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Home , Flexure (embryology), Otic pit

Lesson 4b DEVELOPMENT OF CNS & THE SENSE ORGANS Objectives

By the end of this lesson, you should be able to: 1. Describe the development of the ANS 2. Describe the development of the EYE 3. Describe the development of the

DEVELOPMENT OF THE • The cranial to the fourth pair of the somites develops into the brain. • During the 4th week, three primary brain vesicles are formed: the (prosencephalon), (mesencephalon) and (rhombencephalon). • Two flexures appear simultaneously: the cervical at the junction of the hindbrain and the , and cephalic flexure in the midbrain region. • During the 5th week, the forebrain divides into two vesicles, the telencephalon (future cerebral hemispheres) and diencephalon. • The hindbrain partly divides into the (future and ) and , separated by the pontine flexure. • The cavity of the rhombencephalon is known as the fourth ventricle, that of the diencephalon as the third ventricle, and those of the cerebral hemispheres as the lateral ventricles.

EMBRO – DAY 19 1. 2. Primitive node 3. Primitive streak 4. Cut edge of amnion

EMBRYO – DAY 20 1. Primitive streak 2. Cut edge of amnion 3. 4. 5. Somite 6. Primitive node

EMBRYO – DAY 23 1. Pericardial bulge 2. Anterior neuropore 3. Somite 4. Posterior neuropore

Differentiation of the

1. Neural groove 2. Neural crest 3. Neural fold 4. 5. Intermediate zone 6. Neural tube 7. Surface 8. Mesoderm FORMATION OF THE NEURAL TUBE

1. Notochord 2. Intermediate zone of neural crest 3. Neural groove 4. Neural crest 5. Neural fold 6. Dorsal root ganglion 7. Neural tube 8. Neural tube - 4th week 1. Prosencephalon 2. Mesencephalon 3. Rhombencephalon

Development of the brain - 6th week 1. Telencephalon 2. Lateral ventricle 3. Interventricular foramen of Monro 4. 3rd ventricule 5. 6. 4th ventricle 7. Future aqueduct of Sylvius 8. Central canal

Brain vesicles - 6th week 1. Diencephalon 2. Optic cup 3. Telencephalon 4. Primitive cerebral hemisphere 5. Metencephalon 6. Mesencephalon 7. Rhombencephalic isthmus 8. Myelencephalon 9. Pontine flexure

Brain vesicles (midline section) - 6th week 1. Diencephalon 2. Mesencephalon 3. Myelencephalon 4. Telencephalon 5. Metencephalon 6. Rhombencephalic isthmus 7. Roof of rhombencephalon 8. Central canal

Development of the hypophysis

1. Oral cavity 2. Infundibulum 3. Rathke's pouch 4. Notochord 5. Diencephalon Development of the hypophysis

1. Anterior lobe 2. Lumen of diencephalon 3. Pars nervosa 4. Sphenoid bone 5. Pars intermedia Hypophysis

• The hypophysis or pituitary gland develops entirely from the ectoderm derived from two sources: – ectodermal outpocketing of the , known as Rathke's pouch and a downward extension of the diencephalon, the infundibulum. • Rathke's pouch appears during the fourth week and loses its connection with the oral cavity by the end of the 8th week. • The cells of the Rathke's pouch increase rapidly in number and give rise to the adenohypophysis (the anterior lobe), pars tuberalis and pars intermedia. • The infundibulum gives rise to the stalk, and the neurohypophysis (the posterior lobe).

PERIPHERAL • The peripheral nervous system mostly develops from the neural crest cells. • The neural crest cells are groups of ectodermal cells that temporarily form an intermediate zone between the neural tube and surface ectoderm. • These cells migrate and differentiate into the sensory ganglia, sympathetic , Schwann cells, pigment cells, odontoblasts, meninges and cartilage cells of the branchial arches. • Neuroblasts of the sensory ganglia penetrate the dorsal part of the neural tube and mostly end in the dorsal horn, thus giving rise to the dorsal root neurons. • The peripherally growing processes of the neuroblasts in the sensory ganglia terminate in the sensory receptor organs. DEVELOPMENT OF THE ANS • "The autonomic system can be divided into sympathetic portion, localized in the thoracolumbar region, and parasympathetic portion, localized in the cranial and sacral regions • The postganglionic fibers of parasympathetic ganglia pass to the branchial arches, to the cardiac, pulmonary and intestinal plexus DEVELOPMENT OF SYMPATHETIC SYSTEM

