Development of the Nervous System PDF

Development of the Nervous System Dr Jittima Muensoongnoen  Learning Outcomes By the end of the session (and recommended reading) you should be able to: Describe the neurulation (process of the neural tube formation) Describe the develo...

Development of the Nervous System Dr Jittima Muensoongnoen  Learning Outcomes By the end of the session (and recommended reading) you should be able to: Describe the neurulation (process of the neural tube formation) Describe the development of the neural canal into the ventricular system List and describe some neural tube defects e.g. exencephaly, anencephaly, microcephaly and spinal bifida Describe the development of the neural crest and list its derivatives List and describe the derivatives of the ectoderm Explain the early development of the brain vesicles and list what brain region they will develop into. Explain the early development of spinal cord and be able to relate this to the development of other parts of the CNS Explain the development of neurones and glial cells  Germ layers Week 3: Trilaminar embryonic disc consists of 3 germ layers: Ectoderm Mesoderm Endoderm Head end Tail end  Germ layer derivatives Ectoderm Mesoderm Endoderm  Notochord - Support the embryo: longitudinal support for the body. - stimulates the conversion of overlying surface ectoderm into neuroectoderm. Head end notochord Tail end  Early development of the nervous system Head end Day 17 after fertilization notochord Neural groove Neural fold Neurulation = Process of the neural tube formation. Day 16 Overlying ectoderm induced by notochord to from Tail end neural plate. The ends of the neural plate are known as the neural folds. Neural groove. Fusion of the neural folds to form the neural tube. Neural canal Neural canal is a cavity inside the neural tube. Surface ectoderm Neural tube separated from the surface ectoderm.  Neurulation Fusion proceeds bidirectionally. Cranial and caudal ends left open – neuropores. Day 19 Day 20 Day 22 Day 23 Langman’s Medical Embryology  Neurulation Neurulation starts - day 17 Complete at the end of week 4 – closure of the posterior neuropore. Anterior neuropore - closes on day 24. Posterior neuropore - closes on day 26. Cephalic part of the neural tube – brain Caudal part of the neural tube – spinal cord Langman’s Medical Embryology Day 23  Neurulation Cephalic part of the neural tube – brain Caudal part of the neural tube – spinal cord Neural canal – ventricular system and central canal  Neural Tube Defects Exencephaly (Failure of closure of the anterior neuropore - brain develops outside the skull and the cranial vault is absent or poorly formed --- a precursor to anencephaly (absence of brain). Microcephaly (Gr. Mikros = small) – a small brain in a small cranium. www.cdec.gov Microcephaly Anencephaly Cardiff School of Biosciences  Neural Tube Defects Failure of neural tube closure Spina Bifida - incomplete closing of vertebrae. - spinal cord, nerve roots and meninges may protrude outside the vertebral canal - common in the lumbosacral region www.cdc.gov  Neural crest cells The neural crest cells – located at the border of the neural plate. The closure of the neural tube disconnects the neural crest from the surface ectoderm. The neural crest are now located between the surface ectoderm and notochord neural tube. Migrate to numerous locations and give rise to many different structures. https://www.semanticscholar.org/paper/Insights-into-neural-crest-development-and-from- Simoes-Costa-Bronner/2dde52b10dbf99cb53bbf43631e6e513568622d4/figure/0 Neural crest cells Neural tube  Derivatives of the ectoderm Neuroectoderm: - Neural tube – CNS (brain & spinal cord) and MORE - Neural crest – bones of the face and skull, ganglion of cranial nerves, dorsal root ganglion, Day 16 autonomic ganglion, meninges (pia & arachnoid), Schwann cells (produce myelin sheath in PNS), satellite cells (glial cells found in the ganglia of PNS) and MORE Surface ectoderm: - epidermis (of skin), hair, nail, enamel - anterior lobe of the pituitary gland and More derived from neural crest paraxial mesoderm  Neural tube landmarks Early development of the brain Three primary brain vesicles: 1. Prosencephalon (or forebrain) 2. Mesencephalon (or midbrain) 3. Rhombencephalon (or hindbrain) Five secondary brain vesicles 4. Telencephalon – cerebral cortex and basal ganglia 5. Diencephalon –thalamus, hypothalamus, pituitary gland, pineal gland, optic stalk (CN II), optic cup (retina) 6. Mesencephalon – midbrain 7. Metencephalon – pons & cerebellum 8. Myelencephalon – medulla oblongata  Cranial nerves 12 pairs of cranial nerves: Most arise from the brainstem 5.5 weeks 7 weeks  Optic nerve (CN II) 4th week - evagination from the forebrain form – (hollow) optic vesicles. Distal end of the optic vesicle - optic cups (neuroectoderm): retina etc. Proximal end