NEURULATION NEURAL PLATE NEURAL GROOVE NEURAL TUBE NEUROHISTOGENESIS Formation of Neurons and Glial Cells from Neuroepithelium Layers and Plates of the Neural Tube FORMATION OF CNS Spinal Cord Development Development of Hindbrain (Medulla Oblogata) Development of Hindbrain (Cerebellum) Development of Midbrain Development of Forebrain Diencephalon Telencephalon (Brain) DEVELOPMENT OF PNS Neurolemmocytes (Schwan cells) Afferent Neurons Poıstganglionic Visceral Efferents Neurons Somatic Efferent Neurons and Preganglionic Visceral Efferent Neurons FORMATION OF MENINGES AND VENTRICULER SYSTEM CONTENT
NEURULATION • The notochord induces overlaying ectoderm. • This ectoderm becomesneuroectodermand form a neuraltube. • The following stages of neural tube formation are evident: • Neural plate • Neural groove • Neural tube • Neural Crest forming
DERIVATE of NEURAL TUBE and NEURAL CREST CELLS • Neural tube becomes central nervous system (CNS). • The cavity of the tube (neural cavity) becomesthe ventricular system of the brain and the central canal of the spinal cord. • Neural crest cells become those neurons of peripheral nervous system(PNS) that have their cell bodies located in ganglia. • They also becomeneurolemmocytes (Schwann cells) of the PNS. • Additionally, neural crest cellsbecome adrenal medulla cells, melanocytes of skin and a variety of structures in the face.
Neuroepithelium gives rise to neurons, glialcells (astrocytesand oligodendrocyte), and ependymal cells. Neuroepithelial cells have processes which contact the inner and outersurfaces of the neural tube. NEUROHISTOGENESIS
NEURAL TUBE LAYERS • Ectodermal cells of the early tube develop 3 concentric zones, • 1) germinal (or matrix); Cells near the central canal are called the germinal layer. • Germinal or an internal, columnarependymal layer becomes the ependymal liningandepithelium of choroid plexus, • 2) mantle layer; a middle, denselypacked layer of mantle cells becomes the gray matter of the CNS, • 3) marginal layer; an external marginal layer composed mainly of the processes of cells of the mantle layers becomes the white matter of the CNS.
NEURAL CREST DERIVATIVES • LEGEND: • A:Neural Crest • 1a; Bipolar Neuroblast • 1b; Bipolar Neuroblast (Differentiation) • 1c; Unipolar Spinal Ggl. Neuron • 2a; Unipolar Neuroblast • 2b; Multipolar Neuron (Sympathic Ggl.) • 2c; Medulloblast (Chromaffin Cells) • 3a; Glioblast • 3b; Schwann cell • 3c; Satellite Cell • 4a; Mesenchyme cell • 4b; Leptomeninx cell (Arachnoid & Pia) • 4c; Ectomesenchyme cell • 5a; Melanoblast • 5b; Melanocyte • B: Mantle Zone • C: Marginal Zone • D: Germinal Zone
NEUROHISTOGENESIS; MANTLE LAYER DERIVATIVES • LEGEND: • A:Neural Crest • B: Mantle Zone • 7a; Apolar Neuroblast • 7b; Bipolar Neuroblast • 7c; Unipolar Neuroblast • 7d; Mature Neuron • 8a; Glioblast • 8b Protoplasmic Astrocyte • 8c;Fibrillar Astrocyte • 8d; Oligodendrocyte • C: Marginal Zone • D: Germinal Zone
Additionally, the CNS containsblood vessels and microglial cells derived from mesoderm. NEUROHISTOGENESIS: MARGINAL LAYER DERIVATIVES • LEGEND: • A: Neural Crest • B: Mantle Zone • C:Marginal Zone • 9a Mesenchyme cell • 9b Microglia • D: Germinal Zone
NEUROHISTOGENESIS: GERMINAL LAYER DERIVATIVES • LEGEND: • A:Neural Crest • B: Mantle Zone • C: Marginal Zone • D:Germinal Zone • 10; Ependymocyte • 11; Choroid Plexus Epithelium • 12; Pinealocyte • 13; Pituicyte
Neurons develop from neuroblasts of the neuroepithelium and migrate into the mantle layer. The neuroblast changes into a bipolar cell. Bipolar cell has a primitiveaxon and dendrite. The single dendrite degenerates and is replaced by multiple dendrites forming a multipolar neuroblast. Neurons that fail to make viable contacts are destined to degenerate. Axons have few branches and their axonal growth cones are directed to their targets by tropic factors. NEURON FORMATION
NEURAL TUBE HISTOGENESIS • During week 4, the neural groove closes to form a neural tube beginning in the region of the 4th - 6th somites; • Fusion of neural folds proceeds cranially and caudally forming the brain and spinal cord respectively. • Cells of the original single-layeredtube divide to form a pseudostratifiedneuroepithelium whose cells extend from the neural canal to the tube’s external surface. • They divide rapidly, thickening the walls of the tube and eventually producingneuroblast and glioblast (spongioblast).