• Cells originating from the neural crest of the thoracic region migrate to form bilateral sympathetic chains on each side of the vertebral column. • Some cells give rise to the preaortic ganglia and sympathetic organ plexuses. • Nerve fibers from the intermediate horn of the thoracolumbar spinal cord penetrate the ganglia and the sympathetic chains. FORMATION OF THE SYMPATHETIC GANGLIA 1. Preaortic ganglion 2.Sympathetic ganglioN 3. Organ plexus 4. Developing suprarenal gland 5. Dorsal root ganglion 6. Neural tube 7. Dorsal aorta 8. Urogenital ridge 9. Notochord 10. Gut DEVELOPMENT OF THE SENSE ORGANS - EYE The eye develops from the three sources: of the diencephalon, surface ectoderm of the head and the mesoderm between these layers.

Three tunics surround the eyeball: 1. fibrous coat (sclera and cornea), 2. vascular coat (iris, ciliary body and choroid), 3. retina (neural and pigmented).

These tunics surround three chambers: the anterior chamber (between the cornea and the iris), the posterior chamber (between the iris and the lens), and the vitreous body (behind the lens). STRUCTURE OF THE EYE 1. Retina 2. Choroid 3. Sclera 4. Iris 5. Vitreous body 6. Ciliary body 7. Hyaloid artery 8. Optic papilla 9. Optic nerve 10. Lens 11. Cornea 12. Ciliary process 13. Anterior chamber 14. Posterior chamber DEVELOPMENT OF LENS & RETINA • The eye appears during the 3rd week of development as a lateral evagination of the diencephalon, the optic groove. • It gives rise to the , which is attached to the diencephalon by an . • The optic vesicle grows laterally and comes into contact with the surface ectoderm, inducing its thickening known as the . • This placode subsequently invaginates and develops into the lens vesicle. • The lens vesicle induces the optic vesicle to become invaginated into a double layered optic cup. OPTIC & LENS VESICLES – 4TH WK

1. Forebrain 2. Optic vesicle 3. Lens placode 4. Mesenchyme 5. Surface ectoderm 6. Invaginating lens placode 7. Optic cup 9. Optic stalk DEVELOPMENT OF THE EYE CONT.. • The inner layer of the optic cup develops into the neural retina, while the outer layer becomes the pigmented retina. • The two layers are temporarily separated by the intraretinal space, which is continuous with the lumen of the brain. • Up to the 3rd month, the neural retina has two layers: a nucleated layer and a region of cell processes. • Subsequently, the posterior part of the neural retina develops into specialized neurons (rods and cones, bipolar and ganglion cells) and supporting cells forming the pars optica retinae. • The optic stalk develops into the optic nerve. • Anterior part of the inner layer, pars caeca retinae, remains as single cell layer. It gives rise to the pars ciliaris and pars iridica retinae. DEVELOPMENT OF THE RETINA, CILIARY BODY AND IRIS 1. Pars optica retinae 2. Neural layer 3. Pigment layer 4. Pars caeca retinae 5. Intraretinal space 6. Ciliary process 7. Pars ciliaris retinae 8. Sphincter pupillae 9. Pars iridica retinae 10. Mesenchyme

