Development

Development of Nervous System I. Overview A. Central nervous system (CNS) CNS begins to form in the third week of embryonic development as the neural plate. The neural plate becomes the neural tube, which gives rise to the brain and spinal cord. B. Peripheral nervous system (PNS) PNS consists of spinal, cranial, and visceral nerves and spinal, cranial, and autonomic ganglia. IT is derived from three sources: 1. Neural crest cells -give rise to peripheral ganglia, Schwann cells, and afferent nerve fibers. 2. Neural tube -gives rise to all preganglionic autonomic fibers and all fibers that innervate skeletal muscles. 3. Mesoderm -gives rise to the dura mater and to connective tissue investments of peripheral nerve fibers (endoneurium, perineurium, and epineurium).

II. Development of the Neural Tube

-begins in the third week and is completed in the fourth week. A. Neural plate - is a thickened pear-shaped region of embryonic ectoderm, located between the primitive knot and the oropharyngeal membrane.

B. Neural groove – forms as the neural plate begins to fold inward. -is flanked by neural folds, which are parallel. -deepens as the neural folds begin to close over it. C. Neural folds – -fuse in the midline to form the neural tube. -are the sites of neural crest cell differentiation. D. Neural tube -forms as the neural folds fuse in the midline and separate from the surface ectoderm. -lies between the surface ectoderm and the notochord. -gives rise to the CNS: • The cranial part becomes the brain. • The caudal part becomes the spinal cord. • The cavity gives rise to the central canal of the spinal cord and ventricles of the brain. The two openings in the neural tube connect the central canal with the amniotic cavity: a. Anterior neuropore -closes in the fourth week (day 25) and becomes the lamina terminalis. b. Posterior neuropore- closes in fourth week (day 27)

III. Neural Crest (fig above) –gives rise to a. Pseudounipolar ganglion cells of the spinal and cranial nerve ganglia b. Schwann cells c. Multipolar ganglion cells of the autonomic ganglia d. Leptomeninges e. Chromaffin cells of the suprarenal medulla f. Pigment cells (melanocytes) g. Odontoblasts (dentine-forming cells) h. Aorticopulmonary septum of the heart i. Parafollicular cells (calcitonin-producing C-cells) j. Skeletal and connective components of the pharyngeal arches IV. Placodes -are localized thickenings of the cephalic surface ectoderm. -give rise to cells that migrate into the underlying mesoderm and develop into the sensory receptive organs of the olfactory nerve (CN I) and the vestibulocochlear nerve (CN VIII). A. Olfactory placodes -differentiate into neurosensory cells that give rise to the olfactory nerve -induce formation of the olfactory bulbs. B. Otic placodes -give rise to the following statoacoustic organs: 1. Organ of Corti and spiral ganglion

2. Cristae ampullares, maculae utriculi and sacculi, and vestibular ganglion 3. Vestibulocochlear nerve (CN VIII)

V. Stages of Neural Tube Development A. Vesicle development 1. The three primary brain vesicles and associated flexures

-develop during the fourth week. -give rise to dilations of the primary brain vesicles and two curvatures. a. Prosencephalon (forebrain)

-is associated with the appearance of the optic vesicles. -gives rise to: 1)Telencephalon (endbrain) 2)Diencephalon (between-brain) b. Mesencephalon (midbrain) -remains as the mesencephalon. c. Rhombencephalon (hindbrain) -gives rise to: 1) Metencephalon (afterbrain) -forms the pons and cerebellum. 2) Myelencephalon (medulla oblongata) d. Cephalic flexure (midbrain flexure) -is located between the prosencephalon and the rhombencephalon. e. Cervical flexure -is located between the rhombencephalon and the future spinal cord.

2. The five secondary brain vesicles (with four ventricles) -become visible in the sixth week; the brain vesicles are visible as the primordia of the five major brain divisions.

a. Telencephalon -has lateral outpocketings that form the cerebral hemispheres. -has ventral outpocketings that form the olfactory bulbs. -has visible lateral ventricles. b. Diencephalon -The third ventricle, optic chiasm and optic nerves, infundibulum, and mamillary eminences become visible. c. Mesencephalon --contains a large cavity that will become the cerebral aqueduct. d. Metencephalon -is separated from the mesencephalon by the rhombencephalic isthmus. -is separated from the myelencephalon by the pontine flexure. -contains rhombic lips on the dorsal surface, which give rise to the cerebellum. -becomes the pons and the cerebellum. -contains the rostral half of the fourth ventricle. e. Myelencephalon (medulla oblongata) -lies between the pontine and cervical flexures. -becomes the medulla. -contains the caudal half of the fourth ventricle.