of the optic vesicle - optic stalk (neuroectoderm): optic nerve (CN II) is considered extension of the forebrain. Day 28 Development of the spinal cord Neuroembryology | Neupsy Key During the neural groove stage and immediately after closure of the neural tube - Neuroepithelial cells divides rapidly – neuroepithelial layers or neuroepithelium Once the tube closes – neuroepithelial cells begin to give rise the primitive nerve cells or neuroblasts. Neuroblasts form the mantle zone. The mantle zone later form the grey matter. Outermost layer – marginal layer contains nerve fibres emerging from neuroblasts in the mantle layer Marginal layer – white matter  Development of the spinal cord Marginal layer Mantle layer The wall of the neural tube is divided into: Neuroepithelial layer 1. Neuroepithelial layer 2. Mantle layer – consists of neuroblasts and glial cells -- this layer becomes the grey matter 3. Marginal layer – becomes the white matter White matter Grey matter horn horn horn Spinal cord  Development of the spinal cord Marginal layer As a result of continuous addition of neuroblasts to the Mantle layer mantle layer: Neuroepthelial layer Ventral thickening is known as the basal plates – contain ventral motor horn cells (form the motor areas of the spinal cord) roof plate Dorsal thickening is known as the alar plates – form the sensory areas of the spinal cord. Sulcus limitans (a longitudinal groove) – marks the boundary between the basal and alar plates Ventral and dorsal midline portions of the neural tube are known as floor plate the floor plates and roof plates, respectively. Floor plates and roof plates: no neuroblasts - pathways for nerve fibres crossing from one side to the other. A group of neurones found between the ventral motor horn and dorsal sensory horn – forms the intermediate horn (lateral horn) – contains neurones of sympathetic portion of ANS. Present only at T1-T12 Spinal cord and upper lumbar (L2 or L3) Histological Differentiation Nerve cells: Neuroblasts (primitive nerve cells) arise exclusively by division of the neuroepithelial cells. When neuroblasts migrate into the mantle layer – round and apolar – apolar neuroblasts Further differentiation – 2 new cytoplasmic processes appear on the opposite sides of the cell body – forming bipolar neuroblasts. Multipolar neuroblasts - the process at one end elongates to form the primitive axon - the process at the other end shows a number of cytoplasmic arborisation – primitive dendrites.  Histological Differentiation: Nerve cells Multipolar neuroblasts Adult nerve cells dorsal root (neurones) Axons of neurones in the basal plate break through the marginal layer and become visible on the ventral aspect of the spinal cord – known as the ventral roots. They conduct impulses from the spinal cord to the muscles. Axons of neurones in alar plate penetrate ventral root into the marginal layer – ascend to either White matter higher or lower levels to form association neurones (interneurons) Grey matter Dorsal root ganglion  Histological Differentiation: Glial cells Majority of primitive supporting cells (glial blasts) are formed by neuroepithelial cells after the production of neuroblasts cease. Glial blasts migrate from the neuroepithelial layers to mantle and marginal layers. In the mantle layer – glial blasts differentiate into protoplasmic astrocytes and fibrillar astrocytes – found between blood vessels and neurones where they provide support and serve metabolic functions. Oligodendroglial cells– found primarily in the marginal layer. Forms myelin sheath. Microglail cells appear in CNS in the 2nd half of development: phagocytic cell type. Derived from vascular mesenchyme when blood vessels grow into the nervous system Adult nerve cells (neurones)  Histological Differentiation: Glial cells When neuroepithelial cells cease to produce neuroblasts and glial blasts – they differentiate into ependymal cells lining the central canal of spinal cord (and ventricles) Central canal  Myelination oligodendrocytes Schwann cells: Originate from neural crests Myelinate PNS Each Schwann cell myelinates only a single axon oligodendrocytes Oligodendrocytes: Originate from oligodendroglial cells Myelinate CNS Each cell can myelinate up to 50 axons  Brain and brainstem Brainstem – direct continuation of the spinal cord and may have similar organisation. The higher centres reflect almost none of this basic pattern – show accentuation of the alar plate and regression of the basal plate.  Medulla Opened portion of medulla Development of spinal cord Sulcus limitans Sulcus limitans Medulla differs from the spinal cord - its lateral walls are everted. Alar and basal plates are separated by the sulcus limitans. Basal plates (similar to the spinal cord) contains motor nuclei Alar plate (similar to the spinal cord) contains sensory relay nuclei Questions? www.pinterest.com