DEVELOPMENT OF THE EAR • The ear consists of internal, middle and external parts. The first two parts are concerned with the transference of sound waves from the exterior to the inner ear. • The inner ear concerns with the balance and hearing. Components of the ear: 1. the external ear consists of the auricle, the external auditory meatus and the tympanic membrane; 2. the middle ear components are the auditory tube and the tympanic cavity with auditory ossicles; 3. the inner ear consists of the cochlea (containing the organ of Corti) and the vestibular apparatus (containing the semicircular canals, the saccule and the utricle). 1. External auditory meatus 2. Tympanic membrane 3. Auditory tube 4. Incus 5. Stapes 6. Mastoid cavities 7. Malleus 8. Tympanic cavity 9. Semicircular canals 10. Vestibule 11. Ampullae 12. Utriculus 13. Endolymphatic sac 14. Sacculus 15. Perilymphatic space 16. Oval window 17. Round window 18. Ductus reuniens 19. Helicotrema 20. Cochlea 21. Scala tympany 22. Scala media 23. Scala vestibuli DEVELOPMENT THE INNER EAR • Early in the fourth week, a thickening of the surface ectoderm on each side of the hindbrain (rhombencephalon) forms the . • It invaginates to form the otic (auditory) vesicle, the primordium of the membranous labyrinth. • Each vesicle divides into the ventral component, which gives rise to the saccule and the cochlear duct (scala media), and the dorsal portion which forms the utricle, semicircular canals and endolymphatic duct. • The spiral organ of Corti differentiates from the cells in the wall of the cochlear duct. • The mesenchyme around the auditory vesicle differentiates into a cartilaginous otic capsula, which later ossifies to form the bony labyrinth. • Part of the cartilaginous shell undergoes vacuolization, and form the two perilymphatic spaces (scala vestibuli and scala tympani). DEVELOPMENT OF THE 1. 2. Wall of rhombencephalon 3. Pharynx 4. Mesenchyme 5. Dorsal aorta 6. Endoderm

1. Statoacoustic ganglion 2. Otic vesicle 3. Surface ectoderm 4. Wall of rhombencephalon 5. Tubotympanic recess 6. First pharyngeal cleft 7. Mesenchyme 8. Dorsal aorta DEVELOPMENT OF THE INNER EAR CONT

• Semicircular canals subsequently differentiate as evaginations of the utricular part of the vesicle. • Sensory areas developing from the walls of auditory vesicle form the crista ampullaris (in the semicircular canals) and maculae cristae (in the utricle and saccule). • Small groups of cells break away from the wall of the auditory vesicle to form the statoacoustic ganglion, which suplies the sensory cells of the organ of Corti, the crista ampullaris and the maculae acusticae. DEVELOPMENT OF THE MEMBRANOUS LABYRINTH 1. Scala vestibuli 2. Scala media 3. Scala tympany 4. Mesenchyme 5. Vestibular membrane 6. Organ of Corti 7. Spiral ligament 8. Basilar membrane 9. Tectorial membrane 10. Auditory nerve fibers 11. Spiral ganglion DEVELOPMENT OF THE MIDDLE EAR • The tubotympanic recess of the first pharyngeal pouch grows rapidly and comes into the contact with the floor of the first pharyngeal cleft. • Its distal portion widens and gives rise to the primitive tympanic cavity, while the proximal, unexpanded portion becomes the auditory tube. • The auditory ossicles (malleus, incus and stapes) develop by endochondral ossification of the cartilage of the first two pairs of the pharyngeal arches. • Later in the development, the surrounding mesenchyme dissolves. • The endodermal epithelial lining extends along the wall of the newly formed space and coats the bony ossicles. DEVELOPMENT OF THE MIDDLE EAR & INNER EAR 1. First pharyngeal cleft 2. Wall of rhombencephalon 3. Tubotympanic recess 4. Mesenchymal condensation 5. Endoderm 6. Surface ectoderm 7. Mesenchyme 8. Saccular portion 9. Utricullar portion of otic vesicle 10. Endolymphatic duct DEVELOPMENT OF THE EXTERNAL EAR • The external auditory meatus develops from the dorsal portion of the first pharyngeal cleft. • The epithelial cells at the bottom of the meatus proliferate, forming the meatal plug, which dissolves later on. • The tympanic membrane is made up of the ectodermal epithelial lining at the bottom of the auditory meatus, the endodermal epithelial lining of the tympanic cavity, and an intermediate layer of connective tissue derived from the mesenchyme. • The auricle develops from six mesenchymal proliferations of the first two pharyngeal arches, which surround the first pharyngeal cleft. • Initially, the external are located in the lower neck region, but subsequently ascend to the level of the eyes. DEVELOPMENT OF THE MIDDLE & EXTERNAL EAR 1. External auditory meatus 2. Primitive tympanic cavity 3. Meatal plug 4. Auditory tube 5. Endoderm 6. Wall of inner ear 7. Surface ectoderm 8. Mesenchyme 9. Loose mesenchyme 10. Incus 11. Stapes 12. Malleus Conclusion

• Development of the ANS • Development of the eye • Development o the ear