO | 09/30/2020 Stages of Neuronal Development PREPARED AND EDITED BY: TRANS GROUP 6