PATTERN FORMATION OF NEURAL TUBE • Subdivision of the neural tube are specified through pattern formation which take place in TWO directions: • Dorsoventral;generates Longitudinal Areas: • ALAR ( Roof) PLATES; Sensory, • BASAL (Floor) PLATES; Motor • Rostrocaudal; generatesTransverse Zones: NEUROMERES.
Longitidunal groove of midline regions called the sulcus limitans Sulcus limitans separatesthe developing gray matter into a dorsal (alar-roof)plate and a ventral (basal-floor) plate. MEDIOLATERAL PATTERN FOMATION of NEURAL TUBE
The basal plate contains efferent neuronsthat sendoutputaxons into the PNS. The alar plate contains neurons that receiveinput from the PNS. MEDIOLATERAL PATTERN FOMATION of NEURAL TUBE
Neuromeresare segmentally arranged transverse bulges along the neural tube, particularly evident in the hindbrain. Each neuromere has alar (dorsal) and basal (ventral) components. Neuromeresgradually fade after day 32 (5 Week; stage 15). ROSROCAUDAL PATTERN FOMATION of NEURAL TUBE
Six (6)primary neuromeresappear already at stage 9(18-20 day, week III) when the neural folds are not fused: Prosencephalon (T, D1-D2), Mesomere Four rhombomere (A–D). PRIMARY NEUROMERES Procencephalon LEGEND: T: Telencephalic Neuromere D1-D2: Diencephalic Neuromere M: Mesomere Rh A-D: Rhombomere nch: Notochord mclo:Cloacal Membrane CE: Caudal Eminence
Sixteen (16) secondary neuromerescan be recognized from about four week. Six Prosomere (P1-P6) of forebrain Twomesomeres (M1, M2) of the midbrain, Anisthmicneuromere (I), Eightrhombomeres (Rh1–Rh8). SECONDARY NEUROMERES
PROSOMERES • P6: (T) Telencephalicneuromere; Brain Cortex, Medial and Lateral Ganglionic Eminece • P5: Optic Vesicle • P4 (D1) Diencephalic neuromere; medial ganglionic eminence, hypothalamus various part • P3: (D2-P3):The parencephalonrostralis (prospectiveprethalamus) • P2: (D2-P2): The parencephaloncaudalis(prospective thalamus). • P1: (D2-P1): The synencephalon (prospectivepretectum).
Even on day19; The neural tube becomesbent by two flexures: (1) the mesencephalicflexure at the midbrain level, already evident beforefusion of the neural folds; (2) the cervical flexure, situatedat the junction between the rhombencephalonand the spinal cord, EARLY DEVELOPMENT OF BRAIN (CNS)
The three main divisions of the braincan already be recognized when the neuraltube is not yet closed. These three part: Prosencephalon (Forebrain) Mesencephalon (Midbrain) Rhombencephalon (Hindbrain) EARLY DEVELOPMENT OF BRAIN (CNS)
During the FIFTH WEEK (DAY 32 & Stage 15); Reverse, dorsal flexion (pontine flexure) occurs: Begins at the location of the developing pons Mesencephalon enlarges, The prosencephalon rotates ventrally and then posteriorly around this turning point (hinge) during the fourth and fifth weeks until it is folded back under the mesencephalon. The prosencephalon and rhombencephalon each subdivide into two portion. Converting the three primary brain vesicles into five secondary brain vesicles. EARLY DEVELOPMENT OF BRAIN (CNS)
EARLY DEVELOPMENT OF BRAIN (CNS) • Future cerebral hemispherescan be recognizedby day 32. • The forebrain soon divides into an end portion, the telencephalon, and the diencephalon that can be identified because it gives rise to the optic vesicles. • The hindbrain into a rostral (cranial) part develop into the metencephalon, • Metencephalon becomes the cerebellum, the pons and the trigeminal nerve • The hindbrain into a caudal part becomes the medulla oblongata or myelencephalon. • The junction between the hindbrain and midbrain is relatively narrow and is known as the isthmus rhombencephali.